Winter rains throttled by global warming: Study

As December and February set records for the highest mean temperature in over 100 years, experts and scientists said global warming has changed the dynamics of the 'Western Disturbances' (WD) that play a key role in bringing rain during the Indian winter and keeping the temperatures low.

WDs originate in the west and travel in the upper atmosphere to arrive at the Indian sub-continent during the winter, picking up a lot of moisture all the way from the Atlantic Mediterranean sea. As they move west, they merge with the mid-tropospheric low over India's western coast, picking up more moisture from the Arabian sea. When they hit the Himalayas, moist air is lifted, leading to the formation of precipitation mechanisms. This phenomenon, however, was affected this year with the Indian Meteorological Department (IMD) announcing a 68 per cent deficit in rainfall in February compared to the normal rain of 21.8 mm.

The average maximum temperature in February was 29.64 degrees C, the highest since recording began in 1901, as against the normal 27.80 degrees C.

A study by Climate Trends stated that a number of WDs moved west but turned out to be too feeble. Three of the five WDs in November, six of the seven in December and three of the seven in January were identified as feeble WDs that failed to cause the desired action. The ones that succeeded brought a very little amount of rain, failing to provide the usual respite.

In February, the arrival of the westerly winds in quick succession led to the blocking of the chilly northerly winds from the Himalayas. "As a result, maximum temperatures continue to rise and settle well above the normal average," it said.

Director of the Indian Institute of Geomagnetism Dr A P Dimri attributed the change to global warming.

“In this changing world, WDs are showing distinct structural and dynamical changes. With global warming, WDs are getting lighter due to more convection and heat coming in. With this, they are tracking in the upper atmosphere. Besides, they have become more dynamic also. Off late, it is seen that all WDs do not precipitate. This means there are days when there is no precipitation happening during their passage and contradictorily there are non-WD days when good amounts of precipitation are seen over North India,” he said.

The South Peninsula has been recording some rains on account of a few low-pressure areas and depressions forming in the South Bay of Bengal, but their impact was confined to southern parts only.

The peninsula also missed its date with the rains as La Nina, an oceanic-atmospheric phenomenon that played a role in bringing rains to the region during the end of the northeast monsoon.

The study said the La Nina-led cooling impact was brief and it was not able to reverse the long-term warming trend caused by record levels of heat-trapping greenhouse gases. The condition was similar in 2021 and 2022 when, despite the cooling impact, they were listed among record hot years.

G P Sharma, president, of meteorology and climate change at Skymet, said, "Apparently, all the naturally occurring climate events like El Nino /La Niña /IOD now take place under the shadow of more powerful human-induced climate change. La Niña cooling of 2020-2022 did not suffice and the years still got listed among the hottest on record.”