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

Abstract

Influences of extreme phases of the Southern Oscillation (SO) phenomenon, El Niño and La Niña events, on the seasonal rainfall of Sri Lanka are examined by using composite maps of seasonal rainfall and sea-surface temperature (SST) anomalies. There were 27 El Niño and 22 La Niña events, during the period from 1881 to 1990. Positive and negative rainfall anomalies during the south-west monsoon (SWM) season are associated with La Niña and El Niño events, but negative and positive rainfall anomalies are linked to La Niña and El Niño events during the second intermonsoon (SIM) season. These contrasting patterns are dominant in the dry zone of Sri Lanka. Rainfall anomalies during first intermonsoon (FIM) and north-east monsoon (NEM) seasons do not show clear contrasting patterns as in other seasons and show positive and negative values. 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 anomalies over the Pacific and Indian Oceans were made. As in rainfall patterns, SST anomalies during FIM and NEM seasons do not show clear contrasts between El Niño and La Niña events. During the SWM season, wet (dry) years are associated with negative (positive) SST anomalies over central and eastern Pacific and west Indian Oceans, but opposite SST anomalies are found over the ‘maritime continent’. During the SIM season, wet (dry) years are associated with positive (negative) SST anomalies over central Pacific and west Indian Oceans and opposite SST anomalies over the ‘maritime continent’. Based on the results of this study and previous studies on synoptic circulation patterns, and the dominance of the intraseasonal oscillation, a plausible explanation is given for larger anomalies during the SWM and SIM seasons in Sri Lanka. © 1997 by the Royal Meteorological Society.