Southerly Moisture Surges over the South China Sea in Early-Summer Season: Multiscale Variations and Impacts on Rainfall

Southerly moisture surges over the central South China Sea (SCS) are characterized by the strengthening of low-level southerlies that transport moisture northward from the Pacific or Indian Oceans to South China. These surge events typically occur for days in the early-summer season (from April to June) and can lead to heavy rains in South China. This study categorizes surge events into three types of flow patterns and examines their multiscale variations and impacts on rainfall. The first type occurs mainly in April, with the southeasterlies enhanced by a deepening trough in South China and the western Pacific subtropical high established over the SCS. The second type of surge events mostly appears in June, featuring the prevailing southwesterlies of summer monsoon from the Indian Ocean during the active phases of intraseasonal oscillations. Most surge events exhibit semi-diurnal variations with morning and afternoon peaks of northward moisture fluxes. Specifically, the first type features a dominant afternoon peak, while the second type shows a dominant early-morning peak, which is induced by thermal contrast between the Indochina Peninsula and the SCS. In general, the surge events enhance moisture convergence and increase rainfall downstream in South China, but they show some regional differences. The second type strengthens moisture convergence and rainfall in coastal regions with a morning peak. In contrast, the first type enhances inland rainfall with a morning peak, while moisture divergence dominates coastal regions. The third type of surge events denotes transitional conditions between the first two types, in terms of atmospheric circulations, diurnal cycles, and rainfall patterns. These results highlight a diversity of regional moisture surges and related rainfall ranging from diurnal to sub-seasonal scales.