Seasonal Transition of Summer Rainy Season over Indochina and Adjacent Monsoon Region

The mean onset and withdrawal of summer rainy season over the Indochina Peninsula were investigated using 5-day averaged rainfall data (1975-87). The mean seasonal transition process during onset and retreat phases in Indochina, India and the South China Sea is also examined using 5-day mean OLR (1975-87) and 850 hPa wind (1980-88) data. It was found that the onset of summer rainy season begins earlier in the inland region of Indochina (Thailand) in late April to early May than in the coastal region along the Bay of Bengal. This early onset of rainy season is due to pre-monsoon rain under the mid-latitude westerly wind regime. The full summer monsoon circulation begins to establish in mid-May, causing active convective activity both over the west coast of Indochina and the central South China Sea. In case of withdrawal, the earliest retreat of summer rainy season is found in the central northern part of Indochina in late September. The wind field, on the other hand, already changes to easterlies in the northern South China Sea in early September. This easterly wind system covers the eastern part of Indochina where post-monsoon rain is still active. In late October, the wind field turns to winter time situation, but post monsoon rain still continues in the southern part of the Indochina Peninsula until late November.

Seasonal Transition Features of Large-Scale Moisture Transport in the Asian-Australian Monsoon Region

Using NCEP/NCAR reanalysis data for the period of 1957-2001, the climatological seasonal transition features of large-scale vertically integrated moisture transport （VIMT） in the Asian-Australian monsoon region are investigated in this paper. The basic features of the seasonal transition of VIMT from winter to summer are the establishment of the summertime ＂great moisture river＂ pattern （named the GMR pattern） and its eastward expansion, associated with a series of climatological events which occurred in some ＂key periods＂, which include the occurrence of the notable southerly VIMT over the Indochina Peninsula in mid March, the activity of the low VIMT vortex around Sri Lanka in late April, and the onset of the South China Sea summer monsoon in mid May, among others. However, during the transition from summer to winter, the characteristics are mainly exhibited by the establishment of the easterly VIMT belt located in the tropical area, accompanied by some events occurring in ＂key periods＂. Further analyses disclose a great difference between the Indian and East Asian monsoon regions when viewed from the meridional migration of the westerly VIMT during the seasonal change process, according to which the Asian monsoon region can be easily divided into two parts along the western side of the Indochina Peninsula and it may also denote different formation mechanisms between the two regions.

SEASONAL TRANSITION OF EAST ASIAN SUBTROPICAL MONSOON AND ITS POSSIBLE MECHANISM

The NCEP/NCAR reanalysis datasets and Climate Prediction Center(CPC) Merged Analysis of Precipitation(CMAP) rain data are used to investigate the large scale seasonal transition of East Asian subtropical monsoon(EASM) and its possible mechanism.The key region of EASM is defined according to the seasonal transition feature of meridional wind.By combining the 'thermal wind' formula and the 'thermal adaptation' equation,a new 'thermal-wind-precipitation' relation is deduced.The area mean wind directions and thermal advections in different seasons are analyzed and it is shown that in summer(winter) monsoon period,the averaged wind direction in the EASM region varies clockwise(anticlockwise) with altitude,and the EASM region is dominated by warm(cold) advection.The seasonal transition of the wind direction at different levels and the corresponding meridional circulation consistently indicates that the subtropical summer monsoon is established between the end of March and the beginning of April.Finally,a conceptual schematic explanation for the mechanism of seasonal transition of EASM is proposed.

Alterations of Intracellular Ca^(2+) Concentration and Ultrastructure in Spruce Apical Bud Cells during Seasonal Transition

Potassium antimonite was used to localize Ca2+ in the apical bud cells of spruce from July 1999 to May 2000. During the period of active growth (July 14), Calcium precipitates, an indication of Ca2+ localization, were mainly distributed in vacuoles, intercellular spaces and cell walls. Few Ca2+ deposits localized in the cytosol and nucleus, showing a low level of the cytosolic and nuclear Ca2+ concentration in the warm summer. In August, some Ca2+ deposits appeared in the cytosol and nuclei, indicating that Ca2+ influx occurred in the cytosol and nucleus as the day length became shorter. From September to November, high levels of the cytosolic and nuclear Ca2+ remained. During the mid-winter (December and January), the distribution of Ca2+ deposits and the ultrastructures in the cells were altered dramatically. Plasmolysis occurred in many cells due to the protoplasmic dehydration. In addition plasmalemma invagination and nuclear chromatin aggregation also occurred. A large number of Ca2+ deposits appeared in the space between the plasmalemma and the cell wall. And also some Ca2+ deposits were distributed in the plastids. However, few Ca2+ deposits were observed in the cytosol and nuclei. By spring of the next year (May), when plants were de-acclimated and resumed active growth, Ca2+ subcellular localization essentially restored to that observed in July of the last year, i.e., the cells contained low cytosolic and nuclear Ca2+ concentrations; Ca2+ deposits were mainly distributed in the vacuoles, cell walls and intercellular spaces. The relationships between the seasonal changes of intracellular Ca2+ concentration and the development of dormancy/cold acclimation, as well as plasmolysis associated with dormancy and cold hardiness were discussed.

CHARACTERISTICS OF MIDDLE EAST JET STREAM DURING SEASONAL TRANSITION AND ITS RELATION WITH INDIAN SUMMER MONSOON ONSET

By using the NCEP/NCAR pentad reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream(MEJS) and its thermal mechanism during seasonal transition are studied. Results show that the intensity and south-north location of MEJS center exhibit obvious seasonal variation characteristics. When MEJS is strong, it is at 27.5°N from the 67 th pentad to the 24 th pentad the following year; when MEJS is weak, it is at 45°N from the 38 th pentad to the 44 th pentad. The first Empirical Orthogonal Function(EOF) mode of 200-hPa zonal wind field shows that MEJS is mainly over Egypt and Saudi Arabia in winter and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26th-31 st pentad is the spring-summer transition of MEJS, and the 54th-61 st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500-200 hPa south-north temperature difference(SNTD) well match with 200-hPa zonal wind velocity, indicating that the former leads to the latter following the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger SNTD between 500-200 hPa moves northward earlier, with the westerly jet in the lower troposphere over 40°-90°E appearing earlier than normal, and vice versa.

T_(BB) DATA-REVEALED FEATURES OF ASIAN-AUSTRALIAN MONSOON SEASONAL TRANSITION AND ASIAN SUMMER MONSOOM ESTABLISHMENT

Based on TBB data from Meteorological Institute Research of Japan, study is carried out of the features of seasonal transition of Asian-Australian monsoons and Asian summer monsoon establishment,indicating that the transition begins as early as in April, followed by abrupt change in May-June; the Asian summer monsoon situation is fully established in June. The winter convective center in Sumatra moved steadily northwestward across the "land bridge" of the maritime continent and the Indo-China Peninsula as time goes from winter to summer, thus giving rise to the change in large scale circulations that is responsible for the summer monsoon establishment over SE Asia and India; the South China Sea to the western Pacific summer monsoon onset bears a close relation to the active convection in the Indo China Peninsula and steady eastward retreat of the subtropical TBB high-value band,corresponding to the western Pacific subtropical high.

NUMERICAL STUDY ON INFLUENCE OF QINGHAI-XIZANG(TIBETAN)PLATEAU ON SEASONAL TRANSITION OF GLOBAL ATMOSPHERIC CIRCULATION

It is a worthwhile attempt to address the role of the Qinghai-Xizang Plateau in the seasonal transition of general circulation from a global prospective. In this paper, the CCM1 (R15L7) - LNWP spectral model is used to study the influences of the Qinghai-Xizang Plateau on the seasonal transfer of the general circulation, with the objective analysis form the State Meteorological Center for March 17, 1996 as the initial field. A mid-level heating source in regions on the same latitudes is shown to cause a warming center of 224 K to form on the level of 200 hPa that warms up the atmosphere by more than 7 K and a drop of temperature by about 6 K on most of the 200-hPa layer over the Antarctic continent, with the largest negative center being -8.28 K. It is favorable to the deepening and widening of the polar vortexes in the course of transition from summer to winter. The topographic effect of the plateau plays a vital role in forming and maintaining the mean troughs and ridges of the atmospheric circulation in Northern Hemisphere such that it strengthens (weakens) the south-north positive gradient of temperature on the northern (southern) side of the latitude zone in which the plateau sits and increases the north-south gradient of temperature near 30(N. The seasonal transition is thus favored so that the bulk travel of global westerly at the middle latitudes and the formation of Asian monsoon in early summer are made possible. In the equatorial and low-latitude areas where the geopotential is increased, the effect of the plateau terrain is also evident in that it is favorable for the northern withdrawal of the tropical high ridge in Southern Hemisphere and the northern shift of the subtropical high in Northern Hemisphere. In addition, the effect also helps increase the polar easterly over the Southern Hemisphere and weaken the low zone at 500 hPa. It acts as an increasing factor for the polar vortex around the Ross Sea and contributes to the genesis of the Somali Jet on the equator.

On the Seasonal Transition and the Interannual Variability in Global Kinetic Energy at 500 hPa, Accompanied withAnomalies of Energy during the 1982 / 83 ENSO

Utilizing the material of monthly means of the three primary kinetic energy modes over the whote globe at 500 hPa during the nine years of 1980-1988, both the rapid seasonal changes and the interannual variability in tie general circulation in terms of the energy modes have been investigated, with special attention paid to the unusual year 1983, Two main results are obtained. One, there are remarkable seasonal rapid changes over the Northern Hemisphere, occurring ganerally in April and October. The other, among the nine years of 1980-1988, 1983 is the only one with unusual energy modes and remarkably abnormal seasonal changes.

NUMERICAL STUDY ON THE DYNAMIC AND THERMODYNAMIC EFFECTS OF THE QINGHAI-XIZANG PLATEAU ON THE SEASONAL TRANSITION IN THE EARLY SUMMER IN EAST ASIA

By employing a five-layer seasonal numerical weather prediction model and utilizing the National Meteorological Center (NMC) objective analysis data on May 10,1991 as the initial field,three numerical experiments——diabatic with orography,adiabatic with orography and adiabatic without orography——have been carried out to investigate the dy- namic and thermodynamic effects of the Qinghai-Xizang Plateau (hereafter the Plateau) on the seasonal transition in the early summer in East Asia.The results show that the typical seasonal transition features such as the northward shift of the subtropical westerly jet stream,the adjustment of the long-wave in middle and high latitudes and the changes of the circulation in the lower troposphere can be well simulated,if the dynamic and thermodynamic effects of the Plateau are both considered in the model as in the experiment adiabatic with orography.In other two experiments,it is failed to simulate out the seasonal transition features. The results also show that the thermodynamic effect of the Plateau acts on the atmosphere as a gradually enhanced heating source in the early summer,which makes the air temperature in 500 hPa over the Plateau increase by nearly 10℃ in a month and accelerates the northward shift of the subtropical westerly jet stream about seven latitudes more north- ward in twenty days and helps the shifted jet maintain at the northern periphery of the Plateau.It is also helpful to the formation and maintenance of the Lake Balchas trough and the northward and westward extension of the Pacific subtropical high.On the other hand,the dynamic effect of the Plateau weakens the northward shift of the subtropical westerly jet stream and barricades the westward extension of the Pacific subtropical high.If only the dynamic effect of the Plateau is considered in the model without consideration of the thermodynamic effect of the Plateau,not only the shifted jet withdraws southward but also the simulated Lake Balchas trough is 10 longitudes more eastward than that in the diabatic experiment,and the simulated Pacific subtropical high is more eastward and northward.However,the dy- namic effect of the Plateau strengthens the vertical movements in the west and east of the Plateau and makes the low-level jet maintain in the southeast of the Plateau.The path of the cold air and the tunnel of the water vapor in the lower troposphere during the seasonal transition period in the early summer in East Asia are determined by the coupling of the dynamic and thermodynamic effects of the Plateau.

STUDY ON THE VARIATION IN THE CONFIGURATION OF SUBTROPICAL ANTICYCLONE AND ITS MECHANISM DURING SEASONAL TRANSITION-PART Ⅰ:CLIMATOLOGICAL FEATURES OF SUBTROPICAL HIGH STRUCTURE

Climatological characteristics of subtropical anticyclone structure during seasonal transition are investigated based on NCEP/NCAR reanalysis data.The ridge-surface of subtropical anticyclone is defined by the boundary surface between westerly to the north and easterly to the south (WEB in brief).In Afro-Asian monsoon area,the subtropical high in troposphere whose ridgelines are consecutive in wintertime takes on relatively symmetrical and zonal structure,the WEB tilts southward with increasing height.In summer,the subtropical high ridgelines are discontinuous at low levels and continuous at upper levels,the WEB tilts northward from the bottom up.Under the constraint of thermal wind relation,the WEB usually tilts toward warmer zone.May is the period when subtropical high modality most significantly varies.The structure and properties of subtropical high during seasonal transition are different from area to area.A new concept 'seasonal transition axis' is proposed based on formation and variation of the vertical ridge axis of subtropical anticyclone.The subtropical high of summer pattern firstly occurs over eastern Bay of Bengal in the beginning of May.then stabilizes over eastern Bay of Bengal,Indo-China,and western South China Sea in the 3rd pentad of May,it exists over the South China Sea in the 4th- 5th pentad of May and establishes over central India in the 1st-2nd pentad of June.The three consequential stages when summer modal subtropical high occurs correspond to that of Asian summer monsoon onset,respectively.To a great extent,the summer monsoon onset over the Bay of Bengal depends on the reversal of meridional temperature gradient in vicinity of the WEB in upper troposphere.The meridional temperature gradient at middle-upper levels in troposphere can be used as a good indicator for measuring the seasonal transition and Asian monsoon onset.

A Typical Mode of Seasonal Circulation Transition: A Climatic View of the Abrupt Transition from Drought to Flood over the Middle and Lower Reaches of the Yangtze River Valley in the Late Spring and Early Summer of 2011

In this study, the seasonal transition of precipitation over the middle and lower reaches of the Yang-tze River Valley (YRV) from late spring to early summer is investigated. The results show that the seasonal transition of precipitation exhibits multi-modes. One of these modes is characterized by an abrupt transition from drought to flood (ATDF) over the middle and lower reaches of the YRV in the seasonal transition of precipitation. It is shown that the ATDF event from May to June 2011 is simply one prominent case of the ATDF mode. The ATDF mode exhibits an obvious decadal variability. The mode has occurred more frequently since 1979, and its amplitude has apparently strengthened since 1994. From the climatic view, the ATDF mode configures a typical seasonal circulation transition from winter to summer, for which the winter circulations are prolonged, and the summer circulations with the rainy season are built up early over the YRV.

STUDY ON THE VARIATION IN THE CONFIGURATION OF SUBTROPICAL ANTICYCLONE AND ITS MECHANISM DURING SEASONAL TRANSITION-PART Ⅱ:SEASONAL TRANSITION INDICES OVER THE ASIAN MONSOON REGION

Based on the studies in Part Ⅰ (see Mao et al.2003),this paper further examines the relationship between the Asian summer monsoon onset and variation in meridional position of the warm temperature ridge with a zonal orientation in mid-upper troposphere.During the Asian monsoon bursting consequentially over the Bay of Bengal,South China Sea,and South Asia,in addition to the reversal of winds in the lower and upper troposphere and deep convection before and after the onset,the atmospheric meridional temperature gradient (MTG) in the vicinity of the ridge-surface of subtropical high (WEB defined in Part Ⅰ) exhibits a significant reversal.Since the establishment of temperature structure with higher over north than over south of the WEB in the mid-upper troposphere (200-500 hPa) characterizes the collective essential that the Asian summer monsoon bursts over different areas,the MTG in mid-upper troposphere,based on the thermodynamics associated with the seasonal transition,should be a reasonable index to measure the Asian monsoon onset.The definition for onset date is proposed,and the time series of onset date for different sections are determined.As compared with the onset dates determined by other indices such as 850-hPa zonal wind and OLR.correlation analyses indicate that the 850-hPa zonal wind is only regional index,but the MTG index is applicable universally to the Asian monsoon regime.

STUDY ON THE VARIATION IN THE CONFIGURATION OF SUBTROPICAL ANTICYCLONE AND ITS MECHANISM DURING SEASONAL TRANSITION-PART Ⅲ:THERMODYNAMIC DIAGNOSES

The mechanisms for the variation in the configuration of subtropical anticyclone during seasonal transition are explored from energy budget using the NCEP/NCAR reanalysis data.Based on the seasonal variations of temperature and heating fields,it is found that the significant diabatic heating associated with spring precipitation over southern China has impacts on subsequent Asian seasonal transition.The reversal of meridional temperature gradient in the vicinity of the WEB (westerly-easterly boundary) in the middle and upper troposphere also depends on the latitudinal position where temperature ridge locates.The northward shift of the warm temperature ridge results from the fact that the local temperature increase to the north of the WEB is more than that in its vicinity.The diagnostic results through thermodynamic equation show that physical mechanism responsible for seasonal transition is different from area to area over the Asian monsoon region.The dominant factors responsible for northward shift of the Bay of Bengal warm ridge are the meridional temperature in initial stages of the onset and the descending motion after the onset. The factors for causing the northward jump of the South China Sea warm ridge involve the zonal temperature advection,meridional temperature advection,and diabatic heating associated with the southern China spring rainfall.The subsidence is the factor leading to the northward migration of the South Asia warm ridge.

CHARACTERISTICS OF THE NORTHERN HEMISPHERE SUB-TROPICAL HIGH SEASONAL SPLITTING OVER THE ASIAN MONSOON SECTORS AND ITS POSSIBLE MECHANISM

The splitting of the Northern Hemisphere sub-tropical high (SH) during spring to summer and its possible mechanisms has been analyzed. Results indicate that the splitting of SH occurs over the Bay of Bengal to the Indo-China peninsula. However, remarkable contrast exists in the Hadley cell at the lower and upper levels over these sectors during March to May. The land surface sensitive/latent heating both play an important role, and decay the local Hadley cell over the Indo-China peninsula by enhancing the upwelling. In contrast, the dominant land surface sensitive heating over the Bay of Bengal only damages the low-level Hadley cell. Thus, the splitting of SH should occur over the Indo-China peninsula, rather than the Bay of Bengal at lower levels. In addition, the analysis suggests that the faster seasonal snow melting in the east of Indo-China peninsula can enhance the land surface sensitive heating atmosphere and weaken the local Hadley cell, such seasonal change benefits the splitting of the SH.

An Analysis of the Spring-to-Summer Transition in the West Central Plains for Application to Long Range Forecasting

The spring-to-summer transition is of special importance in long range forecasting, as the general circulation transitions to a less energetic regime. This affects the Midwestern United States in a profound way, since agriculture is very sensitive to the variability of weather and climate. Beginning at the local scale, surface temperature observations are used from a representative station in the West Central Missouri Plains region in order to identify the shift from late spring to early summer. Using upper-air re-analyses as a supplement, the 500-mb height observations are examined to find a spring-to-summer transition date by tracking the location of a representative contour. Each of these is used to identify spring-to-summer transition date and then statistical analysis is performed on this long-term data set. Finally, teleconnections, specifically the influence of El Ni?o Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), and blocking are examined in order to quantify interannual variability. It was found that examining these criteria, developed in an earlier study that covered a much shorter time period, produced similar statistics to this 68-year study of spring-to-summer transitions. It was also found that the onset of La Ni?a was associated with hotter summers in the region, a result first found in the earlier study, but this association was much stronger here.

The Role of Land-sea Distribution and Orography in the Asian Monsoon. Part II： Orography

The role of various mountains in the Asian monsoon system is investigated by AGCM simulations with different mountains. The comparison of the simulation with Asian mountains （MAsia run） with the simulation without mountains （NM run） reveals that the presence of the Asian mountains results in a stronger South Asian summer monsoon （SASM）, characterized by enhanced lower-tropospheric westerly winds, uppertropospheric easterly winds, and stronger water vapor convergence. In East Asia, the southerly winds and water vapor convergence are significantly strengthened in association with the intensified zonal pressure gradient between the East Asian continent and the Pacific Ocean. Both the dynamical and thermodynamic forcing of the Tibetan Plateau play important role in strengthening the Asian summer monsoon. In winter, the presence of Asian mountains significantly strengthens the continental high, which leads to a stronger Asian winter monsoon. The presence of African-Arabian mountains helps to intensify the exchange of mass between the Southern Hemisphere and Northern Hemisphere by strengthening the cross equatorial flows in the lower and upper troposphere over East Africa. Asian mountains also play a crucial role in the seasonal evolution of Asian monsoons. In comparison with the NM run, the earlier onset and later withdrawal of lower-tropospheric westerly winds can be found over South Asia in the MAsia run, indicating a longer SASM period. The African-Arabian mountains also moderately contribute to the seasonal variation of the South Asian monsoon. In East Asia, the clear southto-north march of the southerly winds and subtropical rainfall starts to occur in early summer when the effects of Asian mountains are considered.

The Role of Land–sea Distribution and Orography in the Asian Monsoon. Part I: Land–sea Distribution

A number of AGCM simulations were performed by including various land–sea distributions （LSDs）, such as meridional LSDs, zonal LSDs, tropical large-scale LSDs, and subcontinental-scale LSDs, to identify their effects on the Asian monsoon. In seven meridional LSD experiments with the continent/ocean located to the north/south of a certain latitude, the LSDs remain identical except the southern coastline is varied from 40 ° to 4 ° N in intervals of 5.6° . In the experiments with the coastline located to the north of 21° N, no monsoon can be found in the subtropical zone. In contrast, a summer monsoon is simulated when the continent extends to the south of 21 ° N. Meanwhile, the earlier onset and stronger intensity of the tropical summer monsoon are simulated with the southward extension of the tropical continent. The effects of zonal LSDs were investigated by including the Pacific and Atlantic Ocean into the model based on the meridional LSD run with the coastline located at 21 °N. The results indicate that the presence of a mid-latitude zonal LSD induces a strong zonal pressure gradient between the continent and ocean, which in turn results in the formation of an East Asian subtropical monsoon. The comparison of simulations with and without the Indian Peninsula and Indo-China Peninsula reveals that the presence of two peninsulas remarkably strengthens the southwesterly winds over South Asia due to the tropical asymmetric heating between the tropical land and sea. The tropical zonal LSD plays a crucial role in the formation of cumulus convection.

On the Differences and Climate Impacts of Early and Late Stratospheric Polar Vortex Breakup

The stratospheric polar vortex breakup （SPVB） is an important phenomenon closely related to the seasonal transition of stratospheric circulation. In this paper, 62-year NCEP/NCAR reanalysis data were employed to investigate the distinction between early and late SPVB. The results showed that the anomalous circulation signals extending from the stratosphere to the troposphere were reversed before and after early SPVB, while the stratospheric signals were consistent before and after the onset of late SPVB. Arctic Oscillation （AO） evolution during the life cycle of SPVB also demonstrated that the negative AO signal can propagate downward after early SPVB. Such downward AO signals could be identified in both geopotential height and temperature anomalies. After the AO signal reached the lower troposphere, it influenced the Aleutian Low and Siberian High in the troposphere, leading to a weak winter monsoon and large-scale warming at mid latitudes in Asia. Compared to early SPVB, downward propagation was not evident in late SPVB. The high-latitude tropospheric circulation in the Northern Hemisphere was affected by early SPVB, causing it to enter a summer circulation pattern earlier than in late SPVB years.

NUMERICAL STUDY OF THE EFFECTS OF PERSISTENT WARMER SEA SURFACE TEMPERATURE IN TROPICAL INDIAN OCEAN ON ATMOSPHERIC CIRCULATION IN THE EARLY SUMMER IN EAST ASIA IN 1991

By employing the CCM1(R15L12)long-range spectral model, study is undertaken of the effects of sea surface temperature anomaly(SSTA) for tropical Indian ocean on circulation transformation in the early summer in East Asia in 1991. The results indicate that warmer SSTA contributes to the increasing of the temperature over the Plateau in early summer, resulting in the intensification of tropical easterly jet on 100 hPa and northward shift of Northern Hemisphere subtropical westerly jet in May. It is obviously favorable for the subtropical high enhancement over western Pacific Ocean in May and subtropical westerly jet maintaining at 35～40 °N in June, making the Mei-Yu come earlier and stay over the Changjiang basin in 1991. Furthermore, warmer SSTA is also advantageous to averaged temperature rise in East Asia land region and Nanhai monsoon development. These roles are helpful in accelerating the seasonal transition for East Asia in early summer.

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.