Relationships between the Southern Oscillation and the rainfall of Sri Lanka

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Abstract

The Southern Oscillation (SO) signals in the monthly and seasonal rainfall of Sri Lanka are studied by using statistical analyses. Correlation coefficients (CCs) between the rainfall of the first intermonsoon season (March-April) and the seasonal Southern Oscillation Index (SOI) of August-September-October (ASO) and November-December-January (NDJ) are positive and significant. In particular, March rainfall in region A is significantly correlated with the subsequent SOI of ASO and NDJ. The CC between the rainfall of the southwest monsoon season (May-September) and the SOI is also positive. Cumulative rainfall of the peak monsoon months (July and August) reveals a strong and positive correlation with the seasonal SOI.

The CC between the rainfall of the second intermonsoon season (October-November) and the SOI is negative and significant. The rainfall of this season is also significantly correlated with the SOI (Tahiti-Darwin) of May-June-July (MJJ). The SOI of MJJ serves as a particularly useful predictor for this seasonal rainfall. The CC between the rainfall of the northeast monsoon season (December-February) and the SOI is also negative but weak.

Large signals appear in wind anomalies over Sri Lanka prior to El Niño/Southern Oscillation (ENSO) events. Upper level easterlies (westerlies) and weak (strong) low-level easterlies are dominant in March before the La Niña (El Niño) years. These wind anomalies persist until the ENSO events reach maturity.

Based on the results of correlation analysis and wind anomalies, schematic models of the zonal circulation along the Equator between 60°E and 140°W are presented for La Niña and El Niño phases for northern hemispheric summer and winter seasons.

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