THE INTERANNUAL VARIATIONS OF THE SUMMER MONSOON ONSET OVER THE SOUTHCHINA SEA

Interannual variations of the summer monsoon onset over the South Chin a Sea (SCS) have been studied using data from over seventeen years (19 79-1995) of NMC global analysis and of Outgoing Longwave Radiation (OL R) observed with NOAA polar-orbitting satellites. It was found that th e summer monsoon onset in the SCS occurs abruptly with a sudden change of zonal wind direction from easterly to westerly and an exploding de velopment of deep convection in the whole SCS region in the middle of May. Based on the criteria defined in this paper for the SCS summer mo nsoon onset, the average onset date over the SCS from 1979 to 1995 is around the fourth pentad of May. The airflow and general circulation o ver the SCS changes dramatically after the onset. The ridge of the sub tropical high in the western Pacific in the lower troposphere weakens and retreats eastward from the SCS region with an establishment of wes terly winds over the whole region. During the SCS monsoon onset, the m ost direct impact in the vicinity of the SCS are the equatorial wester lies in the Bay of Bengal through their eastward extension and northwa rd movement. An indirect influence on the SCS onset is also caused by the enhancement of the Somali cross-equatorial flow and the vanishing Arabian High over the sea; the latter may be a signal for the SCS onse t. There are quite significant interannual variations in the SCS onset . In the years of a delayed onset, the most profound feature is that t he easterly winds stay longer in the SCS than on average. Deep convect ion activities are suppressed. The direct cause is the abnormal existe nce of the western Pacific subtropical high over the SCS region. Moreo ver, compared to the average, the equatorial westerlies in the Bay of Bengal are also weaker in the years of a delayed onset. No significant changes for the cross-equatorial flow at 105 degrees E are observed f or these years. It has also been found that the interannual variations of the SCS onset are closely related with the ENSO events. In the yea rs of a delay, the Walker circulation is weaker, and the sea surface t emperature (SST) anomalies in the western Pacific are negative.