This study investigates the meteorological conditions, dynamics, and microphysical characteristics of convective precipitation in Longmen, South China, during the Pre-summer Rainy Season (PRS) from 2016 to 2020, focusing on the influence of the South China Sea summer monsoon (SCSSM) onset. Utilizing the ERA5 reanalysis dataset and observations from the C-band Vertical Pointing Radar (VPR-C) and Two-Dimensional Video Disdrometer (2DVD), we analyzed 4560 Convective Precipitation Features (CPFs) and classified them into shallow convection (SC), middle convection (MC), and deep convection (DC) based on the maximum height of 35 dBZ echo-top. Key findings reveal that the onset of the SCSSM significantly enhances convective rainfall. Specifically, it increases the proportion of convective rainfall by 11 % and intensifies rainfall duration and intensity by approximately 2.2 times. Enhanced moisture convergence and stronger convective instability drive these changes. The microphysical processes are distinct across different CPF types. SCs display warm-rain processes, MCs indicate mixedphase processes, and DCs are associated with ice-phase processes. Each type contributes uniquely to precipitation characteristics, vertical reflectivity profiles, and raindrop size distributions. These insights emphasize the SCSSM's critical role in regional precipitation patterns and provide valuable insights into the underlying processes affecting convective systems in South China, ultimately contributing to improving the capabilities of prediction in atmospheric research.

Previous studies have established a link between the intensity of the monsoon trough (MT) and tropical cyclone (TC) activity. This study investigates the impact of the length of the MT on the interannual variability of multiple TC events (MTCEs) over the western North Pacific (WNP) during the typhoon season (June–October) during 1980–2020. Results show a significant positive correlation between the length of the MT and the occurrence frequency of MTCEs. A longer MT is favorable for the occurrence of more MTCEs. During the long MT years, the MT extends eastwards, resulting in strong low-level relative vorticity, mid-level upward motion, and sufficient moisture over the central-eastern WNP, which are conductive to the occurrence of MTCEs. A diagnosis of genesis potential index confirms the crucial role of positive synoptic-scale disturbances. Barotropic energy conversion analysis further shows that the MT-induced mean flow change can modulate barotropic energy conversion from the mean flow to the synoptic eddies, which is a critical energy source for the development of tropical depressions. In this process, changes in meridional shear of the large-scale zonal winds play a dominant role, followed by their meridional convergence. These changes lead to the increasing synoptic-scale disturbances over the central-eastern WNP during the long MT years, resulting in more MTCEs.

During the winter of 2015-2016, an extraordinary warming event occurred in the troposphere in the Arctic region. The above-normal temperatures extended from the surface to 300 hPa in the Arctic. The number of extremely warm days derived from the surface air temperature revealed that this event was the most significant warming event since 1980. It was found that the tropospheric warming anomaly was not fully dependent on the warming anomalies at lower levels, and it formed earlier than the lower anomalies. In December 2015, there were three obvious warm centers in the troposphere in the mid-high latitude regions in eastern North America, northern Europe, and western Asia. Albeit the strongest warming anomaly occurred in North America, the prominent warm advection to the Arctic region was transported from northern Europe during the initial stage of the Arctic mid-upper tropospheric warming. Further investigation revealed that the teleconnection wave train stretching across North America to the North Atlantic and northern Europe contributed significantly to the existence of a poleward transport channel from northern Europe to the Arctic. The wave train spread from west to east and produced a strong southerly wind disturbance over the warm region in northern Europe. As a result, extraordinary poleward heat and water vapour transport occurred across the polar boundary causing substantial warming in the Arctic mid-upper troposphere. During the extreme Arctic tropospheric warming event, the dynamic processes related to the heat transport over northern Europe directly contributed to the advanced rapid warming in the Arctic mid-upper troposphere; whereas, the dynamic and thermal processes associated with the transport across the Arctic boundary jointly maintained the late stage of Arctic warming.

Limited by the lack of atmospheric observation data over the ocean and the absence of a comprehensive set of track data for monsoon low pressure systems (MLPSs), an in-depth understanding of the activity of East Asian MLPSs has not been acquired. In recent years, advancements in satellite remote sensing and data assimilation techniques have enabled the creation of numerous high-resolution global reanalysis datasets. Additionally, with the improvement of tracking algorithms, two sets of global MLPS track data (HB2015 and VB2020) have been published. This study seeks to understand the fidelity of the two datasets with respect to the East Asian monsoon. The genesis location, movement path, and three-dimensional structure of the East Asian MLPSs obtained using HB2015 and VB2020 are compared, and the atmospheric circulation conditions of typical MLPSs are analyzed. The results show that both datasets are able to generate MLPSs with identical structure for the East Asian Monsoon, and they provide similar results in terms of the location and monthly frequency. Compared to the HB2015, the VB2020 adopts a more stringent set of thresholds for the determination of the MLPS genesis and extinction and a more rigorous tracking algorithm. Therefore, it yields a lower count of MLPSs with significantly shorter lifetimes. However, the MLPSs identified by the VB2020 all have cyclonic circulations in the proximity of their central areas as they continue their movement. In this sense, the results generated by the VB2020 are more consistent with the observed MLPSs and hence are more reliable. However, the tracking can end prematurely with this dataset.

Previous studies suggest that interannual variability of surface air temperature (SAT) over India peaks in June, a month later than its climatology, which is due to the significant year-to-year variability of abrupt summer monsoon onset. However, April is the hottest month over Indochina Peninsula (ICP), and SAT variability is much larger in April than summer monsoon season which starts in mid-May. The first empirical orthogonal function (EOF) mode of daily-mean SAT evolution over ICP from 1 April to 20 May on the interannual timescale captures 49.6

Precipitation over the Tibetan Plateau (TP) significantly affects both the local ecology and the downstream water resources. In this study, it is found that there exists an increasing precipitation trend over the northern TP but not in the southern TP, which is interpreted from the perspective of the activities of Tibetan Plateau vortices (TPVs). In the context of recent warming over the TP, the increasing rate of genesis frequency of TPVs over the northern TP is remarkably larger than that over the southern TP, which is responsible for different precipitation trends in the northern and southern TP, respectively. That is, the most significant warming over the TP appears in the upper troposphere, which intensifies the 200 hPa westerly jet north of the TP, hereby produces more favorable conditions for the genesis of TPVs over the northern TP than over southern TP, and contributes to regional difference in precipitation trends.

An empirical orthogonal function analysis was applied to 34 years of anomalous daily 10-20 day filtered outgoing longwave radiation data to determine the north-westward propagating mode of quasi-biweekly oscillations (QBWOs) over tropical East Asia during boreal summer. The centres of active QBWO convection coincided with positive potential vorticity (PV) anomalies at 500 center dot hPa and cyclonic gyres at 850 center dot hPa, whereas the centres of suppressed convection were superimposed upon negative PV anomalies and anticyclonic gyres. These circulations formed an inclined northwest-southeast wave train. The vertical dynamic and thermodynamic structures of the QBWO mode evince two centres of PV at 500 and 750 center dot hPa, respectively, and an almost upright structure in the troposphere up to 300 center dot hPa. The QBWO's convective cell has a warm-over-cold structure in the troposphere. These features have several similarities to East Asian tropical monsoon depressions (EAMDs), implying a close relationship between the north-westward propagating mode of QBWOs and EAMDs. Further investigation indicates that EAMDs are prone to appear in the region of deep convection and move north-westward with the QBWO circulation. The westernmost position of the western Pacific subtropical high (WPSH), which is linked to the QBWO phase, strongly influences the route of EAMDs. An EAMD generated in the easterly winds to the south of WPSH moves westward under the influence of the horizontal adiabatic advection of PV. More eastward-moving and turning EAMDs appear when WPSH weakens and retreats eastward due to enhancement of QBWO convection over the South China Sea (SCS) and western North Pacific. Most EAMDs reach peak intensity when moving over the SCS, and show a strong relation to the vigorous convection. However, there is no significant relation between the QBWO convection and the points of disappearance of the EAMDs.

Tibetan Plateau vortices (TPVs) are major rain producers over the Tibetan Plateau, some of which can move eastward off the plateau and trigger heavy rainfall over southwestern and eastern China. The infrared black body temperature (TBB) data, derived from geostationary meteorological satellite FY‐2E in the period of May–August of 2010–2014, is utilized to investigate the characteristics of convections associated with the moving‐off TPVs. During May–August, negative TBB anomalies appear over the tracks of the TPVs, but are located further north of the tracks in May and June than in July and August. Relative to the centre of TPVs, low TBB appears in the southeast and extends from southwest to northeast, whose values are smaller and ranges are larger before the TPVs move off than after moving off. Distributions of TBB at different evolution times show a close relationship with the characteristics of TPVs. The intensity, scales and shape of the convections linked to the TPVs are explored in the coordinates whose origin is the composite centre of the convections. Generally, convections associated with the TPVs are elliptic and stretch from southwest to northeast, with average spatial scales of approximately 3° in the zonal and 2° in the meridional direction. Convections in July and August are more intensive and lie southwest‐northeast, and those in May and June are weaker with a quasi‐zonal orientation. The horizontal scales of convections are much larger in May and July than those in June and August. Convections are usually larger and more intense before the TPVs move off than after moving off, corresponding to the changes of TPVs.

Different variables present discrepancy in characteristics of the quasi-biweekly oscillation (QBWO), which is a dominant sub-seasonal signal in the East Asian monsoon regime. However, there is limited investigation about similarities and differences in features of QBWO presented by varying variables. In order to fill this gap, adopting the empirical orthogonal function (EOF) and composite analysis, such variables as outgoing longwave radiation (OLR), 500 hPa potential vorticity (PV), 850 hPa relative vorticity, 850 hPa zonal wind, 850 hPa meridional wind and 750 hPa specific humidity are compared, regarding of the spatial-temporal distribution, intensity and propagation of QBWO over the East Asian monsoon region. It is found that all these variables show significant QBWO across the region with similar spatial and temporal variation. And the strongest QBWO is observed over the South China Sea (SCS) with all variables. QBWO in OLR propagates north-westward over the East Asian monsoon regime. Centres of active (suppressed) QBWO convection correspond to positive (negative) PV anomalies at 500 hPa level and cyclonic (anticyclonic) vortex at 850 hPa level. These circulations form a northwest-southeast tilted wave train. Two leading modes of QBWO in 500 hPa PV, 850 hPa relative vorticity and 850 hPa zonal wind have greater meridional magnitude than those of OLR. QBWO in three variables also propagate north-westward, but spread faster to north. Oppositely, two leading modes of QBWO in 850 hPa meridional wind are characterized by the zonal dipole pattern and the westward propagation is evident. Actually, its speed of northward propagation is the slowest of all. Different from all others, QBWO in 750 hPa specific humidity propagates south-eastward, and variances explained by QBWO of 750 hPa specific humidity is the smallest. As for the intensity of QBWO, except for 750 hPa specific humidity, other variables have consistent inter-annual variation. Totally, affected by such complex physical processes as transformation of precipitation state, release of heat and so on, characteristics of QBWO is hardly captured by 750 hPa specific humidity. However, OLR, 500 hPa PV, 850 hPa relative vorticity, 850 hPa zonal wind and 850 hPa meridional wind can well characterize QBWO over the East Asian monsoon region. Of all variables compared in this analysis, 500 hPa PV and 850 hPa relative vorticity are highly consistent in describing QBWO over the East Asian monsoon region. Specific causes that lead to different characteristics of QBWO over the East Asian monsoon regime presented by different variables need further discussion, which can provide a new reference for selecting monitoring indices for QBWO over the East Asian monsoon region.

Tibetan Plateau vortices (TPVs), generated over the Tibetan Plateau, are important rainfall triggers in southwestern and eastern China when moving off the plateau. The heavy rainfall event that occurred over southwestern China in early July 2018 caused severe floods and landslides, leading to substantial damage to normal production and life. In this work, the roles of TPVs in the rainfall are investigated based on NCEP FNL (final) operational global analysis and forecast data from the Global Data Assimilation System (GDAS). The results show that TPVs over the eastern Tibetan Plateau favor ascending motion to the east of the plateau, which is conducive to precipitation and therefore the genesis of southwest vortices (SWVs). Generally, the ascending motion in southwestern China is stronger when TPVs exist, compared with the other scenarios in which the TPVs are absent. Ascending motion is always observed on the east side of TPVs, which is greatly attributed to the positive vertical vorticity and warm centers in the TPVs. In this fashion, the influence of TPVs is not merely limited around their centers, but further eastward. Therefore, the effects of TPVs may have been underestimated in previous investigations; the TPVs located over the eastern Tibetan Plateau that do not move off the plateau should also be seriously considered in future precipitation predictions.

Investigation into the intraseasonal variation of the South Asian high (SAH) and its relationship with the Indian and East Asian summer monsoon rainfalls indicates that the 10–20 day period dominates the subseasonal zonal oscillation of the SAH between the Iranian mode and the Tibetan mode. The eastward shift of SAH lags a rainfall decrease over northern India by about 1 day, but leads a rainfall increase over the Yangtze River valley (YRV) by about 3 days. Interactive processes are also revealed on quasi‐biweekly time scales. The latent heating over northern India contributes to the southwestward shift of a pair of cyclone‐anticyclone anomalies from the midlatitudes to the SAH region, resulting in the zonal shift of the SAH. An associated upper‐level divergence over eastern China leads to the rainfall increase over the YRV 3 days later. The lead‐lag relationships revealed may benefit the subseasonal rainfall prediction over the two monsoon regions.

The relationships among heavy precipitation in the eastern region of China, the atmospheric heat source over the Tibetan Plateau and its surrounding areas, and atmospheric circulation in East Asia were investigated using the multi-variate empirical orthogonal function (MV-EOF) method and synthetic analysis on daily meteorological data from May through August 2010, which were compared with data from 2013. The MV-EOF decomposition results revealed that the atmospheric heating over the eastern region of the Tibetan Plateau and the Bay of Bengal exhibited opposite trends when heavy precipitation events occurred in South China, West China, and the middle and lower reaches of the Yangtze River. These results indicated that the land–sea thermal contrast between the eastern region of the Tibetan Plateau and the Bay of Bengal was likely one of the key factors leading to the occurrence of heavy precipitation events in the eastern region of China. The results of the synthetic analysis revealed a possible physical mechanism: When the atmospheric heating was weak over the Tibetan Plateau and strong over the Bay of Bengal, there was a strong ascending motion over the Bay of Bengal and its surrounding areas, which was conducive to maintaining the South Asian high and the western Pacific subtropical high (WPSH) in southerly positions. This also resulted in weak water vapor transport in the southwest, thus forming continuous heavy precipitation in South China. After the increase in atmospheric heating over the Tibetan Plateau, the convergence and ascending motion of the lower atmosphere were strengthened, and the South Asian high moved northward to the plateau, with a strengthened eastward extension. The WPSH then lifted northward, and the airflow around it conveyed more water vapor to West China and the middle and lower reaches of the Yangtze River, resulting in heavy precipitation in these regions.

Based on daily precipitation data from the Chinese Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction-Department of Energy, the character of low-frequency precipitation variability during the first rainy season (April–June) over South China and its corresponding atmospheric circulations in the mid-high latitudes are investigated. The results show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation (QBWO) features, with a period of 8–24 days. The influence of wave trains in the mid-high latitudes to low-frequency persistent heavy rain event (PHR-LF event, the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days over South China) is further discussed. During the first rainy season over South China, there are two low-frequency wave trains in the mid-high latitudes associated with the PHR-LF event—the wave train crossing the Eurasian continent and the wave train along the subtropical westerly jet. Analysis of wave activity flux indicates that the wave energy disperses toward eastern China along these two low-frequency wave trains from north to south and from west to east, and then propagates downward over South China. Accordingly, the disturbance of the relative vorticity of the cyclonic anomalies over eastern China is strengthened, which enhances the meridional gradient of relative vorticity. Owing to the transport of low-frequency relative vorticity and geostrophic vorticity by meridional wind, the ascending motion over South China intensifies and lasts for a long time, triggering a PHR-LF event. In addition, the tropical system is also a key factor to PHR-LF event. The QBWO of the convection over the South China Sea provide moisture for PHR-LF events, maintaining persistent rainfall and vertical ascending motion over South China.

Tibetan Plateau vortices (TPVs) are major rainfall producers generated over the Tibetan Plateau, and the ones moving off the Tibetan Plateau can trigger heavy rainfall over eastern China. The structure characteristics of the moving-off TPVs are investigated based on the final operational global analysis data from the National Centers for Environmental Prediction. Generally, the TPVs show different dynamic and thermodynamic structures before and after they move off the Tibetan Plateau. Specifically, the structures of TPVs at the genesis and peak times before they move off the Tibetan Plateau are distinguishing, and different structure characteristics are also found at the times when the TPVs just move off the Tibetan Plateau and when the intensity reaches the peak after moving off. In addition, the moving-off TPVs are divided into two groups according to their lifespans after moving off the plateau, and the structure characteristics of these two groups of TPVs are further compared. Furthermore, on the basis of the evolution features of the moving-off TPVs, the relationship between the structures of TPVs and their intensity is discussed. It is inferred that the evolution of the TPVs is determined by the structures of TPVs themselves to some extent.

利用1979～2013年实时多要素MJO(Madden-Julian Oscillation)监测(RMM)指数,美国NOAA逐日长波辐射资料和NCEP/NCAR再分析资料等,分析了全球变化背景下北半球冬季MJO传播的年代际变化特征.从全球平均气温快速增暖期(1985～1997)到变暖趋缓期(2000～2012),MJO 2～4位相频次减少,5～7位相频次增多,即MJO对流活跃区在热带印度洋地区停留时间缩短、传播速度加快,而在热带西太平洋停留时间加长、传播明显减缓.进一步分析发现,以上MJO的年代际变化特征与全球变化年代际波动有关.当太平洋年代际涛动(PDO)处于负位相时,全球变暖趋缓,热带东印度洋—西太平洋海温异常偏暖,使其上空对流加强,垂直上升运动加强,对流层低层辐合,大气中的水汽含量增多,该区域的湿静力能(MSE)为正异常.当MJO对流活跃区位于热带印度洋地区时,MJO异常环流对季节平均MSE的输送在强对流中心东侧为正、西侧为负,有利于东侧MSE扰动增加,使得MJO对流扰动东移加快;而当MJO对流活跃区在热带西太平洋地区,MJO异常环流对平均MSE的输送形成东负西正的形势,东侧MSE扰动减小,不利于MJO快速东传.因此,全球变化背景下PDO引起的大气中水汽含量及MSE的变化可能是MJO传播年代际变化的重要原因.

Tibetan Plateau vortices (TPVs) are major rain-producing systems over the Tibetan Plateau. Some TPVs can move off the plateau under certain conditions and impact rainfall over Eastern China. Accordingly, the eastward propagation distances of the TPVs moving off the plateau (EPDs) are closely related to the areas of rainfall associated with TPVs. In this study, the moving-off TPVs during May-August of 1998–2015 are classified into two groups according to their EPDs, and the circulations and heating fields at the times when the TPVs move off the plateau (i.e. moving-off times) are investigated based on reanalysis data. The dynamic and thermodynamic conditions to the east of the Tibetan Plateau are found to significantly impact the EPDs. In the middle and lower troposphere, the zonal ranges of negative geopotential height anomalies to the east of the Tibetan Plateau are in accordance with the EPDs of the TPVs, indicating that anomalous lows play a favourable role in the eastward movement of TPVs. In addition, the anomalous highs to the northeast of the Tibetan Plateau and over Southeastern China also benefit the maintenance of cyclonic circulation to the east of the plateau. Meanwhile, in the upper troposphere, the jet stream over Northeast Asia is beneficial for divergence at 200 hPa. Accordingly, ascending motion associated with the upper-level divergence and lower-level convergence is observed, with the zonal extent corresponding well to the EPDs in the two situations. The atmospheric thermodynamic factors also show a remarkable effect on the EPDs. The TPVs move farther away when the unstable stratification and water vapour convergence extend further eastward. The heating ranges above 500 hPa coincide with the EPDs of TPVs, implying a close relationship between the heating fields and the EPDs. These results benefit prediction on EPDs and further on rainfall to the east of the Tibetan Plateau.

Previous observations suggested a connection between the Madden–Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO), yet few of them contrasted the influences between the MJO and the NAO on extreme weather and climate events. In this study, it is found that the impacts of the MJO and the NAO on winter cold wave amplitude (CWA) over China can be distinctive. Time‐lagged correlation analysis indicates that the response of the CWA to the MJO is characterized by a significant anomalous centre over the Tibetan Plateau and its adjacent region, whereas that to the NAO is mainly over western and northeastern China. The ECMWF model from the Sub‐seasonal to Seasonal (S2S) project can approximately reproduce the relationship between CWA and these indices at intraseasonal time‐scale. The MJO‐associated convective activities along the Equator can modulate the local Hadley circulation. When the convection centre of the MJO is located over the Maritime Continent, it can trigger subsidence over the Tibetan Plateau and its adjacent region, warm the surface and consequently decrease the CWA. When descending flows associated with the MJO prevail over the Maritime Continent due to the eastward propagation of the MJO, the situations tend to be opposite. The intraseasonal variations of the NAO will induce eastward‐propagating Rossby wave trains, which usually favour the significantly positive (negative) pressure, temperature and consequently negative (positive) CWA anomalies over western and northeastern China. Thus, the MJO and the NAO might provide two critical predictability sources for sub‐seasonal forecasts of the extreme temperature events.

Tibetan Plateau vortices (TPVs) are major rain producer over the Tibetan Plateau, which trigger heavy rainfall in southwestern and eastern China when moving off the plateau. In this work, two groups of TPVs moving off the plateau are selected according to their eastward moving speeds. The features of the atmospheric dynamic and thermodynamic fields associated with the two groups of TPVs are compared, based on the final (FNL) operational global analysis data from the Global Forecasting System of the National Centers for Environment Prediction (NCEP). The results show that the large-scale circulations and heating fields have a close relationship with the moving speed of the TPVs. The TPVs move eastward faster when wider and stronger convergence at 500hPa, divergence at 200hPa and the related ascending motion are observed to the east of TPVs. In addition, the stronger and further eastward stretching unstable stratification and water vapor convergence, as well as the more intensive heating field above 500hPa to the east of TPVs, correspond to larger eastward moving speed of TPVs. Furthermore, the crucial factors modulating the moving speed of TPVs are explored through potential vorticity (PV) budget analyses, in which the physical variables are partitioned into zonal means and disturbances. The convergence of the mean zonal winds and disturbance winds at 500hPa, as well as the vertical distribution of disturbance heating to the east of TPVs are the crucial factors influencing the eastward moving speed of TPVs, among which the vertical distribution of disturbance heating is the most dominant.

The climatic characteristics of 260 East Asian tropical monsoon depressions (EAMDs) are investigated using the ERA-Interim reanalysis dataset and a tracking dataset of global monsoon low-pressure systems. Most EAMDs form over the South China Sea (SCS) and the western tropical Pacific Ocean in July–October and have an average lifetime of 10 days. The vertical structures of EAMDs are usually upright or tilt slightly westward with height. The warm-over-cold thermal structure is a distinctive characteristic of EAMDs and two potential vorticity (PV) centers are related to the warm core in the upper level and the specific humidity center in the lower level, respectively. We divided the EAMDs into four groups: eastward-moving, westward-moving, turning, and northwestward-moving EAMDs. Most of the eastward-moving EAMDs form over the SCS in May and June, whereas the westward-moving EAMDs form over both the SCS and the western Pacific Ocean in July–October. The turning and northwestward-moving EAMDs are mainly generated over the western Pacific Ocean and have longer lifetimes. The structures of the eastward-moving and turning EAMDs show common characteristics in each stage. Their vertical structures change from upright in the developing and peak stages to northeast tilting with height in the attenuating stage, especially for the specific humidity. By contrast, the structures of westward- and northwestward-moving EAMDs show little change during their lifetime. They are symmetrical relative to the vertical axis of the EAMDs over their whole lifetime and only vary in strength.

The South Asian high (SAH) and the North Pacific subtropical high (NPSH) are two crucial systems affecting the summer rainfall over East Asia. Features of the relative vorticity of the SAH and its dynamic effect on zonal extension of the western NPSH (WNPSH) are investigated on interannual timescales using data diagnosis and numerical model experiments. Results show that two climatological centres of negative relative vorticity are observed along the northern flank of the SAH over the Tibetan Plateau and northern West Asia at 200 hPa during boreal summer. The relatively more intense centre over the Tibetan Plateau (TPV) shows a pronounced southeast-northwest (SE-NW) variation, indicating a SE-NW shift of the SAH. When the SAH shifts southeastward, an anomalous anticyclone occurs over eastern China at 200 hPa. In the middle troposphere, this anomalous anticyclone is located over southern China and the northern South China Sea, leading to a westward extension of the WNPSH. A diagnostic analysis of the vorticity equation indicates that the negative relative vorticity anomalies at 500 hPa are mainly caused by the downward advection of the mean relative vorticity by anomalous sinking motions. When the SAH extends southeastward, intense convergence on the southeastern flank of the anomalous upper-level anticyclone induces descending motions that cause downward advection of mean negative vorticity. Consequently, negative vorticity anomaly is formed at 500 hPa, leading to a westward extension of the WNPSH. Results from the experiments using an idealized anomalous atmospheric general circulation model further demonstrate that the upper-level anomalous anticyclone associated with a southeastward extension of the SAH triggers an anomalous anticyclone at the middle level, causing a westward extension of the WNPSH.