Assessing the Role of Tropical Intraseasonal Oscillations in Occurring Extreme Rainfall Events Over Sri Lanka

ABSTRACT

The weather and climate of the tropical region are significantly modulated by the tropical intraseasonal oscillations. These oscillations are mainly constituent of convectively coupled equatorial waves (CCEWs) and Madden–Julian oscillation (MJO). Though a smaller region and surrounded by oceanic water, the country Sri Lanka faces various types of extreme weather conditions. The extreme rainfall event (ERE) is one of the major disasters and the influence of these tropical waves in the occurrence of extreme events cannot be overlooked. The modulation of EREs by these oscillations is analysed over Sri Lanka during two core monsoon and two inter‐monsoon seasons using very high‐resolution 12 × 12 km rainfall from Indian Monsoon Data Assimilation and Analysis (IMDAA) reanalysis from 1979 to 2020. The probability of ERE is calculated during dry and wet phases of the different waves relative to the climatological probability for all the seasons. These probabilities are not consistent and highly varying across the seasons over Sri Lanka. The modulation of extreme rainfall by the MJO and CCEWs is clearly evident during wet and dry phases of the waves, with a systematic increase (decrease) in the probability of extreme events during convectively active (suppressed) conditions. The results revealed that the wet phase of the equatorial Rossby (ER) and mixed‐Rossby gravity and tropical depressions together (MT) waves during northeast monsoon (NEM) season show maximum increase in the probability of ERE up to 150% while their dry phases reduced the probability of ERE by 100% during first inter‐monsoon (FIM) season over eastern parts of the country. It is important to note that these probabilities are relative to the climatological values. The probability of ERE reduced (increased) during dry (wet) phases for MJO and Kelvin waves during southwest monsoon (SWM) over western regions with lesser quantity. However, MJO has increased the probability of ERE over eastern parts during NEM season. To assess the broader impact over the country, the probabilities are spatially averaged and it is observed that the maximum probability of ERE occurs in the wet phase of ER followed by MT wave during NEM season. The wave filtered anomalies of vertically integrated moisture flux convergence (MFC) linked to the wet phases of the waves play a major role in modulating the ERE over the region.