East Asia and the western Pacific regions are the northernmost and easternmost parts of the Asian summer monsoon region.East Asian region has a wet climate and has a large amount of precipitation especially in the Baiu/Meiyu season.In order to show how the abundant moisture and the wet climate are maintained, seasonal change of the moisture transport from the Indian Ocean to the western North Pacific and East Asia during the Asian summer monsoon is investigated.Zonal and meridional moisture fluxes which calculated from the 6 hourly data of the NCEP/NCAR reanalysis averaged from 1978 to 1998 were examined from May to September.
It is found that there are three typical northward moisture transport areas: the western equatorial Indian Ocean, the Bay of Bengal and the South China Sea.Two typical eastward moisture transport areas were also found: the tropical Indian Ocean and Baiu/Meiyu region in East Asia.Characteristics of seasonal change of these moisture transports are summarized as follows.In May, there are small eastward moisture fluxes around the equatorial Indian ocean and to the south of Japan whereas little northward moisture flux in the Indian ocean and South China Sea.In June, a strong northward moisture flux accompanied with Somali jet are found in the western Indian ocean and it last untill August.It is also found that an intensification and northward spread of the eastward moisture flux in the northern Indian Ocean toward the Arabian sea and the Bay of Bengal around 20N.In the Bay of Bengal, strong northward moisture flux is also found from June to August.In the South China Sea, there are strong eastward and northward moisture fluxes from June to August.They connect to the intense eastward moisture flux area around the eastern China sea and Japan during June and July.In August, the eastward moisture flux increases and reaches to the east of the Philippines, then moisture convergence to the east of the Philippines reaches maximam of the year.On the other hand, northward moisture flux toward the mid-latitude decreases because of the relatively dry southeasterly anticyclonic moisture flux which acompanied by the subtropical Pacific high.In September, the eastward moisture flux in the north Indian Ocean shifts southward and weakens.
This study indicates that the moisture transport from the tropics and subtropics toward the mid-latitude around Asian summer monsoon regions are closely related to the zonal moisture transport around the northern Indian ocean and the South China Sea and that studying variation and interaction of these moisture transports are needed to understand the water circulation of the East Asian summer monsoon.