Linkage of the water vapor transport distribution with the rainy season and its precipitation in the southern regions south of the Yangtze River during the early summer

Based on the NCEP/NCAR reanalysis data from 1961 to 2010, the distribution and evolution of the water vapor transport and its linkage with the early summer rainy season in the southern regions south of the Yangtze River have been studied. The findings are as follows. The water vapor transport increases and moves from south to north along with the summer monsoon front, there are 3 surges of the water vapor flux increasing abruptly at the South China Sea and extending northward to the south of 25°N, 25°-30°N and the north of 30°N successively, and the surges and relevant peaks arising correspond with the beginnings and ends of the pre-flood season in South China, the early summer rainy season in the southern regions south of the Yangtze River, and the Meiyu in the Yangtze-Huaihe River basin, respectively. During the early summer rainy season, the southern regions south of the Yangtze River lie in the north border of the high water vapor flux region, and there is a convergence belt over there. Additionally, the divergence field in the southern side of the Tibetan Plateau is the direct source of water vapor for this rainy season, and the divergence fields at the the areas of northern Australia to Indian Ocean and southern Arab Sea are indirect ones. The meridional water vapor transmission plays a major role for the beginning/end of the early summer rainy season, as the water vapor inflows are impulsively enhanced over the south (north) boundary of these regions on the start (last) day. But the zonal water vapor transmission should not be neglected as well, for the water vapor flux over the west boundary is much greater than that over the south boundary. The intensity indexes of the rainy season have inter-decadal climate variability, and correlate stronger with the anomaly of the total water vapor flux over the west than the south boundary. There are 2 main water vapor transfer paths to the southern regions south of the Yangtze River, one is the north branch, from the north of the Bay of Bengal passing over Burma, Yunnan and Guizhou, the other is the south branch, from the Bay of Bengal passing over the Indo-China peninsula and the South China Sea, and afterwards combined with the water vapor from the west of the Western Pacific subtropical high. Both of the transmissions following the paths mentioned above are enhanced (reduced) for heavy (weak) early summer rainy seasons. The Bay of Bengal, the south of the South China Sea, and the Western Pacific warm pool at the east of the Philippines, which are significant convergence fields, all play the roles of channels rather than the sources of the water vapor transport of the early summer rainy season, and the weak (strong) convergence over there is advantageous (disadvantageous) for the precipitation in the southern regions south of the Yangtze River. The mechanism may be attributable to the convection over the channels that could intercepts the water vapor transport over there.