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This paper introduces the special collection in Geophysical Research Letters and Journal of Geophysical Research: Atmospheres on the exceptional stratospheric polar vortex in 2019/2020. Papers in this collection show that the 2019/2020 stratospheric polar vortex was the strongest, most persistent, and coldest on record in the Arctic. The unprecedented Arctic chemical processing and ozone loss in spring 2020 have been studied using numerous satellite and ground‐based data sets and chemistry‐transport models. Quantitative estimates of chemical loss are broadly consistent among the studies and show profile loss of about the same magnitude as in the Arctic in 2011, but with most loss at lower altitudes; column loss was comparable to or larger than that in 2011. Several papers show evidence of dynamical coupling from the mesosphere down to the surface. Studies of tropospheric influence and impacts link the exceptionally strong vortex to reflection of upward propagating waves and show coupling to tropospheric anomalies, including extreme heat, precipitation, windstorms, and marine cold air outbreaks. Predictability of the exceptional stratospheric polar vortex in 2019/2020 and related predictability of surface conditions are explored. The exceptionally strong stratospheric polar vortex in 2019/2020 highlights the extreme interannual variability in the Arctic winter/spring stratosphere and the far‐reaching consequences of such extremes. Plain Language Summary: The Arctic stratospheric polar vortex—a band of strong winds roughly encircling the pole at about 65°N latitude from about 15 to 50 km above the Earth's surface that forms every winter—was exceptionally strong during the 2019/2020 winter. The strong vortex in the stratosphere was linked to unusual conditions at both higher and lower altitudes. This collection of papers explores the far‐reaching consequences of the exceptionally strong stratospheric polar vortex in 2019/2020, including impacts on Arctic chemical ozone loss and on surface weather conditions. Chemical ozone loss in spring 2020 matched or exceeded the most previously on record (for 2011) and showed some features similar to the larger loss that occurs over the Antarctic every spring. The exceptionally strong stratospheric polar vortex was linked to weather extremes, including record heat, unusual patterns of precipitation, marine cold air outbreaks, and windstorms. Key Points: The stratospheric polar vortex in 2019/2020 was the strongest and longest‐lasting on record as described in this special collectionThis exceptionally strong and cold polar vortex led to unprecedented Arctic ozone loss, approaching that in some Antarctic wintersCirculation anomalies linked to the vortex spanned the mesosphere to the surface with implications for extreme weather and predictability [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]

Arctic cyclone activity is an important component of the local climate, and the frequent occurrence of extreme summer storms has raised widespread scientific interest. In this paper, we investigated the distinctive structural characteristics of intense summer Arctic cyclones by utilizing ERA‐Interim reanalysis data and employing a deep learning algorithm for cyclone detection. We found that the northern edge of Eurasia (i.e., the marginal ice zone (MIZ)) and the Alpha Ridge of Arctic Ocean (AR, i.e. central Arctic) are the two most active regions for intense Arctic cyclone activities in summer (from June to September). However, the surface conditions and coupling frequency between surface cyclone and tropopause polar vortices (TPVs) are distinct over these two regions. By further analysis of 100 intense cyclone activities in these two areas, respectively, we found that cyclones in MIZ are often smaller in size but higher in intensity at their maximum intensity, and their life cycles are generally shorter. MIZ cyclones are typically accompanied by a large Eady growth rate and frontal structure in the lower troposphere and their intensification primarily attributed to the thermal‐baroclinic process. In contrast, cyclones in AR are more frequently associated with higher potential vorticity (PV) values and pronounced PV downward intrusion from the stratosphere, as well as notable "upper warm‐lower cold" structures. The downward intrusion of TPVs and stratosphere vortices contribute to a decrease in the upper and column air mass deficit, leading to the intensification of surface Arctic cyclones in these regions. Plain Language Summary: In this study, we researched intense summer storms in the Arctic. We found that there are two main areas where these storms occur: the marginal ice zone (MIZ) near Eurasia and the Alpha Ridge (AR) in the central Arctic. However, storms in these two areas have different characteristics. In the MIZ, the storms are smaller but stronger, and they do not last as long. They are mainly driven by instability in the lower troposphere. On the other hand, the storms in AR respond more to the downward intrusion of potential vorticity from the stratosphere. These storms have a unique structure where the upper air is warmer than their surroundings, and the lower air is colder than their surroundings, especially in AR. This structure makes them more intense and longer‐lasting. Exploring these differences helps us understand how Arctic storms work, and how they might be affected by climate change. Key Points: Both the marginal ice zone (MIZ) and Alpha Ridge exhibit active summer Arctic cyclone activities, especially for intense stormsIn the MIZ, baroclinic instability plays a more prominent role in the intensification and maintenance of cyclonesCyclones in Alpha Ridge are more commonly accompanied by potential vorticity downward intrusion, and "upper warm‐lower cold" structures [ABSTRACT FROM AUTHOR]

Polar stratospheric clouds (PSCs) play a key role in Arctic amplification and stratospheric ozone destruction in polar regions. In this paper, we used the CALIPSO data to analyze the spatiotemporal distribution of Arctic PSCs from 2006 to 2021. We found that Arctic PSCs mainly appear in December, peak in late December and early January, disappearing in late February and early March. PSCs can extend from heights near the tropopause to over 25 km. However, there is the lowest fraction of PSCs in the 2014/2015 winter. This study found that the temperature in the 2014/2015 winter was warmer than the 15‐year average temperature, with the lowest temperature slightly below the PSCs formation temperature of about 5 K. The formation of the Ural blocking high accompanied by the poleward propagation of the planetary wave caused a sudden stratospheric warming (SSW) event on 3 January 2015, during which the warm air entered the polar vortex and divided it into two lobes. Additionally, a reduction in SO2 column mass density before the SSW event resulted PSCs occurring with a frequency of only 0.148 and dissipating rapidly in December. Moreover, the concentration of H2O and HNO3 in the gravitational settling process of PSCs decreased by 20–50%, the reduction of condensation nuclei made PSCs with the highest frequency of 0.074 in February appear briefly and then disappear. The chemical and dynamic analysis of PSCs formation is needed to further understand the spatiotemporal distribution of Arctic PSCs and to better predict future Arctic amplification and ozone destruction. Plain Language Summary: Polar stratospheric clouds (PSCs) influence polar ozone depletion by providing a reaction interface and also influencing surface temperature changes through longwave radiation effects. Previous studies on polar stratospheric clouds mainly focus on the Antarctic, and few studies on the temporal and spatial distribution characteristics of Arctic PSCs over long timescales. Therefore, by studying the spatial and temporal distribution of PSCs in the Arctic, we found that the spatial and temporal distribution of PSCs in the Arctic has obvious interannual variation compared with that in the Antarctic. The stratospheric sudden warming (SSW) events that occur almost every 2 years in the Arctic cause great interannual variations of the Arctic polar vortex and thus affect the distribution of Arctic PSCs. Moreover, there is the least and almost none occurrence of PSCs in the 2014/2015 winter, the chemical and dynamic analysis found that SSW, decrease of SO2, H2O, and HNO3 concentration are not conducive to the formation of PSCs. It is of great significance to study the influencing factors of Arctic PSCs formation and provide a new basis for further prediction of Arctic amplification and ozone destruction. Key Points: The fraction of Arctic polar stratospheric clouds (PSCs) during 2014/2015 winter was the lowest observed in the past 15 yearsThe splitting of polar vortex and the rise in temperature caused by sudden stratospheric warming (SSW) inhibited the formation of PSCsThe decrease in stratospheric sulfur dioxide, nitric acid, and water was unfavorable for the formation of PSCs before and after the SSW [ABSTRACT FROM AUTHOR]

The Arctic has experienced the frequent and severe fires in recent years. Due to the unique geography and vegetation type in Arctic region, the fire trend and its driving mechanism are different from other regions and still unclear. This study investigated the fire trends across Arctic region based on Moderate‐resolution Imaging Spectroradiometer (MODIS) burned area product MCD64A1 during 2001–2020. In addition, the burned area change was attributed at spatial and temporal scales based on 15 natural and anthropogenic factors. Results show that the average burned area in Arctic region during 2001–2020 is 31.2 × 103 km2, with a maximum of 51.9 × 103 km2 in 2019. The change of burned area in Arctic region is spatially heterogeneous, with a significant increasing trend (Z > 1.96) in central Russia and a significant decreasing trend (Z < −1.96) in western Russia. There is a clear seasonality in the occurrence of fires in the Arctic, with a peak in the summer months and almost no fires in the winter. Relative humidity has the largest explanatory power for the spatial variation of burned area (q = 0.487), followed by vapor pressure deficit (q = 0.475) and land surface temperature (q = 0.367). The combined effect of precipitation and land surface temperature is the most powerful interaction (q = 0.587) to explain the spatial variation of burned area. The significant increase in time‐series burned area is primarily driven by the decrease of relative humidity. The results of this study are helpful to understand the fire trends and the contribution of climate change to Arctic fires. Plain Language Summary: In recent years, the Arctic has experienced frequent and severe fires. Due to the unique geographical location and vegetation type of the Arctic, it is necessary to understand its fire trends and driving mechanisms. This paper explores the spatial and temporal trends of fires in the Arctic from 2001 to 2020 based on the MODIS satellite‐based burned area product MCD64A1, and considers 15 natural and human influences to explore the driving mechanisms. The results show that the Arctic region burned an average of 31.2 × 103 km2 in 2001–2020, with the highest burned area of 51.9 × 103 km2 in 2019, and that burning events were mostly concentrated in summer. The burned area increases significantly in central Russia and decreases significantly in the west. Relative humidity is the most influential factor in the spatial and temporal variability of Arctic fires, and precipitation and land surface temperature have the greatest combined effect on Arctic fires. The results of this study contribute to the understanding of the process of Arctic fire change and the impact of climate change on Arctic fires. Key Points: The burned area significantly increased (decreased) in central (western) RussiaThe interaction between precipitation and land surface temperature has the greatest effect on Arctic firesThe change in burned area is mainly driven by the change in relative humidity [ABSTRACT FROM AUTHOR]

Evaluating the impact of topography on stratospheric ozone is a crucial step toward achieving a deeper comprehension of the role of topography uplift in driving paleoclimatic evolution and stratospheric dynamics and chemistry. This paper quantitatively estimates the contribution of Asian topography to Arctic stratospheric ozone by topography experiments in the Whole Atmosphere Community Climate Model version 6. The results indicate that Asian topography exerts an especially robust influence on Arctic stratospheric ozone during winter. Compared to the flattened topography, Asian topography significantly increases the ozone by 15% in the lower Arctic stratosphere, accompanied by the highest accumulation of total ozone appearing in the polar region north to the North American continent. The intensified residual mean circulation transport is principally responsible for the increase in ozone, and Asian topography has a stronger impact on the vertical residual mean transport than on the meridional residual mean transport. Further examination demonstrates that the robust strengthening of planetary waves caused by Asian topography significantly contributes to the enhancement of the residual mean circulation. Specifically, the upper Arctic stratosphere experiences a 42.7% increase in the amplitude of planetary waves with wavenumber 1, which means the weakened polar vortex. The diagnosis of the mean Lorenz energy cycle further indicates that Asian topography decreases the zonal mean kinetic energy and increases the eddy kinetic energy in the stratosphere. Therefore, the enhancement of residual mean circulation transport and the weakening of the polar vortex both work together to promote the accumulation of Arctic ozone. Plain Language Summary: The stratosphere refers to the atmospheric layer located between 15 to 50 km above the surface, which contains approximately 90% of the global ozone. The stratospheric ozone layer serves as a protective shield for the Earth's ecosystem, safeguarding it against the damaging impacts of ultraviolet radiation. Through numerical simulations with and without the Asian topography (e.g., the Tibetan Plateau), this study evaluates how the Asian topography affects stratospheric ozone, which helps to understand the role of topography uplift in driving paleoclimatic evolution and stratospheric dynamics and chemistry. The results show that the Arctic experiences the strongest influence from Asian topography on stratospheric ozone. In comparison to the absence of the Asian topography, the total ozone over the Arctic can be increased up to 40 DU during winter, with the most significant average increase in the lower stratosphere at 15%. The Asian topography can promote the stratospheric low‐pressure system encircling the Arctic (i.e., polar vortex, the ozone inside it is usually lower) to shift toward Eurasia. Concomitantly, more planetary waves generated by the Asian topography travel poleward and upward, allowing the polar vortex to be weakened and ozone transport from the lower latitudes to be enhanced. Key Points: The Asian topography has a strong impact on Arctic stratospheric ozone during winterIncreasing in Arctic stratospheric ozone is induced mainly by the enhancement of residual mean circulation transport in winterThe more poleward and upward planetary waves generated by the Asian topography are responsible for the intensified residual mean circulation [ABSTRACT FROM AUTHOR]

Natural aerosols and their interactions with clouds remain an important uncertainty within climate models, especially at the poles. Here, we study the behavior of sea salt aerosols (SSaer) in the Arctic and Antarctic within 12 climate models from CMIP6. We investigate the driving factors that control SSaer abundances and show large differences based on the choice of the source function, and the representation of aerosol processes in the atmosphere. Close to the poles, the CMIP6 models do not match observed seasonal cycles of surface concentrations, likely due to the absence of wintertime SSaer sources such as blowing snow. Further away from the poles, simulated concentrations have the correct seasonality, but have a positive mean bias of up to one order of magnitude. SSaer optical depth is derived from the MODIS data and compared to modeled values, revealing good agreement, except for winter months. Better agreement for aerosol optical depth than surface concentration may indicate a need for improving the vertical distribution, the size distribution and/or hygroscopicity of modeled polar SSaer. Source functions used in CMIP6 emit very different numbers of small SSaer, potentially exacerbating cloud‐aerosol interaction uncertainties in these remote regions. For future climate scenarios SSP126 and SSP585, we show that SSaer concentrations increase at both poles at the end of the 21st century, with more than two times mid‐20th century values in the Arctic. The pre‐industrial climate CMIP6 experiments suggest there is a large uncertainty in the polar radiative budget due to SSaer. Plain Language Summary: Aerosols emitted from the ocean, such as sea salt particles (aerosols), are critical for the climate of polar regions. However, there is still uncertainty in their representation in climate models. The purpose of this work is to evaluate the representation of sea salt aerosols (SSaer) in the Arctic and Antarctic in a recent model inter‐comparison initiative, and to assess the consequences for our understanding of present‐day and future polar climate. We find that the models disagree between them and with observations from ground stations and from space. This suggests that the formulation of sea salt emissions in global models is not adapted for polar regions. With sea ice retreat, SSaer will most likely increase in the future, which makes addressing the current uncertainty an important next step for the scientific community. Key Points: CMIP6 models have a large uncertainty in present day sea salt aerosol abundance at the polesModel performance is degraded closer to the poles suggesting inadequate emission sources within the polar regionsBoth present and future radiative balance at the poles is uncertain because of sea salt aerosols [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]

The aerosol direct radiative effect (ADRE) is controlled by both aerosol distributions and environmental factors, making it interesting and important to quantitatively assess their effects on the ADRE inhomogeneity and climate trends. By analyzing the ADRE in the 21st century from a global reanalysis data set, we find that the spatial variability of the ADRE and its trends can be well explained by a linear regression model. In this model, scattering and absorbing aerosol optical depths (AODs) are used, along with critical environmental variables such as surface albedo and cloud radiative effect, as predictors. Based on this model, we find that approximately 70% of the ADRE inhomogeneity is due to the AOD distributions and the remainder is attributable to environmental factors. This study also shows that a stronger cooling effect of the scattering aerosols in the Northern Hemisphere drives northward cross‐equator meridional energy transport, although this transport exhibits a declining trend over the last two decades. The changes in surface albedo and cloud radiative effect strongly influence the trends in the regional ADRE and the meridional energy transport driven by them. In particular, the reduction of surface albedo (sea ice) is primarily responsible for the enhancement of the cooling ADRE, as well as an associated trend in meridional energy transport, in the Arctic. Plain Language Summary: Aerosols are particles produced by natural events such as volcanic eruptions and anthropogenic emissions such as fossil fuel combustion. They can scatter and absorb incoming solar radiation, leading to a strong local cooling or warming effect on the Earth's climate. The impact of aerosols on the Earth's radiative balance is known as the aerosol direct radiative effect (ADRE). Factors that can affect ADRE include the types and amounts of aerosols and the environmental conditions such as surface albedo and clouds. This study proposed a regression model to predict the distributions and trends of global ADRE. According to the results, 70% of the ADRE variability is contributed by aerosols, while the rest is influenced by surface albedo and clouds. The high aerosol concentrations in the Northern Hemisphere require energy to be transported from the South Hemisphere to the North. However, this demand has decreased over the past two decades. Changes in surface albedo and clouds have a significant impact on the ADRE trend. In particular, the retreating sea ice plays a major role in the ADRE trend in the Arctic. Key Points: Nonaerosol factors contribute significantly to the spatial inhomogeneity of the aerosol direct radiative effect (ADRE) and its trends in recent decadesThe hemispheric difference in scattering aerosols drives northward cross‐equator energy transport, which shows a declining trendChanges in the surface albedo due to sea ice melt strongly influence the ADRE trends in, and the energy transport to, the Arctic [ABSTRACT FROM AUTHOR]

This study performs a comprehensive evaluation of the simulated cloud phase in the U.S. Department of Energy, Energy Exascale Earth System Model, atmosphere model version 2 (EAMv2), and version 1 (EAMv1). Enabled by the CALIPSO (Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation) simulator, EAMv2 and EAMv1 predicted cloud phase is compared against the GCM‐Oriented CALIPSO Cloud Product (CALIPSO‐GOCCP) at high latitudes where mixed‐phase clouds are prevalent. Our results indicate that the underestimation of cloud ice in simulated high‐latitude mixed‐phase clouds in EAMv1 has been significantly reduced in EAMv2. The increased ice clouds in the Arctic mainly result from the modification on the WBF (Wegner‐Bergeron‐Findeisen) process in EAMv2. The impact of the modified WBF process is moderately compensated by the low limit of cloud droplet number concentration in cloud microphysics and the new dCAPE_ULL trigger used in deep convection in EAMv2. Moreover, it is found that the new trigger largely contributes to the better cloud phase simulation over the Norwegian Sea and Barents Sea in the Arctic and the Southern Ocean where large errors are found in EAMv1. However, errors in simulated cloud phase in EAMv1, such as the overestimation of supercooled liquid clouds near the surface in both hemispheres and the underestimation of ice clouds over Antarctica, persist in EAMv2. This study highlights the impact of deep convection parameterizations, which has received little attention, on high‐latitude mixed‐phase clouds, and the importance of continuous improvement of cloud microphysics in climate models for accurately representing mixed‐phase clouds. Key Points: EAMv2 largely improves cloud ice phase at high latitude regions, while biases in liquid phase shown in EAMv1 remainUpdated tuning parameters in Wegner‐Bergeron‐Findeisen process and deep convection are important for reduced negative bias in ice phase cloudsThe new dCAPE_ULL trigger in deep convection is responsible for the better cloud phase simulation over high‐latitude oceans [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]

Arctic cyclones are a fundamental component of Arctic climate, influencing atmospheric heat and moisture transport into the region and surface energy, moisture, and momentum fluxes. Arctic cyclones can also drive changes in sea ice and energize ocean waves. Here we investigate a record low sea level pressure (SLP) Arctic cyclone which formed in East Greenland and tracked NE over the Barents and Kara seas between 21 and 27 January 2022. At its peak intensity on 24 January, the cyclone reached an estimated depth of 932.2 mb at 79.5°N 20°E. North of 70°N, this is the lowest SLP in the ERA‐5 reanalysis over 1979 to present. The cyclone resulted in a record (over the period 1979–2022) weekly loss of regional sea ice area and surface wind speeds, and generated ocean waves exceeding 8 m that impinged on sea ice in the Barents sea, observed via satellite altimetry as large waves‐in‐sea ice up to 2 m in amplitude more than 100 km into the ice pack. Surface heat fluxes were strongly impacted by the cyclone, with record atmosphere‐to‐surface turbulent fluxes. However, the direct atmospheric thermodynamic impact on sea ice loss was modest, and the record sea ice changes were likely mainly driven by dynamical and/or ocean processes. While the storm was well predicted up to 8 days in advance, subsequent changes in sea ice cover were not, likely due to biases in the forecasts' sea ice initial conditions and missing physics in the forecast model such as wave‐sea ice interaction. Plain Language Summary: The strongest recorded storm in the Arctic took place in January 2022. This storm was characterized by record wind speeds, sea ice loss and turbulent fluxes, and anomalous but less extreme temperature anomalies. We learned that some of this storm's record‐breaking characteristics were predicted well by model forecasts. However, the effects of this storm on the Arctic sea ice cover were not well predicted. Forecasts underestimated the decrease in sea ice thickness and area from the storm. Satellite data showed large waves traveling into the icepack, which may have contributed to the sea‐ice loss. Such processes may be poorly represented in the model, which hinder its ability to accurately forecast sea‐ice conditions. Improving initial conditions and the representation of interactions between the atmosphere, sea ice, and ocean in models may help improve forecasts of weather and sea ice. Key Points: A record low sea level pressure cyclone was observed in the Arctic in January 2022This low resulted in a record weekly loss of sea ice cover and record surface wind speeds, despite non‐record warm conditionsWhile the storm was well predicted, the rapid decreases in sea ice thickness and area were not [ABSTRACT FROM AUTHOR]

Reanalysis data and a numerical model are employed to explore how diffusion in the upper ocean evolves in different seasons and to understand its contribution to seasonal heat storage (SHS) and Arctic amplification. The numerical simulation results are closely consistent with observations. First, Arctic solar radiation absorption anomaly (maximum in June) due to ice‐loss is mainly stored as SHS (maximum in June and July) or warms the surface (minimum in July) in observations. Furthermore, the numerical simulation suggests that vertical diffusion dominates SHS formation/discharge. Second, surface ocean becomes warmer than surface air in cold season and releases SHS in Arctic. Sea‐ice loss allows more ocean be directly driven by wind; surface high pressure over Arctic may enhance the wind stress; increased water freeze leads to more salt rejection and additional dense water being produced and sinking downward; all of the occurrences strengthen vertical mixing and release more SHS to the atmosphere. As a result, Arctic surface warming reaches its maximum in cold season. In observations, much stronger outgoing longwave radiation due to the warmer surface in October causes Arctic surface warming to reach its maximum in November, although increased carbon dioxide forcing also contributes to maintain longwave radiation in November. Finally, Arctic surface (air) warming in cold season exhibits different spatial patterns from SHS discharge due to heat convergence/divergence induced by anomalous surface wind, which is determined by the variation in surface high pressure over the Arctic. Plain Language Summary: Due to little seasonal variation in global surface warming, seasonality of Arctic amplification (AA) is mainly determined by seasonal variation of Arctic surface warming. Seasonal heat storage (SHS) in the subsurface Arctic Ocean contributes to Arctic surface warming by holding anomalous heat in summer and releasing it in autumn and winter. As a result, there is significant seasonal variation in AA. In this study, we directly estimate the SHS with ocean data and focus on spatial feature discussions. (a) In summer, additional solar radiation is stored as SHS via vertical diffusion in the upper ocean, resulting in weak Arctic warming. (b) In cold season, more current is driven directly by wind due to sea‐ice loss; variation in surface high pressure over the Arctic enhances the wind stress; more dense water, which is produced due to additional water freezes, sink downward; all of which strengthen the vertical mixing and release more SHS, resulting in the Arctic warming maximum. (c) Although SHS discharge dominates Arctic warming in cold season, anomalous surface wind induces heat convergence/divergence and influences the spatial distribution of surface air temperature, which in turn impacts surface warming via air‐sea/air‐land interactions. Finally, the SHS anomaly and surface warming share different spatial patterns. Key Points: With (without) land surface warming occurring in June, the observational (numerical) Arctic warming/Arctic amplification reaches its minimum in July (June)Though seasonal heat storage (SHS) discharge dominates Arctic warming in cold season, anomalous wind makes their spatial patterns differentLess insulation by sea‐ice loss, wind acceleration or more dense water sinking increases vertical mixing and SHS discharge in cold season [ABSTRACT FROM AUTHOR]

Using nine chemistry‐climate and eight associated no‐chemistry models, we investigate the persistence and timing of cold episodes occurring in the Arctic and Antarctic stratosphere during the period 1980–2014. We find systematic differences in behavior between members of these model pairs. In a first group of chemistry models whose dynamical configurations mirror their no‐chemistry counterparts, we find an increased persistence of such cold polar vortices, such that these cold episodes often start earlier and last longer, relative to the times of occurrence of the lowest temperatures. Also the date of occurrence of the lowest temperatures, both in the Arctic and the Antarctic, is often delayed by 1–3 weeks in chemistry models, versus their no‐chemistry counterparts. This behavior exacerbates a widespread problem occurring in most or all models, a delayed occurrence, in the median, of the most anomalously cold day during such cold winters. In a second group of model pairs there are differences beyond just ozone chemistry. In particular, here the chemistry models feature more levels in the stratosphere, a raised model top, and differences in non‐orographic gravity wave drag versus their no‐chemistry counterparts. Such additional dynamical differences can completely mask the above influence of ozone chemistry. The results point toward a need to retune chemistry‐climate models versus their no‐chemistry counterparts. Plain Language Summary: Ozone is a chemical constituent of the atmosphere acting as an absorber of both solar ultraviolet light and infrared radiation emitted by the Earth. It therefore needs to be considered in climate models. Explicit ozone chemistry is a computationally challenging addition to a climate model; hence in most cases ozone is simply prescribed. Especially during relatively cold stratospheric winter/spring seasons, Antarctic and Arctic ozone depletion can be considerable. Such anomalous ozone loss is not reflected in the imposed ozone field, and hence differences in behavior are expected for such situations between chemistry‐ and no‐chemistry models. Indeed for such cold winters/springs, we find an enhanced persistence of such cold spells in a set of chemistry‐climate models, versus their no‐chemistry counterparts; such enhanced persistence generally makes the chemistry model less realistic than its no‐chemistry counterpart. However, if there are substantial further differences between the members of these model pairs, such as regarding their grid configuration or physical processes beyond chemistry, these can obscure the effect of ozone chemistry. We thus claim that adding stratospheric ozone chemistry to a climate model necessitates retuning to counteract a deterioration of the simulated stratospheric climate that can otherwise occur. Key Points: Coupling in ozone chemistry causes an increase in persistence of low temperature anomalies over both polesIn the Antarctic, coupling in chemistry amplifies pre‐existing stratospheric cold biasesThese effects can be masked by other dynamical differences present in some models [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]

Aerosols significantly affect Earth's radiation budget, thus influencing global climate. In the Arctic, sulfate aerosols are thought to have reduced the warming during the twentieth century. However, trends in past sulfate aerosols are poorly known, especially the aerosol sizes and compositions. Here, we analyze a high‐resolution ice core from southeastern Greenland, comparing the seasonal deposition flux of large sulfate salt particles and small sulfur compounds, including non‐neutralized sulfuric acid, between the anthropogenic sulfate maximum (1973–1975) and after sulfur emissions control (2010–2012). Between these periods, we find that the large‐diameter (>0.4 μm) flux remains roughly unchanged, yet the small‐diameter (<0.4 μm) aerosol flux significantly decreases. The results indicate that small sulfates were efficiently activated as cloud condensation nuclei during the 1970s, and thus likely increased cloud albedo, offsetting the warming. Plain Language Summary: Sulfate aerosol compositions and size distributions are important factors in Earth's radiation budget. However, we have little information on past sulfate aerosol sizes and compositions over the Arctic due to a lack of reliable observations, and as such, we have less confidence in modeling historical aerosol composition and transport. We present here the first observational evidence that small sulfate particles increased in the Arctic during the 1970s. With emission controls, anthropogenic SO2 emissions from eastern North America and Western Europe decreased after the 1980s, thus increasing the relative importance of large sulfate particles. Thus, our findings should help to improve the understanding of aerosol‐cloud interactions, and provide new constraints for the parameterization in the model. Key Points: Ice core analyses provide the first evidence of past seasonal deposition flux of large sulfate salt particles and small sulfatesCompared to the 1970s, the small sulfates (<0.4 μm) aerosol flux significantly decreased in the 2010sSmall sulfates were efficiently activated as cloud condensation nuclei during anthropogenic sulfate maximum (1970s) [ABSTRACT FROM AUTHOR]

The term "weather whiplash" was recently coined to describe abrupt swings in weather conditions from one extreme to another, such as from a prolonged, frigid cold spell to anomalous warmth or from drought to heavy precipitation. These events are often highly disruptive to agriculture, ecosystems, and daily activities. In this study, we propose and demonstrate a novel metric to identify weather whiplash events (WWEs) and track their frequency over time. We define a WWE as a transition from one persistent continental‐scale circulation regime to another distinctly different pattern, as determined using an objective pattern clustering analysis called self‐organizing maps. We focus on the domain spanning North America and the eastern N. Pacific Ocean. A matrix of representative atmospheric patterns in 500‐hPa geopotential height anomalies is created from 72 years of daily fields. We analyze the occurrence of WWEs originating with long‐duration events (LDEs) (defined as lasting four or more days) in each pattern, as well as the associated extremes in temperature and precipitation. A WWE is detected when the pattern 2 days following a LDE is substantially different, measured using internal matrix distances and thresholds. Changes in WWE frequency are assessed objectively based on reanalysis and historical climate model simulations, and for the future using climate model projections. Temporal changes in the future under representative concentration pathway 8.5 forcing are more robust than those in recent decades. We find consistent increases in WWEs originating in patterns with an anomalously warm Arctic and decreases in cold‐Arctic patterns. Plain Language Summary: When weather conditions shift abruptly after a long period of the same pattern—for example, from a spell of abnormally mild temperatures to extreme cold, or from a long dry period to a parade of storms—major disruptions to ecosystems and human activities often ensue. These types of shifts have recently been dubbed "weather whiplash" events. In this study, we propose and demonstrate a new approach to measuring the frequency of these events based on continent‐wide atmospheric regimes and their transitions. While the frequency of these events in recent decades has not changed substantially, our analysis of future model projections indicates robust increases and decreases in certain atmospheric patterns, particularly in those featuring an extremely warm or cold Arctic. Key Points: Weather whiplash events (WWEs) are defined as an abrupt transition from one persistent, continental‐scale atmospheric pattern to a different regimeWWEs often cause disruptive shifts in extreme temperatures and precipitation associated with changing atmospheric patternsWarm‐Arctic atmospheric patterns spawn more WWEs than cold‐Arctic patterns, with robust future changes in frequency [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]

Understanding polar amplification (PA) and its underlying processes is key to accurately predicting the climate system's response to increasing anthropogenic forcings. We examine the amplified warming in the Arctic and Antarctic in 17 global climate models from the Coupled Model Intercomparison Project 6 (CMIP6) against satellite data. Large hemispheric differences in PA strength was found in the CMIP6 models. Changes in surface temperature and strength of PA is closely coupled to changes in albedo. The planetary albedo of Earth (αp) is partitioned into a component associated with surface albedo (defined as surface contribution to planetary albedo, αsurfp ${\alpha }_{\mathit{surf}}^{p}$), and a component associated with atmospheric albedo (atmospheric contribution to planetary albedo, αatmp ${\alpha }_{\mathit{atm}}^{p}$). To assess the hemispheric differences in PA strengths, the relative importance of αsurfp ${\alpha }_{\mathit{surf}}^{p}$ and αatmp ${\alpha }_{\mathit{atm}}^{p}$ were investigated. The surface reflection looks different as seen at the surface (defined as surface albedo, αsurf) compared to αsurfp ${\alpha }_{\mathit{surf}}^{p}$ (as seen at the top of the atmosphere). We find a stronger correlation between surface temperature and αsurf in the Arctic than in the Antarctic, with correlation coefficients of −0.94 and −0.88, respectively. Interestingly, the correlation for surface temperature and αsurfp ${\alpha }_{\mathit{surf}}^{p}$ is stronger in the Antarctic than in the Arctic with correlation coefficients of −0.93 and −0.90, respectively. In the southern high latitudes, albedo changes at the surface are more important than changes in the atmosphere, while the opposite applies in the northern high latitudes. Surface temperature changes in the low‐ and mid‐latitudes are strongly associated with changes in αatmp ${\alpha }_{\mathit{atm}}^{p}$, dominated by changes in cloud properties. Plain Language Summary: The temperature is increasing faster in high latitudes compared to other regions of Earth. This is known as polar amplification (PA). The amount of light reflected from a surface is called albedo. Using 17 climate models and satellite data, this study compares the effects of albedo from surface and atmosphere on PA. We find a large spread in PA among models, with stronger PA in the northern hemisphere than in the southern hemisphere. Surface albedo looks different when looking from the surface compared to when looking from the top of the atmosphere (TOA), because of the effects from the atmosphere when looking from the TOA. We find greater correlation between surface temperature and albedo at the surface in the Arctic than the Antarctic. Interestingly, from the TOA we see greater correlation between surface temperature and surface albedo in the Antarctic than in the Arctic. We also find that in Antarctica, changes in surface albedo as seen from the TOA are more sensitive to changes in surface temperature than changes in atmospheric albedo are, while the opposite applies in the Arctic. These differences may provide part of the answer to why there is greater PA in the northern than in the southern hemisphere. Key Points: A strong correlation between CO2‐induced temperature change and planetary albedo is found in all regionsThere is a large spread in polar amplification between modelsCorrelation between CO2‐induced temperature change and contribution from surface albedo to total planetary albedo is clear in the Antarctic, but less clear in the Arctic [ABSTRACT FROM AUTHOR]

Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August–September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ≲70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze‐up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ≳70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous‐phase chemistry during in‐cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole‐air and cloud droplet residual data. These aerosol–cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean. Key Points: Aitken‐mode particles contributed significantly to cloud droplet formation in the high Arctic, especially after the transition to fallResidence time over the pack ice and the relative contribution of organics both increased when average particle size decreasedAddition of aerosol mass due to aqueous‐phase chemistry was probably small over the pack ice [ABSTRACT FROM AUTHOR]

Pacific Decadal Variability (PDV) has enormous influences on North American and Eurasian climate and Pacific ecosystems. Its change under anthropogenic warming is of prodigious societal concern and scientific dispute. We show evidence that the observed PDV has amplified in the northeast Pacific horseshoe‐like region while weakened over the South Pacific and Kuroshio‐Oyashio Extension (KOE) region since the beginning of the 21st century. Congruently, under the influence of PDV, precipitation anomalies have significantly amplified globally and reversed the sign over Africa and East Asia. Analysis of the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical simulations and future projections suggests that greenhouse forcing weakens the PDV in the South and northwestern Pacific and shortens its periodicity. However, the observed PDV's enhancement over the North Pacific eastern boundary is likely due to internal variability. Our findings on the PDV's response to anthropogenic forcing and internal variability shed light on the PDV's future change and decadal prediction. Plain Language Summary: Pacific Decadal Variability (PDV) is one of the primary modes of internal variability. There is an unprecedented change of the PDV after 1999, which reveals a sudden change in the spatial pattern, intensity, and climate impacts of the PDV. The amplitude of PDV has decreased over the South Pacific and Kuroshio‐Oyashio region but strengthened along the west coast of North America and equatorial central Pacific. This current PDV's cycle has amplified the land precipitation anomalies worldwide and reversed the precipitation anomalies over Africa, East Asia, and sub‐Arctic regions. We used the Coupled Model Intercomparison Project Phase 6 multi‐model results to illuminate that the recent unprecedented change of the PDV may be influenced by the combination of greenhouse warming and internal variability. Our findings reflect an advance in understanding the long‐term changes of the PDV and its remarkable impact on global land precipitation. Key Points: Observed Pacific Decadal Variability (PDV) has weakened over the South Pacific and Kuroshio‐Oyashio Extension while amplified over the northeast Pacific horseshoe‐like region since 1999Greenhouse gases forcing has a potential impact on weakening the PDV, which is also suggested by future warming scenarioThe increased PDV over the eastern boundary North Pacific and central Pacific may be associated with internal variability [ABSTRACT FROM AUTHOR]

The amount of ice versus supercooled water in clouds is important for their radiative properties and role in climate feedbacks. Hence, knowledge of the concentration of ice‐nucleating particles (INPs) is needed. Generally, the concentrations of INPs are found to be very low in remote marine locations allowing cloud water to persist in a supercooled state. We had expected the concentrations of INPs at the North Pole to be very low given the distance from open ocean and terrestrial sources coupled with effective wet scavenging processes. Here we show that during summer 2018 (August and September) high concentrations of biological INPs (active at >−20°C) were sporadically present at the North Pole. In fact, INP concentrations were sometimes as high as those recorded at mid‐latitude locations strongly impacted by highly active biological INPs, in strong contrast to the Southern Ocean. Furthermore, using a balloon borne sampler we demonstrated that INP concentrations were often different at the surface versus higher in the boundary layer where clouds form. Back trajectory analysis suggests strong sources of INPs near the Russian coast, possibly associated with wind‐driven sea spray production, whereas the pack ice, open leads, and the marginal ice zone were not sources of highly active INPs. These findings suggest that primary ice production, and therefore Arctic climate, is sensitive to transport from locations such as the Russian coast that are already experiencing marked climate change. Plain Language Summary: Clouds play a critical role in Earth's climate, both reflecting incoming sunlight and trapping outgoing infrared radiation. Hence, even small errors in the representation of clouds in climate models can lead to uncertainty in predictions of, for example, sea ice extent. In the Arctic, liquid clouds often exist below 0°C and cloud water droplets can exist in a supercooled liquid state. In the absence of special particles that can trigger ice formation in droplets, ice‐nucleating particles (INPs), supercooled water droplets can cool well below −35°C before spontaneously freezing. The presence of INPs can reduce the lifetime of a cloud and the amount of supercooled water in clouds, making them less reflective. Based on our knowledge of INPs in other remote oceans, we expected very low INP concentrations in the central Arctic. However, we found very high concentrations of biological INPs in the summertime North Pole. Furthermore, these INPs come from the seas off the coast of Russia, a region already experiencing strong climate change. It is possible that these sources may become even more important as the Arctic becomes increasingly ice‐free, causing changes in Arctic clouds and further changes in climate. Key Points: The concentration of ice‐nucleating particles at the North Pole in summer 2018 was amongst the highest anywhere in the worldBiological ice‐nucleating particles were derived from the Russian seas and perhaps associated with wind‐driven sea sprayThe concentration of ice‐nucleating particles at the surface was often different to that higher in the boundary layer where clouds form [ABSTRACT FROM AUTHOR]

A severe persistent drought occurred in South China during autumn–winter 2020/2021, causing enormous economic losses and severe water shortage in South China. Focusing on this event, this study analyzed the climate anomalies associated with this event, and investigated the underlying mechanisms as well as their relationship with the leading modes of the autumn and winter precipitation in eastern China. The results suggest that the drought in South China during autumn 2020 is mainly attributed to the water vapor divergence anomalies induced by an anomalous anticyclone over eastern China and an anomalous cyclone over the South China Sea, which are caused by an anomalous Eurasian atmospheric wave‐train triggered by the negative sea ice anomalies in the Barents Sea‐Kara Sea region and a La Niña‐like condition in the tropical Pacific sea surface temperatures, respectively. The continued drought in South China during winter 2020/2021 is mainly attributed to the water vapor divergence anomalies induced by an anomalous cyclone over the western North Pacific, which is caused by the continued La Niña‐like condition. The mechanisms of the first two leading modes of autumn precipitation in eastern China both contributed to the drought in South China during autumn 2020, whereas the mechanisms of the first leading mode of winter precipitation in eastern China played an important role in causing the drought in South China during winter 2020/2021. Numerical experiments demonstrate important impact of the ENSO and Arctic sea ice anomalies on the East Asian climate anomalies during autumn–winter 2020/2021. Key Points: The drought in South China during autumn 2020 is attributed to combined effect of El Niño‐Southern Oscillation (ENSO) and Arctic sea ice anomaliesThe drought in South China during winter 2020/2021 is mainly attributed to the La Niña‐like conditionNumerical experiments demonstrate important impact of ENSO and Arctic sea ice on East Asian climate anomalies during autumn–winter 2020/2021 [ABSTRACT FROM AUTHOR]

The presence of clouds in the Arctic regulates the surface energy budget (SEB) over the sea‐ice surface and the ice‐free ocean. Following several previous field campaigns, the cloud‐radiation relationship, including cloud vertical structure and phase, has been elucidated; however, modeling of this relationship has matured slowly. In recognition of the recent decline in the Arctic sea‐ice extent, representation of the cloud system in numerical models should consider the effects of areas covered by sea ice and ice‐free areas. Using an in situ stationary meteorological observation data set obtained over the ice‐free Arctic Ocean by the Japanese Research Vessel Mirai (September 2014), coordinated evaluation of six regional climate models (RCMs) with nine model runs was performed by focusing on clouds and the SEB. The most remarkable findings were as follows: (1) reduced occurrence of unstable stratification with low‐level cloud water in all models in comparison to the observations, (2) significant differences in cloud water representations between single‐ and double‐moment cloud schemes, (3) extensive differences in partitioning of hydrometeors including solid/liquid precipitation, and (4) pronounced lower‐tropospheric air temperature biases. These issues are considered as the main sources of SEB uncertainty over ice‐free areas of the Arctic Ocean. The results from a coupled RCM imply that the SEB is constrained by both the atmosphere and the ocean (and sea ice) with considerable feedback. Coordinated improvement of both stand‐alone atmospheric and coupled RCMs would promote a more comprehensive and improved understanding of the Arctic air‐ice‐sea coupled system. Key Points: Discrepancies remain in surface energy budget (SEB) and cloud properties despite constrained boundary conditionsModels underestimate occurrence of unstable stratification with low‐level water cloudsBiases in hydrometeor partitioning essentially contribute to limited skill in SEB simulation [ABSTRACT FROM AUTHOR]

Black carbon (BC), brown carbon, and light‐absorbing iron oxides (FeOx) aerosols affect Earth's energy budget through their strong absorption of solar radiation. FeOx aerosols can also affect global biogeochemical cycles through their role as a nutrient for oceanic phytoplankton. However, observational data for these aerosols required for evaluating their effects using global models are scarce, especially for FeOx. Here, we summarize and compare the data sets of BC and FeOx from five ground‐based and three aircraft observation campaigns conducted in the East Asian and Arctic regions during the 2009–2018 period acquired using a modified single‐particle soot photometer. In these campaigns, >80% of FeOx‐containing aerosols in the 170–270 nm FeOx core size range had microphysical features indicating an anthropogenic origin. The particle size distribution for each of the BC and FeOx was similar in all of the data sets except for those dominated by fresh urban pollution or pristine Arctic air. The campaign‐averaged mass concentrations of FeOx and BC were ~60–360 ng/m3 and ~240–1,300 ng/m3, respectively, in East Asia, and ~6 ng/m3 and ~20–30 ng/m3, respectively, in the Arctic. The campaign‐averaged FeOx/BC mass concentration ratio varied within a narrow range of 0.2–0.6 in both East Asian and Arctic regions. In every campaign, FeOx, BC, and carbon monoxide were tightly correlated with each other with similar slope to the urban campaigns (around Tokyo), implying the spatiotemporal variation of anthropogenic FeOx emission around northern middle‐to‐high latitudes is similar to those of anthropogenic BC and CO emissions. Key Points: Mass concentrations of anthropogenic iron oxide (FeOx) were at least 20% of those of black carbon (BC) in both East Asia and the ArcticFeOx, BC, and carbon monoxide were tightly correlated with each other in both East Asia and the ArcticParticle size distributions of both FeOx and BC were presented and compared in detail [ABSTRACT FROM AUTHOR]

Arctic precipitation is projected to increase more rapidly than the global mean in warming climates. However, warming‐induced changes in the variability of Arctic precipitation, which are related to surface evaporation and poleward moisture transport (PMT), are currently largely unknown. This study compares the precipitation variability in different quasi‐equilibrium climates simulated by a global climate model (EC‐Earth) and studies the underlying mechanisms. Five quasi‐equilibrium simulations of 400 years length forced with a broad range of CO2 concentrations (0.25, 0.5, 1, 2, and 4 times the current global mean) were analyzed. PMT is the dominant source of Arctic precipitation variability in colder climates when the ocean in the Arctic basin is completely covered by sea ice year‐round. Arctic precipitation variability increases from colder to warmer climates, primarily in summer. In summer, the increasingly stronger relation between Arctic sea level pressure variability and precipitation variability toward warmer climates enhances variability. In winter, the severe increase in mean precipitation (due to enhanced evaporation) exerts a comparatively small increase in variability, and precipitation variability is modulated by both PMT and evaporation, which oppose each other as they both affect the vertical and meridional moisture gradients. Key Points: Increase in precipitation variability does not scale with increase in mean precipitation toward warmer climatesIncrease in mean evaporation toward warmer climates is important for increase in mean precipitation during winterVariability in poleward moisture transport is the main source for precipitation variability in both seasons and across all climate states [ABSTRACT FROM AUTHOR]

This study analyzed summer observations of diurnal and seasonal surface energy budgets across several monitoring sites within the Arctic tundra underlain by permafrost. In these areas, latent and sensible heat fluxes have comparable magnitudes, and ground heat flux enters the subsurface during short summer intervals of the growing period, leading to seasonal thaw. The maximum entropy production (MEP) model was tested as an input and parameter parsimonious model of surface heat fluxes for the simulation of energy budgets of these permafrost‐underlain environments. Using net radiation, surface temperature, and a single parameter characterizing the thermal inertia of the heat exchanging surface, the MEP model estimates latent, sensible, and ground heat fluxes that agree closely with observations at five sites for which detailed flux data are available. The MEP potential evapotranspiration model reproduces estimates of the Penman‐Monteith potential evapotranspiration model that requires at least five input meteorological variables (net radiation, ground heat flux, air temperature, air humidity, and wind speed) and empirical parameters of surface resistance. The potential and challenges of MEP model application in sparsely monitored areas of the Arctic are discussed, highlighting the need for accurate measurements and constraints of ground heat flux. Plain Language Summary: Growing season latent and sensible heat fluxes are nearly equal over the Arctic permafrost tundra regions. Persistent ground heat flux into the subsurface layer leads to seasonal thaw of the top permafrost layer. The maximum energy production model accurately estimates the latent, sensible, and ground heat flux of the surface energy budget of the Arctic permafrost regions. Key Point: The MEP model is parsimonious and well suited to modeling surface energy budget in data‐sparse permafrost environments [ABSTRACT FROM AUTHOR]

The Arctic is experiencing rapid climate change in response to changes in greenhouse gases, aerosols, and other climate drivers. Emission changes in general, as well as geographical shifts in emissions and transport pathways of short‐lived climate forcers, make it necessary to understand the influence of each climate driver on the Arctic. In the Precipitation Driver Response Model Intercomparison Project, 10 global climate models perturbed five different climate drivers separately (CO2, CH4, the solar constant, black carbon, and SO4). We show that the annual mean Arctic amplification (defined as the ratio between Arctic and the global mean temperature change) at the surface is similar between climate drivers, ranging from 1.9 (± an intermodel standard deviation of 0.4) for the solar to 2.3 (±0.6) for the SO4 perturbations, with minimum amplification in the summer for all drivers. The vertical and seasonal temperature response patterns indicate that the Arctic is warmed through similar mechanisms for all climate drivers except black carbon. For all drivers, the precipitation change per degree global temperature change is positive in the Arctic, with a seasonality following that of the Arctic amplification. We find indications that SO4 perturbations produce a slightly stronger precipitation response than the other drivers, particularly compared to CO2. Key Points: Arctic amplification of surface warming is similar between global drivers of climate changeBlack carbon induces differing vertical and seasonal amplification patternsSulfate affects Arctic precipitation responses more strongly than other drivers, particularly in the summer season [ABSTRACT FROM AUTHOR]

Surface temperature data sets for, or including, the Arctic have been derived from various thermal infrared sensors. However, a combined, all surface temperature data set for the Arctic has not been generated previously. Here we present the first combined land, ocean, and ice surface temperature data set for the Arctic produced from Along‐Track Scanning Radiometer ‐ 2 and the Advanced Along‐Track Scanning Radiometer satellite sensors: the Along‐Track Scanning Radiometer Arctic combined Surface Temperature data set. Separate products, produced independently for each sensor and containing quantified uncertainties, together cover the period August 1995 to April 2012. Product validation, utilizing a more extensive in situ database than previous studies, shows that Along‐Track Scanning Radiometer Arctic combined Surface Temperature surface temperatures generally agree with in situ data and are similar to previous validation of input surface temperature retrievals. Biases range from −1.74 to 0.23 K over open ocean, sea ice, snow over land, and the Greenland ice sheet with higher variability over snow/ice. However, there are noticeable outliers in the validation results, particularly over Arctic land in boreal summer for Along‐Track Scanning Radiometer ‐ 2, which are likely due to cloud contamination resulting from a climatologically static snow field being used for that sensor. This study suggests that the Along‐Track Scanning Radiometer Arctic combined Surface Temperature data set presented here is a useful tool for assessment of models in the Arctic. Further work would have clear benefits including improvements to snow cover and cloud clearing to achieve a fully consistently processed, climate quality combined surface temperature data set for the Arctic region. Key Points: This study presents the first combined surface temperature data set for the Arctic, derived here from ATSR thermal infrared satellite dataProduct validation shows data set surface temperatures generally agree with in situ data, but cloud contamination is an issueThis combined surface temperature data set is a useful tool for assessment of models in the Arctic, although further work is required [ABSTRACT FROM AUTHOR]

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The influence of Arctic changes on the weather in the highly populated midlatitude is a controversial topic with little agreements regarding its existence or its mechanism. Utilizing eddy energy as metrics of circulation and temperature fluctuations with weather patterns, this study highlights the robust relationship between the early autumn Arctic sea ice and the wintertime transient activities over northern Eurasia on interannual time scale, by comparing groups of statistical and diagnostical results. With the reduction of sea ice in early autumn in Barents, Kara, Laptev, and East Siberia Seas, the atmosphere over those polar regions exhibits significant increase in eddy energy in the following winter. In the adjacent Eurasia, the wintertime synoptic eddy energy decreases, while the low‐frequency eddy energy, corresponding to persistent weather patterns, exhibits evident and dominant increase. The enhanced southward energy propagation of low‐frequency waves from the polar region to the north of middle‐to‐east Asia suggests a mechanism for the Arctic‐midlatitude linkage, indicating that Arctic sea ice could be a source of predictability for both extended‐range and subseasonal to seasonal forecasts. Plain Language Summary: The Arctic is warming and melting at rapid rate, while its influence on the weathers in the highly populated midlatitude is a controversial topic, with little agreements regarding its existence or its mechanism. Objective metrics from the perspective of atmospheric dynamics and sophisticated diagnostics are exceedingly needed to quantify and understand the Arctic‐midlatitude linkage.This study proposes to use eddy energy as an objective and dynamical meaningful metric of circulation change with weather patterns and finds robust linkage between the Arctic change and eddy energy in Northern Eurasia by comparing groups of statistical and diagnostical results. With the decrease of Arctic sea ice, the change of eddy energy shows that the persistent weathers and quasi‐stationary wave activity increase significantly but synoptic activities decrease. A mechanism for such linkage is also identified by checking the eddy energy propagation. The results suggest a future pathway to quantify and understand the Arctic‐midlatitude linkage via diagnosing energetics and the budget, transfer, and teleconnection of eddy energies. The results also show that Arctic sea ice could be a source predictability for both extended‐range and subseasonal to seasonal forecasts. Key Points: Utilizing eddy energy as metrics, a robust linkage is identified between Arctic sea ice and midlatitude weather pattern within seasonal time scaleEddy energy corresponding to persistent winter weather significantly increases in northern Eurasia with autumn Arctic sea ice lossThe southward propagation of low‐frequency waves from the polar region plays a key role for the linkage [ABSTRACT FROM AUTHOR]