India monsoon 2018: Destructive yes, distributive no

By Shreeshan Venkatesh

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The distribution and incidence of extreme rain events during the season can be read as a clear marker of climate change. Over the past three months, there have been over 150 incidences of more than 125mm of rain falling at one go. This remarkably high number is in keeping with recent trends, but the distribution is skewed more than usual.

Typically, the west coast receives a significant number of extreme rain events. This year though, the concentration of these events in the region is much higher. One reason for this is the atypical distribution of the monsoon this year. While the west coast has received abundant rains, monsoon winds have not penetrated the mainland. According to a senior scientist at the IMD, this is because of poor northward propagation of the winds. “The Madden Julian Oscillation (MJO), which is an intra-seasonal atmospheric oscillation near the equator, affects the movement of the monsoon winds. This year, it has not established a favourable phase for northward propagation. Additionally, we have not had active troughs that direct the winds in the season,” he explains.

While it is fairly common for the MJO to be unfavourable for the monsoon, what is less common is what is happening in the Bay of Bengal. The strength and location of monsoon winds also depends on low pressure systems that develop in the Bay of Bengal. Low pressure systems that evolve into depressions are effective in strengthening and directing monsoonal winds over the sub-continent. The climatological average indicates the development of six well-marked depressions during the four monsoon months—one each in June and September and two in July and August. However, this year, the first depression was formed only in the end of July. With only a little more than a month left, only three depressions have formed so far in the BoB and moved into the Indian mainland. The reduced number of depressions has suppressed distribution of rainfall in the subcontinent and has contributed to the concentration of rainfall over four prolonged spells primarily in the west coast since the onset of monsoon.

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