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.
Highlights
Indian Ocean Dipole (IOD) is an Ocean– atmosphere interaction that may exemplify a characteristic internal mode of the Indian Ocean climate system (Saji et al, 1999; Webster et al, 1999)
The present study examined the influence of IOD on the Second Inter-monsoon (SIM) rains of the Wet zone (WZ) of Sri Lanka
The present study revealed that the quantum of rainfall received within the SIM period in the WZ is in the range of 430 – 867 mm
Summary
Indian Ocean Dipole (IOD) is an Ocean– atmosphere interaction that may exemplify a characteristic internal mode of the Indian Ocean climate system (Saji et al, 1999; Webster et al, 1999). Warm water from the upper equatorial Pacific Ocean is transferred to the Indian Ocean, resulting in a warm SST in the islands of Indonesia Air rises above this area and falls allowing westerly winds to blow along the equator (Rao et al, 2002). Changes in the winds allow cool water to rise up from the deep ocean in the east This sets up a temperature difference across the tropical Indian Ocean resulting in higher rainfall in the countries around the Arabian Sea. Westerly winds intensify along the equator during the negative phase, allowing warmer waters to concentrate near Australia. It is important to ascertain the rainfall variation in the WZ of Sri Lanka under the effect of IOD in order to enhance the skill of weather predictions enabling to make appropriate on-farm decisions well before the onset of the Maha season
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