On the relationship between 100-hPa South Asia High and Mei-yu in Jiangsu province

By analyzing the change of an index for the characteristics of South Asia High and variations of upper-air troughs in 2002–C2005, we studied the impact of South Asia high on the beginning and ending of the Mei-yu in Jiangsu province. Statistic verification is conducted on the relationships between the index and the Mei-yu season in 1991–C2005 to examine the impacts of the SAH characteristics index on a rain intensity index of Mei-yu and regional distribution of a characteristics index. Historical composite is performed of the 100-hPa circulation field using the 100-hPa geopotential height of Northern Hemisphere from 2002 to 2005 and 45-year NCEP reanalysis to study the difference in the circulation. Diagnostic and statistic conclusions have been obtained as follows. (1) The characteristics preceding to and the advancement/retreat of SAH and the movement of westerly troughs can influence the onset time of the Mei-yu season; after the Mei-yu onset, the progression/withdrawal of SAH and how farther east it extends determine how long the Mei-yu lasts and when it ends. (2) During the Mei-yu, the general 100-hPa circulation situation and average characteristics of the SAH are well corresponding to the characteristics of the season and annual patterns of Mei-yu. And the averages of the SAH ridgeline and east-extending index for June, July and the Mei-yu season have some implications to the forecast of the index of Mei-yu intensity. These conclusions can be served as powerful means in determining the starting/ending dates, duration and annual pattern of the Mei-yu season.