EXTREMES OF THE SOUTHERN OSCILLATION PHENOMENON AND THE RAINFALL OF SRI-LANKA

Influences of extreme phases of the Southern Oscillation (SO) phenomen on, El Nino and La Nina events, on the seasonal rainfall of Sri Lanka are examined by using composite maps of seasonal rainfall and sea-surf ace temperature (SST) anomalies. There were 27 El Nino and 22 La Nina events, during the period from 1881 to 1990. Positive and negative rai nfall anomalies during the south-west monsoon (SWM) season are associa ted with La Nina and El Nino events, but negative and positive rainfal l anomalies are linked to La Nina and El Nino events during the second intermonsoon (SIM) season. These contrasting patterns are dominant in the dry zone of Sri Lanka. Rainfall anomalies during first intermonso on (FIM) and north-east monsoon (NEM) seasons do not show clear contra sting patterns as in other seasons and show positive and negative valu es. On the basis of wettest 20 per cent, mid-20 per cent and driest 20 per cent of years of seasonal total rainfall, composite maps of SST a nomalies over the Pacific and Indian Oceans were made. As in rainfall patterns, SST anomalies during FIM and NEM seasons do not show clear c ontrasts between El Nino and La Nina events. During the SWM season, we t (dry) years are associated with negative (positive) SST anomalies ov er central and eastern Pacific and west Indian Oceans, but opposite SS T anomalies are found over the 'maritime continent'. During the SIM se ason, wet (dry) years are associated with positive (negative) SST anom alies over central Pacific and west Indian Oceans and opposite SST ano malies over the 'maritime continent'. Based on the results of this stu dy and previous studies on synoptic circulation patterns, and the domi nance of the intraseasonal oscillation, a plausible explanation is giv en for larger anomalies during the SWM and SLM seasons in Sri Lanka.