Effect of Indian Ocean Dipole (IOD) Events on the Second Inter-monsoonal Rainfall in the Wet Zone of Sri Lanka
A. B. Abeysekera, B. V. R. Punyawardena, B. Marambe, I. M. Shiromani Priyanthika Jayawardena, T. Sivananthawerl, V. N. M. Wickramasinghe
Abstract
The climatic variations in the Indian Ocean have a strong relationship with the rainfall anomaly of Sri Lanka. Indian Ocean Dipole (IOD) is an ocean-atmospheric coupled phenomenon associated with an east-west gradient in the tropical Indian Ocean Sea Surface Temperature (SST) anomalies. Identifying the impact of IOD phenomenon on the spatial and temporal variation of the rainfall pattern is a useful tool for seasonal climate forecasting. The present study examined the influence of IOD on the Second Inter-monsoon (SIM) rains of the Wet zone (WZ) of Sri Lanka. Considering 16 rain-gauge locations scattered in the WZ of Sri Lanka covering nine Agro-ecological Regions (AERs), daily rainfall time series for 44-years (1976-2019) were collected. The IOD-positive and IOD-negative years were extracted based on the Dipole Mode Index (DMI) over the neutral years. Five rainfall indices, namely, cumulative rainfall, the maximum rainfall received within a day, number of wet days, heavy rainfall events and the maximum consecutive dry days were statistically analysed. The results revealed a significant positive anomaly of mean cumulative rainfall in the SIM season during the IOD-positive years while in IOD-negative years this anomaly was negative (p<0.05). An apparent increase or decrease of number of wet days and heavy rainfall events was observed during IOD-positive or IOD-negative years, respectively. However, the mean maximum consecutive dry days showed a distinct negative anomaly with the positive IOD events and positive anomaly with the negative IOD events. Further investigations are suggested to elucidate the <em>El Niño </em>Southern Oscillation (ENSO)-induced anomalous variation over IOD impacts.