Western North Pacific Subtropical High Research Articles

Different Enhancement of the East Asian Summer Monsoon under Global Warming and Interglacial Epochs Simulated by CMIP6 Models: Role of the Subtropical High

AbstractSoutherly wind in the lower troposphere is an essential feature of East Asian summer monsoon (EASM) circulation, which is reported to be enhanced under global warming scenarios and interglacial epochs. Based on an analysis of an ensemble of CMIP6 models, this study shows that the magnitude of intensification of the EASM circulation is much smaller under global warming scenarios than during interglacial epochs. Distinct changes in the western North Pacific subtropical high (WNPSH) are responsible for the different responses of the EASM circulation. The WNPSH is substantially enhanced during interglacial epochs, which acts to strengthen the southerly wind associated with the EASM on the western flank of the WNPSH. However, the change in the WNPSH is insignificant and cannot strengthen the EASM under global warming scenarios, and the weakly enhanced EASM circulation may be a direct response to intensified heating over the Tibetan Plateau. The land–ocean thermal contrast explains the different responses of the WNPSH. During interglacial epochs, the summertime surface warming over the subtropical North Pacific is much weaker than over Eurasia due to the large thermal inertia of the ocean to increased insolation, and the WNPSH is intensified as a response to the suppressed latent heating over the subtropical North Pacific. The fast response of the WNPSH to abrupt quadrupling of CO2 without sufficient ocean warming is an analog to the interglacial epochs, but it is offset by the effect of slow oceanic warming, resulting in an insignificant change of the WNPSH under global warming scenarios.

East Asia Atmospheric River catalog: Annual Cycle, Transition Mechanism, and Precipitation

AbstractAn all‐season East Asia Atmospheric River (EA AR) catalog is constructed based on PanLu2.0. The annual cycle of EA AR exhibits eight stages, synchronizing with the seasonal‐to‐subseasonal variation of precipitation and their associated atmospheric steering. Particularly, extension of the western North Pacific subtropical high (WNPSH) to south China sea, due to the springtime east‐west thermal contrast, steers ARs over Southeast China (SEC), inducing spring precipitation. During the Asian summer monsoon season, the spatial variation of the AR main route echoes the northward displacement of WNPSH, coincides with the prevailing monsoonal wind onset and stepwise propagation of rain belt. The warm moisture‐laden air of the wintertime ARs confronts the dry and cold air over SEC, contributing to the winter precipitation. ARs offer substantial potential predictability of (heavy) precipitation days over the midlatitude from the late spring to early summer, and over SEC and south Japan from late winter to early spring.

Introduction to the Regional Coupled Model WRF4-LICOM: Performance and Model Intercomparison over the Western North Pacific

Regional coupled modeling is one of the frontiers of regional climate modeling, but intercomparison has not been well coordinated. In this study, a community regional climate model, WRF4, with a resolution of 15 km, was coupled with a high-resolution (0.1°) North Pacific Ocean model (LICOM_np). The performance of the regional coupled model, WRF4_LICOM, was compared to that of another regional coupled model, RegCM4_LICOM, which was a coupling of version 4 of the Regional Climate Model (RegCM4) with LICOM_np. The analysis focused on the 2005 western North Pacific summer monsoon rainfall. The results showed that the regional coupled models with either RegCM4 or WRF4 as their atmospheric model component simulated the broad features over the WNP reasonably well. Quantitative intercomparison of the regional coupled simulations exhibited different biases for different climate variables. RegCM4_LICOM exhibited smaller biases in its simulation of the averaged June–July–August SST and rainfall, while WRF4_LICOM better captured the tropical cyclone (TC) intensity, the percentage contributions of rainfall induced by TCs to the total rainfall, and the diurnal cycle of rainfall and stratiform percentages, especially over land areas. The different behaviors in rainfall simulated by the two models were partly ascribed to the behaviors in the simulated western North Pacific subtropical high (WNPSH). The stronger (weaker) WNPSH in WRF4_LICOM (RegCM4_LICOM) was driven by overestimated (underestimated) diabatic heating, which peaked at approximately 450 hPa over the region around the Philippines in association with different condensation-radiation processes. Coupling of WRF4 with LIOCM is a crucial step towards the development of the next generation of regional earth system models at the Chinese Academy of Sciences.

Emergent constraints on future projections of the western North Pacific Subtropical High

The western North Pacific Subtropical High (WNPSH) is a key circulation system controlling the summer monsoon and typhoon activities over the western Pacific, but future projections of its changes remain hugely uncertain. Here we find two leading modes that account for nearly 80% intermodel spread in its future projection under a high emission scenario. They are linked to a cold-tongue-like bias in the central-eastern tropical Pacific and a warm bias beneath the marine stratocumulus, respectively. Observational constraints using sea surface temperature patterns reduce the uncertainties by 45% and indicate a robust intensification of the WNPSH due to suppressed warming in the western Pacific and enhanced land-sea thermal contrast, leading to 28% more rainfall projected in East China and 36% less rainfall in Southeast Asia than suggested by the multi-model mean. The intensification of the WNPSH implies more future monsoon rainfall and heatwaves but less typhoon landfalls over East Asia.

Monsoonal Influences on Offshore Rapid Intensification of Landfalling Typhoons in a Sheared Environment over the South China Sea

Abstract Tropical cyclones (TCs) formed in the western North Pacific and South China Sea can undergo rapid intensification (RI) shortly before making landfall in China. Forecasting such offshore RI is a great challenge in operations. In this study the offshore RI events in a sheared environment are examined for TCs that made landfall in China during 1979–2017. It is found that there were only three offshore RI events in a sheared environment, all of which occurred to the south of Hainan Island within the monsoon trough in early to mid-July, coinciding with the termination of the mei-yu season. The specific geographic location and timing of the occurrence of the offshore RI in the sheared environment is associated with the adjustment of the East Asia summer monsoon system when the mei-yu season terminates in the Yangtze River valley. In addition to the adjustment favorable for TC intensification by enhancing the TC–trough interaction in the upper troposphere, this study suggests that two environmental factors also contribute to the offshore RI over the South China Sea in a sheared environment. One is the intrusion of dry air associated with the western North Pacific subtropical high (WNPSH) and the other is the penetration of the water vapor flux associated with the monsoon surge. The adjustment of the East Asia summer monsoon system allows the water vapor flux of the monsoon surge to penetrate the TC circulation and prevents the dry air of the WNPSH from intruding into the TC circulation.

Evaluation and Projection of Regional Climate over East Asia in CORDEX-East Asia Phase I Experiment

This study investigates the performance of simulated precipitation and estimates future changes in precipitation in CORDEX-East Asia Phase I. In the Historical experiment (1981–2005), a global climate model used as a lateral boundary condition does not realistically simulate the timing and intensity of the East Asian summer monsoon. Hence, it overestimates precipitation over East Asia. Generally, the results of the regional climate models also show similar bias characteristics to that of the large-scale forcing data. The individual biases of the regional climate model vary according to a model configuration, such as physical parameterization schemes. However, when bias correction is applied to data, the spatial variability and spatial correlation of the long-term mean precipitation become similar to the observations, and the annual cycle of precipitation is much improved. The two future experiments in the mid-twenty-first century period (2025–2049), show that mean and extreme precipitation amounts increase over the Korean Peninsula and northern China compared to the frequency of wet days. The increment of the low-level water vapor in all seasons can be attributed to the increased precipitation amounts; moreover, the East Asian summer monsoon is enhanced in mid-latitudes and lasts longer in summer owing to the strengthened western North Pacific Subtropical High. The increasing southerly wind from the East Asian summer monsoon over eastern China and the Korean Peninsula results in favorable conditions for the increase in precipitation.

Summer Monsoon Rainfall Patterns and Predictability over Southeast China

AbstractThis study advances the use of the Self‐organizing Map (SOM) to identify the summer monsoon rainfall patterns over Southeast China (SEC), using 272 gauge records from May to August. Three distinct rain belts over the Huai River basin (HRB), the Lower Yangtze River basin (LYRB) and the South Coast region (SCR) are found. Their subseasonal variability strongly agrees with the northward progression of the East Asian Summer Monsoon (EASM) front in a stepwise fashion. We find that precipitation in the SCR and HRB rain belts exhibit significant changes in the mid‐1990s, while the 1990s is the most active decade for the LYRB rain belt. Promising predictability of average daily rainfall over these three regions is obtained, with about 39% to 50% of the total variance explained by the circulation informed regression models. Both leave‐one‐year‐out cross‐validation and blind prediction verify the regression performance. The western North Pacific Subtropical High phases, mid‐latitude blocking high anomaly over northeast China and upper‐level divergence in SEC are found to best explain the variability of the rain belts. The newly proposed Russia‐China wave patterns (western/central Russia → north of Tibetan Plateau → SEC) and teleconnection between the El Niño‐Southern Oscillation and the rain belts also offer additional predictability. Findings from this work may advance the understanding of the EASM rain belts, and offer insights to the source of bias for numerical simulations of daily summer monsoon rainfall in the region.

Interannual and interdecadal impact of Western North Pacific Subtropical High on tropical cyclone activity

In this study, we analyzed the impacts of Western North Pacific Subtropical High (WNPSH) on tropical cyclone (TC) activity on both interannual and interdecadal timescales. Based on a clustering analysis method, we grouped TCs in the Western North Pacific into three clusters according to their track patterns. We mainly focus on Cluster 1 (C1) TCs in this work, which is characterized by forming north of 15° N and moving northward. On interannual timescale, the number of C1 TCs is influenced by the intensity variability of the WNPSH, which is represented by the first Empirical Orthogonal Function (EOF) of 850 hPa geopotential height of the region. The WNPSH itself is modulated by the El Niño–Southern Oscillation at its peak phase in the previous winter, as well as Indian and Atlantic Ocean sea surface temperature anomalies in following seasons. The second EOF mode shows the interdecadal change of WNPSH intensity. The interdecadal variability of WNPSH intensity related to the Pacific climate regime shift could cause anomalies of the steering flow, and lead to the longitudinal shift of C1 TC track. Negative phases of interdecadal Pacific oscillation are associated with easterly anomaly of steering flow, westward shift of C1 TC track, and large TC impact on the East Asia coastal area.

Atmospheric rivers in the Australia–Asian region under current and future climate in CMIP5 models

Atmospheric rivers (ARs), as long and narrow bands of strong water vapour transport in the lower troposphere, have drawn increasing scientific attention in recent years. Results from a collaborative project between the Australian Bureau of Meteorology and China Meteorological Administration have shown some unique AR characteristics embedded within the Australia–Asian monsoon based on observational analyses. As part of the project, this study focused on assessing the skill of global climate models for simulating ARs in the region under current climate and their projected changes due to global warming. Daily data from 17 Coupled Model Intercomparison Project Phase 5 (CMIP5) models in their historical and Representative Concentration Pathway (RCP) 8.5 simulations were analysed for the periods of 1981–2005 and 2081–2100 respectively. Compared with results derived from European Centre for Medium-Range Weather Forecasts ERA-interim reanalysis data, these model ensemble results showed significant seasonal variations of horizontal water vapour transport as observed, but their magnitudes measured by vertically integrated water vapour transport (IVT) were weaker, particularly for the East Asian summer monsoon. Using an objective AR detection algorithm based on 85th percentile IVT magnitude and its geometry, we showed that multi-model-ensemble (MME) averaged AR occurrence agreed well with the results derived from the reanalysis for their spatial distributions and seasonal variations. Under the RCP8.5 global warming scenario, the model ensembles, overall, showed an enhanced water vapour transport, primarily due to increased atmospheric humidity associated with a warmed atmosphere. Consequently, they simulated increased AR frequency and bigger AR size in most of the region, particularly over north and northeast China and southern Australia. However, the MME results showed a reduced AR frequency and size in July/August in southern and eastern part of China and its adjacent waters. We attributed these results to the response of the Western North Pacific Subtropical High (WNPSH) to global warming. Our analysis showed that westward expansion of WNPSH lead to the shift of ARs more inland in East Asia. In this case, eastern China was directly under the control of WNPSH, which did not favour AR development and penetration into the region. Our analyses of ARs in the A–A monsoon system offers new insight in understanding potential climate changes in the monsoon region under warmed climate.

Effect of Warm SST in the Subtropical Eastern North Pacific on Triggering the Abrupt Change of the Mei-Yu Rainfall over South China in the Early 1990s

AbstractIn the early 1990s, the mei-yu rainfall over South China in early boreal summer exhibited an abrupt change and northward extension. This change altered the pattern of East Asian summer rainfall from a dipole-like to a monopole-like pattern; that is, the out-of-phase relationship between the rainfall in the south and that in the north of the Yangtze and Huaihe River valley changed to an in-phase relationship. The physical processes potentially responsible for triggering this abrupt change were analyzed in this study. Our observations revealed that the western North Pacific subtropical high (WNPSH), sea surface temperature (SST) in the subtropical eastern North Pacific (SENP), and the mei-yu rainfall in South China exhibited an abrupt increase in the early 1990s, suggesting that these factors are correlated. From the observations and results of numerical experiments, we proposed that the abrupt SST warming in the SENP in the early 1990s generated an east–west overturning circulation anomaly in the Pacific Ocean and that the anomalous downward motion in the western North Pacific consequently triggered the abrupt increase and westward extension of the WNPSH in the early 1990s. The enhanced and westward extension of WNPSH created a low-level southeasterly anomaly that transported considerable humid and warm air into East Asia and sequentially triggered the abrupt increase of mei-yu rainfall in the South China in the early 1990s.

The Large‐Scale Circulation Patterns Responsible for Extreme Precipitation Over the North China Plain in Midsummer

AbstractExtreme precipitation events over the North China Plain (NCP) in midsummer during 1979–2016 are classified into two types using objective cluster analysis: a northern pattern with heavy precipitation and a central–southern pattern with relatively moderate precipitation. The large‐scale circulation patterns responsible for the midsummer extreme precipitation are then determined. In the northern NCP type, extreme precipitation accompanies a zonal gradient between an anomalous low‐pressure system at high latitudes and the westward‐ and northward‐extended western North Pacific subtropical high (WNPSH). Anomalous southwesterlies flow is driven by a trough that extended from the high latitudes to the northern NCP, where it encounters southeasterly wind flow induced by an anomalously northward‐extended WNPSH and a southern low‐pressure anomaly at low latitudes. Anomalous amounts of moisture are mainly transported from the tropical western Pacific by southeasterlies. In the central–southern NCP type, remarkable anomalous low‐pressure systems control all of northern China with centers over the Sichuan Basin and northeast China. The westward‐extended WNPSH occupies further south than that of the northern NCP type. The southwesterly low‐level jet (LLJ) is more prevalent in the central–southern NCP type than in the northern NCP type. This southwesterly LLJ plays an important role in extreme precipitation over the central–southern NCP by transporting moisture primarily from the Bay of Bengal and the South China Sea and generating convergence. In addition, the upper‐level anomalous strong divergence that is anchored over the right entrance of the westerly jet makes a greater contribution to extreme precipitation in the northern type than in the central–southern type.

Interdecadal Change of Korea Landfalling Tropical Cyclone Frequency and its Possible Association with PDO

ABSTRACT The present study verified that Korea landfalling tropical cyclone (TC) frequency was rapidly increased since 1981 through the statistical change-point analysis. The increase was due to the formation of anomalous southerlies around Korea from the low latitude ultimately because anomalous cyclone was strengthened in East Asia continent due to the movement of western North Pacific subtropical high (WNPSH) to the east whereas anomalous anticyclone was strengthened in the western North Pacific. The anomalous southerlies played a role in steering flow that moved TCs generated in the subtropical western North Pacific to Korea. In order to determine the cause of strengthening of anomalous cyclone in East Asia continent, a snow depth during the preceding spring (March-May) was analyzed. The analysis result showed that most regions in East Asia had lower snow depth than that before 1981. Thus, anomalous cyclone can be developed in the region during summer due to heating in East Asia continent, which was started from the preceding spring, thereby forming a steering flow of anomalous southerlies that can move TCs into Korea. Furthermore, a correlation analysis between Pacific Decadal Oscillation (PDO) and Korea landfalling TC frequency verified that negative correlation was somewhat stronger after 1981.

Projected changes in the western North Pacific subtropical high under six global warming targets

The summer western North Pacific subtropical high (WNPSH) has large influences on the East Asian summer climate. Many studies have focused on the projected changes in the WNPSH, but little is known...

Synoptic characteristics of extreme heatwaves over the Korean Peninsula based on ERA Interim reanalysis data

AbstractThe characteristics of heatwaves (HWs) in South Korea are studied using data from the European Centre for Medium‐Range Weather Forecasts Reanalysis Interim (ERA Interim) dataset and from automatic surface weather stations. The synoptic conditions of three specific years (1994, 2013, and 2016) during which serious HWs affected South Korea are examined. HWs in South Korea are related to the northwestwardly extension of the western North Pacific subtropical high (WNPSH) toward the Korean Peninsula. Examination of the blocking indices revealed widespread blocking over the northern Pacific Ocean and Asia during South Korean HWs, and blocking is related to stationary weather patterns. The severe temperatures associated with HWs in this region are due to prolonged subsidence. Analysis of the moist static energy (MSE) budget indicated that the energy source of subsidence is derived from both MSE advection and the net heat flux. When compared to the synoptic situation during an HW in South Korea, the relative southward movement of the WNPSH is found before and after the HW. The blocking indices also revealed weak signals and changes in vertical motion due to MSE advection.

Characteristics and circulation background of extreme precipitation over East China

Using the observed daily precipitation data across East China, this paper analyzes spatiotemporal changes of annual and seasonal extreme precipitation indices during 1960–2015. Effects of changes in atmospheric circulation on seasonal extremes were also detected by using the singular value decomposition method. Results show that amount of annual, spring and summer extreme precipitation over East China showed a significant increase with a maximum changing rate of 7.2, 3.2 and 5.4 mm decade−1, and the stations with upward tendency mainly located in the southern part of East China. Preceding sea surface temperature of the Indian Ocean and the Equatorial Pacific Ocean shows great influences on spring extreme precipitation over East China through modulating anomalous anticyclones and winds, especially over the downstream of the south of East China. Summer extreme precipitation is statistically significantly correlated with the western North Pacific subtropical high (WPSH) and positive anomalies of 500 hPa geopotential heights. Anomalously strong WPSH combining with two blocking highs in mid-latitude regions tends to trigger more extreme precipitation events in the Yangtze River Basin but less in the Yellow River Basin.

Why Was the August Rainfall Pattern in the East Asia–Pacific Ocean Region in 2016 Different from That in 1998 under a Similar Preceding El Niño Background?

AbstractPrevious studies have noted that a strong El Niño event occurring in the preceding winter will result in westward stretching of the western North Pacific subtropical high (WPSH) in the following summer, causing anomalously high precipitation in the Yangtze–Huaihe River basin and anomalously low precipitation in southern China. The winters preceding the summers of 1998 and 2016 featured strong El Niño events, which, along with the El Niño event of 1982, represented the strongest El Niño events since 1950. Under these similar El Niño event backgrounds, the July precipitation anomaly in 2016 was similar to that in 1998, but the August precipitation anomalies in the two years featured opposite distributions. According to the atmospheric circulation analysis, we found that an anomalous ascending motion appeared over the Indian Ocean, while an anomalous descending motion appeared over the Pacific Ocean in August 1998. In addition, the WPSH stretched westward over southern China. However, the atmospheric circulation distribution in August 2016 was the opposite of that in 1998, and the WPSH was divided into eastern and western parts by the anomalous western Pacific cyclone. Further analysis showed that the number of tropical cyclones and typhoons over the western Pacific Ocean increased significantly in August 2016, and their activities were concentrated in the South China Sea (SCS)–southern China region and the western Pacific Ocean, resulting in the division of the WPSH. Therefore, the numbers, tracks, and strengths of tropical cyclones and typhoons were responsible for the differences in the anomalous precipitation distributions over the East Asia–Pacific Ocean region between August 2016 and August 1998.

How well do climate models simulate regional atmospheric circulation over East Asia?

AbstractThis study presents a comprehensive and quantitative evaluation of the mean state of summer atmospheric circulation over East Asia. Attention is paid to the South Asian high (SAH), western North Pacific subtropical high (WNPSH) and Indian low (IL) at the upper, middle and lower troposphere, respectively. A total of 31 state‐of‐the‐art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used as examples for the analysis. Most models can basically simulate the three closed‐circulation systems (CCSs), although there is a certain inter‐model spread and an underestimation of their intensity. In terms of geographic location, models demonstrated the best performance for SAH and the poorest performance for WNPSH. The latter shows a generally south‐westward shift compared to the National Centers for Environmental Prediction (NCEP) reanalysis. Five atmospheric fields (zonal and meridional wind at 850 hPa, geopotential height at 500 and 100 hPa and sea level pressure) are inspected and generally well reproduced in models, with Taylor‐S indices all larger than 0.84 for 90% of the models. The best performance is for the geopotential height of 500 hPa with an average Taylor‐S index of 0.98. Models’ skill in simulating the sea level pressure is the lowest. However, a significant positive correlation with models' resolution is observed. Almost all models underestimate the 100 hPa geopotential height over East Asia, mainly due to the common cold bias in the troposphere. As a whole, CCSM4, CNRM‐CM5, CESM1‐CAM5 and NorESM1‐M are identified as high‐skill models for simulating the East Asian atmospheric circulation. High‐skill models also show better simulation of precipitations in East China, with a 21.3% decrease of dry biases in Southeast China. The physical explanation for this linkage is also reported to reside in the central position of the WNPSH, which determines the quality of 850 hPa winds and water vapour transport in Southeast China.

Late onsets of tropical cyclones in the decaying years of super El Niño events

The 2015/2016 El Nino event reached the threshold of super El Nino event, and was comparable to the super events in 1982/1983 and 1997/1998. Interestingly, the tropical cyclones (TCs) were found to have very late onsets in the decaying years of the super El Nino events. This study discusses the causes of late TC onsets related with atmospheric circulation, disturbance sources and trigger mechanisms. The analysis shows that the western North Pacific subtropical high (WNPSH) from January-June during the decaying years of the super El Nino events were stronger than the climatic mean, which resulted in a relatively stable atmospheric state by inhibiting deep convection. As a disturbance source, the April-June intertropical convergence zone (ITCZ) during the decaying years of the super El Nino events were significantly weaker than its climatic mean. The cross-equatorial flow and monsoon trough, as important TC generation triggers, were weaker from April-June during the decaying years of the super El Nino events, which further reduced the probability of TC generation. As for the late TC onsets, the role of atmospheric circulation anomalies (i.e., subtropical-high, the ITCZ, cross-equatorial flow, and monsoon trough) were more important. The cross-equatorial flow may take as predictor of TC onsets in the decaying years of the super El Nino events.

The effect of atmosphere–ocean coupling on the prediction of 2016 western North Pacific tropical cyclones

AbstractWe examine the merit of atmosphere–ocean coupled models for tropical cyclone (TC) predictions in the western North Pacific (WNP), where accurate TC predictions remain challenging. The UK Met Office operational atmospheric global numerical weather prediction (NWP) model is compared with two trial coupled configurations, in which the operational atmospheric model is coupled to a one‐dimensional mixed‐layer ocean model and a three‐dimensional dynamical ocean model. Reforecasts for the 2016 TC season show that the coupled models outperform the NWP model for TC location predictions, with a systematic improvement of 50–100 km over the seven‐day forecasts, but the coupled models amplify the underestimation of TC intensity in the NWP model. Nearly identical TC predictions (for both location and intensity) are found in the two coupled models, indicating the dominance of thermodynamic processes at the air–sea interface for TC predictions on these timescales. The improved prediction of the TC position in the coupled models is associated with an enhanced Western North Pacific Subtropical High (WNPSH), which introduces an anticyclonic steering flow anomaly that shifts TC tracks further west in the southern part of the region and further east in the northern part. Based on sensitivity experiments, we show that these improvements in the coupled models are due mainly to colder initial sea‐surface temperatures (SSTs). Air–sea feedbacks do not change the WNPSH or TC tracks noticeably. Apart from the effect of the initial SSTs, tropical ocean warming due to air–sea interaction in the coupled forecasts can also reduce the predicted TC intensity, presumably due to a stronger regional Hadley circulation with increased subtropical subsidence.

Dynamic effect of the South Asian high on the interannual zonal extension of the western North Pacific subtropical high

AbstractThe 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.