The relation of South China Sea monsoon onset with the subsequent rainfall over the subtropical East Asia

摘要

The relationship between the South China Sea summer monsoon (SCSSM) onset and the subsequent subtropical East Asian monsoon rainfall from May to September (SEAMR) is investigated. The SCSSM onset is found to have a significant positive correlation with the SEAMR stretching from the lower reaches of the Yangtze River valley (LYR) to the southern Japan; e.g. the early onset of the SCSSM tends to be succeeded by less SEAMR, whereas the late onset is likely to be followed by more SEAMR. The positive relationship between the SCSSM onset and the SEAMR is directly controlled by the persistent western North Pacific anti-cyclonic/cyclonic (WNPAC/WNPC) circulation anomaly from boreal spring to summer. The persistence of the large-scale circulation anomalies are accompanied by the decaying phase of El Nino (La Nina) with the pronounced sea-surface temperature anomalies (SSTA) over the tropical Indian Ocean (IO), the western North Pacific and the topical central and eastern Pacific Ocean (PO), respectively. Further numerical experiments indicate that the topical IO SSTA governs the positive relationship between the SCSSM onset and the subsequent SEAMR. On one hand, the wavelength of the Kevin-wave response to tropical IO SSTA forcing is three times that of the Rossby-wave response to the western North Pacific SSTA forcing. The Kevin-wave response directly influences the low level circulation anomalies field throughout the South China Sea region, determining the onset date of the SCSSM. On the other hand, the horizontal wind shear that induced by the persistent Kevin-wave response would also maintain the WNPAC/WNPC and further control the SEAMR from May to September. Thus, it is the topical IO SSTA forcing that consolidates the positive relationship between the SCSSM onset and the SEAMR.