Moisture Transport in the Asian Summer Monsoon Region and Its Relationship with Summer Precipitation in China

The characteristics of moisture transport over the Asian summer monsoon region and its relationship with summer precipitation in China are examined by a variety of statistical methods using the NCEP/NCAR reanalysis data for 1948-2005. The results show that: 1) The zonal-mean moisture transport in the Asian monsoon region is unique because of monsoon activities. The Asian summer monsoon region is a dominant moisture sink during summer. Both the Indian and East Asian monsoon areas have their convergence center,respectively. 2) Most moisture congregates in the lower troposphere primarily from the Bay of Bengal in the mid and upper layers, and the vapor flux comes from mid-latitude westerlies as well as the tropical western Pacific Ocean. 3) The moisture fluxes by the Indian monsoon enhance from May to July mostly in the zonal transport while those by the East Asian monsoon intensify mainly in the 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 Tibetan Plateau across 90°E are strong in spring, while the mid-high latitude and tropical westerly vapor transfers 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 the Asian monsoon region undergo a substantial change after the South China Sea monsoon onset, especially over the Bay of Bengal, Indo-China Peninsula, and South China Sea. The northern boundary of the South China Sea is of great importance in providing abundant moisture for China mainland during summer. 5) The northward progress of the moisture transfer coincides with the seasonal march of the monsoon rainbelts very well. EOF1 of the moisture transport field basically depicts the consistent northward transport anomaly with an obvious decreasing trend over the East Asian monsoon region from 1951 to 2005. Further analyses suggest that this trend owing to the weakening of the 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 the Yangtze River valley shows a slight increasing tendency from the 1980s and it is consistent with the fact of more frequently occurred heavy rainfall over there. The correlation analyses indicate that the interdecadal variation of the East Asian summer monsoon accounts for the main part of the variation.