The Relationship between the East Asian Winter Monsoon Anomaly and the Subsequent Summer Monsoon Onset over the South China Sea and the Impact of ENSO

Linkage between the East Asian winter monsoon (EAWM) and the South China Sea (SCS) summer monsoon onset (SCSSMO) and the impacts of ENSO have been studied by applying the method of correlation analysis to the ERA-interim reanalysis dataset. Anomalous easterly winds and less precipitation in the SCS tend to occur during May after a weak EAWM, which corresponds to the situation of late onset of the summer monsoon. However, this relationship is not robust during the time period of study. We further separate the EAWM into an ENSO-related part (EAWM_EN) and an unrelated part (EAWM_res) by linear regression method. The results indicate that the link of the EAWM_EN to the SCSSMO becomes statistically significant, but the link of the EAWM_res to the SCSSMO is very weak. Hence, the EAWM-SCSSMO relationship is mainly caused by the anomalous winter monsoon associated with ENSO. And this relationship can be explained by the anomalous anticyclone/cyclone over the Philippine Sea (AACPS/ACPS) associated with ENSO. For example, during an El Niño event, anomalous anticyclone over the east of Philippine is induced and the tropical Indian Ocean tends to be warmer. Due to the positive feedback of the local air-sea interaction and the Indian Ocean capacitor effect, this AACPS can be maintained and persist until late spring. The anomalous easterly winds on the south of AACPS may be unfavorable for the onset of summer monsoon. Therefore, the SCSSMO tends to be late in the El Niño year.