INTERANNUAL VARIABILITY OF THE ONSET OF THE INDIAN-SUMMER MONSOON AND ITS ASSOCIATION WITH ATMOSPHERIC FEATURES, EL-NINO, AND SEA-SURFACE TEMPERATURE ANOMALIES

The long-term mean date of the monsoon onset over Kerala (MOK) varies between 30 May and 2 June according to different estimates, with a standard deviation of 8-9 days. The earliest date of MOK, and the most delayed one, during the last 100 years differ by 46 days (7 May and 22 June, respectively). MOK switches on a spatially large and intense convective heat source over south Asia, lasting from June to September, whose moisture supply is made available through the cross-equatorial low-level jet stream. Superposed epoch analysis of 10 years of outgoing longwave radiation (OLR) data shows that MOK is a significant stage in the evolution of the OLR field in the tropics of the eastern hemisphere. At the time of MOK there is increased convection in a band about 5-10 degrees wide meridionally, extending from the south Arabian Sea to south China, and convection is suppressed all around, particularly in the western Pacific Ocean. In 1983 when MOK was delayed by about 3 pentads, OLR data showed that the boreal spring-to-summer migration of the equatorial convective cloudiness maximum (ECCM), both westward and northward, was also delayed. The delayed MOK is accompanied by delays in the northwestward movement of ECCM and is confirmed by an analysis of long-term data of southwest Pacific tropical cyclones. Of the 22 years between 1870-1989 when MOK was delayed by 8 days or more, 16 cases were associated with a moderate or strong El Nino. Of the 13 strong El Ninos during the same period, 9 were associated with moderate-to-large delays in MOK. Delays preferentially occurred in the year +1 of an El Nino, where year 0 is the growing phase of the El Nino in sea surface temperature (SST). Analysis of the SST field has shown that delayed MOK is associated with warm SST anomalies at and south of the equator in the Indian and Pacific oceans and cold SST anomalies in the tropical and subtropical oceans to the north during the season prior to the monsoon onset (i.e., March to May). It is hypothesized that such SST anomalies over the Indian and Pacific oceans (generally found associated with El Nino, either in year 0 or year +1 or in both) cause the interannual variability of the MOK through their action in affecting the timing of the northwestward movement of the ECCM.