Number Theory: Second warmest start to winter since 1951

State-wise averages of minimum temperatures show that of the 30 states and UTs, only J&K and Ladakh have experienced a cooler than normal start to winter

Temperatures across India have started decreasing with the start of the winter season, which officially runs from December to February. However, an HT analysis shows that the season has been unusually warm compared to winter seasons so far by minimum temperatures. While the reason for this warm beginning to winter is unseasonal rain in parts of the country due to Cyclone Michaung, a strong El Niño prevailing currently means that this warming is likely to continue. Here are four charts that explain these trends.

People are going to their work amid cold and smoggy weather at South-West Delhi near Samalkha Chowk (HT File Photo/Parveen Kumar)

Second warmest start to winter since 1951

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  India’s average minimum temperature up to December 12 is 14.8 °C. This is 2.3 °C warmer than the 1981-2010 average, considered normal temperature in India. This has made the first twelve days of the 2023-24 winter the second warmest since 1951, the first year for which the India Meteorological Department (IMD) has published gridded data for temperature. To be sure, this warming in minimum temperatures has decreased in the past three days. The first nine days of the month were all at least 2 °C warmer than normal. The last three days have only been 1.2 °C, 0.5 °C, and 0.7 °C warmer than normal. A similar gradual cooling also happened in the warmest December month on record (2019) in India. The first 12 days of December 2019 were 3.2 °C warmer than normal on average. However, the month ended up only 2.8 °C warmer than normal. To be sure, the trend in maximum temperatures is opposite to that of minimum temperatures so far. India’s average maximum up to December 11 (December 12’s maximum temperatures will only be available on December 13) is 25.6 °C, 0.95 °C cooler than normal and the ninth coolest since 1951.

All states warmer than normal, eastern-central states 4-6 degrees warmer than normal

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  State-wise averages of minimum temperatures show that of the 30 states and union territories (UTs) for which this calculation is possible, only Jammu & Kashmir and Ladakh have experienced a cooler than normal start to winter. However, a warming in the rest of the country does not mean that they are all experiencing a similar departure from normal. The minimum in Madhya Pradesh and Bihar is around six degrees warmer than normal. This trend of high warming is reflected across states in eastern and central India. On the other hand, north-western states such as Rajasthan, Punjab, Haryana, and the UT of Delhi are only 2.5-3 °C warmer than normal and the hilly states of Himachal Pradesh, Nagaland, and Sikkim are less than a degree warmer than normal. A week-wise breakup further shows that the warming has decreased in 21 of 30 states and UTs in the second week of the month, with only southern and northeastern states becoming warmer.

Rain due to Cyclone Michaung one reason behind warmer than normal minimums

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  One reason why minimum temperatures were much warmer than normal in eastern and central states in the first week of December is the unseasonal rain brought by Cyclone Michaung. Rain tends to increase minimum temperatures because clouds trap outgoing heat at night when minimums are recorded. This seems to have played a role in a warm beginning to winter. While southern states such as Tamil Nadu, Karnataka, Kerala, and parts of Andhra Pradesh and Telangana do receive rain in December due to the northeast monsoon (this season runs from October to December), other states usually don’t. However, all big states except Karnataka received surplus rain in the first week of December this year. As is expected, Andhra Pradesh – where Michaung’s landfall happened – received the most rain in absolute terms in the first week of December: 139 mm. Similarly, there was unusually high rainfall along individual places on the east coast. For example, according to IMD’s gridded dataset, Chennai and neighbouring districts (Thiruvallur and Kancheepuram) received 151 mm of rain in the 24 hours ending 8:30 AM on December 4. This is the 19th highest daily rain for any day since January 1, 1901, for these three contiguous districts.

However, El Niño can keep winters warm even after Michaung

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  While warming in minimum temperatures has decreased after Cyclone Michaung, current IMD forecasts show warming in the northern half of the country again in the last week of December. In fact, IMD has forecast a warmer than normal winter as a whole. One big reason for this is the prevailing El Niño situation, which increases global temperatures. El Niño is the periodic warming of the equatorial Pacific Ocean and has been intensifying almost continuously this year. In the week ending May 31, sea surface temperatures in the Nino 3.4 region (a part of the Pacific Ocean monitored for El Niño trends) were 0.8 °C warmer than normal. This warming reached a high of 2.1 °C in the week ending November 22 and was 1.9 °C in the week ending December 6. Since most models predict this warming to remain above the threshold of 0.5 °C up to the April-June season, its warming effect is likely to be active the entire 2023-24 winter season.