Abstract
Changes in temporal and spatial relationships between rainfall of Sri Lanka and the Southern Oscillation Index (SOI) during the 110 year period (1881–1990) are presented. The relationships during the first intermonsoon (FIM) and south-west monsoon (SWM) seasons indicate periods of weak positive and negative correlations. On the basis of major turning points in the summer rainfall over India and Sri Lanka, four distinct epochs were identified which cover the years 1881–1900, 1901–1930, 1931–1960 and 1961–1990. Indian and Sri Lankan summer rainfall shows a weak in-phase relationship from 1881 to 1900, a strong in-phase relationship from 1961 to 1990, and an out-of-phase relationship from 1901 to 1960. The correlation between the SOI and SWM season's rainfall is primarily negative from 1931 to 1960 and positive during other epochs. Strong negative correlations are generally present for the SIM season and strengthening and weakening of negative correlations during this season coincide with changes in the SWM season rainfall and its relationships with the SOI. The north-east monsoon (NEM) season correlations are negative and insignificant. Major changes in spatial patterns of correlations between seasonal rainfall and the SOI have occurred in Sri Lanka during SWM and SIM seasons. The periods of strong positive (negative) correlations during the SWM season coincide with weak (strong) negative correlations during the SIM season. This contrasting pattern is clear when the Indian and Sri Lankan summer monsoon rainfalls were out of phase between 1900 and 1960, but not before 1900, or after 1960. The sudden change in correlations around 1960 suggests a change in the coupled ocean–atmosphere system that dominates the climate of these regions. Changes in temporal and spatial patterns of correlations between the SOI and rainfall have been linked to changes in the location of the active intertropical convergence zone.
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