Roles of oceanic moisture exports in modulating summer rainfall over the middle-lower Yangtze River Basin: Inter-annual variability and decadal transition

Precipitation over the mid-lower reaches of the Yangtze River Basin (YRB), which is a typical East Asian monsoon region in China, is strongly impacted by oceanic moisture exports (OMEs) and features variability at multiple spatial-temporal scales, resulting in frequent floods and droughts. We focus on two key issues of OME and its relevance to the summer rainfall variability over the YRB: determination of the individual contributions of OME from three specific oceanic sub-regions: the Arabian Sea (ARS), Bay of Bengal (BOB), and South China Sea (SCS), and their roles in the inter-annual variability and decadal changes in YRB summer rainfall. Using a Lagrangian forward trajectories tracing approach, we create a catalogue of OME-based diagnosed precipitation fields for three sub-regions spanning the summer seasons (May to August) of 1980-2013 with 6-hourly resolution using ERA-Interim data. The results show that the pattern of the diagnosed OME-based precipitation resembles the observed pattern over the YRB in terms of climatology and temporal variation. Climatologically, the SCS region accounts for the largest relative contribution (60.7%), followed by the ARS (30.8%), whereas the contributions of the BOB (8.5%) are almost negligible. On the inter-annual scale, the variation in OME-based rainfall originated from the ARS, and the BOB is out of phase with that of the SCS. In contrast to climatology, the inter-annual changes in rainfall over the YRB are primarily modulated by the variation in the OME from the ARS. However, for the decadal changes, particularly in 1992/1993 over the YRB, the SCS and ARS are equivalent contributors to the summer precipitation over the YRB. These findings thus provide new insights into our understanding of the multi-scale variability of summer precipitation over the YRB region.