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 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.

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.

DIAGNOSES OF THE SEVERE DROUGHT OVER YUNNAN AREA IN THE EARLY SUMMER OF 2005

High temperature and drought occurred in Yunnan province during the late spring and early summer in 2005, which was the most severe event in this region since 1950’s. Based on the observational data and relevant diagnoses, this extreme weather event was studied and discussed. The results show that the occurrence of this event could be due to the following observational facts that happened in 2005. (1) The seasonal adjustment of middle-high-leveled atmospheric circulation was delayed. (2) The cold air activity center was deviated north. (3) The onset of summer monsoon over South China Sea was delayed. (4) The tropical convection activity was much weaker than usual. (5) The subtropical high over the western Pacific was located southwestwards and relatively strong.

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.