The Relationship between the South China Sea Summer Monsoon Onset and Pacific Meridional SST anomalies

Abstract

Abstract This study investigates the connection between key sea surface temperature (SST) anomalies in the North Pacific during boreal spring (February-April, FMA) and the subsequent South China Sea (SCS) summer monsoon (SCSSM) onset. The SST anomalies, similar to the Pacific meridional mode (PMM), referred to as the PMM + mode, is defined to study the new impact factor on the SCSSM onset. It is found that the (February-March-April, FMA) PMM + has a significant positive correlation with the subsequent May SCSSM onset date, which is weakly affected by the ENSO in previous winter. A strong positive PMM + in boreal spring can be maintained until May via atmosphere-ocean interaction. The cooling area over WNP would reduce in situ precipitation heating, thereby generating descending Rossby waves that reinforce the formation of the anomalous anticyclone over the SCS. As a result, the easterly winds and suppressed convection dominate the SCS, making the SCSSM tend to break out later than normal. In addition, the increase in anticyclonic vorticity anomalies also make the western North Pacific subtropical high (WNPSH) stronger and more western than the normal years, thereby blocking active convection west of the SCS. Given the weakened relationship between El Niño-Southern Oscillation (ENSO) and the SCSSM onset in recent years, the PMM + could be considered as a promising preceding signal for the SCSSM onset, which will be of great importance for the SCSSM prediction.