Climatological Analysis On Medium-range Longitudinal Excursion Of South Asia High

This paper uses the NCEP\NCAR 1997-2007 over 11 years daily reanalysis data and so on, according to the definition of circulation index for Close Pressure Systems, a center position index for daily 100hPa South Asia High and 500hPa Western Pacific Subtropical High was defined, furthermore , the medium-range longitudinal excursion features of South Asia High was analysed, the correlation between the South Asia High and Western Pacific Subtropical High was studied, by using relatively longer data sets, proved that, anomalous longitude position of South Asia High(extend eastwards\ westwards ) in summer with different excursion pattern of Western Pacific Subtropical High, it would extend westwards (withdraw eastwards). During the process, introduced an efficient method for diagnose this phenomenon, analysed the synoptic feature of the medium-range longitudinal excursion of South Asia High, and its correlation with summer precipitation distribution in China. The main conclusions are as follows: (1) It proves suitable to use the index defined in this paper to represents the longitudinal excursion of South Asia High and the Western Pacific Subtropical High in summer. (2) The mainly periods of this phenomenon is 7.6～15.3 days, therefore it represent medium-range features. (3)The medium-range longitudinal excursion features of South Asia High can be classified into two kinds: one should be the single eastwards or westwards excursion, another should be the longitudinal oscillation; Longitudinal excursion is a mainly feature of South Asia High, meanwhile, exists commonly in summer. (4) Anomalous longitude position of South Asia High(extend eastwards\ westwards ) in summer with different excursion pattern of Western Pacific Subtropical High would extend westwards (withdraw eastwards) was 30.3% of all the longitudinal excursion, and it existed in 7years out of the 11 years, 1-2times for one year, so it is not a phenomenon inevitable and high frequency existed in summer. (5) The longitudinal excursion of South Asia High followed by the certain variations of the Western Pacific Subtropical High often happen in from the middle ten days of June to the first ten days of July and from the middle ten days of July to the first ten days of August in summer, when the transition from Meiyu period into midsummer occurs over middle and lower reaches of Yangtse River. (6) When The longitudinal excursion of South Asia High followed by the certain variations of the Western Pacific Subtropical High composite circulation patterns obviously show that the westward (eastward) movements of the centre of the South Asia High go with corresponding eastward (westward) development of the Western Pacific Subtropical High. (7) When The longitudinal excursion of South Asia High followed by the certain variations of the Western Pacific Subtropical High, correlation with summer precipitation anomaly distribution is as follows: When the position of the South Asia High extends westwards, the precipitation increases in mid-and low- basin of Yangtse River and the area from southwest to north China, while decreases in northwest China ; When the position of the South Asia High extends eastwards, the precipitation increases in northwest China, while decreases in mid-and low- valley of Yangtse River, south China and eastern Qinghai-Xizang plateau, and that the wax and wane of rainfall is largely consistent with the longitudinal excursion of the South Asia High.