East Asian Summer Monsoon System Research Articles

Interannual variations of westward extension area of western Pacific subtropical high and its relationship with precipitation in East Asia

The western Pacific subtropical high (WPSH) is viewed as an important part of the East Asian summer monsoon system and its western boundary location significantly affects regional weather and climate. Based on characteristics of the WPSH western boundary in boreal summer, this study proposes a new index of WPSH, the westward extension area (WEA) index, defined as the area enclosed by the 587 dagpm contour in the region (10°–50°N, 105°–130°E), which can reasonably describe the area variations of WPSH when approaching and landing coastal regions. Compared with the canonical area index of WPSH, the WEA index can more effectively represent the WPSH-associated atmospheric circulation, water vapor transport, and convective activities in the boreal-summer East Asia-northwestern Pacific. A big WEA index significantly indicates flood in the middle reaches of Yangtze River basin and drought in South China. The WEA of WPSH has a significant connection with precipitation in Maritime Continent, due to the convection-circulation interaction in Philippine Sea. There, the WPSH-related sea surface temperature (SST) cooling and rainfall deficiency can excite Rossby waves, which induces the anomalous anticyclone development and the westward stretching and intensifying of WPSH that in turn further reduces rainfall in Philippine Sea. Moreover, the WEA index is significantly modulated by the preceding El Niño-type and tropical Indian Ocean SST anomalies, which may provide a highly predictable indicator of the continental precipitation in East Asia.

Different responses of precipitation patterns to the East Asian summer monsoon weakening: The 7.2 and 8.2 ka events

The 8.2 ka abrupt cooling event has been extensively studied, whereas the 7.2 ka event and their comparations rarely mentioned. Here, we present a high resolution (∼4 years) and precisely dated (±25 years) speleothem multi-proxy record from Zhangjia Cave (northern margin of Sichuan basin, China) spanning the 7.2 and 8.2 ka events within the transition of early to middle Holocene. The speleothem δ18O mainly reflects the dynamic facets of the East Asian summer monsoon (EASM) system, while its δ13C and trace elements ratios serve as an indicator of local hydrological conditions. Our results reveal intense dry conditions in southwest China during the 7.2 ka event, despite the 7.2 event was characterized by a relatively stronger monsoon compared to the 8.2 ka event. Furthermore, the hydrological conditions during the 7.2 ka event and the middle Holocene appear to exhibit “north flood and south drought” pattern when compared to different paleoclimate records in EASM domains, while it can be summarized as “spatially consistent drought” in the 8.2 ka event. These different hydrological conditions are a response to the weakening of the EASM, a phenomenon also observed in modern climatic observations and empirical simulations, which underpins our understanding that the speleothem δ18O variations are mainly dynamical manifestations of the EASM system, likely different with its thermodynamical appearances.

The Guiding Role of Rossby Wave Energy Dispersion Theory for Studying East Asian Monsoon System Dynamics

This paper is written to commemorate the 10th anniversary of academician Ye Duzheng (Yeh T.C.) pass away and his great contributions to the development of atmospheric dynamics. Under the inspiration and guidance of the theory of Rossby wave energy dispersion, remarkable progresses have been made in research on planetary wave dynamics and teleconnections of atmospheric circulation anomalies. This paper aims to make a brief review of the studies on the propagating characteristics of quasi-stationary planetary waves in a three-dimensional spherical atmosphere and the dynamic processes of the interannual and interdecadal variabilities of the East Asian summer and winter monsoon systems. Especially, this paper systematically reviews the progresses of the studies on the impacts of the interannual and interdecadal variabilities of the East Asia/Pacific (EAP) pattern teleconnection wave train propagating along the meridional direction over East Asia and the “Silk Road” pattern teleconnection wave train propagating along the zonal direction within the subtropical jet from West Asia to East Asia on the East Asian summer monsoon system and the summer precipitation variability in China, under the guidance of the theory of Rossby wave energy dispersion. Moreover, this paper reviews the dynamic processes of the impact of the interannual and interdecadal oscillations of the propagating waveguides of boreal quasi-stationary planetary waves on the variability of the East Asian winter monsoon system.

Isotope composition of daily precipitation from 2019 to 2020 in Sanming, southeastern China

Introduction: Many studies in southeastern China (SEC) have used stable isotope (δD and δ18O) records from natural archives (e.g., stalagmites, tree-ring cellulose, sediments) to reconstruct past East Asian Summer Monsoon (EASM) climate. However, the interpretation of the meanings of isotope variation in these records has not been fully resolved, and the key to solve this problem is to clarify the referential meaning of modern precipitation isotopes.Methods: In this study, we collected daily precipitation from January 2019 to December 2020 in Sanming to clarify the characteristics of isotope variations and their controlling factors during different seasonal periods [e.g., non-summer monsoon (NSM) and summer monsoon (SM)] in SEC.Results and Discussion: Our results show that the precipitation δ18O (δ18Op) and d-excess values in the SM season (−12.94‰–0.15‰, −4.05‰–21.01‰) were more light than those in the NSM season (−6.91‰–4.37‰, 0.85‰–30.38‰). Combining the findings of backward trajectory and averaged outgoing longwave radiation analyses, the seasonal variation of precipitation isotopes is believed to be determined by a shift in water vapor sources and the intensity of upstream atmospheric convection during water vapor transportation. The consistent variation between the isotope values and convective intensity over the South China Sea in the dragon boat rainy period highlights that δ18Op has the potential to respond rapidly to the upstream convective intensity and can serve as a substitute method for investigating the complicated East Asian summer monsoon system. Therefore, the results of this study imply that the stable isotopes in precipitation and related paleoclimate proxies may not reflect the signal of temperature or precipitation alone but rather reflect changes in moisture sources and upstream convective intensity.

Strong coupling of the East Asian summer monsoon and hydroclimate footprints during 53–47 ka

A multi-proxy (δ18O, δ13C and trace elements) analysis of an absolutely-dated stalagmite in northern China is conducted to reconstruct the East Asian summer monsoon (EASM) intensity and local hydroclimate conditions during the period of 53.8–47.4 ka (BP, before 1950 CE). Our δ18O and δ13C records, with 24-yr resolution, clearly register the Dansgaard-Oeschger (DO) events 14 and 13. A clear double-peak structure for DO13, as previously detected in the NGRIP ice core, is identified in our δ18O and δ13C records, for the first time. This strong correspondence at the finer time-scale indicates a sensitive and rapid response of the EASM system to the North Atlantic climate changes through air-sea coupling. Our δ18O record shows that the EASM intensity deceased abruptly during latter stage of DO14 (51.1–50.3 ka). This abrupt change is different from other stalagmite δ18O records from central and southern China. On the other hand, both the δ13C and trace element records, representing local hydroclimatic changes, match well with the δ18O record, reflecting an integrated EASM system, on the timescale of the DO cycles. The covariance suggests that the hydrological variation in northern China could be modulated by the movement of the large-scale EASM circulation. Further comparison displays an anti-correlated relationship between the millennial precipitation changes in northern and southeastern China during the period of 51.5–47.4 ka. This anti-correlation is analogous to the “North-South” dipole mode in observed present-day precipitation. Overall, this study advances our understanding of the spatial and temporal evolution of the EASM precipitation on the millennial timescale.

Alkane variation in peat reveals palaeohydrological changes since the Little Ice Age in eastern China

The East Asian summer monsoon (EASM) system plays a crucial role in regulating climatic and hydrological variability in East Asia. The hydrological effects of the EASM in eastern China during the Little Ice Age (LIA) remains controversial, however. Here, we established a relatively high-resolution palaeohydrological record over the last 700 years in East China via analyzing n -alkanes from a peat core. Mid- and long-chain (C 21 –C 25 and C 27 –C 31 ) n -alkanes account for more than 95% of the total odd‑carbon-numbered n -alkanes, with short-chain n -alkanes (C 17 –C 19 ) accounting for only 2.3% on average, suggesting that organic matter in the peat core originated primarily from vascular plants. Organic geochemical indices, including P aq (an indicator of contributions from aquatic plants), the average chain length (ACL) of long-chain n -alkanes, and C 31 /C 27 and C 23 /C 29 n -alkane ratios, suggest a transition from dry conditions during the middle LIA to wet conditions in the late LIA. Comparing P aq records from the study profile with previous records from the EASM region, we found that the northern and southern parts of eastern China showed contrasting changes in humidity during the LIA. Specifically, the southern part shifted from mid-LIA dry conditions to late-LIA wet conditions, whereas the northern part shifted from mid-LIA wet conditions to late-LIA dry conditions. This spatial heterogeneity in hydroclimate could be attributed to La Niña-like conditions and intense summer monsoons during the middle LIA and El Niño-like conditions and weak summer monsoons during the late LIA. Our study demonstrates the importance of understanding spatial heterogeneity of hydroclimatic changes to probe the mechanisms whereby EASM modulates regional climatic changes during the LIA. • We analyzed alkane proxies from a peatland in SE China to rebuild palaeohydrology. • Organic debris originate from a mixture of terrestrial and aquatic plants. • Alkanes reveal shifts from moderate wet to dry to wet conditions during the LIA. • Northern and southern parts of the EASM region show humidity contrast in the LIA. • This spatial heterogeneity in palaeohydrology may be linked to EASM and ENSO.

SST‐driven variability of the East Asian summer jet on a decadal time‐scale in CMIP6 models

AbstractThe East Asian summer jet (EASJ) is an important component of the East Asian summer monsoon system and its variability is correlated with precipitation and surface temperature variations over this region. Whilst many studies have considered the interannual variability of the EASJ, less is known about variations on a decadal time‐scale. This study investigates the relationship between decadal EASJ variability and sea surface temperatures (SSTs) and thus the potential predictability that SSTs may provide. Given the relatively short observational record, we make use of the long pre‐industrial control simulations in the Coupled Model Intercomparison Project phase 6 (CMIP6) in addition to a large ensemble of atmosphere‐only experiments, forced with random SST patterns. We then create an SST‐based reconstruction of the dominant modes of EASJ variability in the CMIP6 models, finding a median EASJ–reconstruction correlation for the dominant mode of 0.43. Much of the skill in the reconstruction arises from variations in Pacific SSTs, however the tropical Atlantic also makes a significant contribution. These findings suggest the potential for multi‐year predictions of the EASJ, provided that skilful SST forecasts are available.

Spatiotemporal patterns of synchronous heavy rainfall events in East Asia during the Baiu season

Abstract. Investigating the synchrony and interdependency of heavy rainfall occurrences is crucial to understand the underlying physical mechanisms and reduce physical and economic damages by improved forecasting strategies. In this context, studies utilizing functional network representations have recently contributed to significant advances in the understanding and prediction of extreme weather events. To thoroughly expand on previous works employing the latter framework to the East Asian summer monsoon (EASM) system, we focus here on changes in the spatial organization of synchronous heavy precipitation events across the monsoon season (April to August) by studying the temporal evolution of corresponding network characteristics in terms of a sliding window approach. Specifically, we utilize functional climate networks together with event coincidence analysis for identifying and characterizing synchronous activity from daily rainfall estimates between 1998 and 2018. Our results demonstrate that the formation of the Baiu front as a main feature of the EASM is reflected by a double-band structure of synchronous heavy rainfall with two centers north and south of the front. Although the two separated bands are strongly related to either low- or high-level winds, which are commonly assumed to be independent, we provide evidence that it is rather their mutual interconnectivity that changes during the different phases of the EASM season in a characteristic way. Our findings shed some new light on the interplay between tropical and extratropical factors controlling the EASM intraseasonal evolution, which could potentially help to improve future forecasts of the Baiu onset in different regions of East Asia.

Holocene Moisture Variability and Its Lagged Response to Summer Insolation Revealed by Eolian Deposits in the Lake Qinghai Basin, NE Tibetan Plateau

AbstractThe evolution of the East Asian Summer Monsoon (EASM) and its mechanism since the Holocene remains controversial. Here, we investigate four eolian sediment sequences in the Lake Qinghai Basin (LQB), Northeastern Tibetan Plateau (NETP), in the marginal zone of the modern EASM. The results indicate that paleoclimatological interpretation of the color parameters and CaCO3 in eolian sand‐paleosol sequence from LQB region are different from the loess‐paleosol sequence in Loess Plateau, and a comprehensive analysis that combines lithostratigraphic and other proxy data is required. Changes in the EASM dominated climate change in the NETP, with a gradual increase in moisture from the beginning of the early‐Holocene, however the environment was relatively cold and dry with strong regional eolian sand activity. During 8.5–3.3 ka, the intensity of the EASM and effective moisture levels in the LQB reached a peak. Eolian activity was weak and dunes were effectively fixed in place, while pedogenesis processes were enhanced and paleosols developed widely across the region. Moisture levels decreased after 3.3 ka and the dunes were reactivated, eolian activity was enhanced, and there was extensive accumulation of eolian sand and sandy loess. These records show that moisture variability has been controlled by the EASM system in east Asia since at least the Holocene, and has generally responded to changes in northern hemisphere summer insolation with a lag of 3–5 ka, through mechanisms that are modulated by high‐latitude ice‐volume forcing and large‐scale ocean‐atmospheric circulation patterns.

How Do Extreme Summer Precipitation Events Over Eastern China Subregions Change?

AbstractIn this study, observations from the CN05.1 daily data set and K‐means clustering were used to capture spatiotemporal characteristics of extreme precipitation events (EPEs) during summer over eastern China subregions from 1961 to 2018. Five subregions including South China (SC), the Yangtze River Basin (YRB), the HeTao Area (HTA), North China (NC), and Northeast China (NEC) are identified, which is distinct from previous studies. There were significant rates of EPEs transfer from the YRB‐type to SC‐type (18%), HTA‐type to YRB‐type (16%), HTA‐type to NC‐type (22%), and NC‐type to NEC‐type (25%), and the associated processes of synoptic evolution are addressed. Within these types of regional EPEs, intrinsic relationships exist that have not been recorded by previous studies. The intraseasonal evolution of summer EPEs shows a northward migration of the rainbelt influenced by the East Asian summer monsoon system. Moreover, temporal variations of regional EPEs from interannual timescales to long‐term trends are examined.

Formation of the northern East Asian low: role of diabatic heating

The northern East Asian low (NEAL), characterized by a mid-latitude closed low (or trough) in the lower troposphere, is an important component of the East Asian summer monsoon system. This study investigates formation mechanism of the summer NEAL, with emphasis on the roles of diabatic heating and transient eddy forcing, using a linear baroclinic model (LBM) with prescribed forcing derived from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) dataset during the 1979–2010 period. The LBM simulations show that the formation of NEAL is induced by the diabatic heating, and the effect of transient eddy forcing is negligible. Further investigation indicates that the diabatic heating over northern East Asia (NEA) is the key factor for the NEAL’s formation, in which the latent heating and radiative cooling in concert to play a major role and the vertical diffusion sensible heating plays a secondary role. In response to the NEA diabatic heating (latent and sensible heating), a local northerly is induced that advects cold air to partially balance the diabatic heat source and forms a low to the east. On the other hand, the radiative cooling induces a southerly anomaly over NEA, shaping the position and intensity of the low induced by the latent and sensible heating. The total effect of the diabatic heating and radiative cooling causes a closed NEAL. Results are similar for the NEAL’s formation in June, July, and August, respectively. Moreover, we further investigate sub-seasonal change of the NEAL in summer. The weakened intensity of the NEAL in August, relative to that in June and July, is contributed to mainly by the enhanced offset effect of the radiative cooling over NEA and partially by the weakened impact of the latent and diffusion sensible heating.

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.

Comparison on Relationship between Western Pacific Subtropical High and Summer Precipitation over Dongting Lake Basin Based on Different Datasets

The Western Pacific Subtropical High (WPSH) is a principal component of the East Asian summer monsoon system. This study was to compare the relationship between the WPSH and summer precipitation in Dongting Lake basin (24°35′-30°27′N, 107°13′-114°18′E) based on different datasets by employing the multivariate regression analysis. The observed precipitation data was from China Meteorological Data (CMD) sharing service system and the six gridded precipitation datasets were CFSR, ERA–Interim, CMAP, GPCP, CRU, and PREC/L, respectively. The WPSH exerted different influences on the precipitation of summer months. The indexes including westward extension ridge point index and northern boundary index in June, ridge line index and northern boundary index in July and August had important effect on the precipitation over Dongting Lake Basin. The gridded data could generally reproduce the relationship between the ridge line index and precipitation in July. The relationship between the northern boundary and the precipitation from gridded data was similar to that from observed data in June and July, while the relationship between the western extension ridge index and the precipitation in June could be reproduced by the gridded data except CFSR and ERA-Interim. Only in part area, the relationship between the intensity index and the CMD precipitation could be reproduced by gridded precipitation. The fitness of correlation in some area from the CMD could be reproduced by the gridded data. General speaking, GPCP precipitation data was more applicable to the Dongting Lake Basin in studying on the relationship between the precipitation and a single index of WPSH.

Predictability of East Asian summer monsoon in seasonal climate forecast models

AbstractThe prediction skill and source of the predictability of the East Asian summer monsoon (EASM) system are examined in this work based on four state‐of‐the‐art seasonal climate forecast models including BCC_CSM1.1, ECMWF_SYS4, NCEP_CFS2 and TCC_CPS2. The prediction of the climatology and interannual EASM pattern and the impact on the prediction are further investigated. It is noted that the four models have some skill in predicting summer rainfall in the East Asia, however, the skill is low on average and also largely regional dependence. The interannual variation of EASM measured by monsoon circulation index is well reproduced, implying that the broad‐scale feature/pattern of EASM has higher predictability than the detailed spatial variation of EASM rainfall. The possible sources of predictability of the interannual variability of EASM are associated with the El Niño‐Southern Oscillation (ENSO) and the north Indian Ocean (NIO) sea surface temperature (SST) anomalies. The correlation pattern of rainfall with the NIO SST is characterized by a tripole pattern from south to north of East Asia, which is different from the correlation distribution of the southern‐northern dipole with ENSO, suggesting that NIO SST may exert influence on the EASM independently. The major biases in climatology of EASM in the models are the northward shift of the western Pacific subtropical high (WPSH) and weak monsoonal southerly over the coast of East Asia, which leads to the prediction bias of the Meiyu/Baiu/Changma (MBC) rainfall belt. The prediction of the interannual EASM pattern presents two deficiencies: too weak rainfall variability and northward shift of the dipole rainfall pattern (opposite variation between MBC and the northwestern Pacific), that may be caused by the biases of WPSH in the models.

Intermodel Diversity in the Zonal Location of the Climatological East Asian Westerly Jet Core in Summer and Association with Rainfall over East Asia in CMIP5 Models

The East Asian westerly jet (EAJ), an important midlatitude circulation of the East Asian summer monsoon system, plays a crucial role in affecting summer rainfall over East Asia. The multimodel ensemble of current coupled models can generally capture the intensity and location of the climatological summer EAJ. However, individual models still exhibit large discrepancies. This study investigates the intermodel diversity in the longitudinal location of the simulated summer EAJ climatology in the present-day climate and its implications for rainfall over East Asia based on 20 CMIP5 models. The results show that the zonal location of the simulated EAJ core is located over either the midlatitude Asian continent or the western North Pacific (WNP) in different models. The zonal shift of the EAJ core depicts a major intermodel diversity of the simulated EAJ climatology. The westward retreat of the EAJ core is related to a warmer mid-upper tropospheric temperature in the midlatitudes, with a southwest-northeast tilt extending from Southwest Asia to Northeast Asia and the northern North Pacific, induced partially by the simulated stronger rainfall climatology over South Asia. The zonal shift of the EAJ core has some implications for the summer rainfall climatology, with stronger rainfall over the East Asian continent and weaker rainfall over the subtropical WNP in relation to the westward-located EAJ core.

Physical–Statistical Model for Summer Extreme Temperature Events over South Korea

AbstractExtreme temperature events have a significant impact on human life and property. Since the Korean Peninsula is affected by the high variability of the East Asian summer monsoon system, it is difficult to predict extreme temperature events skillfully. Here, we construct an empirical model to investigate the interannual variation of the frequency of summer extreme temperature events over South Korea by identifying predictors (explanatory variables) from ocean boundary conditions. The selected explanatory variables are sea surface temperature anomalies (SSTAs) over the North Atlantic, the western North Pacific, and the eastern North Pacific. The cross-validated correlation skill of the statistical model constructed using a 23-yr dataset is estimated to be 0.77. A common feature that all three explanatory variables contain is the development of an anticyclonic circulation anomaly over the Korean Peninsula. The North Atlantic SSTA predictor acts as a forcing mechanism for the generation of Rossby wave trains downstream, developing an anticyclonic circulation anomaly in the lower and upper troposphere over the Korean Peninsula. The western North Pacific (WNP) warm SSTA predictor induces a cyclonic circulation anomaly over the WNP and an anticyclonic circulation anomaly over the Korean Peninsula, resembling the Pacific–Japan teleconnection mechanism that represents the northward Rossby wave propagation over the western Pacific. Through air–sea interaction, the tripolar SSTA pattern in the eastern North Pacific representing the North Pacific gyre oscillation induces two opposite precipitation anomalies in the equatorial Maritime Continent and the Philippine Sea. These diabatic anomalies excite northward-propagating Rossby waves that form a cyclonic circulation anomaly in the WNP area and an anticyclonic anomaly over the Korean Peninsula.

Observations on changes in Korean Changma rain associated with climate warming in 2017 and 2018

The period of prolonged summer rain in Korea called Changma was studied. Changma, also known as Meiyu in China and Baiu in Japan, is an integral part of the East Asian Summer Monsoon System. Until the 1980s, the Changma rainy season in Korea occurred from late June and July for 30 to 40 days or approximately 4 to 5 weeks. This study investigated whether changes have been observed in the timing and amounts of precipitation associated with Changma. From the analysis of meteorological data obtained in 2017 and 2018, it is observed that the length of the rainy season has shortened to 2 to 3 weeks in agreement with the earlier studies. Furthermore, during the rainy season, there were many days of no rain and partly cloudy days. Although an active elongated-linear Changma front was common in the past, it was found that an inactive Changma front with a large meridional amplitude has caused intermittent rain showers for several hours from linear convective cloud streaks. In recent years, this has produced large variations in rainfall amounts among regional measuring stations in Korea. Climate warming in the north side of the Changma front has resulted in less contrast with the warm-moist air in the south side of it from the Pacific Ocean. This has resulted in a weaker and inactive quasi-stationary front which has caused a discontinuous broken Changma front and sporadic showery days. It is also observed that maritime polar air mass, mP, over the Okhotsk Sea of far eastern Russia has not affected the Changma front over the Korean Peninsula. Overall, this study found that the characteristics of past Changma fronts have changed significantly in recent years. The present observations imply the need of further studies on climate change and summer rainfall including water management.

A modeling study of the influence of initial soil moisture on summer precipitation during the East Asian summer monsoon

The state-of-the-art WRF model is used to investigate the impact of the antecedent soil moisture on subsequent summer precipitation during the East Asian summer monsoon (EASM) period. The control experiment with realistic soil moisture condition can well reproduce the seasonal pattern from low- to high- atmosphere, as well as the spatial distribution of precipitation belt in East China. Compared with the control experiment, the sensitivity experiment in which the initial soil moisture is reduced generates more precipitation along the East China Sea, and less rainfall over both Central and South China. This suggests that the effect of initial soil moisture on monsoonal precipitation in East China is regionally dependent. The influence on precipitation is mostly attributed to the change in precipitation from mid July to late August. The initial soil moisture condition plays a role in changing the seasonal pattern and atmospheric circulation due to the weak heating and geopotential gradient, leading to a reduction in southeasterly flow and moisture flux from South China Sea. The changes between DRY and CTL runs result in reduced southerly wind over the ocean (south of ˜25 °N) and enhanced northerly wind over the land (north of ∼25 °N). The temperature and associated circulation changes due to drier initial soil moisture anomaly result in reduced southerly winds over East China, and therefore a weakened EASM system. The averaged moisture flux decreases significantly over Central China but increases along the East China Sea. In addition, the drier soil moisture perturbation exerts an effect on suppressing (enhancing) vertical velocity over Central China (along the East China Sea), thus leading to more (less) cloud water and rain water. Therefore, the influence of soil moisture exerts an opposite impact on surface precipitation between these two regions, with more and less accumulation rainfall in Central China and along the East China Sea, respectively.

Role of tropical cyclones over the western North Pacific in the East Asian summer monsoon system

Precipitation observations collected at weather stations in eastern China, the NCEP/NCAR reanalysis data, the tropical cyclone (TC) Best Track Dataset, and a sensitivity numerical experiment were used in the present study to investigate the role in the East Asian summer monsoon (EASM) system played by frequent TC activities over the western North Pacific (WNP). Results indicated that, in active TC years, the EASM is stronger and the southerly winds in the lower troposphere advance farther north and reach higher latitudes. Meanwhile, the monsoon rain belt remains in the lower and middle reaches of the Yangtze River valley for a relatively short period, leading to less precipitation there. Both the western Pacific subtropical high and the South Asian high weaken with the northward shift of the ridgelines for both high-pressure systems as well as the East Asian subtropical upper-level jet. Therefore, the impacts of frequent TC activities over the WNP on each individual component of the EASM are in phase with those of the stronger EASM itself, amplifying features of the already strengthened EASM.

Improving the simulation of East Asian summer monsoon with mesoscale enhancement in an AGCM

In this study, the parameterization of sea surface turbulent heat flux in National Center for Atmospheric Research Community Atmosphere Model version 5 (CAM5) is improved by including the contribution of mesoscale convective motion (i.e., mesoscale enhancement). The effect of mesoscale enhancement on the simulation of the East Asian summer monsoon circulation is further explored with SST-forced experiments that are conducted for the period of January 1979 to December 2015. Experiments suggest that the standard CAM5 exhibits evident biases in simulating the East Asian summer monsoon system. For example, the simulated westerlies at 850 hPa in low latitude of East Asia are too weak, the precipitations in the Bay of Bengal, the South China Sea, and the tropical western Pacific are underestimated, the simulated south Asia and western Pacific subtropical highs are too weak, and the upper and middle troposphere in the low latitudes of East Asia is too cold. By including the mesoscale enhancement in CAM5, these biases are clearly reduced. For example, the simulated summer mean precipitation in the Bay of Bengal is increased from 9.85 to 11.76 mm/day, which is closer to the observed value of 13.80 mm/day. Further analyses show that the improvements in the East Asian summer monsoon precipitations and circulations can be attributed to the enhancement of convections and the increase in water evaporation in the tropical western Pacific Ocean induced by mesoscale enhancement. Correspondingly, the latent heat due to vapor condensation in the atmosphere also increases, which reduces the cold bias in the mid-upper troposphere. Meanwhile, the geopotential height of the upper-middle-level atmosphere is elevated, which makes the simulated intensity of the south Asian and the western Pacific subtropical highs, two important components of the East Asian summer monsoon circulation, closer to the reality.