The relation of cross‐equatorial flow during winter and spring with South China Sea summer monsoon onset

Abstract

ABSTRACTThe relationship between cross‐equatorial flow (CEF) during winter and spring and the South China Sea summer monsoon (SCSSM) onset is investigated by data diagnoses for the period of 1979–2013 and numerical experiments. The SCSSM onset is found to have a significant negative correlation with the CEF nearby the Philippines in preceding January, February and March prior to SCSSM onset. A strong CEF during January, February and March tends to be succeeded by an early onset of SCSSM, whereas a weak CEF is likely to be followed by a late onset. The CEF in preceding months links with the SCSSM onset through the subtropical high over South China Sea and western North Pacific in April. The subtropical high in April is weak and eastward shifted (strong and westward shifted) when the CEF in preceding months is strong (weak). The weak and eastward shifted (strong and westward shifted) subtropical high favours (does not favour) the convection developing (depressing) over South China Sea, resulting in an early (late) onset of SCSSM. The anomalies of CEF nearby the Philippines in preceding months and its close relationship with subsequent subtropical high are caused by the sea surface temperature anomalies (SSTA) and atmospheric heating source over tropical western Pacific. The difference of SSTA between south and north of equator in tropical western Pacific and the corresponding atmospheric diabatic heating result in the anomalies of CEF in January, February and March, and impact the activity of subtropical high in April. The positive SSTA in tropical western Pacific, combining with the wet and warm climatic background over northern South China Sea and coastal areas of Southern China in April are favourable for a cyclonic circulation anomaly occurring over northern South China Sea. As a result, the subtropical high is weak and eastward shifted, and the SCSSM onset is early.