Relationship between South China Sea summer monsoon onset and Southern Ocean sea surface temperature variation

The interannual and interdecadal variability of South China Sea summer monsoon onset date is analyzed, and the relationship between the date and the Southern Ocean sea surface temperature is studied using the 31a (1979-2009) daily mean NCEP-DOE Reanalysis 2 and monthly extended reconstructed sea surface temperatures (ERSST. v2). It is shown that there is significant interannual and interdecadal variability in the monsoon onset date. The mean onset date shows a significant shift from 1979-1993 to 1994-2009. The onset date is positively correlated to Indian Ocean- Southern Ocean (0-80 degrees E, 750 degrees S-50 S) SST in preceding winter (Dec.-Jan.) and Pacific- Southern Ocean (170 degrees E 80 W 75 degrees S-50 S) SST in preceding spring (Feb.-Mar.). The monsoon onset is earlier (later) when SST over Southern Ocean in preceding winter and spring is anomalously low (high). Thus the SST signal over Southern Ocean may be regarded as a predictor for the South China Sea summer monsoon onset. The SST anomaly over Southern Ocean may impact the South China Sea summer monsoon onset through air-sea interaction and atmospheric teleconnection. A strong (weak) ant-arctic oscillation (AAO) during a negative (positive) SST anomaly over Southern Ocean in boreal winter may remotely cause a negative (positive) geopotential height anomaly and a positive (negative) zonal wind anomaly in tropical Indian Ocean-western Pacific. The tropical circulation anomaly is maintained from the winter to April and May, and as a result the cross-equatorial low-level Somali jet from southern hemisphere to northern hemisphere is enhanced (weakened). These provide a favorable circulation condition for the early (late) onset of the South China Sea summer monsoon. The inter-hemispheric oscillation (IHO) resulted from the air mass redistribution contributes to the teleconnection between air-sea interaction over Southern Ocean and tropical monsoon circulation.