A Study On The Features Of Longitudinal Excursion Of South Asia High

This paper uses the monthly mean NCEP/NCAR reanalysis data of 100hPa height field and so on, choose the longitude of east ridge point of 16760gpm line on 100hPa height field during summer(June-July-August) as the eastward index of South Asia High(SAH) to build a 50-yr(l951-2000) series of eastward index of SAH. Morlet wavelet analysis shows that decadal period of longitudinal excursion of SAH during summer is very notable, including a 10-year period and a 25-30-year period. Based on this result we choose 8 years as the east anomouly years and 8 years as the west anomouly years to study the climate characteristics related to anomaly position of SAH and possible factors leading to the anomalies. The decadal variation of the index and the corresponding precipitation field, sea surface temperature, winter accumulated snow depth over the Tibetan Plateau are studied. At last the correlation between 100hPa height field and 500hPa height field, precipitation field, SST field and atmospheric heat source field,etc are studied by using singular value decomposition(SVD). The main conclusions are as follows:1. It proves suitable to use the index of SAH defined in this paper to describe the longitudinal excursion in summer. The cases which are choosed as the anomalous years of SAH during summer is proper.2. While South Asia High on the upper level of troposphere extends eastwards or retreats westwards during summer, 500hPa height field , 850hPa wind anomalies and vertical velocity field show obvious differences, and summer precipitation distribution in China is also different. When SAH extends eastwards, there is much precipitation at the Yangtze River valley while South China is short of precipitation, and vice versa.3. Many factors can lead to the position's anomaly of SAH during summer. Among them previous sea surface temperature of equatorial middle east Pacific Ocean and tropical Indian Ocean, apparent heat source and moisture sink during summer on Tibetan Plateau, East Asia, Indian Ocean and tropical west Pacific, and preceding winter accumulated snow depth over Tibetan Plateau all play important roles in influencing the position of SAH during summer.4. The decadal variation of longitudinal excursion of SAH during summer is very evident. The characteristics of precipitation anomalies in summer, previous andsimultaneous SSTA and preceding winter accumulated snow depth anomalies corresponding to positive and negative phases of 10-yr mode and 25-30-yr mode are discussed respectively. No matter for 10-yr mode or for 25-30-yr mode, reverse characteristics corresponding to positive phases and negative phases appear in the above fields. It proves that the decadal variation of SAH have a good relation to the decadal variation of other key element of climate system and we can regard SAH as a strong signal of the anomalies in the climate system. By analyzing decadal anomalies of SAH we can directly study and forecast the regional climate anomalies.5. The correlation between 100hPa height field and 500hPa height field, precipitation field, SST field and apparent atmospheric heat source and moisture sink field are also studied by using singular value decomposition and the conclusions are similar to those by composite and correlation analysis.