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The Western Pacific jet stream (WPJS) is an essential part of atmospheric circulation in winter, which significantly influences the weather and climate of the North Pacific and North America. In this paper, the characteristics and mechanism of WPJS subseasonal variation in winter are investigated. The upstream atmospheric disturbances in the East Asian polar‐front jet and subtropical jet merge over the Northwestern Pacific to form the subseasonal variability in WPJS, which has a significant period of 40–60 days. During the positive phase events of subseasonal WPJS, the convergence position of the upstream atmospheric disturbances shifts southwardly accompanied with the local enhancement and eastward extension of subseasonal WPJS. On the other hand, the subseasonal WPJS divides into the southern and northern westerly branches during the negative phase events. By the horizontal propagation of local Eliassen‐Palm fluxes in the upper atmosphere, the northward drift of the upstream atmospheric disturbances convergence dominates the delayed acceleration of the northern upper westerly branch. However, the intensification of atmospheric baroclinicity and upward baroclinic energy caused by the leading strong subtropical frontal zone determine the acceleration of the southern upper westerly branch. Plain Language Summary: As an important part of the atmospheric circulation in the Northern Hemisphere, the Western Pacific jet stream (WPJS) has significant impacts on the climate and weather of the North Pacific and North America. However, previous studies more focused on the interannual or synoptic variability of WPJS. The characteristics and mechanism of the subseasonal variability of WPJS have not been appropriately explained, which are very important for the persistent disastrous weather. In this paper, the subseasonal variability characteristics and mechanism of WPJS in winter are investigated by using the daily average reanalysis data from year 1979 to 2022. About 130 WPJS persistent positive and negative phase events are detected, each of which can sustain more than 20 days. Further results show that the subseasonal variability of WPJS is determined by the synergistic effects of upstream atmospheric disturbances and subtropical frontal zone (STFZ). With the southward (northward) convergence of the strong upstream atmospheric disturbances and weaker (stronger) STFZ, the lagging positive (negative) subseasonal phase events of WPJS is more likely to occur and persist. This study is beneficial to the weather and climate forecasts of North Pacific and North America. Key Points: About 130 subseasonal events of winter Western Pacific jet stream (WPJS) are detected from year 1979–2022, each of which persists more than 20 daysWPJS shows local enhancement and extension during the positive subseasonal phase, but meridional separation in the negative oneThe synergistic effects of the upstream atmospheric disturbances and oceanic subtropical front on the subseasonal WPJS are revealed [ABSTRACT FROM AUTHOR]

Black carbon (BC) absorbs solar radiation, increasing the buoyancy and vertical ascent of absorbing aerosol in the atmosphere. This self‐lofting process has been observed for individual plumes in the troposphere and lower stratosphere but here we show it occurring at broader scales through enhanced large‐scale ascent over BC‐rich regions. This is demonstrated in a pair of simulation using the UKESM1 Earth‐System model where BC aerosols were modeled either with or without the ability to absorb radiation. With absorption included the annual global mean concentration of BC in the upper troposphere and lower stratosphere (8–22 km) rose by up to 50% and the column loading over some remote oceanic regions more than doubled. The increase in aerosol height was particularly notable over the southeast Atlantic where biomass burning aerosol from Africa was elevated up to 1 km higher when their absorption was included. Similar effects were seen over the Arctic where the absorbing haze was transported in at higher levels and surface concentrations were halved. The absorption by BC also increased ascent over southern Asia, which tended to thicken the Asian brown cloud during the dry season but in the wet season enhancing ascent promoted deep convection and had the tendency to deplete the aerosol through wash‐out. We conclude that representing aerosol absorption accurately is important in simulating the vertical distribution, transport and abundance of aerosol in the Earth‐system that will affect their interactions with climate. Plain Language Summary: Black carbon is an important component of atmospheric aerosols as it absorbs solar radiation thereby heating the atmosphere and potentially warming climate. The localized heating can also affect clouds and atmospheric motions making the regional and climate effects more complex and uncertain. The vertical distribution and geographic spread of the aerosol is also key to how it interacts with the climate system. In this paper we highlight the fact that the absorption taking place in BC aerosols can affect its vertical ascent in the atmosphere and therefore how it becomes distributed across the globe. This self‐lofting process was demonstrated in a climate model and was particularly important in aiding the elevation and downwind transport of absorbing smoke layers from regions such as central Africa, as well as increasing the amount of BC reaching the upper troposphere and lower stratosphere. Key Points: Absorption of solar radiation helps to raise the altitude of black carbon aerosol by increasing buoyancy and vertical ascentThis self‐lofting mechanism has been captured in a climate model where radiative heating affects the large‐scale ascent and circulationSelf‐lofting increased the altitude and long‐range transport of black carbon to remote regions and the upper troposphere [ABSTRACT FROM AUTHOR]

As a result of their important role in weather and the global hydrological cycle, understanding atmospheric rivers' (ARs) connection to synoptic‐scale climate patterns and atmospheric dynamics has become increasingly important. In addition to case studies of two extreme AR events, we produce a December climatology of the three‐dimensional structure of water vapor and O3 (ozone) distributions associated with ARs in the northeastern Pacific from 2004 to 2014 using MERRA‐2 reanalysis products. Results show that positive O3 anomalies reside in dry intrusions of stratospheric air due to stratosphere‐to‐troposphere transport (STT) behind the intense water vapor transport of the AR. In composites, we find increased excesses of O3 concentration, as well as in the total O3 flux within the dry intrusions, with increased AR strength. We find that STT O3 flux associated with ARs over the NE Pacific accounts for up to 13% of total Northern Hemisphere STT O3 flux in December, and extrapolation indicates that AR‐associated dry intrusions may account for as much as 32% of total NH STT O3 flux. This study quantifies STT of O3 in connection with ARs for the first time and improves estimates of tropospheric ozone concentration due to STT in the identification of this correlation. In light of predictions that ARs will become more intense and/or frequent with climate change, quantifying AR‐related STT O3 flux is especially valuable for future radiative forcing calculations. Plain Language Summary: Long filaments of rapidly moving water vapor in the atmosphere, known as atmospheric rivers (ARs), play a vital role in the Earth's water cycle. Because of this, research continues to expand into ARs' relationship with large‐scale climate patterns. In this paper, we use data from the Modern Era Retrospective analysis for Research Applications to examine several extreme ARs that made landfall on the U.S. West Coast and their relationship to the transport of ozone from the stratosphere to the troposphere. We then combine 11 years of December AR and ozone data in order to study the average trend of ozone transport in connection with ARs. We quantify the AR‐related ozone transport for the first time, and we find ARs with more intense water vapor transport result in the transport of higher concentrations of ozone. Quantifying ozone transport into the troposphere in connection with ARs is important as ARs may become more intense and/or more frequent with climate change, and ozone in the troposphere has consequences for the greenhouse effect. Key Points: Case studies and December climatology using MERRA‐2 reveal positive tropospheric ozone anomalies within dry intrusions associated with ARsAverage excess ozone concentrations are 10–13 ppbv at 400 hPa, and are even greater for increasing intensity of ARsSTT of ozone associated with ARs in the NE Pacific may account for (13 ± 2)% of the total December Northern Hemisphere STT ozone flux [ABSTRACT FROM AUTHOR]

The physicochemical properties and origins of atmospheric aerosols in the Tibetan Plateau (TP) region are a research topic of great interest, but an in‐depth understanding of this topic is challenging, partially due to a lack of intensive in situ observations. Thus, a field campaign was conducted over Yadong, a remote area on the south slope of the TP from June 11 to 31 August 2021. The aerosol loading was low, with a black carbon mass concentration of 147.4 ± 98.4 ng·m−3. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and increased from 450 to 700 nm wavelength. Organic matter (OM) accounting for 69.6% of the total aerosol mass and relatively high secondary organic carbon ratios, highlighting the importance of secondary formation. An interesting phenomenon observed was that the evolution of aerosols was mainly characterized by diurnal variation, which could not be explained by large‐scale atmospheric processes such as Indian summer monsoon. Instead, it was found that regional mountain‐valley winds between the Himalayas and South Asia transported polluted air masses toward the TP, especially in the afternoon when regional valley wind are expected to be the strongest and the boundary layer in South Asia is deepest. Additionally, daytime local valley wind further elevated these aerosols to higher altitudes on the TP. This paper provides insights into the transport mechanisms of aerosols from South Asia to the TP. These findings are of great importance since aerosols exhibit significant diurnal variations in the TP region. Plain Language Summary: Previous studies focused on the analysis of the physical or chemical properties of aerosols on the Tibetan Plateau, but this study provides a comprehensive examination of both. The findings reveal that aerosols on the southern slope of the Tibetan Plateau exhibit strong absorption efficiency. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm), which may be attributed to aerosol secondary generation and coating. Finally, the mechanism of pollutant transport from South Asia to the Tibetan Plateau was analyzed relies on site observations, satellite, and reanalysis data to highlight the link between diurnal variations of pollutants and transport mechanism. The specific transport mechanism be understood uniformly across different scales, including Indian summer monsoon, regional mountain‐valley winds between the Himalayas and South Asia, and local mountain‐valley winds circulation. Key Points: Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and secondary organic matter was the major aerosol componentThe evolution of aerosols was mainly characterized by diurnal variation that was related to transport mechanism over YadongThe Himalayas‐South Asia regional mountain‐valley winds combined with local mountain‐valley winds transport aerosols to the Tibetan Plateau [ABSTRACT FROM AUTHOR]

Self‐organizing maps (SOMs) represent a popular tool for classifying atmospheric circulation patterns. One of their traditional applications has been to link typical synoptic‐scale patterns to large‐scale teleconnections, or modes of low‐frequency circulation variability. However, recently there have been attempts to interpret an array of SOM nodes directly as a continuum of teleconnections, grounded in SOMs' ability to combine two otherwise distinct approaches to data analysis, that is, exploratory projection (or, dimensionality reduction) and classification. This conceptual shift calls for methodological studies that would improve our understanding of how orthogonal modes of variability, typically used to describe teleconnections, relate to SOM outputs. Here, we define three idealized modes of variability and use their various combinations to generate synthetic data sets. Many variants of SOMs are generated for SOMs of various shapes and sizes. The results show that projection of modes on a SOM array is sensitive not only to data structure, but also to various SOM parameters. The leading mode of variability projects rather strongly on SOMs if its explained variance is markedly higher than that of the second‐order mode; the remaining modes project considerably more weakly, and all modes tend to blend when their explained variance is similar, which leads to underrepresentation of some phases of modes and/or combinations of modes among the SOM patterns. Furthermore, we show that some features of SOM topology that were previously considered a proof of data nonlinearity appear even if the underlying modes of variability are strictly linear. Plain Language Summary: One of the tasks of a climatologist is to group together days with similar weather over a region, and to find out how such types of regional weather are influenced by distant locations. These distant influences, also known as teleconnections, are intricate and studying how the methods we use to study them deal with these intricacies is one of the keys that helps us understand the climate. One way to start this task is to apply simple and well‐understood data sets to methods and examine how they interpret the data. In the paper, we studied the popular method of self‐organizing maps in such a way. We found that the method usually identifies the strongest teleconnection, but the images of weaker teleconnections tend to be unclear or even blend together. We use our results to revisit findings of some previous studies and to discuss how self‐organizing maps could be improved in the future to give us more accurate information on the atmosphere. Key Points: Synthetic fields are generated as linear combinations of three idealized functions of spatial co‐variability, or circulation modesSelf‐organizing maps have limited ability to represent the second and third modes, and modes tend to blend when their variance is similarPrincipal component analysis may detect data structures conducive to high sensitivity of self‐organizing maps to the choice of parameters [ABSTRACT FROM AUTHOR]

The most serious heavy precipitation in the past 43 years occurred over Fujian‐Jiangxi region from late May to early June of 2006, causing significant economic losses. Using the ERA5 reanalysis database and daily precipitation collected at 2,479 surface meteorological stations in China, the present study investigates the relationship between the heavy precipitation over Fujian‐Jiangxi region in late spring‐early summer of 2006 and baroclinic wave packets in the upper troposphere. Information flow between the two systems has been diagnosed. Results indicate that different from the disturbance source for Meiyu in the middle and lower reaches of the Yangzi River, the disturbance source for this heavy precipitation originated from areas near the Syrian Desert to the north of the Arabian Peninsula and propagated along the northwest‐southeast direction, reaching Fujian‐Jiangxi region 4 days later. This kind of baroclinic wave packets provide the necessary energy for the occurrence and persistence of heavy precipitation. Analysis of wave activity flux vectors indicates that during the heavy precipitation period, disturbance energy was transported from the upstream westerly belt to Fujian‐Jiangxi region almost every day. Obviously, there existed information transfer between the two regions, re‐confirming that the upstream Rossby wave packets affect the Fujian‐Jiangxi precipitation. The above results provide helpful hints for a better understanding of the mechanisms for heavy precipitation in this region and will be helpful for its effective prediction. Plain Language Summary: In the late spring and early summer of 2006, the strongest precipitation occurred in Fujian‐Jiangxi in China from 1979 to 2021 at the same time. It is of significance to study the causes of such extremely heavy precipitation events to predict future similar events in advance to reduce casualties and economic losses. Atmospheric motions could be regarded as wave motions, and Rossby wave is one of the major atmospheric waves. Multiple Rossby waves together could be simply regarded as Rossby wave packets (RWPs). In this paper, we have used as many methods as possible to find whether RWPs have influence on precipitation and how. A large amount of evidence shows that RWPs have significant influence on this extreme precipitation, especially by providing energy. Four days before the beginning of the heavy precipitation, RWPs began to spread from near the Syrian desert to Fujian‐Jiangxi region. Moreover, a new mathematical method is used to prove that there is a causal relationship between wave packet activities and this heavy precipitation. Nevertheless, the above research is only for such an extreme precipitation event. Whether each extreme precipitation event corresponds to obvious wave packet activities needs further study. Key Points: Rossby wave packets (RWPs) propagated from areas near the Syrian Desert to Fujian‐Jiangxi region before the heavy precipitation beganRWPs at 300 hPa provide disturbance energy daily to heavy precipitation over Fujian‐Jiangxi region during the precipitation periodThere is evident information transfer between RWPs activities from the upstream area with the heavy precipitation in Fujian‐Jiangxi region [ABSTRACT FROM AUTHOR]

This study investigates the use of airborne in situ measurements to derive transit time distributions (TTD) from the boundary layer (BL) to the upper troposphere (UT) over the highly convective tropical western Pacific (TWP). The feasibility of this method is demonstrated using 42 volatile organic compounds (VOCs) measured during the Convective Transport of Active Species in the Tropics (CONTRAST) experiment. Two important approximations necessary for the application are the constant chemical lifetimes for each compound and the representation of the BL source by the local CONTRAST data. To characterize uncertainties associated with the first approximation, we quantify the changes in derived TTDs when chemical lifetimes are estimated using conditions of the BL, UT, and tropospheric average. With the support of a trajectory model study in a companion paper, we characterize the BL source region contributing to the transport to the sampled UT. In addition to the TTDs derived using a regional average, we analyze the potential information content in locally averaged measurements to represent the dynamical variability of the region. Around 150 TTDs, derived using measurements on a ∼100 km spatial scale, show a distribution of mean and mode transit times consistent with the wide range of convective conditions encountered during the campaign. Two extreme cases, with the shortest and the near‐longest TTD, are examined using the dynamical background of the measurements and back trajectory analyses. The result provides physical consistency supporting the hypothesis that sufficient information can be obtained from measurements to resolve dynamical variability of the region. Plain Language Summary: In this study, we use measurements of the atmospheric chemical composition to derive vertical transport timescales over the tropical western Pacific, where convective processes dominate. A large suite of trace gases with a wide range of chemical lifetimes are measured from a research aircraft. The trace gas measurements are used to construct "transit time distributions," which capture the relative contributions of various transport pathways to the chemical composition of the upper troposphere in the study region. Results highlight the wealth of information in atmospheric chemical composition measurements, which is important not only for understanding chemistry of the atmosphere but also for quantifying aspects of atmospheric dynamics and circulation. Key Points: The approximations and uncertainties in deriving airborne chemical observation‐based tropospheric transit time distributions are examinedThe necessary approximation of trace gas lifetimes is found to be the largest source of uncertaintyThe method successfully identified a range of dynamical transport variability from aircraft measurements [ABSTRACT FROM AUTHOR]

This paper examines probability distributions of local wave activity (LWA), a measure of the jet stream's meander, and factors that control them. The observed column‐mean LWA distributions exhibit significant seasonal, interhemispheric, and regional variations but are always positively skewed in the extratropics, and their tail often involves disruptions of the jet stream. A previously derived one‐dimensional (1D) traffic flow model driven by observed spectra of transient eddy forcing qualitatively reproduces the shape of the observed LWA distribution. It is shown that the skewed distribution emerges from nonlinearity in the zonal advection of LWA even though the eddy forcing is symmetrically distributed. A slower jet and stronger transient and stationary eddy forcings, when introduced independently, all broaden the LWA distribution and increase the probability of spontaneous jet disruption. A quasigeostrophic two‐layer model also simulates skewed LWA distributions in the upper layer. However, in the two‐layer model both transient eddy forcing and the jet speed increase with an increasing shear (meridional temperature gradient), and their opposing influence leaves the frequency of jet disruptions insensitive to the vertical shear. When the model's nonlinearity in the zonal flux of potential vorticity is artificially suppressed, it hinders wave‐flow interaction and virtually eliminates reversal of the upper‐layer zonal wind. The study underscores the importance of nonlinearity in the zonal transmission of Rossby waves to the frequency of jet disruptions and associated weather anomalies. Plain Language Summary: Midlatitude weather is characterized by mobile cyclones and anticyclones that cause meandering of the jet stream (transient Rossby waves) in the upper troposphere. The degree of the jet's meander is related to the severity of weather anomaly. Here we use a fluid dynamical metric called local wave activity (LWA) to quantify the waviness of the jet stream, or the amplitude of the Rossby waves. Probability distribution of LWA is positively skewed, that is, the jet is modestly wavy most of the time yet there are less frequent but very wavy conditions that disrupt the jet stream regionally. Using theory and idealized numerical models we show that these very wavy conditions arise from mutual reinforcement between the meander of the jet stream and weakening of its eastward wind. It is also found that a slower jet stream and stronger eddy forcing both increase the probability of the jet disruption events, but the probability of such events is insensitive to the pole‐to‐equator temperature gradient. Key Points: Local wave activity (LWA) measures meandering of the jet stream, large values often linked to disruptions of the jet and extreme weatherObserved probability distribution of LWA is positively skewed due to nonlinearity in the zonal advection (LWA‐zonal wind interaction)Frequency of large LWA events increases with slower jet and stronger eddy forcing but it is insensitive to meridional temperature gradient [ABSTRACT FROM AUTHOR]

The Western North Atlantic Ocean (WNAO) and adjoining East Coast of North America are of great importance for atmospheric research and have been extensively studied for several decades. This broad region exhibits complex meteorological features and a wide range of conditions associated with gas and particulate species from many sources regionally and other continents. As Part 1 of a 2‐part paper series, this work characterizes quantities associated with atmospheric chemistry, including gases, aerosols, and wet deposition, by analyzing available satellite observations, ground‐based data, model simulations, and reanalysis products. Part 2 provides insight into the atmospheric circulation, boundary layer variability, three‐dimensional cloud structure, properties, and precipitation over the WNAO domain. Key results include spatial and seasonal differences in composition along the North American East Coast and over the WNAO associated with varying sources of smoke and dust and meteorological drivers such as temperature, moisture, and precipitation. Spatial and seasonal variations of tropospheric carbon monoxide and ozone highlight different pathways toward the accumulation of these species in the troposphere. Spatial distributions of speciated aerosol optical depth and vertical profiles of aerosol mass mixing ratios show a clear seasonal cycle highlighting the influence of different sources in addition to the impact of intercontinental transport. Analysis of long‐term climate model simulations of aerosol species and satellite observations of carbon monoxide confirm that there has been a significant decline in recent decades among anthropogenic constituents owing to regulatory activities. Key Points: Significant reductions in CO and particulate SO42−, black carbon, and primary organic matter are observed over multiple decadesHigh aerosol extinctions in free troposphere during March‐April‐May and June‐July‐August, whereas aerosols mostly confined to boundary layer at other timesOrganic matter and ammonium sulfate usually dominated PM2.5 composition, whereas sea salt was most abundant in wet deposition [ABSTRACT FROM AUTHOR]

This study assesses the benefit of convective‐scale data assimilation (DA) for model initialization using well‐known functional relationships between lightning flash rate and vertical velocity. Based on the relationships, a lightning DA scheme to update model kinematic states was implemented in the Weather Research and Forecasting Data Assimilation (WRFDA) three‐dimensional variational (3DVar) system. This scheme combines total lightning observations with model‐based prescribed vertical velocity profiles to retrieve kinematic information through a DA scheme. With the availability of space‐borne lightning imagers in recent years, total lightning observations from the Lightning Mapping Imager (LMI) on board the FY‐4A geostationary satellite were assimilated in combination with radar DA. A detailed analysis of the impact of the lightning DA scheme on convective precipitation forecasting was conducted using a squall line case over Beijing on 13 July 2017. The assimilation of LMI data provides added benefits to the assimilation of radar radial winds by reducing wind errors and strengthening convergence along the squall line in the analysis. Although the microphysical states are identical due to the assimilation of reflectivity, the lightning DA scheme helps in promoting updraft developments at lightning observation locations, which improves the representation of mixed‐phase convection. The quantitative verification of short‐term convective forecasts indicated that the lightning DA adds value to the radar DA by improving the precipitation forecast skill. The lightning DA scheme was evaluated further for a heavy rainfall case in 2018 over the Beijing metropolitan area and revealed overall similar forecast improvements for composite reflectivity and accumulated rainfall. Plain Language Summary: Lightning flashes are closely related to the upward air motions in thunderstorms and hence are indicative of strong wind convergence. Currently, lightning imagers on board the geostationary satellites provide increased availability of lightning data over broad regions and can improve weather forecasting accuracy. This paper describes how the space‐borne lightning observations could be employed to update model kinematic states and improve convective precipitation forecasting. Key Points: A lightning data assimilation (DA) scheme to update model kinematic states was developed using a three‐dimensional variational (3DVar) systemThe event data from Lightning Mapping Imager (LMI) aboard the FY‐4A geostationary satellite were assimilated to reflect lightning horizontal dimensionThe new lightning DA scheme improves the convective analysis and storm forecasting in two severe convective cases [ABSTRACT FROM AUTHOR]

The variations of the horizontal phase velocity of an internal gravity wave, generated by wave "blocking" or "reflection" due to an inhomogeneous wind field, have been predicted theoretically and numerically investigated but had yet to be captured experimentally. In this paper, through a collaborative observation campaign using a sodium (Na) Temperature/Wind lidar and a collocated Advanced Mesospheric Temperature Mapper (AMTM) at Utah State University (USU), we report the first potential evidence of such a unique gravity wave process. The study shows that a small‐scale wave, captured by the AMTM, with initial observed horizontal phase velocity of 37 ± 5 m/s toward the northwest direction, experienced a large and increasing headwind as it was propagating in the AMTM field of view. This resulted in significant deceleration along its initial traveling direction, and it became quasi‐stationary before it was "reflected" to the opposite direction at later time. The USU Na lidar measured the horizontal wind and temperature during the event, when the wave was found traveling within a temperature inversion layer and experiencing an increasing headwind relative to the wave. The wind agrees well with the expected value for wave blocking suggested by the wave tracing theory, implying the existence of a large horizontal wind gradient that night near the OH layer altitudes. The study indicates the critical role of horizontal winds and their horizontal gradients in determining propagation in vertical and horizontal directions. Key Points: We observe blocking and reflection of a small‐scale gravity waveThe wave blocking event occurred when the wave was experiencing strong and increasing headwindThe wave was propagating in an upper mesosphere temperature inversion layer formed within the OH layer [ABSTRACT FROM AUTHOR]

The article offers information on the study of atmospheric gravity wave science in the polar regions. Topics discussed include contribution of gravity waves in driving atmospheric circulation; mechanisms in the generation of gravity waves involves instabilities at the edge of the polar vortex, wind flow over topography, and the aurora; and development of Antarctic Gravity Wave Instrument Network to understand gravity wave processes.

The Arctic has experienced step changes in climate conditions, which are still not fully understood. Particularly intriguing was the Early Twentieth‐Century Arctic Warming (ETCAW), which occurred prior to anthropogenic interference in the environment. This paper quantitatively assesses the differences in atmospheric circulation patterns during periods of warming and cooling on Spitsbergen with a particular focus on ETCAW. The patterns of atmospheric circulation were taken from the catalog of day‐by‐day circulation types created for Svalbard. The catalog includes 21 circulation types, assigned to every day between 1920 and 2017. The research focused on winter and autumn, the seasons exhibiting pronounced ETCAW on Spitsbergen. The long‐term variability in autumn and winter air temperatures on Spitsbergen significantly depends on the frequency of air advection from clearly defined directions and reveals some seasonal differentiation. Warming was related in autumn to southern and south‐eastern advection, in winter to southern advection, and in both cyclonic and anticyclonic conditions. These "warm" types explained up to 21% (September–November) or 25% (December–February) of the variability in air temperature. In winter, cooling was significantly related to anticyclonic type with air advection from the northeast (NEa: 24% of variability): in autumn to cyclonic northern (Nc) and northeastern (NEc) types. During ETCAW, the frequency of warm circulation types was significantly higher, and that of "cold" circulation types significantly lower. No relevant differences in patterns were found between ETCAW and Recent Arctic Warming; however, the frequency of warm and cold types was higher and lower, respectively, during ETCAW. Key Points: A 10% change in the circulation types frequency can raise DJF air temperature by 2.5 °C (S types) or reduce them by 2.1 °C (N types)Southern CTs explain up to 46%, and northern up to 41% of variability in DJF air temperaturesETCAW was marked by a significantly higher frequency of warm southern CTs and a lower frequency of cold northern CTs, both by >20% [ABSTRACT FROM AUTHOR]

Abstract: To better understand the potential effects of climate change on atmospheric dynamics, this paper studies Rossby wave breaking and isentropic stratosphere‐troposphere exchange (STE) in the Northern Hemisphere between 320 and 380 K during 1981–2015 using the Modern‐Era Retrospective Analysis for Research and Application version 2 data. The isentropic STE is estimated using Contour Advection. Our results show that anticyclonic wave breaking events have become more frequent, especially in summer at higher isentropic surfaces, and cyclonic wave breaking events have become less frequent at 320 K. The anticyclonic wave breaking has shifted poleward in summer. The isentropic STE is found to be strongest over the regions where Rossby wave breaking activities are most frequent. Both isentropic STE and Rossby wave breaking are shown to be strongest in summer and weakest in winter. Our results do not show any discernable trends during isentropic STE during 1981–2015. [ABSTRACT FROM AUTHOR]

Long‐term changes in the middle atmosphere due to anthropogenic greenhouse gas emissions are examined in relation to the effect of the equatorial Quasi‐Biennial Oscillation (QBO) on the northern midwinter circulation. The examinations are based on the Coupled Model Intercomparison Project Phase 5 simulations for 1979–2100 with the Earth‐System‐Model MPI‐ESM‐MR that generates the QBO internally. In particular, the three‐dimensional residual circulation is used as proxy for the Brewer‐Dobson circulation, revealing an increasing downwelling in the center of the polar low over Northern Europe/Siberia (~5% per decade). The changes in northern midwinter temperature, zonal wind, and residual circulation are much stronger during westerly (QBO‐W) than easterly (QBO‐E) phase of QBO (e.g., for a moderate increase in greenhouse gases, we find maximum decreases in the zonal mean westerly jet at 60°N and 3 hPa of about −14.8 ± 5.4 m/s for QBO‐W but only −4.7 ± 5.2 m/s for QBO‐E). This is due to a change of the extratropical QBO‐W signature toward QBO‐E signature while the equatorial QBO remains nearly unchanged (i.e., a change toward disappearance of the so‐called Holton‐Tan relationship). Similar to the current change from QBO‐W to QBO‐E signature, the changes during QBO‐W include an increase in amplitude and eastward shift in phase of stratospheric stationary Wave 1 at the cost of Wave 2, with decreasing westerlies over North America and increasing downwelling over Siberia. The eastward shift in phase of stationary Wave 1 is related to the associated increase in meridional transport of planetary vorticity. Plain Language Summary: Anthropogenic greenhouse gases impose a warming of the troposphere (0–10 km) and a cooling of the middle atmosphere (10–100 km), and therefore extensive changes in the whole atmospheric circulation system. One important component of the circulation in the stratosphere (10–50 km) is the effect of the regular 28‐month oscillation between westerly and easterly winds in the equatorial stratosphere on the extratropical Northern Hemisphere, where the change from equatorial westerlies to easterlies leads to weaker westerlies in the extratropics and a stronger northward and downward directed mass circulation at high latitudes. The present paper shows that this effect is strongly changing in case of moderate and strong increases in greenhouse gases, as derived from climate model simulations for the time period 1979–2100 with the Earth‐System‐Model MPI‐ESM (which were performed in the framework of the Climate Model Intercomparison Project 5). The extratropical signature of the equatorial westerlies changes toward the signature of the equatorial easterlies; therefore, we find much stronger long‐term changes in the extratropical middle atmosphere during the equatorial westerlies than easterlies, while the equatorial QBO itself remains nearly unchanged. In particular, this change includes a decrease in the westerlies over North America and an increase in the mass circulation over Northern Europe/West Siberia. Key Points: Increasing greenhouse gases impose a change in the response of the northern middle atmosphere to the equatorial quasi‐biennial oscillationModel simulations show that the long‐term change in the extratropics is much stronger during equatorial westerlies than easterliesThe change in the stationary flow patterns includes decreasing westerlies over North America and increasing downwelling over Siberia [ABSTRACT FROM AUTHOR]

Using the extended ERA5 reanalysis and three state‐of‐the‐art models, this study explores how El Niño‐Southern Oscillation (ENSO) can influence the total frequency, seasonal cycle and preconditioning of sudden stratospheric warmings (SSWs). Reanalysis data shows that in the last seven decades, winters with SSWs were more common than winters without, regardless El Niño (EN) or La Niña (LN) occurrence or the ENSO/SSW definitions. In agreement with previous studies, our models tend to simulate a linear ENSO‐SSW relationship, with more SSWs for EN, around mid‐winter (January–February) as in reanalysis, and less for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave‐like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated, emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO in the North Pacific, favoring a stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO‐forced signal relies on modulating the seasonal‐mean polar vortex strength, becoming weaker and more displaced (stronger and more stable) for EN (LN), while ENSO‐unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are distinct sources of variability of the winter atmospheric circulation operating at different time‐scales and may reconcile previous findings in this context. Plain Language Summary: Sudden stratospheric warmings (SSWs) are extreme events of the stratospheric polar vortex that consist in its deceleration and breakdown for several days, with implications on surface weather for several weeks. On the other hand, the atmospheric response to El Niño‐Southern Oscillation (ENSO) has been shown to weaken the stratospheric polar vortex during El Niño and strengthen it during La Niña winters, and hence, potential modulation of the occurrence of SSWs by ENSO is expected. Reanalysis data shows that in the last seven decades, winters with SSWs were more frequent than winters without regardless of ENSO, and instead of having an impact on the SSW total frequency, El Niño seems to favor events in mid‐winter (January–February). The main mechanism driving SSWs is thought to be anomalous upward wave activity from the troposphere into the stratosphere, reaching and perturbing the polar vortex. Independently of the ENSO phase, enhanced wave activity before SSWs is here found over Eurasia, thus pointing to this region as the main precursor of SSWs. While ENSO modulates the polar vortex at monthly/seasonal time‐scales, SSWs do so at daily/weekly time‐scales, representing distinct sources of variability of the polar vortex. Key Points: El Niño‐Southern Oscillation (ENSO) and sudden stratospheric warmings (SSWs) modulate the stratospheric polar vortex at different time‐scalesThe main SSW precursor independently of the ENSO phase is associated with wave activity over EurasiaThe anomalous Eurasian wave pattern can be dominated by different wave numbers depending on the ENSO phase [ABSTRACT FROM AUTHOR]

Ural blocking (UB) is a prominent mode of variability of the Northern Hemisphere atmospheric circulation, particularly in fall. It can persist for several days and exert a lagged influence on the wintertime NH circulation, providing predictability at the subseasonal time scale. Using two atmospheric models, we explore how the early winter atmospheric circulation responds to a 2‐week persistent UB anomaly imposed in early November. Experiments are carried out with two different configurations of Barents‐Kara (BK) sea‐ice concentration to examine whether it plays a role in how UB impacts atmospheric variability. In both models, the UB anomaly is followed by a weakening of the stratospheric polar vortex and a negative phase of the North Atlantic Oscillation (NAO), which lasts up to 2 months after the forcing is released. Interestingly, the response is more persistent under low BK sea‐ice conditions, that is, BK sea ice modulates the atmospheric response to UB. Additional experiments with prescribed sea ice concentration anomalies alone suggest that BK sea ice exerts a limited influence on early winter NH atmospheric variability. The response to UB involves a weakening of the polar vortex that persists longer under low BK sea ice, which explains the more persistent response in that configuration. Our study highlights that UB variability in November is a robust precursor for early winter NAO/polar stratosphere anomalies, and this may be more relevant in the context of declining Arctic sea‐ice extent. Provided that climate models accurately capture this teleconnection, it has the potential to improve subseasonal predictions of the NH wintertime climate. Plain Language Summary: Predicting subseasonal fluctuations of the polar stratosphere in winter (or polar vortex) is critical to anticipate weather extreme events over populated areas of the Northern Hemisphere. In this study, we highlight that a persisting high‐pressure system over the Europe/Ural region in fall (known as Ural blocking [UB]) can force a delayed response at the surface several weeks later, that resembles the negative phase of the North Atlantic Oscillation (NAO). This is found in two different atmospheric models in which the UB pattern is imposed during the first 2 weeks of November, using a regional relaxation of the troposphere. We also investigate the response to fall/early winter Barents‐Kara sea ice decline, which has been hypothesized to have a similar impact on the polar vortex/NAO, but we find that it is of secondary importance compared to the influence of UB. Key Points: Two‐week persistent Ural blocking (UB) anomalies induce a weaker polar vortex and negative North Atlantic OscillationRole of Barents‐Kara (BK) sea ice is secondary compared to the influence of UBHowever, BK sea ice modulates the persistence of the response to UB [ABSTRACT FROM AUTHOR]

A zonal dipole pattern of the interannual variability of Eurasian snow cover during late autumn has attracted growing interest due to its extensive influences on large‐scale atmospheric circulations and climate anomalies, but it remains unknown what influences the formation of this pattern. This study revealed the potential drivers and the related physical processes of the dipole mode of the snow cover variability during 1979–2018. Results showed that the atmospheric circulation anomalies play a crucial role in forming the dipole mode, which could be further attributed to the influences of the atmospheric teleconnection patterns and the variations of sea surface temperature (SST) and sea ice concentration. Specifically, the anomalous circulations related to the teleconnections of the Eastern Atlantic pattern, East Atlantic–Western Russia pattern, and Scandinavian pattern could lead to the anomalies of air temperature and snowfall that directly shape the variability of snow cover. In addition, a dipole structure of the SST anomalies over the North Atlantic exerts an important effect on the snow cover variability through exciting an eastward propagating Rossby wave train prevailing over Eurasia. Moreover, the snow cover variability is closely linked to the sea ice variations over the Barents Sea that can regulate the anomalous circulations over Eurasia by altering the atmospheric dynamic and thermodynamic conditions. These influencing factors jointly account for about 66% of variances in the dipole mode of snow cover variability, suggesting that our findings can substantially improve the understanding of the Eurasian snow cover variations. Plain Language Summary: This study investigated the possible influencing factors responsible for the formation of the west‐east dipole structure of Eurasian snow cover variability during late autumn that has notable impacts on the atmospheric circulations and climate anomalies in the following season. Results suggested that the anomalous circulations associated with the atmospheric teleconnections of the Eastern Atlantic pattern, East Atlantic‐Western Russia pattern, and Scandinavian pattern can effectively regulate the snow cover variability related to the dipole pattern through inducing the anomalies of surface air temperature and snowfall. In addition, the North Atlantic sea surface temperature anomalies play a considerable role in forming the dipole mode by exciting a wave train type anomalous circulation over Eurasia. Furthermore, the variations of sea ice concentration in the Barents Sea could lead to the snow cover variability through modulating the temperature gradient over mid‐high latitudes and the resultant circulation anomalies over Eurasia. The synergetic effect of these influencing factors explains about two‐thirds of the snow cover variability related to the dipole mode, indicating the important contribution of our results to understanding the Eurasian autumn snow cover anomaly. Key Points: The west‐east dipole pattern is the intrinsic mode of the Eurasian snow cover variability during late autumnThe dipole mode is closely associated with the Eastern Atlantic pattern, East Atlantic‐Western Russia pattern, and Scandinavian patternAnomalies of North Atlantic sea surface temperature and Barents sea ice can exert considerable effects on the dipole mode [ABSTRACT FROM AUTHOR]

A distinguishing feature of Arctic sea ice is the rapid declining in the Pacific Arctic Sector (PAS) in recent years. The atmospheric variability during spring (March‐May) plays a critical role in determining the sea ice extent (SIE) during the following summer. The summertime (June‐August) SIE reduction in the eastern part of PAS, including the East Siberian Sea and the Laptev Sea, is strongly related to the first two leading atmospheric circulation modes in spring, the Arctic Oscillation and the Arctic Dipole, retrieved from empirical orthogonal function analysis. Here, we show that the third leading mode of atmospheric circulation in spring, namely the Barents‐Beaufort Oscillation (BBO), can also change the summer SIE, especially on the western regime of the PAS across the Beaufort and Chukchi Seas. We find that a positive phase of springtime BBO (i.e., BBO+), relative to the BBO−, acts to decrease sea ice thickness via advection and generate sea ice deformation through divergence and shear processes, thereby preconditioning for a rapid sea ice loss within the PAS in the following summer. Furthermore, the occurrence of BBO is state‐dependent and potentially modulated by the phase shift of the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO). Since the mid‐1990s, the AMO (PDO) index has changed from a dominant negative (positive) to a positive (negative) phase. These phase shifts in AMO and PDO would have contributed to the subsequently increased appearance of anomalous BBO+ after 2000 and enhanced the summertime sea ice loss in the western regions of the PAS in recent years. Plain Language Summary: Large‐scale atmospheric circulation plays a vital role in regulating the Arctic sea ice extent (SIE) changes in summer. Spring atmospheric circulation anomaly could precondition for a noticeable decline in SIE in the following summer. Arctic Oscillation and Arctic Dipole anomaly modes are well demonstrated in previous studies. In this study, we outline the effectiveness in leading to sea ice reduction by another important circulation pattern, namely the Barents‐Beaufort Oscillation (BBO). The spring BBO in its positive phase can change the summer SIE over the western Pacific‐Arctic sector, including Beaufort and Chukchi Seas. The two seas are the regions undergoing the rapidest shrinkage over the past decades. We also found that the decadal variability of BBO seems to be associated with the ocean activities at low latitudes. The frequent occurrence after 2000 is partly attributable to the phase shift by the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation since mid‐1990s. Key Points: A special connection of summer Arctic sea ice extent shrinkage to spring Barents‐Beaufort Oscillation (BBO) is outlinedThe dynamic impacts on ice thickness decline through diverging and shearing processes linked to BBO are identified over the Pacific‐Arctic sectorThe effectiveness of BBO on Arctic sea ice changes depends on the states of Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation [ABSTRACT FROM AUTHOR]

Methane is thought to have been an important greenhouse gas during the Archean, although its potential warming has been found to be limited at high concentrations due to its high shortwave absorption. We use the Met Office Unified Model, a general circulation model, to further explore the climatic effect of different Archean methane concentrations. Surface warming peaks at a pressure ratio pCH4:pCO2 of approximately 0.1, reaching a maximum of up to 7 K before significant cooling above this ratio. Equator‐to‐pole temperature differences also tend to increase up to pCH4 ≤ 300 Pa, which is driven by a difference in radiative forcing at the equator and poles by methane and a reduction in the latitudinal extend of the Hadley circulation. 3D models are important to fully capture the cooling effect of methane, due to these impacts of the circulation. Plain Language Summary: The Archean is a period in Earth history spanning approximately 4 to 2.5 billion years ago. During this period oxygen concentrations were much lower than today, which in turn allowed methane concentrations to be much higher during the Archean. Methane would have been produced by microbes, and depending on the exact nature of the ecosystem could have led to a wide range of atmospheric concentrations. Using a three‐dimensional climate model and with various methane concentrations in the atmosphere we show that the warming effect of methane first increases and then, at high concentrations, decreases. This decrease is stronger than those found in previous studies using one‐dimensional (single column) atmosphere models. We find that methane can only warm the Archean by up to 7 K. Key Points: Methane warming peaks at a pCH4:pCO2 ratio of 0.1, with global‐mean surface warming <7 KUneven meridional methane radiative forcing and changes to atmospheric circulation determine the equator‐to‐pole temperature difference3D modeling is important to fully capture methane's cooling effect for pCH4:pCO2 ratios greater than 0.1 [ABSTRACT FROM AUTHOR]

Mesospheric gravity‐wave (GW) phase velocity spectra and total powers at two Antarctic stations, Davis and Syowa, were derived using OH airglow data from March to October in 2016. The total powers have similar seasonal variation, that is, maxima in winter at both stations. The average powers at both stations in winter were not significantly different. However, the power at Davis in September was three times smaller than that at Syowa. This lower power at Davis was attributed to GWs with omnidirectional phase velocity. These lower GW activities at Davis could be attributed to a longitudinal variation in wave filtering; a stronger wind at Davis filtered out more GWs than at Syowa. Also, to explore possible sources in the middle atmosphere, we investigated one event, in which GWs with ∼100 ms−1 southeastward phase velocity appeared at Davis on 29 August. The raytracing method was applied, and its result indicated that those GWs with high southeastward phase velocity propagated from ∼45 km altitude or higher over the Southern Ocean. A large residual of the non‐linear balanced equation was found at 50 km on its ray path. GWs, very likely emitted from a tropospheric jet, were also found near the ray path at the termination altitude over the Southern Ocean and possibly appeared saturated between 45 and 50 km. Therefore, the OH imager at Davis probably captured GWs generated by a spontaneous adjustment in the upper stratosphere and/or secondary GWs produced by the breaking of the GWs that have originated from the tropospheric jet. Plain Language Summary: A gravity wave (GW) is a type of atmospheric wave that transports momentum from the Earth's surface to the edge of the atmosphere where it can drive atmospheric circulations. The temporal and spatial variation of GWs is not well understood, and their sources remain unclear. We observed GWs at the upper edge of the atmosphere over two Antarctic stations (Syowa and Davis) and calculated their phase velocity and total power spectra. The total powers have winter maxima at both stations. Although the average powers at both stations in winter were not significantly different, the power at Davis in September was three times smaller than that at Syowa. The lower GW activity at Davis could be attributed to a stronger wind at Davis, which filtered out more GWs than at Syowa. Also, to explore possible sources in the middle atmosphere, we investigated one event at Davis on 29 August 2016. We found that some GWs propagated from ∼45 km altitude or higher over the Southern Ocean. Those GWs could be emitted from the polar night jet (flow imbalance) and/or secondary GWs produced by the breaking of the GWs that have originated from the tropospheric jet. Key Points: Mesospheric gravity‐wave activity at two Antarctic stations was larger in winter than in spring and fallWave activity at Davis was not significantly different with Syowa in winter but smaller in fall, which can be explained by filtering effectWe find that GWs at Davis on 29 August 2016, were very likely generated by a spontaneous adjustment and/or a tropospheric jet wave breaking [ABSTRACT FROM AUTHOR]

North Pacific Atmospheric Rivers (ARs) are affected by many climate modes, including the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). However, such impacts are often examined individually and outside of the context of ARs' own variability. It remains an open question as to the relative importance of each climate mode in affecting the inter‐annual variability of AR occurrences. This work proposes an answer to the above inquiry, via a rotated Empirical Orthogonal Function (EOF) analysis on wintertime AR occurrences. Four major modes of AR occurrence variability are identified: The first mode features a southeast‐northwest shift of the AR tracks, and is associated with the pair of the second leading EOFs of the North Pacific atmospheric circulation and sea surface temperature (SST). The second mode features a large scale suppression of North Pacific ARs and an enhancement west of the California coasts. This is a decadal variability associated with PDO. The third mode features meridionally contracted AR tracks over the Eastern Pacific. This is associated with the leading EOFs of the North Pacific atmosphere/SST, and the inter‐annual component of PDO. These three modes each contributes ∼19% to the total variances. The fourth mode, with a 12.7% variance contribution, carries the complementary information of the non‐linear El Niño ‐ La Niña transition, and features two slanted anomaly bands over boundaries of the basin. Key atmospheric/oceanic driving forces associated with each mode are examined, and certain climate modes exhibit promising predictive power to wintertime AR occurrences. Plain Language Summary: Atmospheric rivers (ARs) are filaments of intense moisture transport in the lower‐mid troposphere that is capable of delivering large amounts of moisture from ocean to land under suitable landfalling conditions. In this work we (a) summarize the most important variability patterns/modes of AR occurrences over the wintertime North Pacific, and (b) associate such modes with their major climate drivers. To achieve these goals, we first detect 6‐hourly occurrences of ARs from a 42‐year (1979–2020) high resolution data set. To summarize the major variability modes of AR occurrences, we devise a new method that allows for better physical correspondences with the climatic context while achieving a dimension reduction. The final result is four major modes of AR occurrences that collectively cover the majority (∼70%) of the total variances, and individually have their associated climate drivers, which have near‐exclusive relationships with an AR mode. AR activities bear significant hydrological implications to the coastal regions of the North Pacific, and results presented here provide insights into their climatic drivers and predictive potentials. Key Points: Summarizes four major North Pacific Atmospheric River (AR) occurrence modes that cover ∼70% of total inter‐annual variancesProvides evidence for two types of non‐linear responses of North Pacific ARs, to the Pacific Decadal Oscillation and El Niño Southern Oscillation modes, respectivelyDevises a new method to rotate the Empirical Orthogonal Function result for better result interpretability [ABSTRACT FROM AUTHOR]

The third generation of the radiative flux profile data product, called ISCCP‐FH, is described. The revisions over the previous generation (called ISCCP‐FD) include improvements in the radiative model representation of gaseous and aerosol effects, as well as a refined statistical model of cloud vertical layer variations with cloud types, and increased spatial resolution. The new product benefits from the changes in the new H‐version of the ISCCP cloud products (called ISCCP‐H): higher spatial resolution, revised radiance calibration and treatment of ice clouds, treatment of aerosol effects, and revision of all the ancillary atmosphere and surface property products. The ISCCP‐FH product is evaluated against more direct measurements from the Clouds and the Earth's Radiant Energy System and the Baseline Surface Radiation Network products, showing some small, overall reductions in average flux uncertainties; but the main results are similar to ISCCP‐FD: the ISCCP‐FH uncertainties remain ≲10 Wm−2 at the top‐of‐atmosphere (TOA) and ≲15 Wm−2 at surface for monthly, regional averages. The long‐term variations of TOA, surface and in‐atmosphere net fluxes are documented and the possible transient cloud feedback implications of a long‐term change of clouds are investigated. The cloud and flux variations from 1998 to 2012 suggest a positive cloud‐radiative feedback on the oceanic circulation and a negative feedback on the atmospheric circulation. This example demonstrates that the ISCCP‐FH product can provide useful diagnostic information about weather‐to‐interannual scale variations of radiation induced by changes in cloudiness as well as atmospheric and surface properties. Plain Language Summary: The article describes the updated version of the International Satellite Cloud Climatology Project (ISCCP) radiative profile flux product, ISCCP‐FH. This version has several important improvements over its previous two versions in its radiation model and input data sets of clouds, aerosol and other atmospheric and surface physical properties as well as ancillary data sets. Its spatial resolution is increased to 110 km. It now has uncertainties ≲10 Wm−2 at the top‐of‐atmosphere (TOA) and ≲15 Wm−2 at surface for monthly, regional averages based on validations against the direct observations at TOA and surface, slightly improved over the previous versions. We also describe long‐term variations of the radiative energy intensity based on the product and give an example to study cloud‐radiation feedback using this product. We expect the new product to be used in climate studies like its previous versions. Key Points: The radiative flux profile data product (called ISCCP‐FH) is described. It benefits from the new ISCCP cloud products (called ISCCP‐H)The product is evaluated against the Clouds and the Earth's Radiant Energy System and the Baseline Surface Radiation Network measurementsThe long‐term variations of top‐of‐atmosphere, surface and in‐atmosphere net fluxes are documented and a possible cloud feedback is investigated [ABSTRACT FROM AUTHOR]

Based on the normalized difference vegetation index (NDVI) and empirical orthogonal function analysis, a "northeast‐southwest" dipole pattern of spring vegetation in mid‐high latitude Asia was detected. This pattern was revealed to be associated with the autumn sea ice concentration (SIC) anomalies in the Barents‐Laptev Seas on the interannual variability. Increased autumn SIC over the Barents‐Laptev Seas tends to increase spring NDVI over the west of Lake Baikal and decrease spring NDVI in the Russian Far East. Further analyses indicated that the sea surface temperature (SST) anomalies persisting from winter to spring over the Northeast Atlantic and the Northwest Pacific play an important role in such a relationship. Increased autumn SIC over the Barents‐Laptev Seas is associated with a teleconnection pattern over the mid‐high latitudes in the following winter, accompanied by anomalous anticyclonic circulations over the Northeast Atlantic and the Northwest Pacific. Such changes in the large‐scale circulations induce downward net heat flux anomalies over the Northeast Atlantic and the Northwest Pacific, thereby contributing to the warming of SST which may persist to spring and in turn influence the atmospheric circulations. The Northeast Atlantic SST warming in spring may evoke a downstream wave train and cause positive geopotential height anomalies over the west of Lake Baikal, which increases local temperature and hence the vegetation. The Northwest Pacific SST warming in spring may lead to negative geopotential height anomalies over the Russian Far East through a meridional wave train, consequently resulting in a decrease in local temperature and hence vegetation. Plain Language Summary: The complex relationship between the climate and vegetation dynamics has been receiving a great deal of research attention. While much is known about how spring vegetation is controlled by the basic conditions such as temperature and precipitation, few studies have examined the response of the vegetation to Arctic sea ice. Our results reveal that the "northeast‐southwest" dipole pattern of spring vegetation in mid‐high latitude Asia is closely related to the preceding autumn sea ice concentration in the Barents‐Laptev Seas. The sea surface temperature anomalies persisting from winter to spring over the Northeast Atlantic and the Northwest Pacific play an important role in bridging such a relationship. These findings contribute to a better understanding of the change in spring vegetation over mid‐high latitude Asia. Key Points: A "northeast‐southwest" dipole pattern of spring vegetation is detected over mid‐high latitude AsiaThe spring vegetation dipole pattern is linked to the preceding autumn sea ice variation in the Barents‐Laptev SeasThe sea surface temperatures in the Northeast Atlantic and Northwest Pacific play an important role in bridging such a lag linkage [ABSTRACT FROM AUTHOR]

The impact of autumn Arctic sea ice reduction on the winter climate in Eurasia has been widely studied, but the role of the atmospheric circulation of the preceding summer has been mostly neglected. This study investigates a linkage between the summer Arctic circulation anomaly and the ensuing East Asian Winter Monsoon (EAWM) variability. The Arctic circulation anomaly, characterized by an anomalous anti‐cyclone occupying the Arctic Ocean and its marginal seas, can strongly melt Arctic sea ice in the Kara Sea‐Laptev Sea‐East Siberian Sea‐Chukchi Sea‐Beaufort Sea through dynamic and thermodynamic processes. The combination of the summer anti‐cyclone (cyclone) anomaly and light (heavy) autumn sea ice leads to an enhanced (weakened) EAWM. By introducing the coupling relationship between summer circulation and autumn sea ice into numerical experiments, the Community Atmospheric Model version 4 model has a good capability for capturing EAWM variability. Nevertheless, the dominant features of EAWM variability cannot be reproduced when the autumn sea ice mismatches the preceding summer circulation in the model. Therefore, the preceding summer circulation may modulate the impact of sea ice on Eurasia to some extent, which may increase uncertainties in the impact of Arctic sea ice loss on winter atmospheric circulation variability. Plain Language Summary: The enhanced East Asian Winter Monsoon (EAWM) leads to cold winter and has a significant impact on the economy, agriculture, and the lives of more than one billion people in East Asia, and the reduced autumn Arctic sea ice is generally believed to influence EAWM variability with great uncertainties due to strong atmospheric internal variability and impacts of other factors. In this study, through observational analysis and large ensemble model simulation experiments, a coupling relationship between the summer Arctic anti‐cyclone anomaly and the Arctic sea ice and their synergistic effect on EAWM variability are investigated. The summer anti‐cyclone anomaly over the Arctic corresponds to light sea ice and enhanced EAWM. Model experiments show that the summer initial conditions may modulate the lag impact of sea ice boundary conditions on winter circulation. Therefore, it is important to consider the synergistic effect of initial conditions and boundary conditions for predicting EAWM and winter surface temperature. Key Points: The Arctic anti‐cyclone (cyclone) anomaly in summer is a great predictor of the East Asian Winter Monsoon (EAWM)The combination of summer anti‐cyclone (cyclone) anomaly and light (heavy) autumn sea ice leads to an enhanced (weakened) EAWMFailure to consider the synergistic effect of initial conditions and sea ice may be one reason for the uncertainty of sea ice effect [ABSTRACT FROM AUTHOR]

Previous studies showed that random atmospheric forcing can stimulate oceanic low frequency variability in midlatitudes, since ocean has huge heat capacity that can redden the atmospheric high frequency variability. However, whether the oceanic stochastic forcing can in turn influence atmospheric variability is still unclear. To answer this question, two parallel 63‐year numerical experiments are conducted with an atmospheric general circulation model. In the control experiment, the atmosphere is forced by observed global sea surface temperature (SST). After adding stochastic forcings that are constrained by observed standard deviations of mesoscale SST anomalies (SSTAs) over the North Pacific in a sensitivity experiment, large‐scale atmospheric variability is greatly increased over the midlatitude northwest Pacific, especially on decadal timescale. Moreover, the anomalous atmospheric circulations associated with the Pacific Decadal Oscillation (PDO), like the enhanced westerly wind in the Kuroshio‐Oyashio and its extension (KOE) region, are more consistent with that resolved by a reanalysis model after the stochastic SSTAs are added. Dynamic diagnostics show that under the background of PDO warm phase, stochastic SSTAs can generate larger diabatic heating over the north of KOE region by inducing more moisture condensation in the mid‐lower troposphere, which is balanced by anomalous cold low‐level air temperature advection. Thus, the atmospheric baroclinicity is increased by the cold temperature advection anomaly, and the transient eddy activity and westerly jet are both enhanced above the KOE region accordingly. Our results suggest that to accurately represent the oceanic effect on large‐scale atmospheric variability and anomalies, oceanic stochastic SSTAs should also be considered. Plain Language Summary: Midlatitude air‐sea interaction is essential for setting climate variability at interannual to decadal timescales, and both atmosphere and ocean have strong small‐scale high‐frequency disturbances that are usually called "weather." While it is known that the atmospheric weather can be transferred to oceanic low‐frequency variability by ocean huge heat capacity, the oceanic weather manifested as mesoscale ocean eddies can influence larger atmospheric variability at longer time scales through its induced systematic atmospheric transient eddy activities, which is the new finding in this study. To verify such idea, stochastic SST forcing is used to represent the statistical effect of mesoscale SST anomalies on the atmosphere, and comparison of two long‐term experiments forced by global observed SST with and without the additional stochastic fluctuations in a coarse resolution model confirm that, the oceanic stochastic forcing can induce stronger atmospheric decadal variability over the midlatitude northwest Pacific, and it is conductive to better simulate the Pacific Decadal Oscillation‐related atmospheric circulations as well as their associated climate variabilities over Eurasian continent. Our results suggest that oceanic stochastic forcing can mimic the influence of oceanic eddies in SST in coarse resolution models and compensate the impact of large‐scale SST anomalies in producing more realistic atmospheric responses. Key Points: Stochastic sea surface temperature anomalies (SSTAs) strengthen the atmospheric decadal variability over the midlatitude northwest PacificAdding stochastic SSTAs conduces to better simulation of Pacific Decadal Oscillation‐related atmospheric circulations over the midlatitude North PacificAtmospheric diabatic heating and transient eddy forcing are the two key processes through which stochastic SSTAs influence the atmosphere [ABSTRACT FROM AUTHOR]

The nocturnal precipitation over the Sichuan Basin (SCB) exhibits obvious spatial variation, most of the existing numerical research of precipitation event over the SCB is based on case studies. In this study, the long‐term climatological evaluation of the prediction of diurnal spatio‐temporal evolution of precipitation and relevant physical mechanism over the SCB by 9 and 3 km operational numerical weather prediction model is carried out. It is found that the SWC‐WARMS model with 9 km grid spacing overestimates the summer precipitation, especially over the western slopes of the SCB. The northeastward timing delay of the precipitation peak over the SCB is well reproduced in the model, with predicted precipitation peaking at midnight over the southwestern SCB and in the early morning over the northeastern SCB. However, the predicted diurnal peak of precipitation appears 2–3 hr earlier than the observed peak. Increasing horizontal resolution to 3 km reduces the wet bias and improves the diurnal phase of precipitation. Compared to reanalysis, the predicted easterly anomalous wind appears earlier and the anomalous wind speed is stronger. Moreover, the predicted atmosphere is too unstable, which is associated with "warm below and cold above" vertical biases in temperature and wet biases in specific humidity. Thus, model precipitation peaks too early in the day over the SCB, and is overestimated. The weakening of the predicted southwesterly flow in the middle troposphere weakens the northeastward propagation of precipitation after midnight over the SCB. The most significant improvements are found for the diurnal variation of low‐level anomalous wind speed in the 3 km resolution model, contributing to a more realistic representation of precipitation over the SCB. Plain Language Summary: Although it is known that the nocturnal precipitation is dominated over the Sichuan Basin (SCB), the diurnal spatial variation of precipitation and the associated atmospheric circulation based on hourly long‐term climatological prediction are not well understood. Our results show that the SWC‐WARMS model can reasonably reproduce the precipitation on both daily and hourly time scales in summer, although the model overestimates the precipitation and peaks earlier than observation over the SCB. In general, the model with higher resolution produces a more realistic representation of precipitation than the coarse resolution. Further analysis suggests that the overestimation of precipitation over the SCB is attributed to the stronger low‐level easterly anomalous wind, much more moisture supply, and intensified atmospheric instability in the model than in ERA5. In addition, the predicted earlier easterly anomalous wind contributes to the earlier peak of precipitation in the day over the SCB. Findings of these basic model biases may be helpful to further understanding of the main cause of nocturnal peak of precipitation over the SCB, and provide a promising way forward for improvement of the model physical processes. Key Points: Increase in model horizontal resolution can reduce the wet bias and improve the diurnal phase of precipitation over the Sichuan Basin (SCB)Predicted earlier and stronger easterly anomalous wind contribute to the earlier peak of nocturnal precipitationThe model predicts a more unstable atmospheric condition which favors the overestimated precipitation over the SCB [ABSTRACT FROM AUTHOR]

The response of the Pacific Walker circulation (WC) to long‐term warming remains uncertain. Here, we diagnose contributions to the WC response in comprehensive and idealized general circulation model (GCM) simulations. We find that the spread in WC response is substantial across both the Coupled Model Intercomparison Project (CMIP6) and the Atmospheric Model Intercomparison Project (AMIP) models, implicating differences in atmospheric models in the spread in projected WC strength. Using a moist static energy (MSE) budget, we evaluate the contributions to changes in the WC strength related to changes in gross moist stability (GMS), horizontal MSE advection, radiation, and surface fluxes. We find that the multimodel mean WC weakening is mostly related to changes in GMS and radiation. Furthermore, the spread in WC response is related to the spread in GMS and radiation responses. The GMS response is potentially sensitive to parameterized convective entrainment which can affect lapse rates and the depth of convection. We thus investigate the role of entrainment in setting the GMS response by varying the entrainment rate in an idealized GCM. The idealized GCM is run with a simplified Betts‐Miller convection scheme, modified to represent entrainment. The weakening of the WC with warming in the idealized GCM is dampened when higher entrainment rates are used. However, the spread in GMS responses due to differing entrainment rates is much smaller than the spread in GMS responses across CMIP6 models. Therefore, further work is needed to understand the large spread in GMS responses across CMIP6 and AMIP models. Plain Language Summary: The Walker circulation (WC), an east‐west circulation over the tropical Pacific, has an uncertain response to climate warming. We diagnose contributions to the WC response in climate models. We find that the spread in WC responses is similar across atmosphere‐only models as across models with both an atmosphere and ocean, implicating the atmosphere in the spread in WC response. We find that the WC response and its spread across models are mostly related to changes in gross moist stability (GMS) and radiation. The GMS measures the propensity of the atmospheric circulation to export energy, and it is influenced by the vertical structure of temperature and winds. Changes in atmospheric radiation, especially those associated with clouds, amplify the effects of changes in GMS on the WC. The GMS is affected by an uncertain parameter in climate models, the entrainment rate. The entrainment rate controls how much clouds mix with their environment. Using an idealized climate model, we learn that the weakening of the WC response is dampened with higher entrainment rates. However, the effect of different entrainment rates is too small to explain the large spread in GMS and WC responses across models; further work is needed to understand this large spread. Key Points: The atmosphere plays an important role in setting the large spread in the Walker circulation (WC) response to warming in coupled modelsEnergetic analysis shows the WC response and its spread are strongly related to the responses of the gross moist stability (GMS) and radiationThe responses of the WC and GMS exhibit some sensitivity to convective entrainment in an idealized general circulation model [ABSTRACT FROM AUTHOR]

Results of two‐dimensional and narrow three‐dimensional (2‐D and 2.5‐D) simulations of a gravity wave (GW) packet localized in altitude and along its propagation direction employing a new, versatile compressible model are described. The simulations explore self‐acceleration and instability dynamics in an idealized atmosphere at rest under mean solar conditions in a domain extending to an altitude of 260 km and 1,800 km horizontally without artificial dissipation. High resolution in the central 2.5‐D domain enables the description of 3‐D instability dynamics accounting for breaking, dissipation, and momentum deposition within the GW packet. 2‐D results describe responses to localized self‐acceleration effects, including generation of secondary GWs (SGWs) at larger scales able to propagate to much higher altitudes. 2.5‐D results exhibit instability forms consistent with previous 3‐D simulations of instability dynamics and cause SGW generation and propagation at smaller spatial scales to weaken significantly compared to the 2‐D results. SGW responses at larger scales are driven primarily by GW/mean flow interactions arising at early stages of the self‐acceleration dynamics prior to strong GW instabilities and dissipation. As a result, they exhibit similar responses in both the 2‐D and 2.5‐D simulations and readily propagate to high altitudes at large distances from the initial GW packet. A companion paper examines these dynamics for an initial GW packet localized in three dimensions and evolving in a representative 3‐D tidal wind field. Key Points: Localized GW packets exhibit strong self‐acceleration dynamics as their amplitudes increaseSelf‐acceleration causes cessation of vertical propagation, local 3‐D instabilities and dissipation, and SGW and AW generationSGW has spatial scales determined largely by the packet scales and thus have larger scales and phase speeds that easily reach higher altitudes [ABSTRACT FROM AUTHOR]

Year‐to‐year forecasts of the Arctic climate conditions have drawn much interest in recent decades. This study investigates the primary drivers responsible for interannual variations (without multi‐decadal trends) of air temperature at 2 m (T2m) and sea ice concentration (SIC) over the Barents‐Kara Seas (BKS) during September from 1950 to 2021. The Arctic Dipole (AD), a leading factor in the BKS variability, plays an important role via influencing large‐scale atmospheric circulations and local thermodynamical feedbacks (i.e., vapor and albedo feedbacks). During a negative AD phase, the positive surface air temperature advection anomalies warm up the BKS region and promote sea ice melting, which in turn maintains the below‐normal sea ice and warmer‐than‐normal conditions via positive vapor and albedo feedbacks. Moreover, our results suggest that the sea surface temperature (SST) variability in the midlatitudes of the North Atlantic in the preceding month could also be an important driver of the SIC variability over the BKS area. The anomalous temperature advections from the surface to the upper troposphere are accompanied by a Rossby wave‐type atmospheric response, which intensifies the energy exchanges between the ocean and the atmosphere. Local SST anomalies associated with the surface heat flux anomalies further persist in the following month and influence sea ice variations due to their long‐lasting memory. A better understanding of the mechanisms of interannual climate variabilities over the BKS region in boreal early autumn is helpful to improving the Arctic regional climate predictions. Key Points: Understanding primary drivers of the interannual climate variabilities over the Arctic is essential to improve the regional prediction skillsThe Arctic Dipole is mainly responsible for driving the Barents‐Kara Seas (BKS) surface air temperature interannual variability in SeptemberThe North Atlantic sea surface temperature variability at 1‐month lead is associated with the early autumn sea ice cover variability in the BKS region [ABSTRACT FROM AUTHOR]

The model uncertainty, one of the major sources of projection uncertainty, is still a challenge in the climate change projection. In this study, the model uncertainty in simulating the East Asian summer monsoon rainfall and its physical link to the upper‐level jet streams and the western North Pacific subtropical high (WNPSH) are investigated from the Coupled Model Inter‐comparison Project Phase 6 (CMIP6) models. The results show that extracted by the inter‐model Empirical Orthogonal Function method, the leading inter‐model mode of the rainfall is characterized as a meridional pattern. The positive mode (a continent‐wet‐ocean‐dry pattern) is closely associated with a northward shift of the oceanic branch of the East Asian subtropical jet and a northward movement of the WNPSH, while the negative mode (a continent‐dry‐ocean‐wet pattern) is highly correlated with an enhanced East Asian polar‐front jet combined with a weakened land branch of the East Asian subtropical jet as well as a weakened and eastward recess of the WNPSH. This systematic link among the atmospheric circulations and the leading inter‐model mode is maintained by the negative phase of the Victoria Mode in the North Pacific and warming over the Atlantic Ocean in the positive mode, and the cooling over the Indian Ocean, Atlantic Ocean and the warming over the central Pacific in the negative mode. Our findings suggest that the physical linkage between the atmospheric circulations and monsoon rainfall is useful to understand the model uncertainty, which would be helpful in providing potential indicators of the East Asia summer monsoon change. Plain Language Summary: Monsoon rainfall has profound economic and social impacts, however large uncertainties lie in the climate model simulations. In this study, the inter‐model spread is used to extract the model uncertainty for the East Asian summer monsoon rainfall. The leading inter‐model mode is a meridional dipole pattern, which is closely associated with the concurrent variations of jet streams and the intensity and meridional movement of the western North Pacific subtropical high (WNPSH), but with the asymmetric response. The associated jet stream and the WNPSH variations are sustained by the oceanic conditions over the Atlantic Ocean, Pacific Ocean, and Indian Ocean, and therefore results in the inter‐model spread of the monsoon rainfall. Our findings suggest a useful metric to understand the traces of model uncertainties, focused on the physical linkage between the atmospheric circulations and the monsoon rainfall. Key Points: The leading inter‐model spread in East Asian summer monsoon rainfall is a meridional pattern from the Coupled Model Inter‐comparison Project Phase 6 modelsThe associated jet streams and western North Pacific subtropical high (WNPSH) variations exhibit asymmetric responses to this inter‐model spreadThe uncertainties in the north Pacific, Atlantic, and Indian Ocean have physically linked the jet stream and WNPSH variations to this inter‐model spread [ABSTRACT FROM AUTHOR]

Based on the Modern‐Era Retrospective analysis for Research Applications Version 2 reanalysis, an upper stratosphere/lower mesosphere (USLM) disturbance in early January and a sudden stratospheric warming (SSW) in early February are both coincidentally captured preceding the anomalously strong polar vortex during the late boreal winters of 2010/2011 and 2019/2020. Our analysis indicates that planetary wave breaking during the USLM disturbance and SSW decelerates the westerly polar night jet (PNJ) in the upper stratosphere, which leads to a continuous downward and poleward propagation of the PNJ during the early and middle winter. As the peak height of the PNJ descends, the region with negative vertical wind shear and negative waveguides also extends downward and reaches an altitude of ∼40 km during the late winter, which significantly limits the upward propagation of planetary waves, inducing a stable polar vortex in the lower stratosphere. In addition, the transformed Eulerian mean diagnostic results based on the specified dynamics simulation of the Whole Atmosphere Community Climate Model eXtended version confirm that the planetary wave‐induced downwelling is the main cause of the stratospheric warming during the two perturbations. More importantly, the simulation results indicate that the strength of the two upper stratospheric perturbations is appropriate to maintain the reflective configuration during the late winter, which suggests that such a specific sequence of perturbations during the early and middle winter, namely a USLM disturbance followed by a minor SSW, is favorable for the development of a late‐winter unusually strong Arctic polar vortex in the lower stratosphere. Plain Language Summary: The ozone depletion in the Arctic is rarely observed, which is usually related to exceptionally strong westerly winds and polar vortices in the Arctic lower stratosphere since they will block the tropics‐to‐pole mass transport. Since the 21st century, only two strong Arctic polar vortex events in the lower stratosphere during late winter have occurred; one in 2010/2011 and the other in 2019/2020. Before the late‐winter strong polar vortex in the two winters, the planetary waves disturb the polar vortex in the upper stratosphere/lower mesosphere twice so that further wave activity becomes reflected away. Such early and middle winter evolution is unique in the 43‐year historical record. The strength of these perturbations is appropriate for reconfiguring the circulation to reflect planetary waves and help maintain a strong polar vortex, which finally results in a less disturbed late winter. This phenomenon to some extent can be regarded as a negative feedback in the middle atmosphere. Our results show the high similarity and uniqueness in the winter evolution preceding the two strong Arctic polar vortex events, and highlight the role of the vertical coupling process, which may improve the predictability of the late‐winter strong polar vortex and ozone depletion in the Arctic. Key Points: A USLM disturbance and an SSW are both captured preceding the late‐winter unusually strong Arctic polar vortex in 2010/2011 and 2019/2020The continuous descent of the peak height of the polar night jet due to the two perturbations is the main cause of the strong polar vortexThe roles of planetary and gravity waves are distinguished in the vertical coupling based on model simulations in the two winters [ABSTRACT FROM AUTHOR]

We investigate the dependence of radiative feedback on the pattern of sea‐surface temperature (SST) change in 14 Atmospheric General Circulation Models (AGCMs) forced with observed variations in SST and sea‐ice over the historical record from 1871 to near‐present. We find that over 1871–1980, the Earth warmed with feedbacks largely consistent and strongly correlated with long‐term climate sensitivity feedbacks (diagnosed from corresponding atmosphere‐ocean GCM abrupt‐4xCO2 simulations). Post 1980, however, the Earth warmed with unusual trends in tropical Pacific SSTs (enhanced warming in the west, cooling in the east) and cooling in the Southern Ocean that drove climate feedback to be uncorrelated with—and indicating much lower climate sensitivity than—that expected for long‐term CO2 increase. We show that these conclusions are not strongly dependent on the Atmospheric Model Intercomparison Project (AMIP) II SST data set used to force the AGCMs, though the magnitude of feedback post 1980 is generally smaller in nine AGCMs forced with alternative HadISST1 SST boundary conditions. We quantify a "pattern effect" (defined as the difference between historical and long‐term CO2 feedback) equal to 0.48 ± 0.47 [5%–95%] W m−2 K−1 for the time‐period 1871–2010 when the AGCMs are forced with HadISST1 SSTs, or 0.70 ± 0.47 [5%–95%] W m−2 K−1 when forced with AMIP II SSTs. Assessed changes in the Earth's historical energy budget agree with the AGCM feedback estimates. Furthermore satellite observations of changes in top‐of‐atmosphere radiative fluxes since 1985 suggest that the pattern effect was particularly strong over recent decades but may be waning post 2014. Key Points: Post 1980 the Earth warmed with feedbacks uncorrelated with—and indicating much lower equilibrium climate sensitivity than—that expected for long‐term CO2 increaseSatellite observations of changes in top‐of‐atmosphere radiative fluxes since 1985 are in agreement with the modelsThe pattern effect may be waning post 2014 [ABSTRACT FROM AUTHOR]

The surface heating on the Tibetan Plateau (TP) exerts large influence on the Asian summer monsoon. Drivers of TP surface heat include the tropical forcings and other climate modes in middle and high latitude of the Northern Hemisphere, however whether the climate modes in Southern Hemisphere influence the TP surface heat is rarely studied. Using multiple source data diagnosis and numerical experiment from an atmospheric general circulation model (AGCM), we found that the Antarctic Oscillation (AAO) in May can efficiently modulate the subsequent surface heat source over the TP in June. When the AAO is in positive phases, a northeastward propagating atmospheric Rossby wave train originates from the Amundsen Sea low. As a part of this wave train, a pair of anomalous cyclone and anticyclone in the Southern Hemisphere accelerates the surface southeasterlies between them, accompanied by cold sea surface temperature (SST) anomaly in the equatorial middle and eastern Indian Ocean induced by the wind‐evaporation‐SST feedback. Due to the large thermal inertia, in June, the cold SST anomaly stimulates anticyclone anomalies over the western TP and eastern Arabian Sea, which increase the moisture transportation toward the TP and are conducive to the formation and maintenance of the precipitation over the middle and eastern TP. In contrast, the surface heat source, which is dominated by the upward sensible heat flux, is reduced substantially. This result indicates that the AAO can be one of the precursors of the TP heat source, and may help improve the prediction of the Asian summer monsoon. Plain Language Summary: The surface heating of the Tibetan Plateau (TP) has vital impacts on the Asian monsoon, and people's production and life. However, previous studies have mainly focused on the study of the Northern Hemisphere, and less attention has been paid to whether and how the Southern Hemisphere signals can affect the surface heating of the TP. Through numerical simulations and various data diagnostic studies, we show that the air pressure anomaly around the Antarctic (Antarctic Oscillation; AAO) in May can effectively regulate the surface heat source over the TP in June. During the positive phase of the AAO, an atmospheric wave train propagates from the Amundsen Sea to the Indian Ocean. Due to the anomalous surface wind, the wind‐evaporation‐SST feedback has been enhanced, resulting in the cold SST anomaly in the equatorial Indian Ocean. Owing to the large thermal inertia of the ocean, the cold SST anomaly lasts until June. The anticyclone anomalies in the western TP and the eastern Arabian Sea are stimulated, which are conducive to the transport of water vapor over the TP and induce the abnormal precipitation over the TP, while the surface sensible heat flux is greatly weakened, which affects the surface heating of the TP. Key Points: A robust linkage exists between the Tibetan Plateau (TP) surface heating and the Antarctic Oscillation (AAO)The AAO influences the Indian Ocean through a Rossby wave trainThe Indian Ocean sea surface temperature prolongs the effect of the AAO and influences the TP surface heating [ABSTRACT FROM AUTHOR]

The May and June precipitation (known as early summer precipitation; ESP) is an important water resource for South China and Taiwan (SCTW). This study explores the similarities and differences between the Community Earth System Model Version 2‐Large Ensemble (CESM2‐LE) and the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi‐model ensemble in simulating the characteristics of ESP over SCTW in the present day, and in projecting the related changes at the end of 21st century. For the present‐day simulation, our results showed that CESM2‐LE and CMIP6 outperformed each other for different examined features. CESM2‐LE was slightly better than CMIP6 in capturing the magnitude of ESP over SCTW, while CMIP6 was more capable of representing the interdecadal shifts in the occurrence timing of the ESP maximum. Both the CESM2‐LE and CMIP6 projections indicated that ESP will be enhanced and the phase of maximum ESP will be delayed, from peaking around mid‐June in the present to late June in the future. These changes can be attributed to an enhanced short‐wave trough over southwest China and a late intensification of the southwesterly wind ahead of the short‐wave trough, which help transport more moisture from the north of the South China Sea to SCTW, particularly in mid‐to‐late June. In addition, relative to CMIP6, CESM2‐LE showed less uncertainty in the projected increase in ESP and phase delay. This finding highlights that model diversity may play a more important role than internal variability in attributing the uncertainty of projected changes in ESP over SCTW. Key Points: Early summer rainfall over South China and Taiwan is projected to be enhanced, with a delayed peak occurred in late June in the futureInternal variability is less important than model diversity in changing the sign (increase or decrease) of projected early summer rainfallRoles of atmospheric circulation modulations on the projected changes in early summer rainfall are identified [ABSTRACT FROM AUTHOR]

Thermal structures at the sea surface are known to affect the overlying atmospheric dynamics over various spatio‐temporal scales, from hourly and sub‐kilometric to annual and O(1,000 km). The relevant mechanisms at play are generally identified by means of correlation coefficients (in space or time) or by linear regression analysis using appropriate couples of variables. For fine spatial scales, where sea surface temperature (SST) gradients get stronger, the advection might disrupt these correlations and, thus, mask the action of such mechanisms, just because of the chosen metrics. For example, at the oceanic sub‐mesoscale, around 1–10 km and hourly time scales, the standard metrics used to identify the pressure adjustment mechanism (that involves the Laplacian of sea surface temperature, SST, and the wind divergence) may suffer from this issue, even for weak wind conditions. By exploiting high‐resolution realistic numerical simulations with ad hoc SST forcing fields, we introduce some new metrics to evaluate the action of the pressure adjustment atmospheric response to the surface oceanic thermal structures. It is found that the most skillful metrics is based on the wind divergence and the SST second spatial derivative evaluated in the across direction of a locally defined background wind field. Plain Language Summary: The ocean surface is characterized by a range of warm and cold structures that are known to influence the overlying atmospheric flow through different mechanisms. One of these mechanisms involves the variation of sea level pressure that can drive secondary wind circulations according to how the sea surface temperature is distributed in space. To assess whether this mechanism is in action, the co‐location of sea temperature maxima (or minima) with zones of wind convergence (divergence) is generally considered. However, the presence of the wind itself has been shown to displace and delay the wind response so that there are cases where the pressure field responds to the sea temperature forcing but this is not detected by the standard metrics. Since pressure variability is generated in all directions, we propose to measure this kind of wind response in the direction perpendicular to the background wind in order to avoid the masking effect of the background wind. Key Points: The standard metrics for the pressure adjustment mechanism is adversely affected by advectionThree new metrics are introduced and testedThe pressure adjustment is detectable in the direction perpendicular to the background wind [ABSTRACT FROM AUTHOR]

In this study, we use idealized aquaplanet simulations of Earth‐like planets to explore the large‐scale atmospheric circulation's response to changes in shortwave radiation, through changes in the planet obliquity, and in longwave radiation, through changes in the prescribed optical depth. We primarily focus on the extratropical circulation and the counterintuitive weakening of extratropical eddy activity, and associated precipitating storm tracks, with increased obliquity. We show that on high‐obliquity planets with a small surface thermal inertia, net energy deficit during the winter months is primarily balanced by the latent energy component of the atmospheric heat capacity, which buffers any significant atmospheric cooling. As temperatures finally start to decrease, condensational latent heat release of atmospheric moisture further slows down the cooling and keeps the winter pole warmer than the midlatitudes until after the winter solstice. This prevents vigorous baroclinic eddy activity, which lives off the potential energy stored in the sloping isopycnals, as well as the development of a Ferrel cell and storm track. For planets with larger surface heat capacity, we see a similar suppression, with the energy storage in the ocean surface, rather than atmospheric latent energy storage, primarily balancing the net energy loss during the winter months. These results suggest that, regardless of surface thermal inertia, water‐covered high‐obliquity Earth‐like planets would not experience significant extratropical storm activity and highlight the need for a proper characterization of polar properties in planetary modeling and observational investigations. Plain Language Summary: We explore how global wind patterns and atmospheric energy transport respond to a change in radiation distribution, using a simple general circulation model of an Earth‐like planet covered by a thin ocean. We examine changes in both the incoming stellar radiation, through changes in obliquity, and the planet's outgoing thermal radiation, by varying the atmosphere's optical thickness. We focus on the high‐latitude effects, where increased obliquity leads to a counterintuitive reduction in eddies. We show that on high‐obliquity planets with strong seasonal cycles, water vapor plays a dominant role in the energy budget and serves as a mechanism for storing energy. The energy released from condensation in early winter helps keep the poles warm for several extra weeks, reducing the temperature differences between the high and low latitudes. This leads to smaller values of potential energy available to fuel eddies, which in turn leads to an absence of the Ferrel cell and storm tracks. For planets with a thicker ocean, energy becomes increasingly stored in the ocean surface, rather than in the atmospheric column. These results suggest that, regardless of ocean surface properties, water‐covered high‐obliquity planets would not experience significant high‐latitude storm activity. Key Points: Atmospheric moisture can mitigate extreme seasonal excursions, reducing summer temperature maxima on high‐obliquity planetsLatent heat release slows polar cooling, keeping the winter pole warm for longer, and weakening baroclinic eddies and storm tracksCombined effects of atmospheric moisture storage and surface heat capacity suggest that high‐obliquity planets tend to have weak baroclinicity [ABSTRACT FROM AUTHOR]

The relationships between the El Niño‐Southern Oscillation (ENSO), Asian‐Pacific region summer precipitation and its corresponding atmospheric circulations exhibit interdecadal changes. From the early 1990s to the early 2000s, these relationships were significant. Therefore, we divided this time frame into three periods for analysis: 1979–1991 (P1), 1992–2005 (P2), and 2006–2019 (P3). The possible mechanism underlying these relationships is as follows: the southern Indian Ocean sea surface temperature (SST) over the east of Madagascar exhibits interdecadal variation. During P2, the SST was relatively cold, which induced an anomalously high mascarene high (MH). The strengthened MH enhanced the Somali jet and Indian monsoon westerlies, which intensified the strong center of the Indian Ocean Walker circulation. Therefore, owing to the strength and location changes of the Indian Walker circulation and the Walker circulation, the connection between the ENSO and the western Pacific vertical motions strengthened, resulting in the close relationship of the ENSO with the atmospheric circulation over the Asian‐Pacific region. Hence, ENSO can influence a north‐south tripole pattern of precipitation over the Asian‐Pacific monsoon region through local vertical activities and meridional Hadley circulation over western Pacific. Numerical experiments using an atmospheric general circulation model, with prescribed three times southern Indian Ocean SST anomalies of 1995–2005 relative to 1979–2019, also lend support to the southern Indian Ocean SST's contribution to modulating the relationship between ENSO and summer precipitation over the Asian‐Pacific monsoon region. Key Points: The linkages of the El Niño‐Southern Oscillation (ENSO), Asian‐Pacific summer precipitation and its corresponding atmospheric circulations exhibit interdecadal changesThe southern Indian Ocean sea surface temperature (SST) is the key role to modulate the relationship between ENSO and Asian‐Pacific climateThe extratropical SST of the southern Indian Ocean can also modulate the relationship between ENSO and Asian‐Pacific climate [ABSTRACT FROM AUTHOR]

A warm Arctic‐cold continent (WACC) pattern has been one of the main characteristics of the Northern Hemisphere (NH) climate variability in winter during the last two decades. However, the factors contributing this pattern remain unclear. We compared the two leading modes of surface temperature in the NH winter on an interdecadal timescale and explored the role of high‐latitude concurrent blockings (HCBs) and the Arctic oscillation (AO) in these two modes. Results show that the first mode resembling the WACC pattern is more related to enhanced HCBs over the Ural Mountains and the North Pacific than the AO. The HCBs induce a warmer Arctic by simultaneously transporting large amounts of moisture and heat. The HCBs excite a tropospheric anticyclonic anomaly near the Arctic Circle, which, in turn, triggers anomalies in the propagation of planetary waves and the meridional circulation, with a subsequent redistribution of momentum and heat. This is accompanied by a weak polar night jet and a poleward shift in the subtropical westerly jet, resulting in mild cold mid‐latitude continents. The second mode represents the AO pattern, which has a different mechanism from WACC. The high‐pressure anomaly over the North Pacific has played an increasingly important part in the WACC mode in recent decades and the WACC pattern with HCBs is more easily reproduced in simulations with human activity. HCBs therefore require further consideration under the current and future conditions of global warming. Plain Language Summary: A rapidly warming Arctic and mild cold mid‐latitude continents have been one of the main characteristics of the Northern Hemisphere winter during the last two decades. However, the factors contributing this warm Arctic‐cold continent (WACC) pattern remain unclear, although anomalies in blocking highs and the Arctic oscillation (AO) are possible factors. The mode with high‐latitude concurrent blockings (HCBs) is more consistent with the WACC pattern than the AO. In the HCB mode, a strong anticyclonic anomalous circulation over the Arctic Circle, excited by the strong HCBs over the Ural Mountains and the North Pacific, changes the polar atmospheric circulation and redistributes both momentum and heat to give a warmer Arctic than in the AO mode. A weak polar night jet and a poleward shift in the subtropical westerly jet result in mild cold mid‐latitude continents. However, these features are not seen in the AO mode, which is characterized by cool continents and a warm Greenland. Besides, human activities may also be contributed to the interdecadal WACC because there is an interdecadal increase in the WACC pattern with HCBs in the CMIP6 simulations with human activities. These results suggest that the role of HCBs should be highly paid attention in winter mid‐high latitude extreme events in recent decades. Key Points: A warm Arctic‐cold continent (WACC) pattern is associated with enhanced high‐latitude concurrent blockings (HCBs) over the Ural Mountains and North PacificThe HCBs excite a tropospheric anticyclonic anomaly near the Arctic Circle, which leads to more warm moist air into the ArcticThe WACC pattern with HCBs is more easily reproduced in CMIP6 simulations with human activity than in simulations without human activity [ABSTRACT FROM AUTHOR]

The summertime North Indian Ocean (NIO) and tropical North Atlantic (TNA) sea surface temperature (SST) anomalies exert important impacts on atmospheric circulation and climate variation. Little attention is paid on the interdecadal change of NIO–TNA SST connection. This study reveals that the NIO–TNA SST relationship experiences an obvious interdecadal change around the early 2000s. The NIO and TNA SST correlation is positive and significant before the early 2000s, while this connection becomes weak and insignificant after that. This interdecadal change is closely associated with the changes in the El Niño‐Southern Oscillation (ENSO) intensity. During 1980–2001, strong ENSO forces a sustained SST warming in the southwest Indian Ocean, which further warms the summertime NIO via inducing antisymmetric circulation and reducing the upward surface latent heat flux. Meanwhile, strong ENSO forces a summertime TNA warming via arousing the Pacific North American (PNA) teleconnection. The PNA weakens the TNA trade winds, then reduces the upward surface heat flux and consequently warms the TNA. Thus, NIO and TNA SST connection is established via ENSO events during 1980–2001. In contrast, during 2002–2020, the summertime NIO SST anomaly is still related to ENSO. However, the connection between the TNA SST anomaly and ENSO is interrupted due to the weak ENSO intensity. Such weak ENSO yields a much weaker PNA teleconnection, which is inefficient to generate the TNA warming. Further analysis shows that the TNA warming during 2002–2020 is generated mainly by the negative North Atlantic Oscillation. Therefore, the connection between NIO and TNA SST anomalies collapses under the weak ENSO condition. Key Points: The summertime North Indian Ocean (NIO) and tropical North Atlantic sea surface temperature (TNA SST) connection experiences an obvious interdecadal change around the early 2000sThe NIO–TNA SST high correlation during 1980–2001 is established by the significantly delayed El Niño‐Southern Oscillation (ENSO) influence on the remote oceansThe NIO–TNA SST weak correlation during 2002–2020 is because of the weakened ENSO, which is inefficient to generate the TNA SST anomalies [ABSTRACT FROM AUTHOR]

Abrupt and large‐scale climate changes have occurred repeatedly and within decades during the last glaciation. These events, where dramatic warming occurs over decades, are well represented in both Greenland ice core mineral dust and temperature records, suggesting a causal link. However, the feedbacks between atmospheric dust and climate change during these Dansgaard–Oeschger events are poorly known and the processes driving changes in atmospheric dust emission and transport remain elusive. Constraining dust provenance is key to resolving these gaps. Here, we present a multi‐technique analysis of Greenland dust provenance using novel and established, source diagnostic isotopic tracers as well as results from a regional climate model including dust cycle simulations. We show that the existing dominant model for the provenance of Greenland dust as sourced from combined East Asian dust and Pacific volcanics is not supported. Rather, our clay mineralogical and Hf–Sr–Nd and D/H isotopic analyses from last glacial Greenland dust and an extensive range of Northern Hemisphere potential dust sources reveal three most likely scenarios (in order of probability): direct dust sourcing from the Taklimakan Desert in western China, direct sourcing from European glacial sources, or a mix of dust originating from Europe and North Africa. Furthermore, our regional climate modeling demonstrates the plausibility of European or mixed European/North African sources for the first time. We suggest that the origin of dust to Greenland is potentially more complex than previously recognized, demonstrating more uncertainty in our understanding dust climate feedbacks during abrupt events than previously understood. Plain Language Summary: Abrupt climate change represents an existential threat to civilization. However, the feedbacks that modulate these abrupt changes are poorly understood, undermining our ability to predict future events. Last glacial Greenland ice core records show abrupt climate events coupled to changes in abundance of atmospheric mineral dust, but how dust impacts these events is unclear as the processes involved in dust emission remain elusive. Here we apply multiple novel tracers of dust provenance as well as regional dust cycle modeling to address this uncertainty. We show that the dominant model of mixed East Asian and Pacific volcanic sources to Greenland dust is not supported. Instead, multiple other source scenarios are plausible, demonstrating far more uncertainty in dust climate feedbacks than previously understood. Key Points: Hf–Nd isotope data demonstrate that Central Greenland last glacial dust is not sourced from mixed SE Asian and circum‐Pacific volcanic dustMost likely sources are the Taklimakan in SE Asia, European and/or North African glacial dust deposits based on Hf–Sr–Nd and D/H isotopesModel simulations show that sourcing of North Greenland Ice Core Project ice core dust from Europe is plausible, the addition of North African dust cannot be excluded [ABSTRACT FROM AUTHOR]

Weather regimes are large‐scale atmospheric circulation states that frequently occur in the climate system with persistence and recurrence, and are associated with the occurrence of specific local weather conditions. This study evaluates the representation of the four Euro‐Atlantic weather regimes in uninitialized historical forcing simulations and initialized decadal predictions performed with the EC‐Earth3 coupled climate model. The four weather regimes are the positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO−, respectively), Blocking, and Atlantic Ridge in winter; and the NAO−, Blocking, Atlantic Ridge, and Atlantic Low in summer. We also analyze the impact that the model initialization toward the observed state of the climate system has on the ability to predict the variability of the weather regimes' seasonal frequency of occurrence. We find that the EC‐Earth3 model correctly reproduces the spatial patterns and climatological occurrence frequencies of the four weather regimes. By contrast, the skill in predicting the inter‐annual to decadal variations of the weather regimes' seasonal frequencies is generally low, and the initialization does not significantly improve such skill. The observed teleconnections between the weather regimes and the North Atlantic sea surface temperatures are generally not reproduced by the model, which could be a reason for the low skill in predicting the temporal variations of the weather regime frequencies. Key Points: The EC‐Earth3 model simulates the spatial patterns and climatological frequencies of the Euro‐Atlantic weather regimes realisticallyCorrelations between simulated and observed frequencies of weather regimes on inter‐annual to decadal time scales are generally lowModel initialization does not significantly alter the skill in predicting the spatial patterns and temporal variations of the weather regimes [ABSTRACT FROM AUTHOR]

Regional transport of air pollutants is modulated by large‐scale synoptic circulation, which has been poorly understood for heavy air pollution in the downstream receptor region. In the present study of 5‐year (2015–2019) observation, we targeted the Twain‐Hu Basin (THB), a region of heavy PM2.5 pollution over central China, to investigate the regulation of synoptic circulation governing regional PM2.5 transport for heavy air pollution. It was found that regional transport of PM2.5 predominated 65.2% of the heavy pollution events (HPEs) over the THB based on the statistics of observational environment and meteorology. By employing the FLEXPART‐WRF model, the regional transport of PM2.5 from upwind areas in central and eastern China (CEC) to receptor region in the THB was identified with three prominent pathways in the northerly, northeasterly, and easterly directions respectively. We then used T‐mode principal component analysis in conjunction with the K‐means cluster method to categorize synoptic circulations, and it was recognized that three regional PM2.5 transport pathways for the HPEs over central China were governed respectively by three patterns of synoptic circulation over CEC with (a) weak high air pressure to the north (13.3%), (b) strong high air pressure to the northeast (46.7%), and (c) weak high air pressure to the east (40.0%). This study reveals a significant modulation of large‐scale synoptic circulation for regional transport of air pollutants in environmental change. Plain Language Summary: Compared to emission source regions (e.g., North China Plain), the modulation of synoptic circulation on regional transport of PM2.5 in heavy air pollution over the downwind receptor region has been poorly understood. In this study, therefore, we investigated the heavy air pollution events (HPEs) in the Twain‐Hu Basin (THB) over central China–a critical regional transport hub–from 2015 to 2019, by using the model FLEXPART‐WRF and the T‐mode principal component analysis (T‐PCA) combined with the K‐means cluster method. The majority of the HPEs in THB were triggered by regional transport of PM2.5 due to the southward invasion of cold air mass from upwind regions driven by East Asian winter monsoonal winds. Based on FLEXPART‐WRF simulations, regional PM2.5 transport was characterized along three routes in the northerly, northeasterly and easterly directions with the averaged contribution of 57.9% to PM2.5 concentrations for HPEs in the THB, which is closely related to evolution of synoptic circulation. By utilizing T‐PCA combined with the K‐means cluster method, three synoptic circulation types were identified to build three regional PM2.5 transport pathways, revealing an important regulation of large‐scale atmospheric circulations on regional air pollutant transport causing HPEs over central China. Key Points: Heavy air pollution events induced by regional PM2.5 transport presents an increasing trend over the past 5 years in central ChinaThree regional transport pathways are identified to central China in the northerly, northeasterly, and easterly directions respectivelySynoptic circulation modulates regional transport in air quality change with the large contribution to air pollutants over central China [ABSTRACT FROM AUTHOR]

Paleoclimatic records reveal spatially asynchronous changes of East Asian summer monsoon (EASM) precipitation during the Holocene, but the underlying mechanisms are not fully understood. This study explores the role of the zonal shift of the Asian westerly jet (AWJ) core on the spatial‐temporal variability of EASM precipitation based on Holocene climate simulations using the KCM and CESM forced by orbital variations. We analyzed the composite differences of summer precipitation and associated atmospheric circulations between the early (middle) and middle (late) Holocene. Simulations suggest a dipole pattern of summer precipitation with increasing (decreasing) precipitation in central (southern) China from the early to late Holocene, which was shaped by the eastward shift of the AWJ core. The eastward shift of the AWJ core and the accompanied eastward movement of the upper‐level convergence over the AWJ's right exit region weakened the low‐level downward motions over central China. Meanwhile, the low‐level upward motions over southern China were weakened by the concurrent eastward (westward) migration of the South Asian High (western Pacific subtropical High) during the Holocene. Consequently, summer precipitation increased (decreased) in central (southern) China from the early to late Holocene. The eastward shift of the AWJ core was primarily driven by amplified meridional insolation gradient at middle latitudes via strengthening upper‐level westerly intensity during the Holocene. In addition, land cover difference between East Asia and central Asia caused a greater enhancement of westerlies intensity in East Asia, further modulating the eastward movement of the AWJ core during the Holocene. Key Points: Our simulations reveal a dipole pattern of summer precipitation between central and southern China during the HoloceneThe dipole precipitation pattern was strongly modulated by zonal shift of the Asian westerly jet coreInsolation variation and land cover caused the eastward shift of the Asian westerly jet core during the Holocene [ABSTRACT FROM AUTHOR]

The representation of the South American Monsoon System (SAMS) by general circulation models (GCMs) is of key relevance for a better understanding of the physical rationale behind the recent climate changes over South Tropical South America (STSA) and their expected changes in a global warming scenario. During the last four decades, STSA experienced a lengthening of the dry season associated with diverse forcings. In this work, a set of 16 GCMs historical Coupled Model Intercomparison Project Phase 6 coupled simulations were evaluated during 1979–2014 in terms of how well they reproduced the atmospheric circulation over STSA through a circulation‐patterns (CPs) approach. Nine CPs were first identified based on low‐level winds from the ERA5 reanalysis. Focus was put on the representation of CPs during the dry‐to‐wet transition season (July‐October). Model performance depended on the seasonal cycle and spatial structure of the CPs. GCMs adequately reproduced the different CPs, with lower skills in the transition seasons. GCMs tended to go from dry to wet conditions too quickly, evidencing deficiencies in the representation of the SAMS onset, related to a poor representation of the southerly wind intrusions to STSA and the variability of the South American low‐level jet. Some GCMs were able to associate the occurrence of anomalous dry and wet years with specific CPs, suggesting well‐represented physical mechanisms controlling precipitation variability. This study could identify a few GCMs that adequately simulated the CPs in STSA (among them, CESM2, CMCC‐CM2‐HR4 and MPI‐ESM1‐2‐HR), which is relevant for driving high‐resolution models and the analysis of future projections. Key Points: Some general circulation models (GCMs) adequately represented both the seasonal cycle and spatial structure of circulation patternsGCMs often struggled in representing the monsoon initiation and the variability of the South American low‐level jetGCMs tended to go from dry to wet conditions too quickly [ABSTRACT FROM AUTHOR]

The hydroclimate of the Tibetan Plateau (TP) and Central Asia (CA) plays a crucial role in sustaining surface water reservoirs and thus water resources in the respective regions. In this study, we investigate the changes in Asian hydroclimate and its driving forces during specific time intervals in the last 3 Ma. We conduct high‐resolution (∼0.75° per grid cell) general circulation model ECHAM‐5 experiments with boundary conditions for the mid‐Pliocene (∼3 Ma), the Last Glacial Maximum (LGM; ∼21 ka), the mid‐Holocene (∼6 ka), and the pre‐industrial. Results suggest that seasonally relatively high precipitation rates (>1 mm day−1) were longer in the mid‐Pliocene and shorter in the LGM, relative to the pre‐industrial. We calculate different monsoon indices to detect changes in the intensity, strength and duration of the East Asian summer monsoon (EASM), South Asian summer monsoon (SASM), and the Indian summer monsoon (ISM), and construct climatologies of mid‐latitude high‐level westerly jet (WJ) stream occurrences based on the ECHAM5 wind fields. Our results suggest that in warm periods (e.g., mid‐Pliocene or interglacial), the WJ migrates northward earlier in the year (April) and reaches higher latitudes than in the pre‐industrial, resulting in a wetter TP and CA. During cooler periods (e.g., LGM or glacial), the WJ migrates northward later in the year (June) and remains over lower latitudes, resulting in a drier TP and CA. Increased/decreased local precipitation in TP and CA for the mid‐Pliocene/LGM experiments correlates strongly with (a) intensity, strength and duration of the EASM, SASM, and the ISM and (b) WJ latitudinal position. Key Points: High‐resolution general circulation model (ECHAM5) is used to study past changes in hydroclimate in the Tibetan Plateau and Central AsiaSeasons with high precipitation rates were longer in the mid‐Pliocene and shorter in the Last Glacial Maximum compared to the pre‐industrialLate Cenozoic hydroclimate of the region is linked to the dynamics of the mid‐latitude jet stream and the Indo‐Asian monsoon circulation [ABSTRACT FROM AUTHOR]

By employing a state‐of‐the‐art global atmospheric general circulation model from the Global Monsoons Model Inter‐comparison Project, this study investigates the mechanical and thermal impacts of the Tibetan‐Iranian Plateau (TIP) on the North Pacific storm track (NPST). When the TIP‐associated mechanical forcing is removed, the winter NPST strengthens remarkably on the northern flank of its climatological maximum and becomes inconspicuously separated from storm track activities over the Asian continent. The midwinter suppression of the NPST is almost inapparent but still exists. In contrast, when TIP‐associated thermal forcing is absent, the winter NPST amplitudes weaken prominently on the north and west sides, and the midwinter suppression remains obvious. The mechanical and thermal impact of the TIP on the NPST are robust not only in winter but also in other seasons. Without the TIP‐associated mechanical forcing, the intensified winter NPST may be dependent on the weakened and widened upper‐level jet, strengthened atmospheric baroclinicity on the north side of climatological winter upper‐level jet, enhanced baroclinic energy conversion, and attenuated East Asian trough. In contrast, without the TIP‐associated thermal forcing, the weakened winter NPST may be determined by the strengthened and narrowed upper‐level jet, reduced atmospheric baroclinicity on the northern flank of upper‐level jet, decreased baroclinic energy conversion, and intensified East Asian trough. Key Points: Winter North Pacific storm track (NPST) are intensified (weakened) in response to the removal of the mechanical (thermal) forcing of the Tibetan‐Iranian Plateau (TIP)The absence of TIP‐associated mechanical and thermal forcings also significantly impact the NPST in other seasonsAltered baroclinic energy conversion, East Asian trough and stationary eddy heat flux may explain the TIP‐induced winter NPST anomalies [ABSTRACT FROM AUTHOR]

The surface sensible heating over the Tibetan Plateau (TP) is a significant, elevated heating source in spring (March‐April‐May, MAM). It exerts considerable thermal effects on both regional and global climate. During MAM, the surface sensible heating over the TP triggers a giant saddle‐like circulation anomaly in the upper troposphere at 200 hPa. Although this structure was observed a long time ago and it contributes to the stationary wave, its formation mechanism remains unclear. Using the Linear Baroclinic Model, we reveal the evolution of this structure in detail. In the initial state, a typical baroclinic anticyclone with a vertical phase tilt forms around the heating source. When the wave propagates downstream due to strong relative vorticity advection and favorable lower level circulation, an anticyclone with a barotropic structure forms downstream. Our analysis indicates that both the barotropic process and baroclinic process contribute to the formation of the saddle‐like structure, and the latter is associated with the spring unique background circulation and vertical heating structure over the TP. Plain Language Summary: Due to the special geographical and topographical features, the surface sensible heating over the Tibetan Plateau (TP) regulates atmospheric circulation considerably, especially in boreal spring. Many early studies find that the spring sensible heat over TP triggers a saddle‐like circulation anomaly in the upper troposphere. However, this phenomenon is not well understood. This study explores the phenomenon by a simplified model. The result indicates that the saddle‐like pattern evolution can be divided into initial state, adjustment process, and stationary state, in which interaction between different vertical levels is critical. Both the special background circulation in spring and the heating structure over the TP are essential to the formation of the saddle‐like response. Key Points: This study shows how an upper‐troposphere saddle‐like pattern forms as a result of surface sensible heating over the Tibetan PlateauDifferent from traditional stationary wave theory, the baroclinic process is concerned and its role in the pattern formation is emphasizedIn the 3 stages of stationary pattern formation, the forcing structure and westerly jet shape are both essential for the response pattern [ABSTRACT FROM AUTHOR]

Land surface albedo plays a critical role in climate, hydrological and biogeochemical modeling, and weather forecasting. It is often assigned in models and satellite retrievals by albedo climatology look‐up tables using land cover type and other variables; however, there are considerable differences in albedo simulations among models, which partially result from uncertainty in obsolete albedo climatology. Therefore, this study introduces a new global 500 m daily blue‐sky land surface albedo climatology data set under both actual and snow‐free surface conditions utilizing 20‐year Moderate Resolution Imaging Spectroradiometer products from Google Earth Engine. In situ measurements from 38 long‐term‐maintained sites were utilized to validate the accuracies of different albedo climatology datasets. The root‐mean‐square error, bias, and correlation coefficient of the new climatology are 0.031, −0.003, and 0.96, respectively, which are more accurate than the Global Land Surface Satellite, GlobAlbedo, and 16 model datasets. Data intercomparison suggests that ERA5 exhibits better performance than Modern‐Era Retrospective Analysis for Research and Applications Version 2 (MERRA2) and 14 Coupled Model Intercomparison Projects Phase 6 models. However, it contains positive biases in the snow‐free season, while MERRA2 underestimates the snow albedo. Global albedo variation associated with basic surface plant functional types was also characterized, and snow impact was considered separately. Temporal variability analysis indicates that traditional climatology datasets with coarser temporal resolutions (≥8 days) cannot capture albedo variation over areas with distinct snow seasons, especially in central Eurasia and boreal regions. These results confirm the high reliability and robustness of the new albedo climatology in model assessment, data assimilation, and satellite product retrievals. Plain Language Summary: Surface albedo represents the Earth's surface ability to reflect solar energy, and thus directly influences atmospheric and water circulations by controlling available energies. As an essential variable estimated in current climate models, surface albedo is assigned by searching look‐up tables based on the land cover and soil types. However, there are large differences in simulated albedo among various models, partially because the albedo climatology in these tables was generated in the 1980s and needs to be updated. As global surface albedo has been accurately retrieved from satellite data for decades, we were able to produce a new albedo climatology data set under actual and snow‐free conditions with high spatiotemporal resolutions. Ground validation and data comparison indicate that the new data set performs with higher accuracy compared with other datasets. Moreover, assessment by the new climatology data set shows that albedo from advanced climate models and reanalysis datasets has clear uncertainties, especially over high latitudes and the Tibetan Plateau. Temporal variation in global albedo is better characterized at basic vegetation types. The new albedo climatology data set shows great potential for use in model assessment, modeling improvement, and satellite retrieval of other variables. Key Points: A global 500 m daily blue‐sky land surface albedo climatology data set in actual and snow‐free surface conditions was produced from Moderate Resolution Imaging SpectroradiometerSite validation and data comparison reveal its reliability and robustness in model assessment, data assimilation, and satellite retrievalsSimulated albedo data by state‐of‐art reanalysis and Coupled Model Intercomparison Projects Phase 6 models still have a clear bias in polar regions and the Tibetan Plateau [ABSTRACT FROM AUTHOR]

Since the late 1970s, observations have shown a widening of the tropical Hadley cell (HC) circulation. State‐of‐the‐art climate models reproduce the general trend along with a projected continuous expansion. Discrepancies in expansion rates of observation‐ and model‐based studies have been attributed to differences in applied methods, natural variability and model shortcomings. Furthermore, the driving influence of tropical or extratropical processes on these changes is not well understood. All of this highlights the dynamical mechanisms and the region of origin controlling the tropical width are still insufficiently understood. Here we examine the influence of systematic model biases of the atmosphere‐only Unified Model (UM) onto the simulation of the Southern Hemisphere (SH) tropical edge. We utilize nudged experiments with prescribed sea surface temperatures, where potential temperature and horizontal winds are relaxed back to ERA‐Interim reanalysis for a 20‐year period in selected regions. Correcting model biases in the tropics and extratropics separately allows us to dissect the dominant remote impacts of present model errors onto the SH tropical edge simulation. The experiments are applied to established tropical width metrics ranging from near‐surface to upper‐level metrics capturing the poleward flank of the HC. We find both regions work remotely to reduce errors in the UM fields and location of the tropical edge. Surprisingly, correcting the extratropical biases, south of 45°S, more consistently improves the tropical width across the metrics and seasons than nudging the tropics (10°N–10°S). These findings demonstrate the substantial role of extratropical influences in locating the SH tropical edge. Plain Language Summary: The width of the Tropics has been observed to expand polewards in the Southern Hemisphere over the satellite era. We compare and contrast experiments of an atmospheric general circulation model to determine the most influential regions on setting the width of the Tropics, using various measures to locate the tropical edge. In these experiments, typical model errors in the winds and temperature are fixed in specific regions and the remote impact is assessed. We find that corrections both north and south of the subtropics can improve the simulation of the tropical edge, however correcting model errors south of 45°S have the largest impact. Key Points: Tropical and extratropical biases influencing the climatological SH tropical edge position are analyzed using regional nudging experimentsCorrecting both tropical and extratropical biases have a remote impact by improving the model fields and the SH tropical edge positionNudging extratropical circulation errors most consistently improves the SH tropical edge position across different metrics [ABSTRACT FROM AUTHOR]