Monsoon arrives over Bay of Bengal and Andaman & Nicobar archipelago

The winds of the South West Monsoon (SWM) arrived in the Bay of Bengal on May 19, 2024, three days ahead of the normal date of May 22, according to data from the India Meteorological Department (IMD).

The weather agency had earlier forecast that the onset of the SWM over Kerala would happen on May 31, one day earlier than the normal date of June 1.

The IMD declares the onset of the SWM over the south Bay of Bengal, south Andaman Sea and Andaman and Nicobar Islands when certain parameters are met. These include the increase in strength of westerly winds, prevalence of south westerly winds, increase in cloudiness and continuous rainfall over the region.

On April 15, IMD had forecast an above normal rainfall for the SWM season, mainly owing to the expected La Nina phenomenon in the equatorial Pacific Ocean, positive Indian Ocean Dipole conditions in the Indian Ocean and reduced snow cover over the northern hemisphere in the spring and winter months of 2024.

The Department expects the seasonal rainfall for SWM to be 106 per cent of the long period average (LPA) of 870 mm. The LPA is the average rainfall that occurred during the monsoon season (June-September) between 1971 and 2020.

There is also a 30 per cent chance of the season ending up with excess rainfall (>110 per cent). There is only a 10 per cent chance that the season receives below normal or deficient rainfall.

With regard to the distribution of the rainfall during the season, the IMD says that except some eastern parts of the country and most of the Northeast, all regions may receive good rainfall.

The winds may be encountering a low pressure area forming in the Bay of Bengal in the next few days. There is a chance of intensification of the low pressure area into a cyclone.

The only “restricting factor could be a quick northward progression of the monsoon, suppressing the vertical formation of the cyclone”, wrote climate scientist Roxy Mathew Koll on the social media platform Facebook.

IMD has not put out an alert regarding the cyclone as yet. Tamil Nadu and Kerala have been put on red alert for extremely heavy rainfall between May 19 and May 22 due to a cyclonic circulation in south interior Tamil Nadu and an extended low pressure region from central Maharashtra to the cyclonic circulation.

The last record breaking triple dip La Nina phenomenon that occurred between 2020 and 2023, led to extended SWM seasons in 2021 and 2022. This, in turn, resulted in floods in many Indian states.

La Nina is the cooler than normal phase of the El Nino Southern Oscillation (ENSO) phenomenon in the equatorial Pacific Ocean and is generally responsible for higher than normal rainfall over India. The opposite happens during El Nino, which is the warmer than normal phase of the ENSO.

A map released by the IMD

The IMD said in the press release dated April 15:

The latest Monsoon Mission Climate Forecast System (MMCFS) as well as other climate model forecasts indicate that the El Nino condition is likely to weaken further to neutral ENSO conditions during early part of the monsoon season and La Nina conditions are likely to develop during second half of monsoon season.

The agency added: “At present, neutral Indian Ocean Dipole (IOD) conditions are prevailing over the Indian Ocean and the latest climate model forecasts indicate that the positive IOD conditions are likely to develop during the latter part of the southwest monsoon season.”

During a positive Indian Ocean Dipole, the sea surface temperatures in the western parts of the Indian Ocean become warmer than normal and the eastern parts become colder than normal. This generally increases rainfall over India, especially during the SWM season.

Another factor that affects monsoon rainfall over India is the extent of the snow cover in the northern hemisphere, especially Eurasia. IMD said this has an inverse relationship with SWM rainfall and further added that “the Northern hemisphere snow cover extent during the last three months (January to March 2024) was below normal.”

For the past few years, the distribution of rainfall over India has been a big concern. Many places have received excessive rainfall for short time periods, interspersed with long dry spells. The torrential rains in short durations, symptomatic of global warming and consequent climate change, have often caused debilitating floods and flash floods. What the SWM season, which is responsible for 70 per cent of annual rainfall for most of India, brings with it in 2024, is yet to be seen.

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