This year, the southwest monsoon season has shown unusual movement of its main rainfall causing low pressure region (known as the monsoon trough) and drastic variations in rainfall including extreme rainfall events and floods across the country.

Apart from the initial stalling of winds in June, the monsoon has not taken its usual break and has also produced a rare cyclone in the Arabian Sea in August, which almost completely intensified on land.

In the coming months the extreme rainfall may continue, even into late October.

The conditions, including the expected La Niña event, seem to be eerily similar to the year 1999 when back to back cyclones struck Odisha in October, the most destructive amongst them being the Odisha super cyclone that claimed as many as 10,000 lives.

Monsoon rainfall distribution so far

Currently, the country has an overall excess rainfall of seven per cent with deficient rainfall in eight of the states between June 1 and September 1. The highest deficit in terms of percentage of normal rainfall has been recorded in Nagaland and Manipur, both at 28 per cent.

Bihar, Punjab and Chandigarh are facing a deficit of 25 per cent. One fourth of the districts of the country are under deficient (between 20 and 59 per cent less rainfall than normal) or large deficient (between 60 and 99 percent less rainfall than normal) rainfall.

For many periods in the last few months, the monsoon trough, which is the main rain causing low pressure region, has remained in a skewed position with one end of the trough at a normal position while the other end being south or north of its normal position or vice versa.

As on September 2, the western end of the monsoon trough is at its normal position and its eastern end is south of its normal position.

When the monsoon trough is south of its normal position the rainfall shifts towards southern India while the core monsoon zone and northern and eastern India do not get enough rainfall.

When the trough moves north of its normal position, the monsoon is said to be on a break. During such a period there is rainfall in the foothills of the Himalayas and in northeast India, while the rest of the country does not get much rainfall.

This year’s similarities with 1999

The similarities with 1999 start off with the extreme heat in the month of April.

The April month of 1999 was the hottest April of the 20th century with much of northwest and central India recording 40°C or above for a fortnight.

April 2024 was the hottest April on record with humid heat waves in large parts of the country. The year 1999 was also a La Niña year and followed a strong El Niño in 1997-98, similar to 2024.

After a deficit rainfall in August, 1999, September experienced above normal rainfall. A similar prediction has been made for September, 2024, by the India Meteorological Department (IMD), though the month of August was not without rainfall this year and in fact witnessed catastrophic flash floods in places like Himachal Pradesh, Gujarat and Telangana.

The IMD has also made a prediction of warmer than normal maximum and minimum temperatures across most of the regions of the country which could lead to a hot and humid September with continuing extreme rainfall events.

“Well, I think Cyclone Remal affected the onset and it seemed like an on time onset but June was mostly deficient. July and August did better but the distribution showed some familiar patterns and some new hotspots”, Raghu Murtugudde, professor of climate studies at Indian Institute of Technology, Bombay and emeritus professor at the University of Maryland told Down To Earth.

“The central-east part (such as Bihar) has remained deficit as it has done in the last few years and it may again get more rain as late season extremes could mean October rains,” Murtugudde added.

“But a hotspot of dry anomalies persisted over eastern Gujarat and western Madhya Pradesh which were quite hard to understand. And the northern region and the Himalayan foothills (Himachal Pradesh and Jammu and Kashmir have 23 percent deficits) are stubbornly dry as well,” Murtugudde explained.

The expert noted that this is most likely related to the record heat of 2023-24 and the widespread extreme weather patterns across the planet.

“The expected transition to La Niña is likely delayed as the ENSO Transition Mode is not favourable,” said Murtugudde.

According to the IMD, La Niña, the cooler than normal phase of the El Niño Southern Oscillation (ENSO) phenomenon, is expected to develop in the equatorial Pacific Ocean towards the end of the monsoon season.

Interestingly, Murtugudde further remarked that ‘we keep on looking for external forcing of the monsoon and forget that the monsoon itself is a significant heat source with a mind of its own'.

Other climatic factors that influence rainfall during the monsoon season such as the Indian Ocean Dipole (IOD) are minor and most probably related to the monsoon itself and the ENSO and not vice versa, according to Murtugudde.

During a positive IOD event, the sea water in the western Indian Ocean is warmer than usual and it is cooler than usual in the eastern Indian Ocean which generally increases monsoon rainfall over India.

“So given the warm oceans and land, the monsoon has danced on its own and is likely to have an influence on the oceans. The monsoon surprises us every year because we have no clue what to expect. We need to understand its intrinsic variability better, rather than looking at ENSO and IOD and so on,” Murtugudde concluded.

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