Abstract:
Indian Ocean Dipole (IOD) is an ocean-atmospheric coupled phenomenon
associated with the east-west gradient of Sea Surface Temperature (SST)
anomalies in the tropical Indian Ocean. The positive (warm) phase of IOD leads
to enhanced rainfall in the western Indian Ocean and diminished rainfall in the
south eastern Indian Ocean, while the opposite is true for the negative (cold)
phase. Any ocean atmospheric variations in the Indian Ocean is strongly
associated with the rainfall anomaly of Sri Lanka. Present study examined the
influence of IOD on the Second Inter-monsoonal (SIM) rainfall (October and
November) as a useful observation for seasonal climate forecasting of Maha
seasons. Fifteen agro-meteorological stations, scattered in the Dry zone, covering
six agro-ecological regions, were selected. Daily rainfall time series for 44-years
(1976-2019) were collected. The Dipole Mode Index (DMI), i.e. SST gradient
between the western equatorial Indian Ocean and the south eastern equatorial
Indian Ocean, were used to identify the positive (>+0.4 °C) and negative
(<−0.4 °C) IOD events. Eight positive years and four negative years were
identified during the SIM season for the study period and compared them with
remaining neutral years. Means of five rainfall indices, namely, cumulative
seasonal rainfall, maximum rainfall received within a day, number of wet days
(rainfall > 1mm), heavy rainfall events (> 90th percentile) and maximum count of
consecutive dry days were statistically analysed. The results revealed a
significant increase in mean cumulative rainfall, number of wet days and heavy
rainfall events during the IOD-positive years (p<0.05) and an apparent negative
anomaly of those events during the IOD negative years. The mean maximum
consecutive dry days showed a distinct negative (positive) anomaly with the
positive (negative) IOD events. However, further investigations are suggested to
elucidate the anomalous variation of rainfall by the other climatic drivers over
IOD impact.
