Western North Pacific Subtropical High Research Articles

Evaluating boreal summer circulation patterns of CMIP6 climate models over the Asian region

Our confidence in future climate projection depends on the ability of climate models to simulate the current climate, and model performance in simulating atmospheric circulation affects its ability of simulating extreme events. In this study, the self-organizing map (SOM) method is used to evaluate the frequency, persistence, and transition characteristics of models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) for different ensembles of daily 500 hPa geopotential height (Z500) in Asia, and then all ensembles are ranked according to a comprehensive ranking metric (MR). Our results show that the SOM method is a powerful tool for assessing the daily-scale circulation simulation skills in Asia, and the results will not be significantly affected by different map sizes. Positive associations between each two of the performance in frequency, persistence and transition were found, indicating that a good ensemble of simulation for one metric is good for the others. The r10i1p1f1 ensemble of CanESM5 best simulates Z500 in Asia comprehensively, and it is also the best of simulating frequency characteristics. The MR simulation of the highest 10 ensembles for the Western North Pacific Subtropical High (WNPSH) and the South Asia High (SAH) are far better than those of the lowest 10. Such differences may lead to errors in the simulation of extreme events. This study will help future studies in the choice of ensembles with better circulation simulation skills to improve the credibility of their conclusions.

Record-breaking summer rainfall in South Korea in 2020: Synoptic characteristics and the role of large-scale circulations

AbstractIn the summer of 2020, South Korea experienced record-breaking rainfall due to 15 consecutive heavy rainfall events (HREs) from mid-June to early September. Among them, 11 HREs occurred in late June to mid-August with distinct synoptic characteristics depending on the occurrence period. All HREs from 29 June to 27 July (P1) were triggered by extratropical cyclones, while those from 28 July to 15 August (P2) mainly occurred along monsoon rainband. We argue that their transition is associated with atmospheric teleconnections. During P1, the western North Pacific subtropical high (WNPSH) anomalously extended westward, but its northward expansion was hindered by the meridional wave train from the suppressed convection over the South China Sea. This condition prevented a northward migration of the monsoon rainband but allowed more extratropical cyclones to pass over the Korean Peninsula, resulting in four HREs. During P2, the South China Sea convection was enhanced, and its circulation response prompted an abrupt northward expansion of the WNPSH with a large pressure gradient along its northern boundary. With intensified southwesterly moisture transport, a monsoon rainband was activated over the Korean Peninsula, producing six HREs. The opposite phases of the summer North Atlantic Oscillation, i.e., negative in P1 but positive in P2, further contributed to the anomalous monsoon circulation by modulating the mid-latitude circulation response to the South China Sea convection. This study demonstrates that the nature of summertime HREs in East Asia can be strongly modulated by remote forcings.

The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020

During summer 2020, the Yangtze River basin suffered from extreme Meiyu rainfall and catastrophic flooding in June and July. This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event, based on forecasts from the Met Office GloSea5 operational forecast system. The forecasts successfully predicted above-average rainfall over the Yangtze River basin, which arose from the successful reproduction of the anomalous western North Pacific subtropical high (WNPSH). Our results indicate that both the Indian Ocean warm sea surface temperature (SST) and local WNP SST gradient were responsible for the westward extension of the WNPSH, and the forecasts well captured these tropical signals. We explore extratropical drivers but find a large model spread among the forecast members in the meridional displacements of the East Asian mid-latitude westerly jet (EAJ). The forecast members with an evident southward displacement of the EAJ, favored more extreme Yangtze River rainfall. However, the forecast Yangtze River rainfall anomaly was weaker compared to that observed and no member showed such strong rainfall. In observations, the EAJ displayed an evident acceleration in summer 2020, which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin, and favor more Meiyu rainfall. The model forecast failed to well reproduce these processes. This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall, hindering a better forecast of the intensity of the event and disaster mitigation.

A Recent Increase in Long-Lived Heatwaves in China Under the Joint Influence of South Asia and Western North Pacific Subtropical Highs

AbstractLong-lived (≥6 days) heatwaves (HWs) have strong social impacts with serious health implications. Using homogenized historical daily temperatures from China and ECMWF reanalysis data, this study investigates its frequency between 1979 and 2018 and driving mechanisms. It is found that the occurrence of HWs is strongly associated with the joint actions of the South Asian high and the western North Pacific subtropical high, which can be described by a synergy index measured by the boundary distance between the two subtropical high-pressure systems. When the synergy index is positive, there are more long-lived HWs occurrence in the east of the Tibetan Plateau, the lower reaches of the Yangtze River and the southern region in China, and vice versa. A Mann-Kendall test shows a significant interdecadal shift around 2004/2005 towards increased occurrence that is consistent with enhanced subtropical high systems. This study shows the important roles of large-scale dynamic systems in regional climate extremes and their future changes.

Possible mechanisms for persistent anomalous rainfall over the middle and lower reaches of Yangtze River in winter 2018/2019

AbstractThe middle and lower reaches of Yangtze River (MLYR) experienced a historically high number of persistent cloudy and rainy days throughout winter 2018/2019. The persistent cold and rainy weather caused severe social impacts and economic losses. Climatic monitoring shows that a weak central Pacific (CP) El Niño event initiated in November 2018 with an increase in the sea surface temperature (SST) in the central and eastern tropical Pacific. During the mature phase of the CP El Niño, the western North Pacific Subtropical High (WNPSH) was stronger than normal and the western boundary moved westward. However, the weak CP El Niño event alone is not enough to explain the abnormally strong and northward displacement of WNPSH and associated heavy rainfall in winter of 2018/2019. This study highlights the eastward propagating Rossby wave train in the middle‐upper troposphere along with the Asian westerly jet‐induced anomalous high pressure and anticyclonic circulation over East Asia, which contributed to the intensification and northward displacement of WNPSH. The synergic influence of both the CP El Niño and Rossby wave train over Eurasia resulted in an intensified and northward shifted WNPSH, which, together with the intensification of the India Burma trough, strengthened the transport of water vapour into the MLYR basin. Accompanied by frequent outbreaks of cold air and confluence of cold and warm air over the MLYR basin, cold and rainy weather persisted in winter 2018/2019 in this region.

Improving the Accuracy of Subseasonal Forecasting of China Precipitation With a Machine Learning Approach

Precipitation change, which is closely related to drought and flood disasters in China, affects billions of people every year, and the demand for subseasonal forecasting of precipitation is even more urgent. Subseasonal forecasting, which is more difficult than weather forecasting, however, has remained as a blank area in meteorological service for a long period of time. To improve the accuracy of subseasonal forecasting of China precipitation, this work introduces the machine learning method proposed by Hwang et al. in 2019 to predict the precipitation in China 2–6 weeks in advance. The authors used a non-linear regression model called local linear regression together with multitask feature election (MultiLLR) model and chosen 21 meteorological elements as candidate predictors to integrate diverse meteorological observation data. This method automatically eliminates irrelevant predictors so as to establish the forecast equations using multitask feature selection process. The experiments demonstrate that the pressure and Madden–Julian Oscillation (MJO) are the most important physical factors. The average prediction skill is 0.11 during 2011–2016, and there are seasonal differences in forecasting skills, evidenced by higher forecast skills of winter and spring seasons than summer and autumn seasons. The proposed method can provide effective and indicative guidance for the subseasonal prediction of precipitation in China. By adding another three factors, Arctic Oscillation (AO) index, Western North Pacific Monsoon (WNPM) index and Western North Pacific Subtropical High (WNPSH) index into the MultiLLR model, the authors find that AO can improve the forecast skill of China precipitation to the maximum extent from 0.11 to 0.13, followed by WNPSH. Moreover, the ensemble skill of our model and CFSv2 is 0.16. This work shows that our subseasonal prediction of China precipitation should be benefited from the MultiLLR model.

Recent Intensified Influence of the Winter North Pacific Sea Surface Temperature on the Mei-Yu Withdrawal Date

AbstractThe mei-yu withdrawal date (MWD) is a crucial indicator of flood/drought conditions over East Asia. It is characterized by a strong interannual variability, but its underlying mechanism remains unknown. We investigated the possible effects of the winter sea surface temperature (SST) in the North Pacific Ocean on the MWD on interannual to interdecadal time scales. Both our observations and model results suggest that the winter SST anomalies associated with the MWD are mainly contributed to by a combination of the first two leading modes of the winter SST in the North Pacific, which have a horseshoe shape (the NPSST). The statistical results indicate that the intimate linkage between the NPSST and the MWD has intensified since the early 1990s. During the time period 1990–2016, the NPSST-related SST anomalies persisted from winter to the following seasons and affected the SST over the tropical Pacific in July. Subsequently, the SST anomalies throughout the North Pacific strengthened the southward migration of the East Asian jet stream (EAJS) and the southward and westward displacement of the western North Pacific subtropical high (WPSH), leading to an increase in mei-yu rainfall from 1 to 20 July. More convincingly, the anomalous EAJS and WPSH induced by the SST anomalies can be reproduced well by numerical simulations. By contrast, the influence of the NPSST on the EASJ and WPSH were not clear between 1961 and 1985. This study further illustrates that the enhanced interannual variability of the NPSST may be attributed to the more persistent SST anomalies during the time period 1990–2016.

Different Configurations of Interannual Variability of the Western North Pacific Subtropical High and East Asian Westerly Jet in Summer

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

Investigating the influence of synoptic circulation patterns on regional dry and moist heat waves in North China

Summer (June–August) heat waves in North China are found to be either primarily dry or moist, based on surface meteorological observations. This study characterizes synoptic circulation patterns (i.e., 500 hPa geopotential height) using the self-organizing map (SOM) method and investigates the influence of synoptic circulation patterns on these two types of heat waves. Results show that regional dry and moist heat waves are associated with different circulation patterns, which significantly modulate the advection of water vapor within the low-level atmosphere, and soil moisture and evaporation conditions at the surface. Regional dry heat waves are associated with times when a continental high pressure ridge is situated to the northwest of North China, and when the northern edge of the western North Pacific subtropical high (WNPSH) is south of 30° N. Regional moist heat waves are associated with a northward shift of the WNPSH. Long term variations of dry and moist heat wave occurrences correlated significantly with the occurrences of their associated circulation patterns at 0.38 (p = 0.02) and 0.71 (p = 0.00), respectively. On sub-seasonal time scales, the dominant heat wave type transforms from dry in June to moist in late July, which is in accordance with summer north–south WNPSH shifts. In addition, training the SOM with absolute geopotential height results in representative circulation patterns that are closely related to surface heat wave conditions in North China rather than the anomaly field, which mixes different circulation regimes.

Distinct Features of Atmospheric Rivers in the Early Versus Late East Asian Summer Monsoon and Their Impacts on Monsoon Rainfall

AbstractAtmospheric river (AR) and its impact on monsoon rainfall in East Asia are investigated by considering their month‐to‐month variations during the East Asian summer monsoon (EASM). The AR in the EASM, defined as an anomalously enhanced plume‐like water vapor transport, frequently forms over eastern China, Korea and western Japan. However, its characteristics vary from the early (June‐July) to the late (August‐September) period of the EASM. In the early EASM, AR is typically characterized by a quasi‐stationary monsoon southwesterly along the northern boundary of the western North Pacific subtropical high (WNPSH), which is further intensified by a migrating extratropical cyclone in the north. In contrast, the late‐EASM AR, which is less frequent than the early EASM AR, is primarily organized by a migrating extratropical cyclone. The quasi‐stationary monsoon southwesterly is less influential as the northern boundary of the WNPSH shifts northward, being decoupled from the subtropical ocean. Both the early‐ and late‐EASM ARs contribute substantially to monsoon rainfall, especially to heavy rainfall events. In the early EASM, 35%–70% of total rainfall amount and 60%–80% of heavy rainfall events in eastern China, Korea and western Japan are associated with AR. Although weakened, AR‐related rainfall is still significant in the late EASM in Korea and western Japan. These results indicate that AR is a key ingredient of EASM precipitation and its subseasonal variations should be taken into account to better understand and predict AR‐related extreme precipitation in East Asia.

Prediction of the Western North Pacific Subtropical High in Summer without Strong ENSO Forcing

The western North Pacific subtropical high (WNPSH) is one of the deterministic predictors of the East Asian summer climate, and a better prediction of the WNPSH favors more reasonable forecast of the East Asian summer climate. This study focuses on seasonal prediction of the WNPSH during neutral summers without strong El Niño-Southern Oscillation (ENSO) forcing, and explores the associated predictable sources, using the one-month lead time retrospective forecasts from the Ensembles-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) project during 1960–2005. The results indicate that the ENSEMBLES atmosphere-ocean-land coupled models exhibit considerable prediction skill for the WNPSH during neutral summers, with successful reproduction of the WNPSH in the majority of neutral summers. The anomalous WNPSH in neutral summers, which corresponds to cyclonic/anticyclonic anomalies in the lower troposphere, is highly correlated with an east-west dipole local sea surface temperature (SST) distribution over the tropical WNP, suggesting an intimate local air-sea coupling. Further diagnosis of the local SST-rainfall relationship and surface heat flux indicates that the anomalous local SST plays an active role in modulating the variation of the WNPSH during neutral summers, rather than passively responding to the atmospheric change. The local SST anomalies and relevant air-sea coupling over the tropical WNP are reasonably well reproduced in the model predictions, and could act as primary predictable sources of the WNPSH in neutral summers. This could aid in forecasting of the East Asian rainband and associated disaster mitigation planning.

Human-Perceived Temperature Changes in South Korea and Their Association with Atmospheric Circulation Patterns

AbstractThis study investigates the spatiotemporal characteristics of human-perceived temperature (HPT) data, which describe the joint effects of temperature and humidity on the human body, and examines the related large-scale atmospheric circulation patterns for the summer season (July–August) in South Korea using trend and composite analyses. The increasing trend of HPT was stronger than that of the maximum, mean, and minimum temperatures during 1981–2018. There was an abrupt change in HPT between 1981–2009 and 2010–18, which is likely caused by the northward upper-level subtropical jet, strengthened downward motion, anomalous anticyclones around South Korea, and increased sea surface temperature over the western North Pacific Ocean, which are related to the enhancement and western expansion of the western North Pacific subtropical high (WNPSH). These results highlight the importance of the activity of the WNPSH in the variability of HPT in South Korea. When the western edge of the WNPSH is located in the northwest, a positive geopotential height anomaly at 500 hPa is centered over South Korea, which is associated with high temperatures and low relative humidity. The southwestern extension of the WNPSH modifies the wind circulation pattern and brings warm and moist air from the West (Yellow) Sea along the ridge line of the WNPSH. Eventually, it leads to extreme HPT, associated with high relative humidity and temperature over South Korea, particularly in the southern part of the country. Therefore, we concluded that monitoring and predicting the location of WNPSH and understanding the mechanism and factors influencing the movement of WNPSH under global warming are necessary for predicting and coping with extreme HPT.

A climatological analysis of the monsoon break following the summer monsoon onset over Luzon Island, Philippines

AbstractThis study investigates the climatology of the monsoon break following the onset of the summer rainy season over Luzon Island (120–122.5°E, 13–22°N) in the Philippines from 1979–2017. The first post‐onset monsoon break is remarkable in stations located over the north and central Luzon Island and occurs climatologically in early June. Composite analysis of the large‐scale circulation features during the monsoon break period shows that this break is associated with the westward extension of the western North Pacific Subtropical High (WNPSH), which weakened the monsoon southwesterlies and induced enhanced low‐level divergence over Luzon Island. The westward extension of the WNPSH may be facilitated by the phase change of the boreal summer intraseasonal oscillation (BSISO). About 59% (23/39) of the monsoon break cases occurred when suppressed convection, associated with the dry phases of the BSISO, is apparent over the western North Pacific. This suppressed convection favours the westward expansion of the WNPSH.With the occurrence of the monsoon break in early summer, the seasonal march of the early summer monsoon over the Philippines can be divided into three phases: (1) the monsoon onset phase, which occurs between mid to late May under the influence of the westerly/southwesterly low‐level winds, (2) the monsoon break phase, when rainfall decreases over Luzon Island in early June, and (3) the monsoon revival phase, when rainfall increases again due to the intrusion of monsoon southwesterlies over the Philippines. This study highlights the complex features of the summer monsoon onset and the impact of the WNPSH on the local climate of the Philippines in early summer.

Inter‐model Spread of the Changes in the East Asian Summer Monsoon System in CMIP5/6 Models

AbstractAmong the state‐of‐the‐art Coupled Model Inter‐comparison Project Phase 5 and 6 (CMIP5/6) models, there is considerable inter‐model spread in the projected changes in the East Asian summer monsoon (EASM) and its components, including the three‐dimensional atmospheric circulation, western North Pacific subtropical high (WNPSH), and East Asian upper‐tropospheric westerly jet (EAJ). In this study, we extracted the leading inter‐model mode of the EASM changes based on the atmospheric circulation, and investigated its relationship with the WNPSH and EAJ. The leading mode of circulation, characterized as a low‐level cyclonic circulation change, is closely correlated with a weakened or eastward‐retreated WNPSH and a southward‐shifted EAJ. This systematic link between them is maintained by the stronger tropospheric warming over East Asia and the western North Pacific, which originates from the greater surface warming over the equatorial western Pacific.

Performance of the IPCC AR6 models in simulating the relation of the western North Pacific subtropical high to the spring northern tropical Atlantic SST

AbstractThis study examines the relationship between the spring northern tropical Atlantic (NTA) sea surface temperature (SST) and its following summer western North Pacific subtropical high (WNPSH) in historical simulations of 20 coupled models of the IPCC AR6. These models well simulate the spring NTA SST, as well as the WNPSH‐related atmospheric and precipitation anomalies over the Asian monsoon region. Ensemble mean of these models reproduces the observed relation and processes linking the spring NTA SST to the summer WNPSH. In the ensemble mean, spring NTA SST warming persists to the following summer, and induces an anomalous Walker circulation with an ascent over the tropical Atlantic and a descent over the tropical central Pacific. The associated precipitation decrease over the tropical central Pacific induces an anomalous low‐level anticyclone over the WNP via a Rossby wave atmospheric response. Meanwhile, spring NTA SST warming leads to easterly wind anomalies over the Indian Ocean (IO) via a Kelvin wave atmospheric response and causes SST warming there via reducing wind speeds. Then, the precipitation anomalies induced by the IO SST warming result in an enhancement of the WNPSH. There exists a large diversity of the NTA SST‐WNPSH relationship among the models. This diversity is related to the difference in the climatological mean IO precipitation. The models with large mean IO precipitation provide a favourable condition for the IO SST warming to bring large local precipitation anomalies and trigger strong easterly wind anomalies over the tropical WNP, and thus exert a strong impact on the WNPSH.

Revisiting the relationship between Indo‐Pacific heat content and South China Sea summer monsoon onset during 1980–2020

AbstractThe relationship of the tropical Indo‐Pacific upper ocean heat content (HC) and the South China Sea (SCS) summer monsoon (SCSSM) onset during 1980–2020 was investigated. It was found that when HC anomalies in the preceding winter–spring are positive (negative) in the eastern Indian–western Pacific Ocean or negative (positive) in the southwestern Indian Ocean and eastern Pacific Ocean, SCSSM onset is usually early (late). Empirical orthogonal function (EOF) analysis suggested that the primary EOF modes of HC anomaly in both tropical Pacific and Indian Oceans are associated with El Niño–Southern Oscillation (ENSO). It is through modulating the westerly over the north Indian Ocean–SCS and the western north Pacific subtropical high (WNPSH) via the Walker circulation over the Indo‐Pacific basin, monsoon circulation in the Indian Ocean that the ENSO related Indo‐Pacific HC variation connects with the SCSSM onset. Both the second EOF modes for the tropical Pacific and Indian Ocean also show significant correlations with the SCSSM onset. The former related vertical motion over the SCS–far northwestern Pacific can influence the WNPSH and convection, thus occurrence of the SCSSM. The latter represents the intrinsic Indian Ocean HC variability; the air temperature variation associated with it can modulate the atmospheric circulation in the Indian Ocean–SCS through thermal wind balance, and thus the SCSSM onset by acting on the westerly over the SCS and the WNPSH. A western Pacific index and an Indian Ocean dipole index were defined to represent the leading HC modes in the tropical Pacific and Indian Ocean, respectively. A binary linear regression model was constructed using these two indices to improve SCSSM onset prediction.

Changing Relationship Between Tibetan Plateau Temperature and South China Sea Summer Monsoon Precipitation

The present study documents the changes in the relationship between the Tibetan Plateau summer surface temperature (TPST) and the South China Sea summer monsoon precipitation (SCSSMP). A stepwise regression model is used to exclude the signals of global warming, El Niño–Southern Oscillation (ENSO), western North Pacific Subtropical High (WNPSH), Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO). The results indicate that the relationship between TPST–SCSSMP changes with time, going from a negative correlation during the period of 1980–1994 to an obvious positive correlation during 1998–2016 in the South China Sea. Meanwhile, the negative correlation between TPST and the East Asia subtropical front (Meiyu) is enhanced during 1998–2016. This change in the TPST–SCSSMP relationship is associated with the change of the atmospheric circulation, which is mainly due to TPST interdecadal variation. A wave–like structure at the low latitude moves eastward along the low–level monsoon flow, and a strong cyclonic circulation is apparent in the southwestern part of the Plateau, including the Indochina Peninsula, South China Sea, and the ocean to the east of the Philippines, which is consistent with the negative correlation between TPST and Outgoing Longwave Radiation (OLR). The increase in water–vapor convergence and more favorable convection conditions lead to more precipitation in the region after the late 1990s. The present results suggest that, in a changing climate, we should be cautious when using predictor with interdecadal variations.

The Potential of Using Tree-Ring Chronology from the Southern Coast of Korea to Reconstruct the Climate of Subtropical Western North Pacific: A Pilot Study

Tree-ring width (TRW) chronologies have successfully been used as climate proxies to infer climate variabilities over the past hundreds to thousands of years worldwide beyond observational records. However, these data are scarce over parts of subtropical East Asia, and especially over the Korean Peninsula. In this pilot study, Korean red pine (Pinus densiflora Siebold and Zucc.) TRW chronologies from Mt. Mudeung and Mt. Wolchul, South Korea, were developed, and their local- to large-scale climatic responses were investigated. Mt. Mudeung TRW had a positive association with local temperature in the preceding December and April. Mt. Wolchul TRW had a positive association with local temperature in the preceding December and most of the early summer to autumn months, and with local precipitation in February and October. On a large scale, both TRWs retained meaningful temperature and monsoon precipitation signals over East Asia and sea surface temperature signals over the Western North Pacific. The results suggest that the subtropical trees from South Korea can be used to infer past long-term climate variability at both local and large scales over East Asia and the Western North Pacific, such as the East Asian summer monsoon, the Kuroshio Current, the Western North Pacific Subtropical High, and El Niño–Southern Oscillation.

Predictability of the Western North Pacific Subtropical High Associated with Different ENSO Phases in GloSea5

The western North Pacific subtropical high (WNPSH) dominates the summer climate over East Asia. The intensity, position, and shape of WNPSH influence the spatiotemporal distributions of precipitation, temperature, and tropical cyclone activities in this region. This paper intends to investigate the performance of the UK Met Office Global Seasonal forecast system version 5 (GloSea5) in simulation/prediction of the WNPSH based on a hindcast dataset. Analyses of the hindcast data show a systematic bias in the mean circulation over West Pacific, with negative geopotential height anomalies over the western North Pacific (WNP) and cyclonic anomalies in the 850-hPa winds and water vapor transport, indicating a weakening and eastward shift of the WNPSH. Despite the model’s bias in the climatology, it well captured the interannual variability of the monthly and seasonal-mean intensity of the WNPSH and the position of its ridge line in boreal summer from 1993 to 2015. The seasonal hindcasts indicate that there is significant prediction skill at up to three-month lead time for both the intensity and position of the WNPSH ridge line. The relationship between the WNPSH and different phases of the El Niño-Southern Oscillation (ENSO) in both the observational data and GloSea5 hindcasts was then investigated. The model captured the summer WNPSH anomalies well during most of the ENSO phases, except in the La Niña decaying and neutral summers. The intensity of the anticyclone in the WNP is weak in the decaying phase of El Niño in the GloSea5 hindcasts compared with the reanalysis data. GloSea5 is capable of representing the lagged teleconnection between El Niño events in the previous winter and the intensity of the WNPSH in the following summer. Regression analysis reveals weakened negative sea surface temperature anomalies (SSTAs) over the WNP in GloSea5, which reduced the gradient between the tropical western Pacific and the tropical Indian Ocean, resulting in a weaker easterly anomaly and stronger westerly anomaly, contributing to the weak anomalous anticyclone over the WNP and the weakened WNPSH relative to the reanalysis data.

Breakdown of the Summertime Meridional Teleconnection Pattern over the Western North Pacific and East Asia since the Early 2000s

AbstractThe meridional teleconnection over the western North Pacific and East Asia (WNP–EA) plays a predominant role in affecting the interannual variability of East Asian climate in summer. This study identified a breakdown of the meridional teleconnection since the early 2000s. Before the early 2000s, there are close tropical–extratropical relationships in light of both circulation and rainfall anomalies. For instance, the westward extension of the western North Pacific subtropical high (WNPSH) is closely associated with the southward shift of the East Asian westerly jet (EAJ), and more rainfall in the tropical WNP closely corresponds to less rainfall in the subtropical WNP–EA. However, after the early 2000s, the tropical–extratropical relationships are absent. Particularly, the tropical WNP precipitation anomalies can induce WNPSH anomalies, but the WNPSH anomalies cannot induce subtropical precipitation in the latter period, due to the absence of EAJ-related extratropical circulation anomalies. Further results indicate that in the latter period, the westward extension of the WNPSH is associated with the decay of central Pacific-like El Niño, and simultaneous summer sea surface temperature (SST) anomalies in the central eastern Pacific favor the northward shift of the EAJ, resulting in the disruption of the WNPSH–EAJ relationship. This evolution of tropical SSTs is sharply different from the decay of canonical El Niño and simultaneous summer tropical Indian Ocean warming, which favor the WNPSH–EAJ correspondence in the former period.