A Study On The Influence Of South Asia High On The Climatic Change At The Yangtze River Upper Valley In Summer

The South Asia High(SAH) is the strongest upper level high-pressure systemof northern hemisphere during summer,and the chief member of summer monsoonin Asia.It is closely related to the general circulation in northern hemisphereand to the weather and climate in Asia.This paper uses the monthly mean NECP/NCARreanalysis data of 100hPa height field and so on, and the observationalprecipitation data and air temperature data in Sichuan-chongqing basin at theYangtze River upper valley.A series of eastward index and ridge line index aswell as area index of the SAH were built,also the precipitation data and airtemperature data in Sichuan-Chongqing basin.Morlet wavelet analysis andresultant as well as correlation analysis were used to study the influence ofthe SAH on the climatic change in Sichuan-chongqing basin.The main conclusionsare as follows:(1) The interdecadal variation of eastward index of the SAH during summeris very evident,including a 10 year period and a 30 year period. Also the ridgeline index of the SAH,including a 15 year period and a 23 year period.Theirtransition all occurs in late 1970s. But their interannual variation is not veryevident. There is a 10-20 year period of the precipitation and air temperaturein Sichuan-Chongqing basin,and their transition happens in late 1970s,whichare consistent with that of the SAH.But their interannual period is 2-3 year.(2) The correlation analysis shows that while the SAH extendseastwards(westwards) , and Western Pacific subtropical high extendswestwards(eastwards),at the same time there is an anomaly wave train in mid-highlatitude at the northern Hemisphere with the mark "+-+(-+-)",there is more(less) precipitation in The western Sichuan-Chongqing basin.While the SAH andWestern Pacific subtropical high extend all southwards(northwards),at the sametime there is an anomaly wave train in mid-high latitude with the mark "-+-(+-+)",there is more(less) precipitation in the eastern Sichuan-Chongqing basin.While the SAH and Western Pacific subtropical high extend allnorthwards(southwards),at the same time there is an anomaly wave train inmid-high latitude with the mark "+-+(-+-)",the air temperature in theSichuan-Chongqing basin is high (low).(3) While the SAH extends eastwards or retreats westwards,and southwardsor northwards during summer,5OOhPa height field,and 850hPa wind anomalies showobvious interannual and interdecadal differences,summer precipitationdistribution in the western and eastern Sichuan-Chongqing basin is alsodifferent,and the air temperature in the Sichuan-Chongqing basin. It provesthat the interannual and interdecadal variation of the SAH has a good relationto the interannual and interdecadal variation of other key element of climatesystem and we can regard the SAH as a strong signal of the anomalies in the climatesystem. By analyzing the anomalies of the SAH we can directly study and forecastthe regional climate anomalies.(4) While the SAH extends eastwards,the center of anticyclonic circulationon the wind anomaly field at 200hPa lies on the Tibetan Plateau.But while theSAH retreats westwards, the center lies on the Iran Plateau.And while the SAHextends eastwards,the circulation anomaly over the north area of east Asia isvertically baroclinic.(5) It was pointed out that Western Pacific subtropical high would extendwestwards(withdraw eastwards) if the center of the SAH extendedeastwards(withdraw westwards) in the 1960s,on the synoptic scale. The resultsof the paper suggest that Western Pacific subtropical high would not only extendwestwards(withdraw eastwards) if the SAH extended eastwards(withdrawwestwards),but also would extend northwards(withdraw southwards)if the SAHextended northwards(withdraw southwards),on the interannual and interdecadalscale.(6) While the SAH extends northwards during summer on the interannual andinterdecadal scale, there are prevailing southerly anomalous monsoon inSichuan-Chongqing basin,leading to little precipitation in its eastern partand high air temperature in the basin.(7) The results of the paper suggest that due to the special geographiclocation of Sichuan-Chongqing basin,the anomaly of the SAH has more direct impacton climatic variation of it.Thus we propose that the SAH should be consideredas a strong anomalous signal for the climatic variation in this area.