Influence of convectively coupled equatorial waves and intra-seasonal oscillations on rainfall extremes over India

摘要

Convectively coupled equatorial waves and intra-seasonal oscillations are known to modulate rainfall over the Tropics significantly. The effect of these oscillations on the frequency distribution of extreme precipitation events (defined based on percentile threshold) over Indian region is investigated for the monsoon months of June-September. Based on the variance and phase of the wave, any given day is classified into wet phase and dry phase. Modulation of extreme event probability by the presence of these phases over a given region is quantified for different waves, namely Madden-Julian Oscillation (MJO), equatorial Rossby wave (ER), equatorial Kelvin wave and mixed Rossby gravity and tropical depressions (MT). It is shown that the presence of wet phase of the waves increases the probability of extreme precipitation occurrence by as much as 150% of the baseline probability while the presence of dry phase can decrease the probability by half over certain regions depending on the wave. Averaged over the whole country, wet phase increases the probability by 30-60% while dry phase decreases by 20-40% except for MT waves where there is increased extreme activity during dry phase. However, the intensity of extreme precipitation does not show significant sensitivity to the changes in phase of the waves.