Southwesterly Water Vapor Transport Induced by Tropical Cyclones over the Bay of Bengal during the South Asian Monsoon Transition Period

Based on best-track, outgoing longwave radiation, sea surface temperature, and reanalysis data during 1979–2018, statistical and composite analyses were performed to investigate characteristics of the southwesterly water vapor transport (WVT) induced by Bay of Bengal (BoB) tropical cyclones (TCs) during the South Asian summer monsoon (SASM) transition period. The results show that the BoB TCs mainly occur several days before/after the date of SASM onset (retreat) in May (October–November), thereby imposing an important impact on southwesterly WVT to China in those two periods. The WVT is significant in the middle and lower troposphere during the bimodal peak periods of BoB TCs, with large values over the east–central BoB, southeastern Tibetan Plateau, and Southwest and South China. The WVT is located more northward at 500 hPa than at 700 hPa, reaching close to 35°N and covering the southeastern Tibetan Plateau owing to weakening of the plateau’s blocking effect at upper levels. The BoB TCs mainly increase the northward and eastward WVT anomalies compared to the climatological mean. Furthermore, the large southerly WVT anomalies are located in the lower troposphere in low-latitude areas, while the large westerly WVT anomalies appear mainly in the middle troposphere in high-latitude areas. This indicates an enhanced WVT channel where the southwesterly moisture jet first climbs northwards to the southeastern Tibetan Plateau and then turns eastwards to East China under the influence of the BoB TCs. Besides, the southwesterly WVT during the TC period in May is stronger and more widespread than that in October–November, being about twice the latter in value. However, their maximum contributions to the climatological average do not differ much, with maximums of 12% and 19% occurring in Southwest China during the bimodal periods, respectively, implying an important role played by the BoB TCs in the WVT.