East Asian Subtropical Summer Monsoon Research Articles

Combined Modification of Urbanization and Monsoon in Meiyu Precipitation Changes in the Megacity Shanghai, China

The Meiyu season is a typical rainy season in East Asia that is controlled by summer monsoon. Despite extensive research on its impact, it is unclear how urbanization modifies precipitation during the Meiyu season in the background of the monsoon influence. To address this gap, this study investigated the effects of urbanization and monsoon on the modification of precipitation during the Meiyu season (PDM) in the megacity of Shanghai, China. Through homogenization analysis of the original observational data, we assessed the temporal and spatial variation in PDM in Shanghai during two stages of urbanization. Our findings revealed that both total precipitation and extreme daily precipitation during the Meiyu season in Shanghai have significantly increased since 1961. The spatial heterogeneity of PDM has also enhanced during the rapid urban process that has occurred since 1986. The long-term trend of increasing precipitation in Shanghai showed a synchronous variation with the East Asian subtropical summer monsoon (EASM) in 1961–2021. Over the interannual time scale, the significant positive correlation between PDM and EASM during the slow urbanization period (Stage 1: 1961–1985) changed to a non-significant correlation during the rapid urbanization period (Stage 2: 1986–2021), which was associated with the enhanced convective precipitation in Shanghai during the Meiyu season. Urbanization induced more convective precipitation and further weakened the association between PDM and EASM over the central city and nearby areas during Stage 2. The rapid urbanization process also resulted in increased differences in near-surface wind between urban and non-urban areas, which facilitated more PDM over the central city due to the urban friction effect and wind shear in Stage 2. Furthermore, our analysis suggests that the increase in precipitation may be associated with the enhanced coupling of cold air intrusion with the warmer climate background due to the urban heat effect occurring in Stage 2. These findings contribute to a better understanding of how urbanization and monsoons affect PDM in East Asian megacities and serve as a unique reference for climate prediction in this region.

A Diagnostic Study of the Influence of Early Spring Soil Moisture in Southeastern China on Interannual Variability of the East Asian Subtropical Summer Monsoon Onset

A Diagnostic Study of the Influence of Early Spring Soil Moisture in Southeastern China on Interannual Variability of the East Asian Subtropical Summer Monsoon Onset

The Spring Heat Source Over the Qinghai–Tibetan Plateau Linked With the Winter Warm Arctic–Cold Siberia Pattern Impacting Summer Drought in China

The atmospheric heat source over the Qinghai–Tibetan Plateau (QTP) in spring has an important impact on the climate of the surrounding regions. However, there have been few systematic studies of the dominant mode of the heat source and the cross-seasonal connections with the preceding winter and following summer. Using a distinct empirical orthogonal function (DEOF) decomposition method, we obtained the leading mode of the spring heat source over the QTP and the surrounding regions and analyzed its precursors in the previous winter and lagging effects in the following summer. Our results show that the first mode (DEOF1) was characterized by a warm plateau and cold surrounding regions. The positive phase was significantly associated with the warm Arctic–cold Siberia (WACS) pattern (r = 0.39, p = .01) and the La Niña-like SST anomaly in the Pacific in the preceding winter and the following East Asian subtropical summer monsoon (r = –0.44, p = .01), resulting in a widespread drought in China during the following summer. The cold anomaly in Siberia and the warm anomaly at mid-to low latitudes in winter associated with the WACS pattern coincide with the DEOF1 mode of the heat source over the QTP and its surroundings through change of meridional temperature gradient and wave-flow interactions. A mid-latitude wave train excited by the WACS and the thermal difference in the meridional direction of the spring DEOF1 mode caused high-pressure anomalies over the QTP and the mid-latitude region of East Asia, influencing central and eastern China. This anomaly was not conducive to the northward advancement of the East Asian summer monsoon, resulting in drought in most of China in spring and summer. The cross-seasonal relationship between the main mode of the spring heat source on the QTP and the preceding winter WACS pattern and the following East Asian summer monsoon can be used as a reference in climate prediction studies.

Middle Eocene East Asian monsoon prevalence over southern China: Evidence from palynological records

Abstract The initiation of Asian monsoon systems is a complex and controversial issue, and the existence of monsoon systems in the Eocene has not yet been confirmed by commonly applicable proxy data. Eocene palynological records from three basins in southern China provide an ideal opportunity to investigate the early stage of the Asian monsoon due to their unique geographical location. The palynological assemblages suggest that the middle Eocene vegetation in southern China was composed of mixed evergreen and deciduous broad-leaved forests, accompanied by abundant subtropical to tropical evergreen taxa, thus bearing strong resemblance to the modern vegetation in southern China, and indicating a warm-humid subtropical climate. By analysing the palynological data using a coexistence approach (CA), we quantitatively reconstructed seven climatic parameters, which indicate a subtropical monsoon climate with a mean annual temperature (MAT) in the range 16.5 to 23.5 °C and a mean annual precipitation (MAP) in the range 1035 to 1724 mm, similar to those of the modern subtropical monsoon climate in southern China although the MAT was surprisingly lower than present. Significant seasonal variations in precipitation and temperature and as well as the spatial distribution of MAP during the middle Eocene were most similar to those of the modern East Asia Monsoon (EAM) climate of southern China, clearly indicating that the Eocene climate of southern China was mainly influenced by the EAM, rather than the Indian monsoon (IM) or Inter-tropical Convergence Zone (ITCZ) monsoons. By combining the above results with previously published geological evidence, we infer that the EAM developed in southern China as early as the middle Eocene, albeit with an intensity that may have been weaker than present. The establishment of the middle Eocene EAM in southern China was most likely driven by the northward drift of the Indian Subcontinent and its collision with the Eurasian continent, associated with the uplift of the Central-Southern Tibetan Plateau, which enlarged the land-sea thermal contrast and deepened the low pressure over continental Asia in summer, thus greatly intensifying the East Asian subtropical summer monsoon over southern China.

Relationship over southern China between the summer rainfall induced by tropical cyclones and that by monsoon

This study investigates the relationship between tropical cyclone–induced precipitation (P TC) and summer monsoon–induced precipitation (P SM) in southern China (SC) during June–August. The spatial patterns of the first interannual mode are uniform in sign over SC, with positive anomalies for P TC and negative anomalies for P SM. The background of an increase in cyclonic vorticity, an increase in RH, and a decrease in vertical wind shear over the South China Sea (SCS)–western north Pacific (WNP) provides favorable conditions for more TC genesis. The positive equatorial central Pacific SST anomaly and negative North Indian Ocean SST anomaly contribute to the anomalous cyclone over the SCS–WNP, which causes decreasing P SM in SC together with an anomalous anticyclone over eastern China–Japan. By contrast, whilst the spatial patterns of the first interdecadal mode are also uniform in sign over SC, there are positive anomalies for both P TC and P SM. The first interdecadal principal component features significant positive correlation with the number of TCs forming in the SCS. There is a significant increase in P TC and P SM after early 1990s. A positive tropical Indian Ocean (TIO) SST anomaly persists from the preceding winter to summer. During the preceding winter and spring after the early 1990s, a positive western Pacific SST anomaly can result in TIO SST warming through vertical circulation. Then, the positive TIO SST anomaly triggers an anomalous WNP anticyclone and contributes to the interdecadal increase in SC P SM in the succeeding summer. The persistent heating source over SC from May to summer related to an earlier onset of the SCS monsoon may strengthen the East Asian subtropical summer monsoon.

The East Asian subtropical summer monsoon: Recent progress

The East Asian subtropical summer monsoon (EASSM) is one component of the East Asian summer monsoon system, and its evolution determines the weather and climate over East China. In the present paper, we firstly demonstrate the formation and advancement of the EASSM rainbelt and its associated circulation and precipitation patterns through reviewing recent studies and our own analysis based on JRA-55 (Japanese 55-yr Reanalysis) data and CMAP (CPC Merged Analysis of Precipitation), GPCP (Global Precipitation Climatology Project), and TRMM (Tropical Rainfall Measuring Mission) precipitation data. The results show that the rainy season of the EASSM starts over the region to the south of the Yangtze River in early April, with the establishment of strong southerly wind in situ. The EASSM rainfall, which is composed of dominant convective and minor stratiform precipitation, is always accompanied by a frontal system and separated from the tropical summer monsoon system. It moves northward following the onset of the South China Sea summer monsoon. Moreover, the role of the land–sea thermal contrast in the formation and maintenance of the EASSM is illustrated, including in particular the effect of the seasonal transition of the zonal land–sea thermal contrast and the influences from the Tibetan Plateau and midlatitudes. In addition, we reveal a possible reason for the subtropical climate difference between East Asia and East America. Finally, the multi-scale variability of the EASSM and its influential factors are summarized to uncover possible reasons for the intraseasonal, interannual, and interdecadal variability of the EASSM and their importance in climate prediction.

The impact of the direct effects of sulfate and black carbon aerosols on the subseasonal march of the East Asian subtropical summer monsoon

AbstractAerosol emissions have rapidly increased in East Asia since the late 1970s. During the same period, the East Asian summer monsoon has shown a weakening trend. In this work, the direct effects (DE) of sulfate and black carbon (BC) aerosols on the subseasonal (pentad mean) march of the East Asian subtropical summer monsoon (EASSM) are investigated using an interactive global climate‐chemistry model. The simulation results suggest that the DE of sulfate aerosols have a notable effect on the cooling of the low troposphere across the continent in spring and autumn, hence, changing the time of the seasonal transition of the zonal land‐sea thermal contrast (ZTC). The DE of BC result in cooling of the low troposphere and heating of the middle troposphere, leading to a different impact than that caused by sulfates. The cooling of the surface and troposphere by sulfates leads to a delay in the warming of East Asian continent in spring and the EASSM onset time; it also accelerates the process of the continent turning colder and advances the retreat of the EASSM. The deeper heating in the middle‐upper troposphere than the cooling in the low troposphere due to the DE of BC or the combination of both lead to an advance in the onset time of the monsoon caused by the continent turning warmer earlier in spring. In autumn, the same cooling effect by sulfates leads to the continent turning colder earlier, resulting in an advance in the retreat time.

Meridional Propagation of the 30- to 60-day Variability of Precipitation in the East Asian Subtropical Summer Monsoon Region: Monitoring and Prediction

ABSTRACTThe meridional propagation of the 30- to 60-day intraseasonal variability (ISV) of precipitation in the East Asian subtropical summer monsoon (EASSM) region and its monitoring and prediction are investigated in the current study. Based on a multivariate empirical orthogonal function (MV-EOF) analysis of precipitation and relative vorticity at 700 hPa in East Asia, a bivariate index referred to as the EASSM-ISV index is designed using the two leading MV-EOF modes, with the objective of real-time monitoring of the 30- to 60-day variability of precipitation in the EASSM region. It is found that this index, with its eight phases, can explain the meridional propagation of the 30- to 60-day ISV in precipitation and circulation in the EASSM region. Based on a singular value decomposition technique, a statistical forecast model is developed in which the EASSM-ISV indices from the preceding five pentads are used to predict the indices in five pentads in the future. Meanwhile, the indices are used to predict the meridional propagation of the 30- to 60-day precipitation anomaly in the EASSM region. This model thus provides a useful tool for intraseasonal prediction of precipitation during the rainy season in China.

Correlation between the Onset of the East Asian Subtropical Summer Monsoon and the Eastward Propagation of the Madden–Julian Oscillation

Abstract Using the daily outgoing longwave radiation (OLR), the pentad Climate Prediction Center Merged Analysis of Precipitation (CMAP), and the 6-h Climate Forecast System Reanalysis (CFSR) dataset from 1979 to 2010, a composite analysis along with space–time wave filtering is performed to examine the linkage between the Madden–Julian oscillation (MJO) and the onset of the East Asian subtropical summer monsoon (EASSM) (over 20°–30°N, 110°–120°E). The onset of the EASSM is shown to be best characterized by the reversal of the mean meridional wind shear related to the rapid reestablishment of the South Asian high (SAH) over the southern Indochinese Peninsula in the upper troposphere. The mean date of EASMM onset is near the end of April, which is about a month earlier than the typical onset of the East Asian summer monsoon. Further analysis indicates that the onset of the EASSM and the reestablishment of SAH are often associated with the arrival of the wet phase of the tropical MJO over the central and eastern Indian Ocean.

Characteristics and evolution of atmospheric circulation patterns during Meiyu over the Jianghuai valley

This study focuses on the evolution of large-scale circulations before and after the beginning of the Meiyu and analyzes the formations of the typical vertical circulation pattern associated with Meiyu and its relationship with the Meiyu rainband. Results show that the typical vertical circulation pattern during the Meiyu season is characterized by the “two-leg”-type pattern in the vorticity field, a typhoon eye-like structure of the equivalent potential temperature field, as well as the sharp gradient of the equivalent potential temperature zone, i.e., the Meiyu front are typically presented in the Meiyu season. Tracking its evolution process, we find that the typical vertical circulation pattern is built at late March and early April along with the rainband locating at the area south to the Yangtze River. This typical pattern and the rainband both advance northward affecting Jianghuai valley since the beginning of Meiyu. Moreover, the typical vertical circulation pattern derived from Meiyu season has been formed in April and corresponds to the reverse of the land-sea thermal contrast between the Eastern Asia and western Pacific Ocean, demonstrating the close relationship of the movement between the rainband and the march of the East Asian subtropical summer monsoon.

Onset of East Asian subtropical summer monsoon and rainy season in China

Here we use harmonic analyses to examine seasonal variations of China land rainfall, low-level winds, and atmospheric heating over East Asia during spring to summer and the associated subtropical summer monsoon activities. Our results indicate that the South China spring rainfall (SCSR) in March is the prophase of East Asian sub-tropical summer monsoon (EASSM), and the onset of EASSM and China summer rainy season starts in early April, characterized by the enhanced rainfall in South China and the seasonal reverse of zonal land-sea thermal contrast in sub-tropical East Asia. The EASSM onset is earlier than that of South China Sea summer monsoon, and it is active in east of 100°E and north of 20°N. Our analyses suggest that the subsequent heating appears over India-China Peninsula in March and South China in April and causes the low-level atmospheric warming and the zonal land-sea thermal contrast seasonal reverse in East Asian subtropics. The atmospheric heating over South China is the main force to drive the southwesterly winds, updrafts and strengthen the summer precipitation in South China.

Seasonal cycle of the zonal land-sea thermal contrast and East Asian subtropical monsoon circulation

Based on analysis of the climatic temperature latitudinal deviation on middle troposphere, its seasonal cycle suggests that due to the rapid warming from eastern China continent to the east of Tibetan Plateau and the heating of Tibetan Plateau in spring, seasonal transition of the thermal difference between East Asia continent and West Pacific first takes place in the subtropical region with greatest intensity. On the accompanying low troposphere, the prevailing wind turns from northerly in winter to southerly in summer with the convection precipitation occurring at the same time. This maybe indicates the onset of the East Asian subtropical summer monsoon. Consequently, we advice that the seasonal cycle formed by the zonal thermal contrast between Asian continent and West Pacific may be an independent driving force of East Asian subtropical monsoon.

Vertical circulation structure,interannual variation features and variation mechanism of western pacific subtropical high

The paper investigates the vertical circulation structure of the western Pacific subtropical high (STH) and its interannual variation features in relation to East Asian subtropical summer monsoon and external thermal forcing by using the high-resolution and good-quality observations from the 1998 South China Sea Summer Monsoon Experiment (SCSMEX), the NCEP 40-year reanalysis data and relevant SST and the STH parameters. It is found that the vertical circulation structures differ greatly in features between quasi-stationary and transient components of the western Pacific STH. When rainstorms happen in the rainband of East Asian subtropical monsoon on the STH north side, the downdrafts are distinct around the ridge at a related meridian. The sinking at high (low) levels comes from the north (south) side of the STH, thereby revealing that the high is a tie between tropical and extratropical systems. The analyses of this paper suggest that the latent heat release associated with subtropical monsoon precipitation, the offshore SST and East Asian land-sea thermal contrast have a significant effect on the STH interannual anomaly. Our numer-ical experiment shows that the offshore SSTA-caused sensible heating may excite an anomalous anticyclonic circulation on the west side, which affects the intensity (area) and meridional position of the western Pacific STH.