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Increasing the urban tree cover percentage (TCP) is widely recognized as an efficient way to mitigate the urban heat island effect. The cooling efficiency of urban trees can be either enhanced or attenuated on hotter days, depending on the physiological response of urban trees to rising ambient temperature. However, the response of urban trees' cooling efficiency to rising urban temperature remains poorly quantified for China's cities. In this study, we quantify the response of urban trees' cooling efficiency to rising urban temperature at noontime [∼1330 LT (local time), LT=UTC+8] in 17 summers (June, July, and August) from 2003–19 in 70 economically developed cities of China based on satellite observations. The results show that urban trees have stronger cooling efficiency with increasing temperature, suggesting additional cooling benefits provided by urban trees on hotter days. The enhanced cooling efficiency values of urban trees range from 0.002 to 0.055°C %−1 per 1°C increase in temperature across the selected cities, with larger values for the low-TCP-level cities. The response is also regulated by background temperature and precipitation, as the additional cooling benefit tends to be larger in warmer and wetter cities at the same TCP level. The positive response of urban trees' cooling efficiency to rising urban temperature is explained mainly by the stronger evapotranspiration of urban trees on hotter days. These results have important implications for alleviating urban heat risk by utilizing urban trees, particularly considering that extreme hot days are becoming more frequent in cities under global warming. [ABSTRACT FROM AUTHOR]

Seasonal rainfall plays a vital role in both environmental dynamics and decision-making for rainfed agriculture in Ethiopia, a country often impacted by extreme climate events such as drought and flooding. Predicting the onset of the rainy season and providing localized rainfall forecasts for Ethiopia is challenging due to the changing spatiotemporal patterns and the country's rugged topography. The Climate Hazards Group InfraRed Precipitation with Station Data (CHIRPS), ERA5-Land total precipitation and temperature data are used from 1981–2022 to predict spatial rainfall by applying an artificial neural network (ANN). The recurrent neural network (RNN) is a nonlinear autoregressive network with exogenous input (NARX), which includes feed-forward connections and multiple network layers, employing the Levenberg Marquait algorithm. This method is applied to downscale data from the European Centre for Medium-range Weather Forecasts fifth-generation seasonal forecast system (ECMWF-SEAS5) and the Euro-Mediterranean Centre for Climate Change (CMCC) to the specific locations of rainfall stations in Ethiopia for the period 1980–2020. Across the stations, the results of NARX exhibit strong associations and reduced errors. The statistical results indicate that, except for the southwestern Ethiopian highlands, the downscaled monthly precipitation data exhibits high skill scores compared to the station records, demonstrating the effectiveness of the NARX approach for predicting local seasonal rainfall in Ethiopia's complex terrain. In addition to this spatial ANN of the summer season precipitation, temperature, as well as the combination of these two variables, show promising results. [ABSTRACT FROM AUTHOR]

The East Asian summer monsoon in Northeast Asia (NEA) has experienced an increase in summer rainfall and a delayed end to the rainy season after 2000, suggesting a trend of enhancement. Based on the data analyses spanning 1979–2022, our results show that the increased rainfall amounts are associated with a more pronounced Mongolian cyclone (MC) in July–August, a manifestation of a portion of the Eurasian barotropic Rossby wave train. Sea surface temperature (SST) anomalies in the North Atlantic (NA) regulate this wave train, with SST increases leading to its amplification. Somewhat independently, a delayed end to the rainy season in September is related to an enhanced anticyclone over the Kuril Islands (ACKI) in the Russian Far East. This anticyclone originates in the Arctic region, possibly induced by the loss of sea ice in the East Siberian Sea, a condition that can be detected two months in advance. The stronger MC and ACKI jointly contribute to the observed enhancement in the East Asian summer monsoon in NEA since 2000 by facilitating ascending motion and moisture transport. Therefore, the SST anomaly in the NA, which is responsible for the intensified rainfall in the rainy season in NEA, coupled with the sea ice conditions in the East Siberian Sea, provides a potential prediction source for the retreat of the rainy season. [ABSTRACT FROM AUTHOR]

Using surface and balloon-sounding measurements, satellite retrievals, and ERA5 reanalysis during 2011–20, this study compares the precipitation and related wind dynamics, moisture and heat features in different areas of the South China Sea (SCS) before and after SCS summer monsoon onset (SCSSMO). The rainy sea around Dongsha (hereafter simply referred to as Dongsha) near the north coast, and the rainless sea around Xisha (hereafter simply referred to as Xisha) in the western SCS, are selected as two typical research subregions. It is found that Dongsha, rather than Xisha, has an earlier and greater increase in precipitation after SCSSMO under the combined effect of strong low-level southwesterly winds, coastal terrain blocking and lifting, and northern cold air. When the 950-hPa southwesterly winds enhance and advance northward, accompanied by strengthened moisture flux, there is a strong convergence of wind and moisture in Dongsha due to a sudden deceleration and rear-end collision of wind by coastal terrain blocking. Moist and warm advection over Dongsha enhances early and deepens up to 200 hPa in association with the strengthened upward motion after SCSSMO, thereby providing ample moisture and heat to form strong precipitation. However, when the 950-hPa southwesterly winds weaken and retreat southward, Xisha is located in a wind-break area where strong convergence and upward motion centers move in. The vertical moistening and heating by advection in Xisha enhance later and appear far weaker compared to that in Dongsha, consistent with later and weaker precipitation. [ABSTRACT FROM AUTHOR]

The spring atmospheric heat source (AHS) over the Tibetan Plateau (TP) has been suggested to affect the Asian summer monsoon and summer precipitation over South China. However, its influence on the summer precipitation in Northeast China (NEC) remains unknown. The connection between spring TP AHS and subsequent summer precipitation over NEC from 1961 to 2020 is analyzed in this study. Results illustrate that stronger spring TP AHS can enhance subsequent summer NEC precipitation, and higher soil moisture in the Yellow River Valley–North China region (YRVNC) acts as a bridge. During spring, the strong TP AHS could strengthen the transportation of water vapor to East China and lead to excessive rainfall in the YRVNC. Thus, soil moisture increases, which regulates local thermal conditions by decreasing local surface skin temperature and sensible heat. Owing to the memory of soil moisture, the lower spring sensible heat over the YRVNC can last until mid-summer, decrease the land–sea thermal contrast, and weaken the southerly winds over the East Asia–western Pacific region and convective activities over the South China Sea and tropical western Pacific. This modulates the East Asia–Pacific teleconnection pattern, which leads to a cyclonic anomaly and excessive summer precipitation over NEC. [ABSTRACT FROM AUTHOR]

The 2015/16 El Niño event ranks among the top three of the last 100 years in terms of intensity, but most dynamical models had a relatively low prediction skill for this event before the summer months. Therefore, the attribution of this particular event can help us to understand the cause of super El Niño–Southern Oscillation events and how to forecast them skillfully. The present study applies attribute methods based on a deep learning model to study the key factors related to the formation of this event. A deep learning model is trained using historical simulations from 21 CMIP6 models to predict the Niño-3.4 index. The integrated gradient method is then used to identify the key signals in the North Pacific that determine the evolution of the Niño-3.4 index. These crucial signals are then masked in the initial conditions to verify their roles in the prediction. In addition to confirming the key signals inducing the super El Niño event revealed in previous attribution studies, we identify the combined contribution of the tropical North Atlantic and the South Pacific oceans to the evolution and intensity of this event, emphasizing the crucial role of the interactions among them and the North Pacific. This approach is also applied to other El Niño events, revealing several new precursor signals. This study suggests that the deep learning method is useful in attributing the key factors inducing extreme tropical climate events. [ABSTRACT FROM AUTHOR]

This study compares the summer atmospheric water cycle, including moisture sources and consumption, in the upstream, midstream, and downstream regions of the Yarlung Zangbo River Basin in the southern Tibetan Plateau. The evolutions of moisture properties under the influence of the westerly and summer southerly monsoon are examined using 5-yr multi-source measurements and ERA5 reanalysis data. Note that moisture consumption in this study is associated with clouds, precipitation, and diabatic heating. Compared to the midstream and downstream regions, the upstream region has less moisture, clouds, and precipitation, where the moisture is brought by the westerly. In early August, the vertical wet advection over this region becomes enhanced and generates more high clouds and precipitation. The midstream region has moisture carried by the westerly in June and by the southerly monsoon from July to August. The higher vertical wet advection maximum here forms more high clouds, with a precipitation peak in early July. The downstream region is mainly affected by the southerly-driven wet advection. The rich moisture and strong vertical wet advection here produce the most clouds and precipitation among the three regions, with a precipitation peak in late June. The height of the maximum moisture condensation is different between the midstream region (325 hPa) and the other two regions (375 hPa), due to the higher upward motion maximum in the midstream region. The diabatic heating structures show that stratiform clouds dominate the upstream region, stratiform clouds and deep convection co-exist in the midstream region, and deep convection systems characterize the downstream region. [ABSTRACT FROM AUTHOR]

In the boreal summer and autumn of 2023, the globe experienced an extremely hot period across both oceans and continents. The consecutive record-breaking mean surface temperature has caused many to speculate upon how the global temperature will evolve in the coming 2023/24 boreal winter. In this report, as shown in the multi-model ensemble mean (MME) prediction released by the Institute of Atmospheric Physics at the Chinese Academy of Sciences, a medium-to-strong eastern Pacific El Niño event will reach its mature phase in the following 2–3 months, which tends to excite an anomalous anticyclone over the western North Pacific and the Pacific-North American teleconnection, thus serving to modulate the winter climate in East Asia and North America. Despite some uncertainty due to unpredictable internal atmospheric variability, the global mean surface temperature (GMST) in the 2023/24 winter will likely be the warmest in recorded history as a consequence of both the El Niño event and the long-term global warming trend. Specifically, the middle and low latitudes of Eurasia are expected to experience an anomalously warm winter, and the surface air temperature anomaly in China will likely exceed 2.4 standard deviations above climatology and subsequently be recorded as the warmest winter since 1991. Moreover, the necessary early warnings are still reliable in the timely updated medium-term numerical weather forecasts and sub-seasonal-to-seasonal prediction. [ABSTRACT FROM AUTHOR]

Assessment of past-climate simulations of regional climate models (RCMs) is important for understanding the reliability of RCMs when used to project future regional climate. Here, we assess the performance and discuss possible causes of biases in a WRF-based RCM with a grid spacing of 50 km, named WRFG, from the North American Regional Climate Change Assessment Program (NARCCAP) in simulating wet season precipitation over the Central United States for a period when observational data are available. The RCM reproduces key features of the precipitation distribution characteristics during late spring to early summer, although it tends to underestimate the magnitude of precipitation. This dry bias is partially due to the model's lack of skill in simulating nocturnal precipitation related to the lack of eastward propagating convective systems in the simulation. Inaccuracy in reproducing large-scale circulation and environmental conditions is another contributing factor. The too weak simulated pressure gradient between the Rocky Mountains and the Gulf of Mexico results in weaker southerly winds in between, leading to a reduction of warm moist air transport from the Gulf to the Central Great Plains. The simulated low-level horizontal convergence fields are less favorable for upward motion than in the NARR and hence, for the development of moist convection as well. Therefore, a careful examination of an RCM's deficiencies and the identification of the source of errors are important when using the RCM to project precipitation changes in future climate scenarios. [ABSTRACT FROM AUTHOR]

Three cases of microphysical characteristics and kinematic structures in the negative temperature region of summer mesoscale cloud systems over the eastern Tibetan Plateau (TP) were investigated using X-band dual-polarization radar. The time-height series of radar physical variables and mesoscale horizontal divergence δ ¯ derived by quasi-vertical profiles (QVPs) indicated that the dendritic growth layer (DGL, −20°C to −10°C) was ubiquitous, with large-value zones of KDP (specific differential phase), ZDR (differential reflectivity), or both, and corresponded to various dynamic fields (ascent or descent). Ascents in the DGL of cloud systems with vigorous vertical development were coincident with large-value zones of ZDR, signifying ice crystals with a large axis ratio, but with no obvious large values of KDP, which differs from previous findings. It is speculated that ascent in the DGL promoted ice crystals to undergo further growth before sinking. If there was descent in the DGL, a high echo top corresponded to large values of KDP, denoting a large number concentration of ice crystals; but with the echo top descending, small values of KDP formed. This is similar to previous results and reveals that a high echo top is conducive to the generation of ice crystals. When ice particles fall to low levels (−10°C to 0°C), they grow through riming, aggregation, or deposition, and may not be related to the kinematic structure. It is important to note that this study was only based on a limited number of cases and that further research is therefore needed. [ABSTRACT FROM AUTHOR]

This study explores the linkage between summertime temperature fluctuations over midlatitude Eurasia and the preceding Arctic sea ice concentration (SIC) by utilizing the squared norm of the temperature anomaly, the essential part of local eddy available potential energy, as a metric to quantify the temperature fluctuations with weather patterns on various timescales. By comparing groups of singular value decomposition (SVD) analysis, we suggest a significant linkage between strong (weak) August 10-to-30-day temperature fluctuations over mid-west Asia and enhanced (decreased) Barents-Kara Sea ice in the previous February. We find that when the February SIC increases in the Barents-Kara Sea, a zonal dipolar pattern of SST anomalies appears in the Atlantic subpolar region and lasts from February into the summer months. Evidence suggests that in such a background state, the atmospheric circulation changes evidently from July to August, so that the August is characterized by an amplified meridional circulation over Eurasia, weakened westerlies, and high-pressure anomalies along the Arctic coast. Moreover, the 10-to-30-day wave becomes more active in the North Atlantic–Barents-Kara Sea–Central Asia regions and manifests a more evident southward propagation from the Barents-Kara Sea into the Ural region, which is responsible for the enhanced 10-to-30-day wave activity and temperature fluctuations in the region. [ABSTRACT FROM AUTHOR]

Sea ice, one of the most dominant barriers to Arctic shipping, has decreased dramatically over the past four decades. Arctic maritime transport is hereupon growing in recent years. To produce a long-term assessment of trans-Arctic accessibility, we systematically revisit the daily Arctic navigability with a view to the combined effects of sea ice thickness and concentration throughout the period 1979–2020. The general trends of Navigable Windows (NW) in the Northeast Passage show that the number of navigable days is steadily growing and reached 89±16 days for Open Water (OW) ships and 163±19 days for Polar Class 6 (PC6) ships in the 2010s, despite high interannual and interdecadal variability in the NWs. More consecutive NWs have emerged annually for both OW ships and PC6 ships since 2005 because of the faster sea ice retreat. Since the 1980s, the number of simulated Arctic routes has continuously increased, and optimal navigability exists in these years of record-low sea ice extent (e.g., 2012 and 2020). Summertime navigability in the East Siberian and Laptev Seas, on the other hand, varies dramatically due to changing sea ice conditions. This systematic assessment of Arctic navigability provides a reference for better projecting the future trans-Arctic shipping routes. [ABSTRACT FROM AUTHOR]

This study identifies break events of the South China Sea (SCS) summer monsoon (SCSSM) based on 42 years of data from 1979 to 2020, and investigates their statistical characteristics and associated atmospheric anomalies. A total of 214 break events are identified by examining the convection evolution during each monsoon season. It is found that most events occur between June and September and show a roughly even distribution. Short-lived events (3–7 days) are more frequent, accounting for about two thirds of total events, with the residual one third for long-lived events (8–24 days). The SCSSM break is featured by drastic variations in various atmospheric variables. Particularly, the convection and precipitation change from anomalous enhancement in adjoining periods to a substantial suppression during the break, with the differences being more than 60 W m−2 for outgoing longwave radiation (OLR) and 10 mm d−1 for precipitation. This convection/precipitation suppression is accompanied by an anomalous anticyclone in the lower troposphere, corresponding to a remarkable westward retreat of the monsoon trough from the Philippine Sea to the Indochina Peninsula, which reduces the transportation of water vapor into the SCS. Besides, the pseudo-equivalent potential temperature (θse) declines sharply, mainly attributable to the local specific humidity reduction caused by downward dry advection. Furthermore, it is found that the suppressed convection and anomalous anticyclone responsible for the monsoon break form near the equatorial western Pacific and then propagate northwestward to the SCS. [ABSTRACT FROM AUTHOR]

Extreme summer heat can have serious socioeconomic impacts in North China. Here, we explore the decadal variability of the number of extreme heat days in early-to-mid summer (June and July) and a related potential mechanism consistent with the major seasonal occurrence period of extreme heat events in North China (NCSH). Observational analyses show significant decadal variability in NCSH for 1981–2021, potentially linked to the Indo-Pacific warm pool and Northwest Pacific Ocean dipole (IPOD) in early-to-mid summer. Dynamic diagnostic analysis and the linear baroclinic model (LBM) show that the positive IPOD in early-to-mid summer can excite upward vertical wind anomalies in the South China-East China Sea region, shifting the position of the western Pacific subtropical high (WPSH) to the east or weakening the degree of its control of the South China-East China Sea region, thus generating a positive geopotential height quadrupole (EAWPQ) pattern in the East Asia-Northwest Pacific region. Subsequently, the EAWPQ can cause air compression (expansion) over North China by regulating the tropospheric thickness anomalies in North China, thus increasing (decreasing) NCSH. Finally, an empirical model that incorporates the linear trend can better simulate the decadal NCSH compared to an empirical model based solely on the IPOD index, suggesting that the decadal variability of NCSH may be a combined contribution of the decadal IPOD and external linear forcing. [ABSTRACT FROM AUTHOR]

The interannual meridional displacement of the upper-tropospheric westerly jet over the eastern portion of East Asia in summer has been well documented. This study, however, investigates the interannual meridional displacement of the westerly jet over the western portion of East Asia in summer, which is distinct from its eastern counterpart. The results show that the meridional displacement of the western East Asian jet shows a clear asymmetric feature; that is, there are remarkable differences between the southward and northward displacement of the jet. The southward displacement of the jet corresponds to suppressed convection in the tropical western North Pacific and Maritime Continent and enhanced convection in the equatorial Pacific, which can be explained by the warmer sea surfaces found in the northern Indian Ocean and equatorial eastern Pacific. These tropical anomalies somewhat resemble those associated with the eastern East Asian jet variability. However, the northward displacement of the western East Asian jet does not correspond to significant convection and SST anomalies in the entire tropics; instead, the northward displacement of the jet corresponds well to the positive phase of the Arctic Oscillation. Furthermore, the meridional displacement of the western jet has asymmetric impacts on rainfall and surface air temperatures in East Asia. When the western jet shifts northward, more precipitation is found over South China and Northeast China, and higher temperatures appear in northern China. By contrast, when the jet shifts southward, more precipitation appears over the East Asian rainy belt, including the Yangtze River valley, South Korea, and southern and central Japan and warmer temperatures are found South and Southeast Asia. [ABSTRACT FROM AUTHOR]

The Sichuan-Tibet Railway, mainly located in the southeastern Qinghai-Tibet Plateau, is affected by summertime extreme precipitation (SEP). Using daily rain-gauge observations and ERA5 reanalysis data for the summers of 1979–2020, the spatiotemporal distribution characteristics of SEP in the key region of the Sichuan-Tibet Railway (28°–33°N, 90°–105°E, hereafter KR) are revealed, and the mechanism for SEP amount (SEPA) variation in the KR is investigated. The results show that SEPA in the KR contributes nearly 30% to the total summer precipitation. Regional differences are evident in SEP, justifying thresholds higher in the plateau-dominated central-western KR (CWKR) and lower in the basin-dominated eastern KR (EKR). In addition, SEP in the CWKR is less intense but more frequent than SEP in the EKR. During 1979–2020, the SEPA in the KR increased slightly while the SEPA in the CWKR increased significantly and peaked in the last decade. When anticyclonic circulation (AC) anomalies dominate the 500 hPa pattern over the Bay of Bengal and Mongolia, the southerly flow and cyclonic shear over the southeastern plateau will be strengthened, favoring more SEPA in the CWKR. When an AC anomaly dominates the 500 hPa pattern over the Bohai Sea, the low-level easterly wind over the basin will be strengthened, favoring more SEPA in the EKR. The strengthening of the ascent, water vapor convergence, and convective instability is conducive to more SEPA in the KR. Our results deepen the understanding of the characteristics and the physical mechanisms responsible for extreme precipitation in the KR. [ABSTRACT FROM AUTHOR]

The South China Sea Summer Monsoon (SCSSM) onset is characterized by an apparent seasonal conversion of circulation and convection. Accordingly, various indices have been introduced to identify the SCSSM onset date. However, the onset dates as determined by various indices can be very inconsistent. It not only limits the determination of onset dates but also misleads the assessment of prediction skills. In 2021, the onset time as identified by the circulation criteria was 20 May, which is 12 days earlier than that deduced by also considering the convection criteria. The present study mainly ascribes such circulation-convection inconsistency to the activities of tropical cyclones (TCs) modulated by the Madden-Julian Oscillation (MJO). The convection of TC "Yaas" (2021) acted as an upper-level diabatic heat source to the north of the SCS, facilitating the circulation transition. Afterward, TC "Cho-wan" (2021) over the western Pacific aided the westerlies to persist at lower levels while simultaneously suppressing moist convection over the SCS. Accurate predictions using the ECMWF S2S forecast system were obtained only after the MJO formation. The skillful prediction of the MJO during late spring may provide an opportunity to accurately predict the establishment of the SCSSM several weeks in advance. [ABSTRACT FROM AUTHOR]

The northeastern China cold vortex (NCCV) plays an important role in regional rainstorms over East Asia. Using the National Centers for Environmental Prediction Final reanalysis dataset and the Global Precipitation Measurement product, an objective algorithm for identifying heavy-precipitation NCCV (HPCV) events was designed, and the climatological features of 164 HPCV events from 2001 to 2019 were investigated. The number of HPCV events showed an upward linear trend, with the highest frequency of occurrence in summer. The most active region of HPCV samples was the Northeast China Plain between 40°–55°N. Most HPCV events lasted 3–5 days and had radii ranging from 250 to 1000 km. The duration of HPCV events with larger sizes was longer. About half of the HPCV events moved into (moved out of) the definition region (35°–60°N, 115°–145°E), and half initiated (dissipated) within the region. The initial position was close to the western boundary of the definition region, and the final position was mainly near the eastern boundary. The locations associated with the precipitation were mostly concentrated within 2000 km southeast of the HPCV systems, and they were farther from the center in the cold season than in the warm season. [ABSTRACT FROM AUTHOR]

The Northeast China cold vortex (NCCV) during late summer (from July to August) is identified and classified into three types in terms of its movement path using machine learning. The relationships of the three types of NCCV intensity with atmospheric circulations in late summer, the sea surface temperature (SST), and Arctic sea ice concentration (SIC) in the preceding months, are analyzed. The sensitivity tests by the Community Atmosphere Model version 5.3 (CAM5.3) are used to verify the statistical results. The results show that the coordination pattern of East Asia-Pacific (EAP) and Lake Baikal high pressure forced by SST anomalies in the North Indian Ocean dipole mode (NIOD) during the preceding April and SIC anomalies in the Nansen Basin during the preceding June results in an intensity anomaly for the first type of NCCV. While the pattern of high pressure over the Urals and Okhotsk Sea and low pressure over Lake Baikal during late summer—which is forced by SST anomalies in the South Indian Ocean dipole mode (SIOD) in the preceding June and SIC anomalies in the Barents Sea in the preceding April—causes the intensity anomaly of the second type. The third type is atypical and is not analyzed in detail. Sensitivity tests, jointly forced by the SST and SIC in the preceding period, can well reproduce the observations. In contrast, the results forced separately by the SST and SIC are poor, indicating that the NCCV during late summer is likely influenced by the coordinated effects of both SST and SIC in the preceding months. [ABSTRACT FROM AUTHOR]

Clouds are a dominant modulator of the energy budget. The cloud shortwave radiative effect at the surface (CRE) is closely related to the cloud macro- and micro-physical properties. Systematic observation of surface irradiance and cloud properties are needed to narrow uncertainties in CRE. In this study, 1-min irradiance and Total Sky Imager measurements from 2005 to 2009 at Xianghe in North China Plain are used to estimate cloud types, evaluate cloud fraction (CF), and quantify the sensitivities of surface irradiance with respect to changes in CF whether clouds obscure the sun or not. The annual mean CF is 0.50, further noting that CF exhibits a distinct seasonal variation, with a minimum in winter (0.37) and maximum in summer (0.68). Cumulus occurs more frequently in summer (32%), which is close to the sum of the occurrence of stratus and cirrus. The annual CRE is −54.4 W m−2, with seasonal values ranging from −29.5 W m−2 in winter and −78.2 W m−2 in summer. When clouds do not obscure the sun, CF is a dominant factor affecting diffuse irradiance, which in turn affects global irradiance. There is a positive linear relationship between CF and CRE under sun-unobscured conditions, the mean sensitivity of CRE for each CF 0.1 increase is about 1.2 W m−2 [79.5° < SZA (Solar Zenith Angle) < 80.5°] to 7.0 W m−2 (29.5° < SZA < 30.5°). When clouds obscure the sun, CF affects both direct and diffuse irradiance, resulting in a non-linear relationship between CF and CRE, and the slope decreases with increasing CF. It should be noted that, although only data at Xianghe is used in this study, our results are representative of neighboring areas, including most parts of the North China Plain. [ABSTRACT FROM AUTHOR]

This study depicts the sub-seasonal prediction of the South China Sea summer monsoon onset (SCSSMO) and investigates the associated oceanic and atmospheric processes, utilizing the hindcasts of the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). Typically, the SCSSMO is accompanied by an eastward retreat of the western North Pacific subtropical high (WNPSH), development of the cross-equatorial flow, and an increase in the east-west sea surface temperature (SST) gradient. These features are favorable for the onset of westerlies and strengthening of convection and precipitation over the South China Sea (SCS). A more vigorous SCSSMO process shows a higher predictability, and vice versa. The NCEP CFSv2 can successfully predict the onset date and evolution of the monsoon about 4 pentads (20 days) in advance (within 1–2 pentads) for more forceful (less vigorous) SCSSMO processes. On the other hand, the climatological SCSSMO that occurs around the 27th pentad can be accurately predicted in one pentad, and the predicted SCSSMO occurs 1–2 pentads earlier than the observed with a weaker intensity at longer leadtimes. Warm SST biases appear over the western equatorial Pacific preceding the SCSSMO. These biases induce a weaker-than-observed WNPSH as a Gill-type response, leading to weakened low-level easterlies over the SCS and hence an earlier and less vigorous SCSSMO. In addition, after the SCSSMO, remarkable warm biases over the eastern Indian Ocean and the SCS and cold biases over the WNP induce weaker-than-observed westerlies over the SCS, thus also contributing to the less vigorous SCSSMO. [ABSTRACT FROM AUTHOR]

This study investigates the influences of boreal summer intraseasonal oscillation (BSISO), which originates from the equatorial Indian Ocean and prevails over the Indo-Pacific region, on precipitation over Southeast China, including South China and Yangtze River Valley. The results indicate that the BSISO-related precipitation anomalies are remarkably different between early summer (May–June) and late summer (July–August). The BSISO-related precipitation anomalies tend to appear more northward in late summer in comparison with early summer. Accordingly, the BSISO is significantly related to precipitation anomalies over South China during many phases in early summer but related to very weak anomalies during all the phases in late summer. Such northward shifts of precipitation anomalies from early summer to late summer are clearest during phases 4 and 7, when the lower-tropospheric anticyclonic and cyclonic circulation anomalies dominate over the subtropical western North Pacific, respectively. Finally, we explain the differences between early and late summers through the seasonal northward migration of climatological equivalent potential temperature gradient, which is located in the South China during early summer but migrates northward to the YRV during late summer. [ABSTRACT FROM AUTHOR]

Seasonal minimum Antarctic sea ice extent (SIE) in 2022 hit a new record low since recordkeeping began in 1978 of 1.9 million km2 on 25 February, 0.17 million km2 lower than the previous record low set in 2017. Significant negative anomalies in the Bellingshausen/Amundsen Seas, the Weddell Sea, and the western Indian Ocean sector led to the new record minimum. The sea ice budget analysis presented here shows that thermodynamic processes dominate sea ice loss in summer through enhanced poleward heat transport and albedo-temperature feedback. In spring, both dynamic and thermodynamic processes contribute to negative sea ice anomalies. Specifically, dynamic ice loss dominates in the Amundsen Sea as evidenced by sea ice thickness (SIT) change, while positive surface heat fluxes contribute most to sea ice melt in the Weddell Sea. [ABSTRACT FROM AUTHOR]

The non-hydrostatic global variable resolution model (MPAS-atmosphere) is used to conduct the simulations for the South Asian Summer monsoon season (June, July, and August) in 2015 with a refinement over the Tibetan Plateau (TP) at the convection-permitting scale (4 km). Two experiments with different topographical datasets, complex (4-km) and smooth (60-km) topography, are designed to investigate the impacts of topographical complexity on moisture transport and precipitation. Compared with the observations and reanalysis data, the simulation can successfully capture the general features of key meteorological fields over the TP despite slightly underestimating the inflow through the southern TP. The results indicate that the complex topography can decrease the inward and outward moisture transport, ultimately increasing the total net moisture transport into the TP by ∼11%. The impacts of complex topography on precipitation are negligible over the TP, but the spatial distributions of precipitation over the Himalayas are significantly modulated. With the inclusion of complex topography, the sharper southern slopes of the Himalayas shift the lifted airflow and hence precipitation northward compared to the smooth topography. In addition, more small-scale valleys are resolved by the inclusion of complex topography, which serve as channels for moisture transport across the Himalayas, further favoring a northward shift of precipitation. Overall, the difference between the two experiments with different topography datasets is mainly attributed to their differing representation of the degree of the southern slopes of the Himalayas and the extent to which the valleys are resolved. [ABSTRACT FROM AUTHOR]

The influences of interannual surface potential vorticity forcing over the Tibetan Plateau (TP) on East Asian summer rainfall (EASR) and upper-level circulation are explored in this study. The results show that the interannual EASR and associated circulations are closely related to the surface potential vorticity negative uniform leading mode (PVNUM) over the TP. When the PVNUM is in the positive phase, more rainfall occurs in the Yangtze River valley, South Korea, Japan, and part of northern China, less rainfall occurs in southern China, and vice versa. A possible mechanism by which PVNUM affects EASR is proposed. Unstable air induced by the positive phase of PVNUM could stimulate significant upward motion and a lower-level anomalous cyclone over the TP. As a result, a dipole heating mode with anomalous cooling over the southwestern TP and anomalous heating over the southeastern TP is generated. Sensitivity experiment results regarding this dipole heating mode indicate that anomalous cooling over the southwestern TP leads to local and northeastern Asian negative height anomalies, while anomalous heating over the southeastern TP leads to local positive height anomalies. These results greatly resemble the realistic circulation pattern associated with EASR. Further analysis indicates that the anomalous water vapor transport associated with this anomalous circulation pattern is responsible for the anomalous EASR. Consequently, changes in surface potential vorticity forcing over the TP can induce changes in EASR. [ABSTRACT FROM AUTHOR]

The current work examines the impact of the snow cover extent (SCE) of the Tibetan Plateau (TP) on the interannual variation in the summer (June-July-August) surface air temperature (SAT) over Central Asia (CA) (SAT_CA) during the 1979–2019 period. The leading mode of the summer SAT_CA features a same-sign temperature anomalies in CA and explains 62% of the total variance in SAT_CA. The atmospheric circulation associated with a warming SAT_CA is characterized by a pronounced high-pressure system dominating CA. The high-pressure system is accompanied by warm advection as well as descending motion over CA, favoring the warming of the SAT_CA. Analysis shows that the interannual variation in the summer SAT_CA is significantly positively correlated with the April SCE over the central-eastern TP. In April, higher than normal SCE over the central-eastern TP has a pronounced cooling effect on the column of the atmosphere above the TP and can persist until the following early summer. Negative and positive height anomalies appear above and to the west of the TP. In the following months, the perturbation forcing generated by the TP SCE anomalies lies near the western center of the Asian subtropical westerly jet (SWJ), which promotes atmospheric waves in the zonal direction guided by the Asian SWJ. Associated with this atmospheric wave, in the following summer, a significant high-pressure system dominates CA, which is a favorable condition for a warm summer SAT_CA. [ABSTRACT FROM AUTHOR]

Traditionally, a delayed (early) onset of the South China Sea summer monsoon (SCSSM) has been observed to follow a warm (cold) El Niño-Southern Oscillation (ENSO) event in winter, supporting high seasonal predictability of SCSSM onset. However, the empirical seasonal forecasting skill of the SCSSM onset, solely based on ENSO, has deteriorated since 2010. Meanwhile, unexpected delayed onsets of the SCSSM have also occurred in the past decade. We attribute these changes to the Northwest Indian Ocean (NWIO) warming of the sea surface. The NWIO warming has teleconnections related to (1) suppressing the seasonal convection over the South China Sea, which weakens the impacts of ENSO on SCSSM onset and delays the start of SCSSM, and (2) favoring more high-frequency, propagating moist convective activities, which enhances the uncertainty of the seasonal prediction of SCSSM onset date. Our results yield insight into the predictability of the SCSSM onset under the context of uneven ocean warming operating within the larger-scale background state of global climate change. [ABSTRACT FROM AUTHOR]

Convectively coupled equatorial Rossby waves (ERW) modulate tropical cyclone activities over tropical oceans. This study presents a survey of the statistical relationship between intraseasonal ERWs and tropical cyclone genesis (TCG) over major global TC basins using four-decade-long outgoing longwave radiation (OLR) and TC best-track datasets. Intraseasonal ERWs are identified from the OLR anomalies using an empirical orthogonal function (EOF) analysis method without imposing equatorial symmetry. We find that westward-propagating ERWs are most significant in four tropical ocean basins over the summer hemisphere and that ERWs exhibit similar northeast-southwest (southeast-northwest) tilted phase lines in the northern (southern) hemisphere, with an appreciable poleward advance of wave energy in most TC basins. The EOF-based ERW indices quantitatively show that ERWs significantly modulate TC genesis. The convectively active (suppressed) phases of ERWs coincide with increased (reduced) TCG occurrences. The TCG modulation by ERWs achieves the maximum where the ERWs propagate through the climatological TCG hotspots. As a result, the total number of TCG occurrences in the TC basins varies significantly according to the ERW phase. The ERW-TCG relationship is significant over the northwestern Pacific Ocean, northeastern Pacific Ocean, and the northern Indian Ocean during the northern summer seasons. In the southern summer season, the ERW-TCG relationship is significant over the southern Indian Ocean, Indonesian-Australia basin, and the southwestern Pacific Ocean. However, ERW activities are weak in the main TC development region of the Atlantic Ocean; and the impact on Atlantic TCG appears to be insignificant. [ABSTRACT FROM AUTHOR]

East Africa is particularly vulnerable to precipitation variability, as the livelihood of much of the population depends on rainfed agriculture. Seasonal forecasts of the precipitation anomalies, when skillful, can therefore improve implementation of coping mechanisms with respect to food security and water management. This study assesses the performance of Nanjing University of Information Science and Technology Climate Forecast System version 1.0 (NUIST-CFS1.0) on forecasting June–September (JJAS) seasonal precipitation anomalies over East Africa. The skill in predicting the JJAS mean precipitation initiated from 1 May for the period of 1982–2019 is evaluated using both deterministic and probabilistic verification metrics on grid cell and over six distinct clusters. The results show that NUIST-CFS1.0 captures the spatial pattern of observed seasonal precipitation climatology, albeit with dry and wet biases in a few parts of the region. The model has positive skill across a majority of Ethiopia, Kenya, Uganda, and Tanzania, whereas it doesn't exceed the skill of climatological forecasts in parts of Sudan and southeastern Ethiopia. Positive forecast skill is found over regions where the model shows better performance in reproducing teleconnections related to oceanic SST. The prediction performance of NUIST-CFS1.0 is found to be on a level that is potentially useful over a majority of East Africa. [ABSTRACT FROM AUTHOR]

The contribution of tropical cyclones (TCs) to the East Asia—Pacific (EAP) teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to 2018. The results showed that the TCs over the western North Pacific (WNP) correspond to a strengthened EAP pattern: During the summers of strong convection over the tropical WNP, TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere, an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere, and a more northward shift of the East Asian westerly jet in the upper troposphere. Further analyses indicated that two types of TCs with distinctly different tracks, i.e., westward-moving TCs and northward-moving TCs, both favor the EAP pattern. The present results imply that TCs over the WNP, as extreme weather, can contribute significantly to summer-mean climate anomalies over the WNP and East Asia. [ABSTRACT FROM AUTHOR]

Record-breaking heavy and persistent precipitation occurred over the Yangtze River Valley (YRV) in June-July (JJ) 2020. An observational data analysis has indicated that the strong and persistent rainfall arose from the confluence of southerly wind anomalies to the south associated with an extremely strong anomalous anticyclone over the western North Pacific (WNPAC) and northeasterly anomalies to the north associated with a high-pressure anomaly over Northeast Asia. A further observational and modeling study has shown that the extremely strong WNPAC was caused by both La Niña-like SST anomaly (SSTA) forcing in the equatorial Pacific and warm SSTA forcing in the tropical Indian Ocean (IO). Different from conventional central Pacific (CP) El Niños that decay slowly, a CP El Niño in early 2020 decayed quickly and became a La Niña by early summer. This quick transition had a critical impact on the WNPAC. Meanwhile, an unusually large area of SST warming occurred in the tropical IO because a moderate interannual SSTA over the IO associated with the CP El Niño was superposed by an interdecadal/long-term trend component. Numerical sensitivity experiments have demonstrated that both the heating anomaly in the IO and the heating anomaly in the tropical Pacific contributed to the formation and maintenance of the WNPAC. The persistent high-pressure anomaly in Northeast Asia was part of a stationary Rossby wave train in the midlatitudes, driven by combined heating anomalies over India, the tropical eastern Pacific, and the tropical Atlantic. [ABSTRACT FROM AUTHOR]

Summer precipitation over the Yangtze River basin (YRB) in 2020 experienced a strong subseasonal and synoptic fluctuation in addition to contributing to an exceptionally large seasonal mean precipitation. The cause of this higher-frequency fluctuation is examined based on observational analyses. Apart from the continuous northward movement of the climatological mei-yu rainband, the mei-yu rainbelt in the summer of 2020 experienced multiple northward and southward swings. The cause of the swings was attributed to the subseasonal variability of southerly winds to the south and northeasterly winds to the north of the YRB. In addition, synoptic-scale variability, characterized by the eastward propagation of low-level cyclonic vorticity and precipitation anomalies, was also commonplace in the summer of 2020. While the strengthening of both the subseasonal and synoptic variabilities in the summer of 2020 was attributed to the increase of the background mean moisture, the synoptic variability was greatly affected by the subseasonal rainfall variability. As a result, both the synoptic-scale and subseasonal variabilities contributed to the north-south swings of the rainbelt. The large-scale modulations by both the seasonal mean and subseasonal anomalies provide insight regarding the optimization of issuing accurate, extended-range forecasts of extreme weather events. [ABSTRACT FROM AUTHOR]

Seasonal forecasts for Yangtze River basin rainfall in June, May–June–July (MJJ), and June–July–August (JJA) 2020 are presented, based on the Met Office GloSea5 system. The three-month forecasts are based on dynamical predictions of an East Asian Summer Monsoon (EASM) index, which is transformed into regional-mean rainfall through linear regression. The June rainfall forecasts for the middle/lower Yangtze River basin are based on linear regression of precipitation. The forecasts verify well in terms of giving strong, consistent predictions of above-average rainfall at lead times of at least three months. However, the Yangtze region was subject to exceptionally heavy rainfall throughout the summer period, leading to observed values that lie outside the 95% prediction intervals of the three-month forecasts. The forecasts presented here are consistent with other studies of the 2020 EASM rainfall, whereby the enhanced mei-yu front in early summer is skillfully forecast, but the impact of midlatitude drivers enhancing the rainfall in later summer is not captured. This case study demonstrates both the utility of probabilistic seasonal forecasts for the Yangtze region and the potential limitations in anticipating complex extreme events driven by a combination of coincident factors. [ABSTRACT FROM AUTHOR]

In the summer of 1998, heavy rainfall persisted throughout the summer and resulted in a severe prolonged flooding event over East Asia. Will a similar rainy summer happen again? To date, many studies have investigated projected changes in the seasonality or daily extreme precipitation events over East Asia; however, few studies have focused on the changes in extreme summer-averaged East Asian rainfall. This type of summer is referred to as a "heavy rainy summer (HRS)" in this study, and an investigation of future changes in its probability is performed by analyzing CMIP5 model outputs in historical climate simulation (HIST) and under RCP4.5 and RCP8.5. All models project increased probabilities of HRS by a factor of two to three. The projected East Asian summer rainfall (EASR) (EASRRCPs−EASRHIST) in both climatology and HRS is expected to intensify significantly. The increased EASR could be attributed to significantly intensified water vapor transport (WVT) originating from the tropical Indian Ocean (TIO) and the eastern subtropical North Pacific (SNP), which is a result of the thermodynamic component. The WVT from the TIO would supply more moisture for EASR because of its stronger intensity and faster rate of increase. Meanwhile, the EASR anomaly in HRS relative to climatology (EASRHRS−EASRCLM) would increase by approximately 11%–33%. In HIST, the associated WVT anomaly, caused only by the dynamic component, converges moisture from adjacent land and ocean. However, under the RCPs, the WVT anomaly from the TIO, resulted from the thermodynamic component, would appear and increase by a factor of three to be comparable to the WVT anomaly from the eastern SNP. The latter would result from the dynamic component but increase by only half. [ABSTRACT FROM AUTHOR]

Photochemical smog characterized by high concentrations of ozone (O3) is a serious air pollution issue in the North China Plain (NCP) region, especially in summer and autumn. For this study, measurements of O3, nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), nitrous acid (HONO), and a number of key physical parameters were taken at a suburban site, Xianghe, in the NCP region during the summer of 2018 in order to better understand the photochemical processes leading to O3 formation and find an optimal way to control O3 pollution. Here, the radical chemistry and O3 photochemical budget based on measurement data from 1–23 July using a chemical box model is investigated. The daytime (0600–1800 LST) average production rate of the primary radicals referred to as ROx (OH + HO2 + RO2) is 3.9 ppbv h−1. HONO photolysis is the largest primary ROx source (41%). Reaction of NO2 + OH is the largest contributor to radical termination (41%), followed by reactions of RO2 + NO2 (26%). The average diurnal maximum O3 production and loss rates are 32.9 ppbv h−1 and 4.3 ppbv h−1, respectively. Sensitivity tests without the HONO constraint lead to decreases in daytime average primary ROx production by 55% and O3 photochemical production by 42%, highlighting the importance of accurate HONO measurements when quantifying the ROx budget and O3 photochemical production. Considering heterogeneous reactions of trace gases and radicals on aerosols, aerosol uptake of HO2 contributes 11% to ROx sink, and the daytime average O3 photochemical production decreases by 14%. The O3-NOx-VOCs sensitivity shows that the O3 production at Xianghe during the investigation period is mainly controlled by VOCs. [ABSTRACT FROM AUTHOR]

Independent datasets consistently indicate a significant correlation between the sea ice variability in the Bering Sea during melt season and the summer rainfall variability in the Lake Baikal area and Northeastern China. In this study, four sea ice datasets (HadISST1, HadISST2.2, ERA-Interim and NOAA/NSIDC) and two global precipitation datasets (CRU V4.01 and GPCP V2.3) are used to investigate co-variations between melt season (March–April–May–June, MAMJ) Bering Sea ice cover (BSIC) and summer (June–July–August, JJA) East Asian precipitation. All datasets demonstrate a significant correlation between the MAMJ BSIC and the JJA rainfall in Lake Baikal-Northeastern China (Baikal-NEC). Based on the reanalysis datasets and the numerical sensitivity experiments performed in this study using Community Atmospheric Model version 5 (CAM5), a mechanism to understand how the MAMJ BSIC influences the JJA Baikal-NEC rainfall is suggested. More MAMJ BSIC triggers a wave train and causes a positive sea level pressure (SLP) anomaly over the North Atlantic during MAMJ. The high SLP anomaly, associated with an anti-cyclonic wind stress circulation anomaly, favors the appearance of sea surface temperature (SST) anomalies in a zonal dipole-pattern in the North Atlantic during summer. The dipole SST anomaly drives a zonally orientated wave train, which causes a high anomaly geopotential height at 500 hPa over the Sea of Japan. As a result, the mean East Asian trough moves westward and a low geopotential height anomaly occurs over Baikal-NEC. This prevailing regional low pressure anomaly together with enhanced moisture transport from the western North Pacific and convergence over Baikal-NEC, positively influences the increased rainfall in summer. [ABSTRACT FROM AUTHOR]

This study investigates the circulation and precipitation anomalies associated with different configurations of the western North Pacific subtropical high (WNPSH) and the East Asian westerly jet (EAJ) in summer on interannual timescales. The in-phase configuration of the WNPSH and EAJ is characterized by the westward (eastward) extension of the WNPSH and the southward (northward) shift of the EAJ, which is consistent with the general correspondence between their variations. The out-of-phase configuration includes the residual cases. We find that the in-phase configuration manifests itself as a typical meridional teleconnection. For instance, there is an anticyclonic (cyclonic) anomaly over the tropical western North Pacific and a cyclonic (anticyclonic) anomaly over the mid-latitudes of East Asia in the lower troposphere. These circulation anomalies are more conducive to rainfall anomalies over the Yangtze River basin and south Japan than are the individual WNPSH or EAJ. By contrast, for the out-of-phase configuration, the mid-latitude cyclonic (anticyclonic) anomaly is absent, and the lower-tropospheric circulation anomalies feature an anticyclonic (cyclonic) anomaly with a large meridional extension. Correspondingly, significant rainfall anomalies move northward to North China and the northern Korean Peninsula. Further results indicate that the out-of-phase configuration is associated with the developing phase of ENSO, with strong and significant sea surface temperature (SST) anomalies in the tropical central and eastern Pacific which occur simultaneously during summer and persist into the following winter. This is sharply different from the in-phase configuration, for which the tropical SSTs are not a necessity. [ABSTRACT FROM AUTHOR]

Two types of three-dimensional circulation of the East Asian summer monsoon (EASM) act as the coupling wheels determining the seasonal rainfall anomalies in China during 1979–2015. The first coupling mode features the interaction between the Mongolian cyclone over North Asia and the South Asian high (SAH) anomalies over the Tibetan Plateau at 200 hPa. The second mode presents the coupling between the anomalous low-level western Pacific anticyclone and upper-level SAH via the meridional flow over Southeast Asia. These two modes are responsible for the summer rainfall anomalies over China in 24 and 7 out of 37 years, respectively. However, the dominant SST anomalies in the tropical Pacific, the Indian Ocean, and the North Atlantic Ocean fail to account for the first coupling wheel's interannual variability, illustrating the challenges in forecasting summer rainfall over China. [ABSTRACT FROM AUTHOR]

The classification of the Northeast China Cold Vortex (NCCV) activity paths is an important way to analyze its characteristics in detail. Based on the daily precipitation data of the northeastern China (NEC) region, and the atmospheric circulation field and temperature field data of ERA-Interim for every six hours, the NCCV processes during the early summer (June) seasons from 1979 to 2018 were objectively identified. Then, the NCCV processes were classified using a machine learning method (k-means) according to the characteristic parameters of the activity path information. The rationality of the classification results was verified from two aspects, as follows: (1) the atmospheric circulation configuration of the NCCV on various paths; and (2) its influences on the climate conditions in the NEC. The obtained results showed that the activity paths of the NCCV could be divided into four types according to such characteristics as the generation origin, movement direction, and movement velocity of the NCCV. These included the generation-eastward movement type in the east of the Mongolia Plateau (eastward movement type or type A); generation-southeast longdistance movement type in the upstream of the Lena River (southeast long-distance movement type or type B); generation-eastward less-movement type near Lake Baikal (eastward less-movement type or type C); and the generation-southward less-movement type in eastern Siberia (southward less-movement type or type D). There were obvious differences observed in the atmospheric circulation configuration and the climate impact of the NCCV on the four above-mentioned types of paths, which indicated that the classification results were reasonable. [ABSTRACT FROM AUTHOR]

El Niño-Southern Oscillation (ENSO) events have a strong influence on East Asian summer rainfall (EASR). This paper investigates the simulated ENSO-EASR relationship in CMIP6 models and compares the results with those in CMIP3 and CMIP5 models. In general, the CMIP6 models show almost no appreciable progress in representing the ENSO-EASR relationship compared with the CMIP5 models. The correlation coefficients in the CMIP6 models are relatively smaller and exhibit a slightly greater intermodel diversity than those in the CMIP5 models. Three physical processes related to the delayed effect of ENSO on EASR are further analyzed. Results show that, firstly, the relationships between ENSO and the tropical Indian Ocean (TIO) sea surface temperature (SST) in the CMIP6 models are more realistic, stronger, and have less intermodel diversity than those in the CMIP3 and CMIP5 models. Secondly, the teleconnections between the TIO SST and Philippine Sea convection (PSC) in the CMIP6 models are almost the same as those in the CMIP5 models, and stronger than those in the CMIP3 models. Finally, the CMIP3, CMIP5, and CMIP6 models exhibit essentially identical capabilities in representing the PSC-EASR relationship. Almost all the three generations of models underestimate the ENSO-EASR, TIO SST-PSC, and PSC-EASR relationships. Moreover, almost all the CMIP6 models that successfully capture the significant TIO SST-PSC relationship realistically simulate the ENSO-EASR relationship and vice versa, which is, however, not the case in the CMIP5 models. [ABSTRACT FROM AUTHOR]

This study investigated the large-scale circulation anomalies, in both the upper and lower troposphere, associated with the interannual variation of rainfall in Pakistan during summer, using the station observation data in this country and circulation data of the NCEP-NCAR reanalysis from 1981 to 2017. Results showed that the upper- and lower-tropospheric circulation anomalies associated with monthly rainfall variability exhibit similar features from June to August, so analyses were performed on June–August circulation and Pakistan rainfall data. The analyzed results indicated that summer rainfall in Pakistan is enhanced when there is an anticyclonic anomaly to the northwest of Pakistan in the upper troposphere and easterly anomalies along the southern foothills of the Himalayas in the lower troposphere, and vice versa. These upper- and lower-tropospheric circulation anomalies were found to be related, but show unique features. The upper-tropospheric anticyclonic anomaly is closely related to the Silk Road Pattern along the Asian westerly jet, while the lower-tropospheric easterly anomalies are related to the cyclonic anomaly to the south of Pakistan, i.e., intensified South Asian monsoon trough. The results presented here suggest that the interannual variability of summer rainfall in Pakistan is a combined result of upper- and lower-tropospheric circulation anomalies, and of extratropical and tropical circulation anomalies. [ABSTRACT FROM AUTHOR]

The variation in the precipitation phase in polar regions represents an important indicator of climate change and variability. We studied the precipitation phase at the Great Wall Station and Antarctic Peninsula (AP) region, based on daily precipitation, synoptic records and ERA-Interim data during the austral summers of 1985–2014. Overall, there was no trend in the total precipitation amount or days, but the phase of summer precipitation (rainfall days versus snowfall days) showed opposite trends before and after 2001 at the AP. The total summer rain days/snow days increased/decreased during 1985–2001 and significantly decreased at a rate of −14.13 d (10 yr)−1/increased at a rate of 14.31 d (10 yr)−1 during 2001–2014, agreeing well with corresponding variations in the surface air temperature. Further, we found that the longitudinal location of the Amundsen Sea low (ASL) should account for the change in the precipitation phase since 2001, as it has shown a westward drift after 2001 [−41.1° (10 yr)−1], leading to stronger cold southerly winds, colder water vapor flux, and more snow over the AP region during summertime. This study points out a supplementary factor for the climate variation on the AP. [ABSTRACT FROM AUTHOR]

Using a high-resolution precipitation dataset, the present study detected that the mountainous area of central China (MACA) is a hotspot of ENSO's impact on the summer rainfall variability. Further analysis suggests that both ENSO and atmospheric forcing make contributions to the summer rainfall variability in MACA. The dominant rainfall-related SST mode features as a seasonal transition from an El Niño-like warming in the preceding winter to a La Nina-like cooling in the following autumn, and it explains about 29% of the total variance of the rainfall during 1951–2018. It indicates that ENSO with a rapid phase transition is responsible for inducing summer rainfall anomalies in MACA. Besides, an upper-level circumglobal wave mode in the Northern Hemisphere during summer also explains about 29% of the summer rainfall variance. Contributions of both the SST and the atmospheric modes have experienced interdecadal changes. The influence of the SST mode gradually increases and plays a dominant role in the recent decades, suggesting that ENSO with a rapid phase transition becomes more important for rainfall prediction in MACA. [ABSTRACT FROM AUTHOR]

This study investigates the vertical structure of variability in the cross-equatorial flows (CEFs) over the Maritime Continent and Indian Ocean in boreal summer, based on three reanalysis datasets: ERA-Interim, JRA-55 and NCEP-2. The results show a high consistency in the interannual variability among the reanalysis datasets, especially between ERA-Interim and JRA-55, while great uncertainty exists in the decadal or long-term changes of CEFs. Further analyses on the interannual variability in CEFs indicate that there is a significant negative relationship between the upper- and lower-level CEFs over the Maritime Continent—that is, the northerlies at the upper level and southerlies at the lower level are both enhanced or weakened. This seesaw pattern is also significantly related to the CEFs over the Indian Ocean at the upper level and lower level (i.e., the Somali jet). This close relationship between the upper- and lower-level CEFs and between the Maritime Continent and Indian Ocean is manifested as the leading mode of equatorial meridional winds in the vertical-zonal section over the Maritime Continent and Indian Ocean. Finally, it is found that ENSO is closely related to the vertical leading mode, and to all the CEFs at the upper and lower levels over the Maritime Continent and Indian Ocean. [ABSTRACT FROM AUTHOR]

We demonstrate that there is significant skill in the GloSea5 operational seasonal forecasting system for predicting June mean rainfall in the middle/lower Yangtze River basin up to four months in advance. Much of the rainfall in this region during June is contributed by the mei-yu rain band. We find that similar skill exists for predicting the East Asian summer monsoon index (EASMI) on monthly time scales, and that the latter could be used as a proxy to predict the regional rainfall. However, there appears to be little to be gained from using the predicted EASMI as a proxy for regional rainfall on monthly time scales compared with predicting the rainfall directly. Although interannual variability of the June mean rainfall is affected by synoptic and intraseasonal variations, which may be inherently unpredictable on the seasonal forecasting time scale, the major influence of equatorial Pacific sea surface temperatures from the preceding winter on the June mean rainfall is captured by the model through their influence on the western North Pacific subtropical high. The ability to predict the June mean rainfall in the middle and lower Yangtze River basin at a lead time of up to 4 months suggests the potential for providing early information to contingency planners on the availability of water during the summer season. [ABSTRACT FROM AUTHOR]

The differences in the influences of the North Atlantic Oscillation (NAO) on the air-sea CO2 fluxes (fCO2) in the North Atlantic (NA) between different seasons and between different regions are rarely fully investigated. We used observation-based data of fCO2, surface-ocean CO2 partial pressure (pCO2sea), wind speed and sea surface temperature (SST) to analyze the relationship between the NAO and fCO2 of the subtropical and subpolar NA in winter and summer on the interannual time scale. Based on power spectrum estimation, there are significant interannual signs with a 2–6 year cycle in the NAO indexes and area-averaged fCO2 anomalies in winter and summer from 1980 to 2015. Regression analysis with the 2–6 year filtered data shows that on the interannual scale the response of the fCO2 anomalies to the NAO has an obvious meridional wave-train-like pattern in winter, but a zonal distribution in summer. This seasonal difference is because in winter the fCO2 anomalies are mainly controlled by the NAO-driven wind speed anomalies, which have a meridional distribution pattern, while in summer they are dominated by the NAO-driven SST anomalies, which show distinct zonal difference in the subtropical NA. In addition, in the same season, there are different factors controlling the variation of pCO2sea in different regions. In summer, SST is important to the interannual variation of pCO2sea in the subtropical NA, while some biogeochemical variables probably control the pCO2sea variation in the subpolar NA. [ABSTRACT FROM AUTHOR]

As the first leading mode of upper-tropospheric circulation in observations, the meridional displacement of the East Asian westerly jet (EAJ) varies closely with the East Asian rainfall in summer. In this study, the interannual variation of the EAJ meridional displacement and its relationship with the East Asian summer rainfall are evaluated, using the historical simulations of CMIP5 (phase 5 of the Coupled Model Intercomparison Project). The models can generally reproduce the meridional displacement of the EAJ, which is mainly manifested as the first principal mode in most of the simulations. For the relationship between the meridional displacement of the EAJ and East Asian rainfall, almost all the models depict a weaker correlation than observations and exhibit considerably large spread across the models. It is found that the discrepancy in the interannual relationship is closely related to the simulation of the climate mean state, including the climatological location of the westerly jet in Eurasia and rainfall bias in South Asia and the western North Pacific. In addition, a close relationship between the simulation discrepancy and intensity of EAJ variability is also found: the models with a stronger intensity of the EAJ meridional displacement tend to reproduce a closer interannual relationship, and vice versa. [ABSTRACT FROM AUTHOR]

The East Asian westerly jet (EAJ), an important midlatitude circulation of the East Asian summer monsoon system, plays a crucial role in affecting summer rainfall over East Asia. The multimodel ensemble of current coupled models can generally capture the intensity and location of the climatological summer EAJ. However, individual models still exhibit large discrepancies. This study investigates the intermodel diversity in the longitudinal location of the simulated summer EAJ climatology in the present-day climate and its implications for rainfall over East Asia based on 20 CMIP5 models. The results show that the zonal location of the simulated EAJ core is located over either the midlatitude Asian continent or the western North Pacific (WNP) in different models. The zonal shift of the EAJ core depicts a major intermodel diversity of the simulated EAJ climatology. The westward retreat of the EAJ core is related to a warmer mid-upper tropospheric temperature in the midlatitudes, with a southwest-northeast tilt extending from Southwest Asia to Northeast Asia and the northern North Pacific, induced partially by the simulated stronger rainfall climatology over South Asia. The zonal shift of the EAJ core has some implications for the summer rainfall climatology, with stronger rainfall over the East Asian continent and weaker rainfall over the subtropical WNP in relation to the westward-located EAJ core. [ABSTRACT FROM AUTHOR]

This study examines associations between California Central Valley (CCV) heat waves and the Madden Julian Oscillation (MJO). These heat waves have major economic impact. Our prior work showed that CCV heat waves are frequently preceded by convection over the tropical Indian and eastern Pacific oceans, in patterns identifiable with MJO phases. The main analysis method is lagged composites (formed after each MJO phase pair) of CCV synoptic station temperature, outgoing longwave radiation (OLR), and velocity potential (VP). Over the CCV, positive temperature anomalies occur only after the Indian Ocean (phases 2–3) or eastern Pacific Ocean (phases 8–1) convection (implied by OLR and VP fields). The largest fractions of CCV hot days occur in the two weeks after onset of those two phase pairs. OLR and VP composites have significant subsidence and convergence above divergence over the CCV during heat waves, and these structures are each part of larger patterns having significant areas over the Indian and Pacific Oceans. Prior studies showed that CCV heat waves can be roughly grouped into two clusters: Cluster 2 is preceded by a heat wave over northwestern North America, while Cluster 1 is not. OLR and VP composite analyses are applied separately to these two clusters. However, for Cluster 2, the subsidence and VP over the CCV are not significant, and the large-scale VP pattern has low correlation with the MJO lagged composite field. Therefore, the association between the MJO convection and subsequent CCV heat wave is more evident in Cluster 1 than Cluster 2. [ABSTRACT FROM AUTHOR]