Research On Features Of Asian Summer Monsoon Moisture Transport And Their Relations To Rainfall In China

Based on the 1948-2005 NCEP/NCAR reanalysis data and the station observationsof precipitation and temperature of China from 1951-2005, the features of Asiansummer monsoon and their relations with the rainfall in China are investigated, themain results from this study are as follows:1)The Asian summer monsoon region is a major moisture sink in summer with twoconvergent centers in East Asian and Indian summer monsoon region, respectively.The prominent westerly moisture transfer north of the Equator over Asian summermonsoon region is in reverse with other places of similar geographic latitudes, and thezonally averaged meridional moisture transfer across the equator is around 2 timeslarger than that averaged along the equator. The southwest moisture transport fromIndian monsoon increases from May to July, and the southwest transport from thewestern Pacific enhances from June to July.2)The moisture transports from various latitudes in East Asian summer monsoonchange with season. In winter and spring, moisture primarily comes from subtropicaland mid-latitude westerlies in South China, the subtropical westerly moisture flux isstrongest in spring. From the end of spring to summer, the tropical westerly fluxreinforced greatly with its peak in July, while the magnitude of the mid-high latidudewesterly moisture flux is only about a quarter of the tropical westerly flux.3)The rainy season rainfall of North China bears obvious interannual andinterdecadal variabilities, and have good agreement with moisture influx from itssouth and west boundaries, particularly on interdecadal time scale, a 17-year period isevident throughout 1951-2005. But the 11-year period is more conspicuous after1990's. The rapid weakening of moisture transport by monsoonal currents andmid-latitude westerlies occurred in the middle of 1970's, and the former, which holdthe keys of the rainfall in North China, decreased more drasticly than the latter.4) Close relationship between summer moisture transfer and rainfall in the east partof China is revealed on multi-scale. For the interdecadal time scale, the North China,as well as Changjiang River basin appear significant correlation with the distributionof moisture flux. As to the interannual time scale, the moisture flux and rainfall bothhave a 2-3year period. The moisture flux closely relate to the rainfall inChangjiang-Huaihe River valley, the Huanghe River valley and the most part of southof Changjiang River. On the 30～60-day time scale, a 40-day period is discoveredexplicitly. The three SVD leading modes demonstrate the relationship between moisture transport and summer rainfall in China, specifically the third modeillustrates the effect of typhoon activities on the rainfall in coastal areas of China.5)Water balance estimate indicates that in summer, water recycling is mostvigorous over South China, then the North China, and most inactive inChangjiang-Huaihe River valley. During non-rainy season, especially in winter, thenet moisture convergence is mainly a result of evaporation offsetting the moisturedivergence in the regions, which provides moisture for the rainfall during the season.Evidence also shows that the moisture transfer by the mean flow and that by transienteddy are out of phase in annual cycle. The intensive belt of transient eddy flux shiftswith season, and characterized by distinct meridional transport. In rainy season, themean flow moisture transport is decisively responsible for the regional water vaporconvergence, in contrast, the transient eddies play an opposite role to the mean flow.As for non-rainy season, the transient moisture transfer becomes relatively crucial.6)Calculations of the violent rainfall cases in different regions demonstrate that theanomalous moisture essential for the rainfall comes from different sources. For thecase of 1963, the anomalous moisture is chiefly from the subtropical western Pacific,and the regional meridional transport is dominant. While for the cases in theChangjiang River valley and South China, the anomalous moisture sources are Bay ofBengal and the South China Sea, and the zonal moisture transport is noticeable. Thegrowth of the meridional moisture convergence caused by the influx across thesouthern boundary principally accounts for the augmentation of the rainfall in eachregion.