Southwest monsoon has set in over Kerala, aided by Cyclone Nisarga: IMD

The southwest monsoon season has set in over Kerala on June 1, according to the India Meteorological Department (IMD). This is the normal date for the onset of the monsoon season over the state and is four days earlier than the date IMD predicted earlier.

The season brings around 70 per cent of the total rainfall that falls over India.

The reason for this is the development of a low-pressure area in the Arabian Sea which is touted to intensify into a cyclone by June 2 and make landfall along the Maharashtra coast, close to Mumbai.

The monsoon winds are south westerly in nature and move towards the north and north west directions.

This might help the cyclone gain in strength and move northwards and further inland after making landfall and decreasing in intensity, according to Raghu Murtugudde, climate scientist at the University of Maryland in the United States. This might cause rainfall in the interior districts of Maharashtra and Madhya Pradesh.

“In recent years, we have seen quite a number of cyclones forming in the Arabian Sea close to the monsoon onset. We don’t know yet if this is part of a rising trend,” Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, Pune, said.

“We were expecting that the monsoon wind conditions (wind shear) would suppress the low-pressure system, despite the conducive ocean conditions,” he explained.

However, it seems the ocean has won in terms of triggering a cyclone. Now, since the track is geared towards Maharashtra-Gujarat coast, these regions would receive the rains earlier than usual.

This is because of the cyclonic system pulling the monsoonal moisture inland. Generally, after a cyclone the ocean-atmospheric conditions take time to normalise, and it can create a temporary (few days) lull in the monsoon progression, according to Koll.

For farmers in the north, north west and central parts of India suffering from crop damage due to hailstorms and excess rainfall in March and April, a nation-wide lockdown due to the COVID-19 pandemic since March 24 and locust attacks since the first week of May, the timely rains could be a welcome sign.

The IMD has also issued its second long-range forecast for the monsoon season. It has reiterated its earlier prediction that the rainfall over the country will be normal.

It predicts a country-wide rainfall of 103 per cent of the long period average (LPA). The LPA of the monsoon season rainfall over the country as a whole for 1961-2010 was 880 mm.

The distribution over various regions of the country will be a bit uneven. The rains will be most intense in the north western region (107 per cent of LPA) of the country and least intense in the north eastern parts (96 per cent of LPA) of the country.

The rainfall in the central (103 per cent of LPA) and southern regions (102 per cent of LPA) of the country will be closer to the national average.

In 2019, the central and southern parts of the country had received maximum rainfall while the north, north western and north eastern parts of the country had received less rainfall.

For instance, Manipur in the north east had received 56 per cent less rainfall than normal while Haryana in the north had received 42 per cent less than normal rainfall between June 1 and September 30.

These are the normal onset and withdrawal dates for the southwest monsoon season over India. The month of July will be rainier in general, with rainfall intensity being 103 per cent of LPA while August will experience less rains with rainfall intensity being 97 per cent of LPA. 

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