Western North Pacific Subtropical High Research Articles

Comparison of Intraseasonal Variation of the Meridional Displacement of the Western North Pacific Subtropical High in Early and Late Summer

Abstract The meridional displacement of the western North Pacific subtropical high (WNPSH) on an intraseasonal time scale is investigated, with emphasis on differences between early (May–June) and late (July–August) summer. The intraseasonal variation (ISV) of the meridional displacement of the WNPSH is dominated by the 10–30-day period, and the variation amplitude is larger in late summer. The ISV of the WNPSH is attributed mainly to the evolution of an anomalous cyclone/anticyclone north of the WNPSH in early summer, whereas it is due to a south-to-north dipole of an anomalous anticyclone and cyclone over East Asia in late summer. Moreover, the WNPSH tends to shift westward when it moves northward, and vice versa, especially in early summer. Both tropical convection and mid- to high-latitude teleconnection across Eurasia are responsible for the ISV of the meridional displacement of the WNPSH in early and late summer. The role of mid- to high-latitude teleconnection is more important in early summer, whereas tropical convection over the South China Sea is more crucial in late summer, through triggering a Pacific–Japan (PJ) pattern. In early summer, as the WNPSH shifts northward, rainfall increases over the Yangtze River valley and decreases over Southeast China, and vice versa. In late summer, deficient rainfall over North China persists when the WNPSH is at its southernmost location and during its northward shift, and vice versa. The characteristics, underlying processes, and impacts of the 10–30-day meridional displacement of the WNPSH are significantly different in early and late summer.

Large-scale atmospheric features favoring the tropical cyclone activity affecting the Guangdong–Hong Kong–Macao Greater Bay Area of China

This study analyzes the large-scale circulation associated with the tropical cyclones (TCs) affecting the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) in southern China during the peak TC season (July–September) from 1980 to 2019. The results show that the frequency of the TCs affecting the GBA is closely tied to a meridional Rossby wave train from Southeast Asia to Northeast Asia, manifested by the important roles of the variability of the monsoon trough (MT) and the western North Pacific subtropical high (WNPSH). In the active GBA TC years, intensified and westward-shifted MT and northeastward-shrunk WNPSH cause anomalous southeasterly wind over the southeast coast of China, and hence more TCs move northwestward to the GBA. Furthermore, the cyclonic convergent conditions related to the tacit-and-mutual configurations of MT and WNPSH favor more TC genesis in the west of 140° E over the WNP via significant larger meridional shear vorticity of the zonal winds, ascending motions, and positive vorticity. Opposite features of wind anomalies and TC genesis are seen in the inactive GBA TC years. Both statistical and numerical analyses confirm that the meridional Rossby wave train is associated with a Matsuno–Gill response to the anomalous convective activity over the western Pacific warm pool, which is maintained by the ascending branches of the La Niña-type Walker circulation over the Pacific and the monsoon-enhanced Walker circulation over the Indian Ocean.

Combined Effects of Tropical Indo-Pacific–Atlantic SST Anomalies on Record-Breaking Floods over Central-North China in September 2021

Abstract The rainy season typically terminates over the area of central northern China (CNC) in late August with the rapid retreat of the East Asian summer monsoon (EASM). However, the CNC suffered continuous heavy rainfall in September 2021. The monthly accumulative precipitation amount broke its historical record since 1961 and caused severe floods. The present study ascribes this extreme rainfall event to the westward inland intrusion of the western North Pacific subtropical high (WNPSH). Both case study and statistical analysis indicate that westward extension of the WNPSH is significantly positively correlated with convection over the Maritime Continent (MC) and northern Indian Peninsula (NIP) under the influence of sea surface temperature anomalies (SSTAs). Atmospheric general circulation model (AGCM) experiments validate the individual and combined effects of SSTAs in the tropical Indo-Pacific–Atlantic Oceans on WNPSH and CNC rainfall. In particular, the warm SSTAs in the Indo-Pacific warm pool increase the probability of extremely westward extension of the WNPSH and above-normal rainfall over the CNC. The cold SSTAs in the equatorial central-eastern Pacific enlarge the zonal SSTA gradient, strengthen the anomalous convection over the MC and NIP, and intensify the WNPSH and CNC rainfall anomalies. Meanwhile, the warmer tropical Atlantic anchors the anomalous WNPSH and CNC rainfall in their observed positions. A statistical model based on the tropical SSTAs in August can skillfully predict the westward extension of the WNPSH in September 2021, suggesting that the combined effects of tropical Indo-Pacific–Atlantic SSTAs in late summer possibly tend to prolong rainy season of the EASM until early autumn.

Interdecadal Change in the Interannual Variation of the Western Edge of the Western North Pacific Subtropical High During Early Summer and the Influenceof Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific (WNP) subtropical high (WNPSH) in early summer experienced an interdecadal decrease around 1990. Correspondingly, the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP (WNPHA) in abnormal years possess smaller ranges after 1990. The different influences of the tropical SSTAs are important for this interdecadal change, which exhibit slow El Niño decaying pattern before 1990 while rapid transformation from El Niño to La Niña after 1990. The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences. Before 1990, the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation, respectively. Meanwhile, the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response, which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific. After 1990, the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation, respectively. Overall, the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990. Consequently, the WNPHA features narrower zonal range and less eastward extension after 1990, corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH. On the other hand, the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990, contributing to the change of the El Niño SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.

Dynamical Seasonal Prediction of the Asian Summer Monsoon in the China Meteorological Administration Climate Prediction System Version 3

Based on the 20 years of ensemble hindcast data, we evaluated the performance of the new version climate prediction system developed by the China Meteorological Administration (CMA CPSv3) on the Asian summer monsoon (ASM) seasonal prediction in this study. Many major features of the ASM are well predicted by CPSv3, including the intensity and location of the heavy precipitation centers, large-scale monsoon circulations, monsoon onset, and the interannual variation of dynamical monsoon indices. The model captures realistically interannual variability of the summer western North Pacific subtropical high (WNPSH) and is highly skillful for the WNPSH index. Compared with its predecessor, the prediction skill of summer precipitation over Asia in CPSv3 is obviously improved, especially over eastern China. The improvement mainly benefits from skillful predictions of the tropical Pacific Ocean and tropical Indian Ocean sea surface temperatures and ocean–atmosphere coupling associated with them.

An objective approach to predict the spatial property of anomalous rain-belt of Meiyu

The Meiyu 2020 with record-breaking amount of precipitation, widespread area coverage and anomalous northern location brought about great social and economic impacts in China. However, the spatial property of anomalous rain-belt for such a catastrophic Meiyu is not well predicted. In this study, high-resolution in-situ observation and reanalysis data are used to explore key signals and possible prediction method. Results show that anomalous rain-belt of Meiyu 2020 is mainly influenced by the westward extended western North Pacific subtropical high (WNPSH) in the low-latitude and enhanced meridional circulation in mid-high latitudes. However, the distribution of composite anomalous circulation, especially in the mid-high latitude, cannot be revealed for 2020 if only considering the influence from the tropical Indo-Pacific thermal conditions. Through additional consideration of North Atlantic Oscillation (NAO), which has great contribution to circulation in the Northern Hemisphere mid-high latitudes, the anomalous key circulation features in 2020 can be appropriately captured. Also, the spatial property of anomalous rain-belt like 2020 could be accurately simulated by the established prediction model including combined effects of tropical Indo-Pacific thermal conditions and NAO. Specifically, quantified analyses prove that the warmed Indian Ocean plays a dominant role and anomalous NAO also has great importance for record-breaking area coverage and anomalous northern location of Meiyu 2020. Thus, for a more accurate prediction for anomalous rain-belt such as Meiyu 2020, the combined effects of both low- and mid-high-latitudes should be comprehensively considered.

Characterising the interaction of tropical and extratropical air masses controlling East Asian summer monsoon progression using a novel frontal detection approach

Abstract. The East Asian summer monsoon (EASM) is a complex phenomenon, influenced by both tropical and mid-latitude dynamics and by the presence of the Tibetan Plateau. The EASM front (EASMF) separates tropical and extratropical air masses as the monsoon marches northwards. Although the different factors behind EASM progression are illustrated in a number of studies, their interactions, in particular between tropical and extratropical air masses, still need to be clarified. In this study we apply Eulerian and Lagrangian methods to the ERA5 reanalysis dataset to provide a comprehensive study of the seasonal progression of and interannual variability in the EASM, and we highlight the dynamics of the air masses converging at its front. A frontal detection algorithm is used to perform a front-centred analysis of EASM progression. The analysis highlights the primary role of the subtropical westerly jet (STWJ) and of the western North Pacific subtropical high (WNPSH) in controlling the strength and the poleward progression of the EASMF, in particular during Mei Yu, the primary stage of EASM progression. These forcings act to steer the southerly advection of low-level moist tropical air, modulated by the seasonal cycle of the Asian monsoon. The Mei Yu stage is distinguished by an especially clear interaction between tropical and extratropical air masses converging at the EASMF. The analysis of composites based on the latitude of the EASMF during Mei Yu reveals the influence exerted by the STWJ on the cool extratropical flow impacting on the northern side of the EASMF, whose progression is also dependent on the location of the WNPSH. In turn, this affects the extent of the warm moist advection on its southern side and the distribution and intensity of resultant rainfall over China. This study shows the validity of an analysis of EASM progression focused on its front and on the related low- and mid-level airstreams, at least in the Mei Yu stage. The framework highlighted shows how the regional flow over East Asia drives the low-level airstreams that converge at the EASMF, thus controlling the shape of EASM progression. This framework provides a basis for studies of climate variability and extreme events and for model evaluation.

Different mechanisms for daytime, nighttime, and compound heatwaves in southern China

Heatwaves often cause immense stress on human society and the natural environment. While heatwaves can be classified into daytime, nighttime, and compound daytime-nighttime types, the specific processes associated with different heatwave types remain poorly understood. In this paper, we identify different mechanisms operating in compound (i.e., extreme heat during both day and night) and independent daytime and nighttime heatwaves in southern China. Compound heatwaves generally exhibit stronger temperature increases than either daytime or nighttime types. Daytime heatwaves are accompanied by increased downward shortwave radiation under a clear sky with reduced cloud cover and moisture. Nighttime heatwaves are characterized by more cloudy and moist conditions, and increased downward longwave radiation at the surface at night. A combination of these conditions for daytime and nighttime heatwaves prevail during compound heatwaves. All three types of heatwaves are associated with strengthening and eastward extension of the South Asian high (SAH) in the upper troposphere, and strengthening and westward extension of the western North Pacific subtropical high (WNPSH) in the lower and middle troposphere. Further examinations suggest that compound heatwaves are accompanied by the strongest intensification of SAH and WNPSH. Compared with daytime heatwaves, nighttime events are associated with a stronger amplitude of SAH and WNPSH, and both highs tend to extend more southward when nighttime heatwaves occur. This southward extension induces an anomalous lower-level anticyclone that drives a southwesterly wind anomaly over southeastern China. This circulation feature transports warmer and more humid air towards southern China. The enhanced concentration of water vapor leads to increased absorption of outgoing longwave radiation, and increased re-emission of longwave radiation to the surface, thus resulting in surface warming at night.

Long‐Lead Predictability of Western North Pacific Subtropical High

AbstractWe present a new assembly of three steppingstones in thorough inquiry of the complex chain reaction from the diverse equatorial sea surface temperature (SST) pattern two‐season ahead to the East Asia summer weather condition, by regarding the western North Pacific subtropical high (WNPSH) as an anchor. We identified three predictors that are discordant with conventional climatic indices and achieve a good skill in the summer WNPSH prediction (temporal correlation coefficient of 0.78) with a two‐season long lead in the leave‐1‐year‐out cross‐validation during 1981–2020, despite the diversity of El Niño‐Southern Oscillation (ENSO) and Indian ocean dipole (IOD) patterns. Specifically, the first predictor captures the initiation and development of both IOD and El Niño via the SST warming over central equatorial Pacific in autumn; the second one serves as a better delegate of IOD by capturing the easterly acceleration over tropical Indian ocean; the third one illustrates the unprecedented informative role of upper‐level high pressure over the entire tropics in the WNPSH prediction with 5months lead time. They mediate the discordance between conventional climate indices and the diverse WNPSH response and caution people to revisit and rethink the complex atmosphere‐ocean coupled response from the diverse tropical SST patterns two seasons ahead to EA summer weather condition.

Medium-Range Predictability of Boreal Summer Western North Pacific Subtropical High and Its ENSO Modulation

In boreal summer, variations of intensity and location of the western North Pacific subtropical high (WNPSH) have significant impacts on weather and climate in East Asia. In this study, the medium-range prediction of WNPSH is comprehensively evaluated with various scores based on reforecast data of the National Centers for Environmental Prediction - Global Ensemble Forecast System, and the predictability source of the WNPSH medium-range forecasting is further analyzed by examining how well the model can reproduce the modulation of El Niño-Southern Oscillation (ENSO) on WNPSH as observed. The results show that this system has a systematic bias in the WNPSH forecasts, mainly manifested by the weak strength and the southeastward shifted position, and such a bias further increases with lead time. Effective prediction skills of WNPSH are 10–11 days for its intensity and area, 7 days for its ridge line, but only 1–3 days for its western boundary ridge point in terms of different scores, respectively, which can be improved through developing a bias correction method of prediction. It is demonstrated that the medium-range predictability of WNPSH is mainly originated from ENSO and its significant lagged effects on WNPSH well revealed in observation can be realistically reproduced by this system within the effective prediction lengths. A strong ENSO modulation of the WNPSH prediction skills has been clearly found in terms of the different indices, which depends on the ENSO’s developing and decaying phases. The intensity and area of WNPSH are usually highly predictable due to the ENSO effect being reproduced by the system well while the location indices of WNPSH have relatively low predictability, which are mainly affected by internal variability and difficultly captured by the model. Predictability analysis of WNPSH as modulated by ENSO shows good potential for medium-range forecasting with high skills.

Detecting Interdecadal Change in Western North Pacific Tropical Cyclone Genesis Based on Cluster Analysis Using pHash + Kmeans

Previous studies have noted an abrupt decrease in western North Pacific (WNP) tropical cyclone (TC) genesis frequency and a westward shift in genesis location since the late 1990s. The recent application of cluster analysis in TC research shows the effect of detecting the contribution of the Western North Pacific Subtropical High (WNPSH) and the interdecadal Pacific oscillation (IPO) on interdecadal change in WNP TCs. In this work, we also apply a clustering algorithm called pHash + Kmeans to group WNP TCs into three classes based on their genesis environmental conditions. The clustering results show that an abrupt decrease after 1998 is related primarily to a decrease in the dominant class (Class3, located mainly in the southern and eastern WNP), and an increase after 2010 occurs because of a new dominant class (Class1, located mainly in the northwestern WNP), which indicates that the WNP environment suppresses Class3 genesis after 1998 and enhances Class1 genesis after 2010. Three periods (P1: 1979–1997, P2: 1998–2010, and P3: 2011–2020) and three regions (SCS: 100°E-120°E, EQ-30°N; WNP1: 120°E-140°E, EQ-30°N; and WNP2: 140°E-160°W, EQ-30°N) are divided to further confirm the above findings. In P1, high (low) mid-level relative humidity (RH), intense (weak) low-level vorticity, and weak (strong) vertical wind shear (VWS) are distributed in WNP2 (SCS and WNP1), indicating suitable environmental conditions for TC genesis in WNP2 but unsuitable conditions in SCS and WNP1. This situation is the opposite in P2, leading to a decrease in genesis frequency and a westward shift in genesis location. In P3, strong low-pressure vorticity and thermodynamic conditions occur in SCS and WNP1, contributing to an increase in TC genesis frequency.

Physical processes of summer extreme rainfall interannual variability in eastern China: Part I—observational analysis

Extreme precipitation can have catastrophic effects in China by triggering floods, landslides, and other natural disasters. We measure extreme precipitation over eastern China by the maximum of five-day precipitation amount (Rx5day) in June, July, and August (JJA), which contributes more than 20% of the climate mean of JJA regional total precipitation. Based on the empirical orthogonal teleconnection (EOT) method, this work identifies four dominant regions of observed Rx5day interannual variability in eastern China: north-eastern China (EOT1), the southern lower reaches of the Yangtze valley (EOT2), southern China (EOT3) and the northern lower reaches of the Yangtze valley (EOT4). EOT1 extreme precipitation is related to a strong East Asian Summer Monsoon (EASM), a weak monsoon front and a northward displaced upper-tropospheric westerly jet. EOT2 and EOT4 extreme precipitation are related to an enhanced and stable monsoon front and a strong western North Pacific subtropical high (WNPSH). The WNPSH associated with EOT4 is stronger than that associated with EOT2, which pushes the monsoon front further north. EOT3 represents extreme precipitation that is related to anomalous southerlies around the western ridge of the WNPSH. The southerlies transport warm and moist air to southern China and increase precipitation there. The four key regions and the related mechanisms are not sensitive to the EOT technique, as the EOT-based extreme precipitation patterns and circulation anomalies are confirmed using Self-Organising Maps (SOMs).

Contributions of Anomalous Large‐Scale Circulations to the Absence of Tropical Cyclones Over the Western North Pacific in July 2020

AbstractNo tropical cyclone (TC) formed over the western North Pacific (WNP) in July 2020, which is record‐breaking. Some recent studies suggested that this extreme event was mainly caused by the strong WNP subtropical high (WNPSH) in July 2020, which might not be the only reason. In this study, results from statistical and composite analyses indicate that the strong WNPSH, the strong South Asian high, the westward extended tropical upper‐tropospheric trough, and the weak South Asian summer monsoon are all detrimental to TC genesis over the WNP and might contribute to the absence of TCs in July 2020. Results from sensitivity experiments using a regional atmospheric model that can resolve TCs further demonstrate that it was the collective effect, not any of the individual large‐scale circulation anomalies, that induced the extremely unfavorable conditions for TC genesis and thus led to the record‐breaking inactive TC month.

Mechanisms responsible for interdecadal variation of the thermodynamic component of East Asian summer monsoon moisture transport

AbstractIn this study, the East Asian summer (June–July–August) monsoon (EASM) atmospheric moisture transport variability under global warming are examined using ERA‐Interim reanalysis data from 1979 to 2019. The total EASM atmospheric moisture transport is decomposed into the thermodynamic (δTH) and dynamic (δMCD) components, which are related to specific humidity and wind velocity, respectively. Based on empirical orthogonal function (EOF) analysis, both the second and the fourth modes of δTH are linked to δMCD, indicating that δMCD can impact the moisture distribution and consequently influence δTH indirectly. Both the first (δTH_EOF1) and the third (δTH_EOF3) modes of δTH show considerable interdecadal changes. It is noted that their principal components exhibit significant increasing trends in recent decades. The response of EASM moisture transport to global warming is probably reflected in these two modes. δTH_EOF1 is associated with negative phases of the Interdecadal Pacific Oscillation (IPO)/Pacific Decadal Oscillation (PDO). This causes the western North Pacific subtropical high (WNPSH) to extend westward and the strengthening of Pacific trade winds. As a result, positive precipitable water anomalies dominate East Asia and δTH_EOF1 is characterized by anomalous southwesterly moisture transport along the East China Sea to southern Japan. δTH_EOF3 corresponds to anomalous moisture transport over the Indochina Peninsula and Japan. The precipitable water associated with δTH_EOF3 in general represents a meridional dipole. Anomalous southward wind induced by the eastward extension of the WNPSH is the main reason for positive (negative) precipitable water anomalies in southern (northern) East Asian. The anomalous circulation associated with the anomalous east–west sea surface temperature gradient over the Indian Ocean affects atmospheric moisture in southern East Asian and can dramatically impact δTH_EOF3.

Summertime Moisture Sources and Transportation Pathways for China and Associated Atmospheric Circulation Patterns

Based on the Lagrangian particle dispersion model, HYSPLIT 4.9, this study analyzed the summertime atmospheric moisture sources and transportation pathways affecting six subregions across China. The sources were: Midlatitude Westerly (MLW), Siberian-Arctic regions (SibArc), Okhotsk Sea (OKS), Indian Ocean (IO), South China Sea (SCS), Pacific Ocean (PO), and China Mainland (CN). Furthermore, the relative contributions of these seven moisture sources to summertime precipitation in China were quantitatively assessed. Results showed that the CN precipitation source dominates the interannual and interdecadal variation of precipitation in most subregions, except Southwest and South China. The Northeast China vortex and Pacific–Japan (PJ) teleconnection, which transport water vapor from the MLW, OKS and PO sources, are crucial atmospheric systems and patterns for the variation of precipitation in Northeast China. The interannual variation of precipitation in Northwest and North China is mainly dominated by mid–high-latitude Eurasian wave trains, which provide the necessary dynamical conditions and associated moisture transport from the MLW and SibArc sources. In addition, an enhanced western North Pacific subtropical high (WNPSH) accompanied by the East Asian–western North Pacific summer monsoon and PJ teleconnection, transports extra moisture to North China from the SCS and PO sources, as well to the Yangtze River Valley and South China. The Indian summer monsoon (ISM) is also critically important for the interdecadal change in precipitation over the Yangtze River Valley and South China, via the southwesterly branch of moisture transport from the IO source. The interdecadal changes in precipitation over Southwest China are determined by the IO and SCS sources, via enhanced WNPSH coupling with a weakened ISM. These results suggest that the interdecadal and interannual variations of moisture sources contribute to the attendant variation of summertime precipitation in China via large-scale circulation regimes in both the mid–high and lower latitudes.

Joint effects of three oceans on the 2020 super mei‐yu

An unexpected super mei-yu struck in 2020 in the Yangtze–Huaihe River basin, southern Korea, and southern Japan (hereafter referred to as the mei-yu regions), causing many casualties and huge economic losses. The super mei-yu was characterized by a remarkably early onset (around 1 June), late withdrawal (around 1 August), and intense rainfall during the mei-yu season. The precipitation in the early onset and late withdrawal stages contributed more than half of the total mei-yu-period precipitation over the mei-yu regions in 2020. In this study, the authors explored the dominant remote forcing of the mei-yu early onset and late withdrawal to understand the mechanisms of this super mei-yu. The early onset can mainly be attributed to an early northward-shifted East Asian jet stream (EAJS). The late withdrawal mainly resulted from the stagnant EAJS and the western North Pacific subtropical high (WPSH) during 10 July to 1 August. Specifically, North Atlantic sea surface temperature anomalies (SSTAs) excited a Rossby wave, which was steered by atmospheric anomalies related to the western North Pacific SSTAs, causing the early northward-shifted EAJS and generating an early onset. The record-breaking warm SSTAs over the North Indian Ocean to South China Sea and the reduced sea-ice concentration (SIC) over the Laptev–East Siberian Sea played important roles in causing the stagnant WPSH and EAJS during July, which led to the late withdrawal. Meanwhile, the SIC anomalies may have caused the inhomogeneous rainfall distribution in the mei-yu regions. Furthermore, projection results suggest that the probability of a late mei-yu withdrawal similar to the 2020 case will increase in the future. Finally, potential predictors of an extreme mei-yu are discussed.摘要2020年发生在江淮流域, 朝鲜半岛和日本南部 (简称梅雨区) 的暴力梅造成了巨大的人员伤亡和经济损失. 此次暴力梅的主要特征为: 入梅早 (6月1日) , 出梅晚 (8月1日) 以及较强的梅雨期降水. 2020年异常早入梅和晚出梅时期的降水占梅雨期总降水的一半以上. 因此, 为了深入解析2020暴力梅的机制, 本文将分析2020异常早入梅和晚出梅的主导遥强迫机制. 研究表明, 东亚急流提前北跳是造成入梅偏早的主要原因, 而西北太平洋副热带高压 (西太副高) 的作用并不显著. 具体来说, 5月北大西洋海表温度 (SST) 异常激发出东传的罗斯贝波, 同时西北太平洋的暖SST异常改变该罗斯贝波的传播. 进而在两者共同的作用下造成东亚急流的提前北跳, 并导致入梅偏早. 2020年7月破纪录的北印度洋-南海SST暖异常和拉普特夫-东西伯利亚海的海冰密集度 (SIC) 的偏少阻止西太副高和东亚急流的北跳.推迟北跳的西太副高和东亚急流有利于梅雨降水增多, 最终导致出梅偏晚. 同时SIC异常可能是梅雨雨带偏北的原因. 此外, 预估结果表明, 类似于2020出梅偏晚的概率在未来将会增加. 最后, 本文也探讨了暴力梅的潜在预测因子.

Atmospheric Rivers over the Indo-Pacific and its Associations with Boreal Summer Intraseasonal Oscillation

AbstractRecent works have revealed that the wintertime atmospheric river (AR) activity is closely related to the 30–60-day tropical intraseasonal variability, yet it remains unclear whether summertime AR activity is also significantly influenced by the intraseasonal variability, often referred to as the boreal summer intraseasonal oscillation (BSISO). Diagnosing the 40-year (1979–2018) ERA5 reanalysis dataset, the present study examines the climatological features of ARs over the Indo-Pacific region during June to October and its associations with the BSISO. Results suggest that the western North Pacific Subtropical High (WNPSH) provides a favorable circulation background for the summertime AR activity, which conveys the moisture from the tropics to midlatitude North Pacific along its periphery. Our analysis reveals that the BSISO has substantial impacts on the occurrence and distribution of ARs. More ARs are found over the western North Pacific (WNP) when the BSISO convective envelope propagates northward to the subtropical regions, while fewer ARs can be seen when convection is suppressed there. Specifically, in phases 7–8, the active BSISO convection over the Philippine Sea induces a low-pressure anomaly and the corresponding anomalous cyclonic circulation, leading to the enhanced poleward moisture transport and more frequent AR activity over the WNP. Moreover, the WNP ARs tend to be longer and have larger sizes during these two phases. It is also found that more frequent occurrence of tropical cyclones in phases 7–8 can significantly enhance the moisture transport and AR occurrence over the WNP.

Divergent Responses of Summer Precipitation in China to 1.5°C Global Warming in Transient and Stabilized Scenarios

AbstractResponses of summer precipitation in China to global 1.5°C warming with transient and stabilized pathways are investigated through the analysis of the Common Earth System Model climate simulations. Precipitation increases more significantly in southeast China in the stabilized scenario compared with the transient one. The difference is primarily attributable to dynamic factors related to circulation changes and driven by differences of land‐ocean thermal contrast between the two scenarios. The stabilized warming favors conditions of larger ocean warming and smaller land warming, leads to enhanced meridional temperature gradient and ultimately speeds up the jet stream over East Asia. Ageostrophic wind at the entrance of the accelerated jet strengthens ascending motion and precipitation along the coast of Asia from southeast China to India. The enhanced heating over India further induces a Kelvin‐wave response with an anomalous anticyclone that contributes to the enhanced westward shifted of the western North Pacific Subtropical High. Southerly winds at the west flank of the anticyclone transport moisture northward from tropical oceans, and strengthen precipitation in southeast China.

Enhanced Turbulent Heat Fluxes Improve Meiyu‐Baiu Simulation in High‐Resolution Atmospheric Models

AbstractBased on 10 pairs of atmospheric general circulation models from the Coupled Model Intercomparison Project Phase 6 High Resolution Model Intercomparison Project, we find that high‐resolution models (HRMs, ∼50 km) perform better than low‐resolution models (LRMs, ∼100 km) in simulating the East Asian summer monsoon (EASM) rain belt. We attribute the increased rainfall to enhanced vertical advection of water vapor in the HRMs which is mainly contributed by enhanced vertical motion. Based on moist static energy analyses, the stronger vertical motion in the HRMSs is mainly balanced by enhanced net energy flux into the atmosphere column in the “Meiyu‐Baiu” (MB) region due to the increase of air‐sea turbulent latent heat fluxes along the Kuroshio and sensible heat fluxes over land. The increased net energy flux in the HRMs drives stronger ascending motion, resulting in more realistic rainfall in the MB region. Higher saturated specific humidity determined by finer sea surface temperature, less near‐surface specific humidity along the Kuroshio, and stronger near‐surface winds on western side of the western North Pacific Subtropical High strengthened by the enhanced pressure gradient together contribute to higher ventilation efficiencies and enhanced ocean forcing along the Kuroshio, leading to more turbulent latent heat fluxes in the HRMs. Additionally, drier and less cloudy atmosphere in the HRMs result in more downward shortwave radiation heating over the land, which partly contributes to the increased sensible heat flux. This understanding of the sensitivity of simulated MB to model resolution has implications for precipitation simulation and prediction over East Asia.

Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s

Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s