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.

Discussion of Some Problems as to the East Asian Subtropical Monsoon

Based on NCEP/NCAR gridded reanalysis, TRMM precipitation data, CMAP, and rainfall observations in East China, a study is conducted with focus on the timing and distinctive establishment of the rainy season of the East Asian subtropical monsoon （EASM） in relation to the South China Sea （SCS） tropical summer monsoon （SCSM）. A possible mechanism for the EASM is investigated. The results suggest that 1） the EASM rainy season begins at first over the south of the Jiangnan region to the north of South China in late March to early April （i.e., pentads 16-18）, and then the early flooding period in South China starts when southerly winds enhance and convective rainfall increases pronouncedly; 2） the establishment of the EASM rainy season is earlier than that of its counterpart, the SCSM. The EASM and the SCSM each is featured with its own independent rain belt, strong southwesterly wind, intense vertical motion, and robust low-level water vapor convergence. The SCSM interacts with the EASM, causing the EASM rainy belt to move northward. The two systems are responsible for the floods/droughts over the eastern China; and 3） in mid-late March, the eastern Asian landmass （especially the Tibetan Plateau） has its thermal condition changing from a cold to a heat source for the atmosphere. A reversal of the zonal thermal contrast and related temperature and pressure contrasts between the landmass and the western Pacific happens. The argument about whether or not the dynamic and thermal effects of the landmass really act as a mechanism for the earlier establishment of the EASM rain belt is discussed and to be further clarified. Finally, the article presents some common understandings and disagreements regarding the EASM.

Impact of Urban Land-Use Change in Eastern China on the East Asian Subtropical Monsoon:A Numerical Study

The effect of urban land-use change in eastern China on the East Asian subtropical monsoon（EASTM） is investigated by using the Community Atmosphere Model version 5.1.Comparison of the results between the urban expansion and reference experiments shows that with the urban expansion,the land surface energy balance alters：surface net radiation and sensible heat fluxes enhance while the latent heat fluxes reduce.As a result,a significant increase in surface air temperature over eastern China is detected.The urban land-use change contributes to a change in the zonal land-sea temperature difference（LSTD）,leading to a delay in the time when LSTD changes from positive to negative,and vice versa.Additionally,the onset and retreat dates of the EASTM are also delayed.Meanwhile,the rise in surface air temperature leads to formation of abnormal northerly air flows,which may be the reason for the slower northward movement of the EASTM and a more southward location of its northern boundary.

Definition and characteristics of the south edge of the subtropical winter monsoon in East Asian

Based on observational daily data of 730 meteorological stations in China, the south edge of the subtropical winter monsoon is defined according to relevant criterion and its variation characteristics are analyzed. Results show that this south edge has obvious inter-annual variation characteristics and shows a northward moving tendency as a whole, but since the 21 st century it has moved southwards and date of the south edge entering winter becomes earlier. Wind fields of the anomalously northward south edge of the subtropical winter monsoon in East Asia has an obvious southerly wind component which prevents cold air from moving southward. The index of this south edge and winter temperature has a positive correlation. Climate warming might be the main reason for the northward movement of the south edge of the subtropical winter monsoon.

THE EAST ASIAN SUBTROPICAL SUMMER MONSOON INDEX AND ITS RELATION WITH THE CLIMATE ANOMALIES IN CHINA

A new East Asian subtropical summer monsoon circulation index is defined, where the barotropic and baroclinic components of circulation are included. Results show that this index can well indicate the interannual variability of summer precipitation and temperature anomalies in China. A strong monsoon is characterized by more rainfall in the Yellow River basin and northern China, less rainfall in the Yangtze River basin, and more rainfall in south and southeast China, in association with higher temperature in most areas of China. Furthermore, comparison is made between the index proposed in this paper and other monsoon indexes in representing climate anomalies in China.

A STUDY ON RELATION BETWEEN EAST ASIAN WINTER MONSOON AND CLIMATE CHANGE DURING RAINING SEASON IN CHINA

Based on the data of NCEP, OLR and rainfall of China, we studied the influences of the East Asian winter monsoon activities on the precipitation during the raining season over China by correlation analysis and composite analysis. The result shows that annual and interdecadal change of East Asian winter monsoon is distinct. It is strong from 1950s to the middle of 1980s but weak after the middle of 1980s. The effect of abnormal winter monsoon on the precipitation during raining season is significant over the middle and lower reaches of the Changjiang River basin. It is revealed that the precipitation will increase when preceding winter monsoon is weak but decrease when preceding winter monsoon is strong. In this paper, some appropriate reasons are given to explain the abnormal rainfall by analyzing the distribution of SSTA and the variation of summer circulation. It is pointed out definitely that the variation of SSTA and summer circulation is a primary cause for abnormal rainfall over the middle and lower reaches of the Changjiang River.

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 effect of soil moisture(SM)on the onset of East Asian subtropical summer monsoon(EASSM)is investigated based on multiple sets of reanalysis data in the period of 1981–2010.It is found that the EASSM is characterized by persistent 2-m s^(−1) southerly winds for about 3 months in spring at 850 hPa over the subtropical region of East Asia.Considering this feature of the meridional winds,we define the EASSM onset date,and obtain that the climatological onset date is pentad 17.7,around 26 March.On the interannual timescale,the onset date of EASSM exhibits statistically significant correlation with the SM in southeastern China in the month preceding the onset,with wetter(drier)conditions being associated with later(earlier)onset.The physical process by which the preceding SM affects the EASSM onset is further explored by examining the surface energy balance as well as its impacts.Positive(negative)SM anomalies in southeastern China in the month before onset may induce negative(positive)surface temperature anomalies.The decreased(increased)surface temperature in southeastern China before the EASSM onset weakens(strengthens)the zonal sea–land thermal contrast in the surface and low-level atmosphere in the subtropical East Asia.The zonal sea–land thermal contrast in wetter(drier)years induces anomalous northerly(southerly)winds over southeastern China,which tends to delay(advance)the zonal thermal seasonal transition in spring and is conducive to a later(earlier)onset of EASSM.These results are helpful for understanding and prediction of the variability of EASSM and the EASSM onset.

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.

Changed Relationships Between the East Asian Summer Monsoon Circulations and the Summer Rainfall in Eastern China

In previous statistical forecast models,prediction of summer precipitation along the Yangtze River valley and in North China relies heavily on its close relationships with the western Pacific subtropical high（WPSH）,the blocking high in higher latitudes,and the East Asian summer monsoon（EASM）.These relationships were stable before the 1990 s but have changed remarkably in the recent two decades.Before the 1990 s,precipitation along the Yangtze River had a significant positive correlation with the intensity of the WPSH,but the correlation weakened rapidly after 1990,and the correlation between summer rainfall in North China and the WPSH also changed from weak negative to significantly positive.The changed relationships present a big challenge to the application of traditional statistical seasonal prediction models.Our study indicates that the change could be attributed to expansion of the WPSH after around 1990.Owing to global warming,increased sea surface temperatures in the western Pacific rendered the WPSH stronger and further westward.Under this condition,more moisture was transported from southern to northern China,leading to divergence and reduced（increased） rainfall over the Yangtze River（North China）.On the other hand,when the WPSH was weaker,it stayed close to its climatological position（rather than more eastward）,and the circulations showed an asymmetrical feature between the stronger and weaker WPSH cases owing to the decadal enhancement of the WPSH.Composite analysis reveals that the maximum difference in the moisture transport before and after 1990 appeared over the western Pacific.This asymmetric influence is possibly the reason why the previous relationships between monsoon circulations and summer rainfall have now changed.