Moisture transpotr in Asian summer monsoon region and its relationship with summer precipitation in China

The characteristics of moisture transport over Asian summer monsoon region and its relationship with China summer precipitation are examined by a variety of statistical methods using NCEP-NCAR reanalysis data for 1948-2005. Results show that: 1)The zonal-mean moisture transport in Asian monsoon region is unique because of the monsoon activities. Asian summer monsoon region is a dominant moisture sink during summer. Both the Indian monsoon and East Asian monsoon areas have their convergence center, respectively. 2) Most column moisture congregates in the lower troposphere primarily from the Bay of Bengal and in the mid and upper layers, the vapor flux comes from mid-latitude westerlies as well as the tropical West Pacific Ocean. 3)The moisture fluxes by Indian monsoon enhance from May to July mostly in zonal transport while those by East Asian monsoon intensify mainly in meridional transport from June to July. Both reach their maxima in July and then decrease from August. The sub-tropical westerly moisture fluxes south to the Tibet Plateau across 90°E is strong in spring, while the mid-high latitude and tropical westerly vapor transfer change in phase and increase from January to July. The tropical westerly transport accounts for about 80% of the total moisture transport in July and only 18% from mid-high latitudes. 4)The moisture transfer and budgets over Asian monsoon region undergo a substantial change after the South China Sea monsoon onset, especially in the Bay of Bengal and IndoChina peninsula and the South China Sea. The northern boundary of South China Sea is of great importance in providing abundant moisture for China continent during summer. 5)The northward progress of the moisture transfer coincides with the seasonal march of the rainbelts very well. The EOF1 of moisture transport field basically depicts the consistent northward transport anomaly with an obvious decreasing trend over East Asian monsoon region from 1951 to 2005.Further analyses suggest that this trend owing to the weakness of East Asian Summer monsoon is largely responsible for the decline of rainfall over North China. The EOF2 reveals that moisture flux convergence from northeast and southwest over Yangtze River Valley shows a slight increasing tendent from 1980’s and it is consistent with the fact of more frequently occurred heavy rainfall over there.The correlation analyses denote that the interdecadal variation of the East Asian summer monsoon accounts for the main part of the variation.