Changes in water vapor transport during the Meiyu season after 2000 and their relationship with the Indian ocean SST and Pacific-Japan pattern

Euler's method of water vapor analysis and the Lagrangian trajectory analysis for 1979-2009 show the water vapor associated with Meiyu rainfall over the Yangtze-Huaihe River Valley (YHRV) mainly comes from the South China Sea (SCS) and the Bay of Bengal (BOB), and the local evaporation from the lower and middle troposphere over the Pacific Ocean and Eurasia are secondary. Compared with that before 2000, the contribution of water vapor from the BOB to Meiyu rainfall doubles in the low level but decreases in the mid-level after 2000, while that from the SCS decreases and mid-level transport path shifts to north of the Indochina Peninsula. The increased water vapor transport from the BOB and north of the Indochina Peninsula is the primary cause for the water vapor budget increase at the southern boundary of the YHRV. It is further noted that the water vapor from the SCS mainly influences precipitation over the region to the south of the mid-lower reaches of the Yangtze River, while water vapor transport from the BOB can cause opposite rainfall variations between north and south of Yangtze River over Meiyu rainfall region. After 2000, the decreased/increased SCS/BOB water vapor transport jointly influences the Meiyu rain belt shift northward. Singular value decomposition is used to study the relationship of changes in the water vapor transport with the Indian Ocean sea surface temperature (SST) and Pacific-Japan (P-J) pattern. Enhancement of the coupling between the Indian Ocean SST basin-wide warming and P-J pattern is an inner mechanism of the changes in water vapor transport and the northward shift of the rain belt.