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The impact of global warming on lightning flash rates remains relatively unknown. In this study, the South China Sea (SCS) and the surrounding areas within Southeast Asia were selected to examine the long‐term trend and future projection of lightning activity based on the currently longest satellite‐based lightning data set available and climate models. Our study revealed a reduction in the observed lightning flash rates around the SCS, with a linear trend of −0.11 fl km−2 yr−2 during 1996–2013. In contrast, the precipitation around the SCS exhibited an increasing trend and was negatively correlated with the local lightning flash rate. The sea surface temperature gradient over equatorial Pacific Ocean, latent heat flux over the equatorial Indian Ocean, local convective available potential energy, precipitation and aerosol changes collectively accounted for 82% of the variance in the lightning fluctuations over the SCS and Southeast Asia. Multiple linear regression proxies of lightning flash rates were constructed and applied to the climate models. The models indicated that lightning activity around the SCS is projected to intensify by 10% and 12% by the end of the 21st century under SSP245 and SSP370, respectively. Plain Language Summary: The projection of future changes in lightning activity is uncertain owing to various factors, and different regions exhibit distinct trajectories. In this study, the focus was on the South China Sea (SCS) and Southeast Asia, and past variations in lightning activity was examined and used to predict future variations in lightning activity. Employing a multiple linear regression approach, we identified the large‐scale thermodynamic drivers and local environmental factors that influence lightning activity around the SCS. The research findings indicated a reduction in lightning activity over the past few decades over the SCS and Southeast Asia region. The simulation results indicate that in the future under ongoing global warming, regional lightning activity will intensify. Key Points: Lightning activity and precipitation show opposing regional trends around the South China Sea (SCS)Remote thermodynamic and local environmental factors both contribute to the variability in lightning around the SCSUnder future climate change scenarios, lightning activity around the SCS is projected to gradually increase [ABSTRACT FROM AUTHOR]

This study discovers that the interannual synchronization of winter temperature anomalies between northern Eurasia and North America is closely related to variation in the second leading mode of the 300 hPa meridional wind anomalies (i.e., V300‐EOF2) over the North Atlantic‐Arctic Ocean. The V300‐EOF2 mode modulates the large‐scale Rossby wave propagating across the mid‐high latitudes of the Eurasian and North American continents at the mid‐high level of troposphere, and it also interferes with the climatological quasi‐stationary wavenumber‐2 to affect the stratosphere circulation, which facilitates the southward intrusion of cold air into Eurasia and North America simultaneously. This study also evaluated whether the CMIP6 high‐top (HT) models could more accurately represent the V300‐EOF2 mode and the associated modulation mechanism on synchronized temperature variation between Eurasia and North America than the low‐top (LT) models. The results demonstrate that although the multi‐model ensemble means (MME) of both the HT and LT models can reproduce the V300‐EOF mode, they all underestimate its modulation mechanism on atmospheric circulation and coherent temperature changes over mid‐latitude Eurasia and North America, with minor differences in simulation skill capability. Most importantly, although containing a full stratosphere, the HT models do not significantly improve their simulation ability to depict the observed linear interference between the anomalous wave‐2 and the climatological stationary wave‐2 compared to the LT models. Therefore, comparing to the LT models, the CMIP6 HT models present no clear advantages in reproducing the V300‐EOF2 mode and its associated low‐frequency climate anomalies. These results also have implications for the evaluation of the predictability of concomitant Eurasian‐North American cooling in the Subseasonal‐to‐Seasonal Prediction Project. [ABSTRACT FROM AUTHOR]

In August 2020, the historical warmest upper‐ocean thermal condition occurred in the western North Pacific (WNP), along with three tropical cyclones (TCs, namely Bavi, Maysak, and Haishen) generated in the WNP and affected Northeast China and the Korea peninsular (KP), within an unprecedentedly short span of 12 days from late August to early September of 2020. Meanwhile, prior to the formation of Bavi, a series of extreme warm mesoscale oceanic eddies were also observed over the WNP, and clearly sea surface cooling was detected after the passage of the three TCs. However, it remains unknown whether the warmest WNP with extreme warm mesoscale oceanic eddies has a modulation effect on intensity change of the three TCs. The results of sensitive experiments with fixed atmospheric initial conditions revealed that the historically warmest upper‐ocean thermal condition in the WNP in August 2020 might have contributed to increasing the intensity of three TCs by 5.3–10.0 hPa compared to normal ocean thermal conditions. The simulation results indicated that the extreme warm mesoscale eddy conglomeration might assist in intensifying the three TCs when they passed over these warm mesoscale eddies. Whereas the cold wake over the East China Sea caused by preceding TCs, Bavi and Maysak, might partly aid in weakening subsequent TCs in the decaying period. Furthermore, the simulated bias in the intensity evolution of the three TCs and the possible cause of minor differences in the simulated tracks and large‐scale steering flow between the sensitive and control experiments were discussed. Plain Language Summary: In summer 2020, three typhoons hit Northeast Asia in an unprecedented 2‐week period. Using the COWAST model we investigated the possible effect of the historical warmest upper‐ocean thermal condition with preexisting extreme warm mesoscale eddies in the western North Pacific (WNP) on the three tropical cyclones, as well as the cold wake induced by preceding tropical cyclones (TCs) on subsequent TCs. The results indicated that the WNP's warmest upper ocean thermal condition might enhance the three TCs, whereas the preexisting warm mesoscale eddy conglomeration and cold wake caused by preceding TCs might partly affect the intensification and decay of Maysak and Haishen. Key Points: Three typhoons affected Northeast Asia within a short span of 2 weeks in 2020 over the warmest upper ocean of the western North Pacific in historyThe historical warmest upper ocean thermal condition in 2020 may increase the intensity of the three tropical cyclones (TCs)Pre‐storm warm‐core eddy conglomeration and cold wake induced by preceding TCs partly influenced intensity evolution of Maysak and Haishen [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Mode (AMM) is the dominant mode of interannual climate variability in the tropical Atlantic, maintained primarily by the positive wind–evaporation–sea surface temperature (SST) feedback in which the wind anomalies lead the SST anomalies by ~2 months. A previous study revealed that climate models from Coupled Model Intercomparison Project Phase 5 (CMIP5) show poor performance in simulating the AMM-related wind–SST relationship, but the possible causes remain unclear. This study assesses the representation of the AMM and associated wind–SST relationship in the climate models from CMIP6. Results show that most of the CMIP6 models can reasonably reproduce the observed spatial pattern of the AMM, with significant SST and wind anomalies in the northern tropical Atlantic and weak anomalies in the equatorial–southern oceans. However, the simulated wind–SST relationship associated with the AMM varies among the models. In particular, several models fail to capture the observed wind–SST relationship; that is, the simulated wind anomalies peak in boreal spring as in the observations, but no obvious peak occurs in the corresponding SST anomalies. Further analysis suggests the models that fail to capture the observed wind–SST relationship tend to simulate a stronger mean trade wind and a thicker mixed layer in the northern tropical Atlantic, leading to a weaker ocean–atmosphere coupling and, thus, a weaker SST response to the wind forcing. Moreover, there exists a significant out-of-phase relationship between the strength of ocean–atmosphere coupling and mean mixed layer depth among the models, supporting the impact of mean state biases on the AMM variability in the models. [ABSTRACT FROM AUTHOR]

The North Atlantic Oscillation (NAO) generally has an in-phase relationship with surface air temperature (SAT) anomalies over Northeast China in late winter. The present study shows that such an NAO–SAT relationship becomes stronger during easterly phases of the quasi-biennial oscillation (QBO), but is relatively weak during westerly phases. Observational evidence reveals that the modulation effect of the QBO on the NAO–SAT relationship over Northeast China is attributable to QBO-induced changes in the spatial structure of the NAO and associated stratosphere–troposphere coupling. During easterly QBO phases, the NAO has a strong connection with the Northern Hemisphere stratospheric polar vortex, facilitating a hemisphere-wide structure of the NAO and thus a downstream extension of NAO signal from the Euro–Atlantic sector toward Northeast Asia. During westerly QBO phases, however, the NAO has a limited connection with the stratospheric polar vortex. In this case, the NAO features a classically regional mode, with the signal in the SAT field mainly confined to the Euro-Atlantic sector. By examining historical simulations from six climate models participating in CMIP6 and including stratospheric processes, it is found that none of these models captures a significant difference in the spatial structure of the NAO and its connection with the stratospheric polar vortex between the easterly and westerly QBO phases in late winter. A key reason may be related to the poor performance of the models in simulating the Holton–Tan effect, which is critical for linking the QBO and NAO. Significance Statement: The QBO is the dominant mode of equatorial stratospheric variability and important for seasonal forecasting. This study aims to better understand the surface influence of the QBO by examining the relationship between the NAO and temperature anomalies over China according to the phases of the QBO during boreal winter. Our results highlight the importance of the QBO in modulating the spatial structure and thus climate impact of the NAO via stratosphere–troposphere coupling in observations, while state-of-the-art climate models perform poorly in simulating the QBO-related stratosphere–troposphere coupling and thus the QBO–NAO connection. These findings have important implications for seasonal prediction and model development in the future. [ABSTRACT FROM AUTHOR]

The leading mode of the atmospheric background flow over the North Atlantic–Arctic Ocean (i.e., 300-hPa meridional wind; V300-EOF1) exhibited notable decadal variations during 1979–2019, with intensified V300-EOF1 since the early-2000s. The modulation of the interdecadal change in the East Asian winter monsoon (EAWM) by the V300-EOF1 mode and its link with the concurrently warmer North Atlantic and low Arctic sea ice was explored through observational analysis and numerical simulations. The results indicated that the intensified V300-EOF1 mode enhanced the modulation of Rossby wave trains propagating from the North Atlantic–Arctic region to East Asia, which interfered with the climatological quasi-stationary waves to induce a stronger EAWM. The widespread North Atlantic–Arctic warming tended to modulate and interact with this V300-EOF1 background flow, which further enhanced the quasi-stationary Rossby waves along the midlatitude and sub-polar waveguides, resulting in a strengthened EAWM and cold East Asia. This finding was further verified by numerical experiments conducted with an atmospheric model. The model responses generally captured the observed physical processes, albeit weaker in magnitude, suggestive of a modulating role played by external forcings in the enhancement of the V300-EOF1 mode and the EAWM. Noting also the large internal variability in the atmospheric responses, with approximately half of the model years capturing a stronger-than-average East Asian cooling along with deep and strong Arctic warming. This further indicates the importance of atmospheric internal variability in the development of the V300-EOF1 mode. These results have implications for future changes in the EAWM and concomitant East Asian cooling on interdecadal timescales. [ABSTRACT FROM AUTHOR]

In recent decades, unprecedented extreme summer heat waves have occurred in Europe, and they have exhibited an increasing trend since 1970s. Although previous studies have suggested that these recent hot European summers could have been instigated by the underlying surface thermal conditions, the possible influence of shrinking Arctic sea ice and Eurasian snow cover on heat waves are not well understood. Herein, we present evidence obtained via observational analyses and numerical experiments indicating that the interdecadal increase in European heat waves is closely linked to the reductions in Arctic sea ice concentration (ASIC) and Eurasian snow cover fraction (EASC) across mid–high latitudes via the excitation of the anomalous Eurasian wave train. The combined effects of declined ASIC and EASC, accompanied by the drier soil and the stronger heat flux, tend to weaken the poleward temperature gradient at mid–high latitudes and affect the midlatitude jet stream and transient eddy activities. These dynamic and thermodynamic circulations increase the likelihood of more persistent European blocking events that favor frequent and strengthened heat waves. Further projection analysis of simulations from 13 CMIP5 climate models suggests that Europe may experience more hot summers as the ASIC and EASC continue to decline over the next century. [ABSTRACT FROM AUTHOR]

Previous studies have revealed that warm (cold) sea surface temperature (SST) anomalies in the northern tropical Atlantic (NTA) can enhance (weaken) the anomalous low-level anticyclone over the western North Pacific (WNP) during boreal summer. This study assesses the ability of current atmospheric general circulation models (AGCMs) to simulate such an NTA–WNP connection by using Atmospheric Model Intercomparison Project experiments from 23 Coupled Model Intercomparison Project Phase 5 (CMIP5) and 35 CMIP6 climate models. It is shown that both the CMIP5 and CMIP6 multimodel ensemble (MME) averages and the majority of the individual AGCMs can reasonably reproduce the observed pattern of the NTA-related anomalous anticyclone over the WNP during boreal summer. Overall, the performance of the CMIP6 AGCMs in representing the NTA–WNP connection is similar to that of the CMIP5 AGCMs, except that the former tends to have a smaller spread than the latter among models. Additionally, both the CMIP5 and CMIP6 MME averages as well as the individual models can reasonably represent the mechanism responsible for the boreal summer NTA–WNP connection, which involves a zonally westward-extending overturning circulation over the Pacific–Atlantic Oceans. Furthermore, the intensity of the NTA-related WNP anomalous anticyclone is positively correlated with that of the WNP local climatological convection activity for both the CMIP5 and CMIP6 AGCMs, implying that better representation of the WNP climatological convection activity may be crucial for improving the skill of AGCMs to simulate the boreal summer NTA–WNP connection. However, model bias in the simulation of climatological convection activity over the WNP remains large for the current CMIP6 AGCMs, although the bias is reduced over most of the tropical and subtropical Pacific–Atlantic regions compared to that for the CMIP5 AGCMs during boreal summer. [ABSTRACT FROM AUTHOR]

In recent decades, unprecedented extreme summer heat waves have occurred in Europe, and they have exhibited an increasing trend since 1970s. Although previous studies have suggested that these recent hot European summers could have been instigated by the underlying surface thermal conditions, the possible influence of shrinking Arctic sea ice and Eurasian snow cover on heat waves are not well understood. Herein, we present evidence obtained via observational analyses and numerical experiments indicating that the interdecadal increase in European heat waves is closely linked to the reductions in Arctic sea ice concentration (ASIC) and Eurasian snow cover fraction (EASC) across mid–high latitudes via the excitation of the anomalous Eurasian wave train. The combined effects of declined ASIC and EASC, accompanied by the drier soil and the stronger heat flux, tend to weaken the poleward temperature gradient at mid–high latitudes and affect the midlatitude jet stream and transient eddy activities. These dynamic and thermodynamic circulations increase the likelihood of more persistent European blocking events that favor frequent and strengthened heat waves. Further projection analysis of simulations from 13 CMIP5 climate models suggests that Europe may experience more hot summers as the ASIC and EASC continue to decline over the next century. [ABSTRACT FROM AUTHOR]

An east–west dipole mode of winter Eurasian snow water equivalent (SWE) is found during the period of 1979–2015. It accounts for about 23.4% of the total variance, and displayed a significant decadal change in the early-2000s. The basin warming footprint of the North Atlantic likely exerted an influence on this decadal change, and the observation-based evidence is reproduced by numerical experiments using the Community Atmosphere Model (CAM3.1). A basin-wide warming of North Atlantic sea surface temperature induced atmospheric anomalies by exciting a stationary Rossby wave train, which prorogates from the subtropical North Atlantic to the mid-to-high latitudes of the Eurasian continent. Along with the Rossby wave train, an enhanced upper-level ridge occurs over the Ural Mountain, and a deepened upper-level trough appears over the eastern Siberian Plateau, which promotes heavy snowfall over the eastern Siberian Plateau and light snowfall to its west. Thus, it is plausible that the North Atlantic warming plays a role in exciting the Rossby wave train to modulate the decadal change in the east–west dipole SWE mode of the extratropical Eurasian continent. The possible moisture transport paths associated with the decadal change in the east–west dipole SWE mode are also discussed. [ABSTRACT FROM AUTHOR]

An anomalous "north‐south" dipole mode of the snow water equivalent (SWE) persisting from winter to spring is detected over the Eurasian mid‐to‐high latitudes in this study. Using observational data sets and numerical experiments of the Community Atmospheric Model (5.0), we show that this mode contributes to prolonged winter‐springtime coldness in midlatitude Eurasia and is closely linked to the declining November Arctic sea ice concentration. The decline in the sea ice concentration over the Barents‐Laptev Seas can induce a teleconnection pattern over the mid‐to‐high latitudes in the following winter, accompanied by an anomalous ridge over the Ural Mountains and an anomalous trough over Europe and East Asia. Such changes in the large‐scale circulation lead to more cold surges and heavy snowfall in the midlatitudes and light snowfall in the high latitudes, forming an anomalous north‐south dipole mode of the SWE, which further reduces the temperature through thermodynamic feedback. Due to seasonal memory, this SWE pattern can persist into the following spring and can lead to springtime midlatitude coldness via thermodynamic and dynamic processes. For the thermodynamic process, the anomalous SWE condition can lead to anomalous wet soil, reduced incoming surface solar radiation, and cooling air in the midlatitudes. This phenomenon induces an enhanced Siberian High and a deepened East Asian trough via the snow‐Siberian high‐feedback mechanism, which favors a cold spring in northern East Asia. Further analysis suggests that an empirical seasonal prediction model based on the SWE can reasonably predict East Asian spring temperature anomalies. Key Points: An anomalous "north‐south" dipole mode of the snow water equivalent over the Eurasian mid‐to‐high latitudes can persist from winter to springThe dipole mode of snow water equivalent tends to link prolonged winter‐springtime Eurasian midlatitude cooling to autumn time sea ice retreat [ABSTRACT FROM AUTHOR]

This study uses both observations and numerical modeling experiments to investigate the lead-lag relationship and the associated physical mechanism of the western North Pacific anomalous anticyclone (WNPAC) with sea surface temperature (SST) anomalies over the northern tropical Atlantic (NTA). The results show that the WNPAC from late spring to the middle of autumn has a significant in-phase relationship with the NTA SST anomalies up to two seasons ahead. This relationship reaches a peak when the NTA SST leads by approximately 1-2 months and is nearly independent of the El Niño-Southern Oscillation variability. Diagnosis based on observations and numerical experiments using the Community Atmospheric Model version 5.3 reveals that the NTA warming favors intensified local convection activity during the spring-autumn seasons, causing enhanced low-level convergence and upper-level divergence (i.e., ascending motion) over the NTA and opposite flow anomalies over the central tropical Pacific. The enhanced subsidence over the central tropical Pacific, in turn, triggers an anomalous low-level anticyclone over the western North Pacific. Moreover, the intensity of the anomalous local convection activity that is associated with the NTA SST is closely related to the seasonal migration of the Atlantic intertropical convergence zone (ITCZ). As the Atlantic ITCZ migrates northward, the NTA SST-induced local convection activity extends northward from a narrow band near the equator during early spring to nearly the entire NTA region during the middle of summer, leading to the strongest remote effect of the NTA SST anomalies on the WNPAC during late summer and early autumn. [ABSTRACT FROM AUTHOR]

Maps of observed ground surface temperature (GST) in China generally contain inhomogeneities due to relocation of the observation site, changes in observation method, transition to automatic instruments, and so on. By using the observations of collocated manual and automatic weather stations in China, bias in daily GST caused by the transition to automatic observation systems is corrected for the first time in the present work. Then, the inhomogeneities caused by nonclimatic factors (e.g., relocation of the station and change of observation time) in the historical records of monthly GST are further reduced by using the penalized maximal F-test method. Analysis based on this new homogenized dataset reveals that the trend of annual-mean GST in China is approximately 0.273°C decade−1 during 1961–2016. The warming trend is stronger in winter (0.321°C decade−1) and spring (0.312°C decade−1) and weakest in summer (0.173°C decade−1). Spatially, all the stations in China, except for a few stations in southern China, present warming trends in the annual mean and in spring, fall, and winter seasons. In summer, cooling trends are observed in central and southern China. Moreover, we assess the monthly GST from five reanalysis products of the Global Land Data Assimilation System (GLDAS) during 1980–2016. The warming trends of Noah and the Catchment Land Surface Model (CLSM) from GLDAS-V2.0 are the closest to those of the homogenized observation, while the linear trends in the other three products (Noah, CLM, and MOS) from GLDAS-V1 are obviously different from those of the homogenized observation. Also, it is found that the spatial distribution of the warming trend is substantially overestimated in central China but underestimated in the other regions of China in these five GLDAS reanalysis products. [ABSTRACT FROM AUTHOR]

The influence of Arctic sea ice concentration (SIC) on the inter‐annual variations of the frequency of summertime Ural blocking (UB) during the period of 1980–2013 is investigated using observational and reanalysis data sets and version 5.0 of the Community Atmospheric Model. The results reveal that the variations in the UB frequency display a statistically significant association with a persistent spring–summer SIC pattern in the Barents Sea. Related to high UB frequencies, heavy SICs exert a dynamic influence by increasing the meridional temperature gradient (MTG) in the lower troposphere and cause stronger (weaker) zonal winds in high‐latitude (mid‐latitude) areas through the thermal wind balance. This zonal wind pattern establishes the background conditions for the blocking activity and thus helps to initiate summertime UB events. Moreover, persistent heavy SICs tend to enhance the low‐level atmospheric baroclinicity to the south and decreases in mid‐latitude areas, inducing weakened synoptic‐scale transient eddy activity (STEA) that stretches from eastern Europe to the Ural Mountains. This reduced STEA is accompanied by a locally intensified eddy‐vorticity forcing that may exert a downstream influence on the onset of UB events. In terms of thermodynamic processes, heavy SICs‐induced water vapour content anomalies are expected to cause deficits in precipitation over the East European Plateau in late spring and subsequently desiccate the underlying soil. Both of these effects are expected to increase surface heat fluxes and thickness of the lower‐middle troposphere, thus favouring anomalous anticyclonic circulation over the Ural Mountains. On the other hand, the opposite dynamic and thermodynamic effects are expected to result from light SICs with respect to low UB frequencies. Therefore, these two effects identified in this study each contribute to an increased probability of more frequent (rare) UB events in summer as the spring–summer sea ice within the regions surrounding the Barents Sea expands (disappears). [ABSTRACT FROM AUTHOR]

Two dominant modes of the winter temperature over East Asia, a northern mode and a southern mode, and their links with Arctic climate conditions are analyzed. The relationships of the two modes with Arctic sea ice are different. The northern mode is closely linked to variations in sea ice of the Arctic Barents-Laptev Sea in previous autumn and most of the Arctic in concurrent winter. The southern mode seems independent from the Arctic sea ice variations, but is associated with sea surface temperature (SST) anomalies in the equatorial central-eastern Pacific. Results suggest an effect of Arctic sea ice variation on the northern mode and an influence of tropical SST anomalies on the southern mode. Reduced sea ice over the Arctic increases 1000-500-hPa thickness over the high-latitudes of Eurasian continent, which reduces the meridional thickness gradient between the middle and high latitudes and thus weakens the extratropical upper-level zonal wind. The weakened zonal wind provides a favorable dynamic condition for the development of a high-latitude ridge around the Ural Mountain. Reduced Arctic sea ice also tends to enhance the Siberian high through both thermodynamic and dynamic processes. The above atmospheric circulation patterns provide a favorable condition for the intrusion of cold air to northern East Asia. [ABSTRACT FROM AUTHOR]

ABSTRACT Analysis of observational and reanalysis data shows that the autumn precipitation in South China is closely related to the east-west sea surface temperature ( SST) contrast in the tropical Pacific. An east-west contrast ( EWC) index, defined as the difference of the normalized area-averaged SST between the tropical eastern Pacific (150°W-90°W, 5°S-5°N) and the tropical western Pacific ( WP; 110°E-180°E, 5°S-5°N), is proposed to delineate this feature. When the EWC index is positive, the tropical eastern (western) Pacific is warmer (colder) than normal. It would cause significant southerly wind anomalies from the Indochina Peninsula to eastern China, leading to enhanced water vapour convergence and anomalous ascending motion over South China. Therefore, enhanced autumn precipitation is observed in South China. Further analysis suggests that the SST of the tropical WP dominates the lower-tropospheric meridional wind and the resultant water vapour transport and convergence over subtropical East Asia, and that the SST of both the tropical eastern and the tropical WP contributes to the anomalous ascending motions over South China. These results highlight the combined effects of tropical eastern and WP on the autumn precipitation in South China and specifically emphasize the role of tropical WP SST. Given the performance of the EWC index in describing the autumn precipitation as well as the leading time and the persistence of the EWC index, it could be used as a good indicator in the monitoring and prediction of the autumn precipitation in South China, which is demonstrated by a simple statistical prediction model. [ABSTRACT FROM AUTHOR]

A strong (weak) East Asian summer monsoon (EASM) is usually concurrent with the tripole pattern of North Atlantic SST anomalies on the interannual timescale during summer, which has positive (negative) SST anomalies in the northwestern North Atlantic and negative (positive) SST anomalies in the subpolar and tropical ocean. The mechanisms responsible for this linkage are diagnosed in the present study. It is shown that a barotropic wave-train pattern occurring over the Atlantic-Eurasia region likely acts as a link between the EASM and the SST tripole during summer. This wave-train pattern is concurrent with geopotential height anomalies over the Ural Mountains, which has a substantial effect on the EASM. Diagnosis based on observations and linear dynamical model results reveals that the mechanism for maintaining the wave-train pattern involves both the anomalous diabatic heating and synoptic eddy-vorticity forcing. Since the North Atlantic SST tripole is closely coupled with the North Atlantic Oscillation (NAO), the relationships between these two factors and the EASM are also examined. It is found that the connection of the EASM with the summer SST tripole is sensitive to the meridional location of the tripole, which is characterized by large seasonal variations due to the north-south movement of the activity centers of the NAO. The SST tripole that has a strong relationship with the EASM appears to be closely coupled with the NAO in the previous spring rather than in the simultaneous summer. [ABSTRACT FROM AUTHOR]

Using NCEP/NCAR reanalysis data for the period of 1957–2001, the climatological seasonal transition features of large-scale vertically integrated moisture transport (VIMT) in the Asian-Australian monsoon region are investigated in this paper. The basic features of the seasonal transition of VIMT from winter to summer are the establishment of the summertime “great moisture river” pattern (named the GMR pattern) and its eastward expansion, associated with a series of climatological events which occurred in some “key periods”, which include the occurrence of the notable southerly VIMT over the Indochina Peninsula in mid March, the activity of the low VIMT vortex around Sri Lanka in late April, and the onset of the South China Sea summer monsoon in mid May, among others. However, during the transition from summer to winter, the characteristics are mainly exhibited by the establishment of the easterly VIMT belt located in the tropical area, accompanied by some events occurring in “key periods”. Further analyses disclose a great difference between the Indian and East Asian monsoon regions when viewed from the meridional migration of the westerly VIMT during the seasonal change process, according to which the Asian monsoon region can be easily divided into two parts along the western side of the Indochina Peninsula and it may also denote different formation mechanisms between the two regions. [ABSTRACT FROM AUTHOR]