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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]

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]

The influence of Arctic sea ice concentration (SIC) on the interannual variations of the frequency of summertime Ural blocking (UB) during the period of 1980–2013 is investigated using observational and reanalysis datasets 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 vapor 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 favoring anomalous anti-cyclonic 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]