Quantitative diagnosis of moisture sources and transport pathways for summer precipitation over the mid-lower Yangtze River Basin

Using a moisture tracking model with 32-year reanalysis data and station precipitation observations, we diagnosed the sources of moisture for summer (June 1-August 31) precipitation in mid-lower reaches of the Yangtze River Basin (YRB). Results indicate the dominant role of oceanic evaporation compared to terrestrial evapotranspiration, and the previously overlooked southern Indian Ocean, as a source region, is found to contribute more moisture than the well-known Arabian Sea or Bay of Bengal. Terrestrial evapotranspiration appears to be important for summer precipitation, especially in early June when moisture contribution is more than 50%. The terrestrial contribution then decreases and is generally less than 40% after late June. The Indian Ocean is the most important oceanic source before mid-July, with its largest contribution during the period of heavy precipitation, while the Pacific Ocean becomes the more important oceanic source after mid-July. To quantitatively analyze paths of moisture transport to YRB, we proposed the Trajectory Frequency Method. The most intense branch of water vapor transport to YRB stretches from the Arabian Sea through the Bay of Bengal, the Indochina Peninsula, the South China Sea, and South China. The other main transport branches are westerly moisture fluxes to the south of the Tibetan Plateau, cross-equatorial flows north of Australia, and separate branches located in the north and equatorial Pacific Ocean. Significant intraseasonal variability for these branches is presented. Additionally, the importance of the South China Sea for moisture transport to YRB, especially from the sea areas, is emphasized.