Sri Lankan Rainfall Climate and its Modulation by El Nino and La Nina Episodes

Seasonal and inter-annual rainfall variations have profound influence on sectors of engineering concern such as water resources, agriculture, energy, fisheries, environment and construction. One of the dominant mechanisms of global inter-annual climate variability is the El Nino-Southern Oscillation (ENSO). ENSO is a shift in the pattern of oceanic warming and atmospheric circulation centered in the Pacific Ocean with implications on the climate across the tropics. Anomalously warm sea surface temperatures in the equatorial Eastern Pacific is referred to as El Nino phase and its cold analogue is referred to as La Nina. Here, the historical relationship of ENSO to Sri Lankan rainfall is described. Ranking both the seasonal Sri Lankan rainfall and its contemporaneous ENSO index shows a modest but significant association in the January to March, May, July to August and October to December periods. El Nino leads to wetter conditions during May, October, November and December and to drier conditions during January, February, March, July and August. Except for January to March the impacts of La Nina are inverse of that for El Nino. The rainfall during this period declines for both El Nino and La Nina. This relationship between rainfall and ENSO during October to December, January to March and July to August are statistically significant. This association can be used to predict the rainfall during the planting phase and the harvesting phase of the Maha (October to March) and during the harvest season of Yala (April to August). Rainfall predictions can be provided 3-6 months in advance based on the availability of ENSO predictions. The predictability has implications for agriculture, irrigation engineering, hydro-electricity generation and demand, construction planning and disaster risk for planning, operational management and policy formulation.