Explained | Why did Ooty have such a cold Pongal?

At around 7 am on January 14, the ground temperature was measured to be –6.3º C in Fingerpost.

Updated - January 16, 2023 10:09 pm IST

Published - January 16, 2023 02:06 pm IST

The ground temperature was below freezing point at several places in Udhagamandalam, January 14, 2022.

The ground temperature was below freezing point at several places in Udhagamandalam, January 14, 2022. | Photo Credit: Sathyamoorthy M.

At around 7 am on January 14, as the day before Pongal dawned on Udhagamandalam, a local temperature gauge measured an unusually frigid ground temperature of –6.3º C in the town’s Fingerpost locality. The Government Botanical Garden said it was sprinkling water on the ground and covering flowering plants with another bushy plant to remove the rime.

The Hindu also reported that the ground in other parts of Udhagamandalam had reached subzero temperatures as well. The lowest ambient temperature in the day was a relatively more tolerable 1.7º C. What had caused the mercury to dip so low in Fingerpost?

The answer, as is often the case with the weather in the 21st century, begins somewhere else on the planet. In this case, it’s the equatorial Pacific Ocean. “We are in a La Niña winter,” Raghu Murtugudde, a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland, said. This means heady winds blow warm water on the sea surface away from the South American mainland, roughly off the coast of Ecuador.

This heat movement across the Pacific has global consequences. Over India, the La Niña can intensify the summer monsoons and bring excess rainfall, and cause colder winters. At the start of 2022, the World Meteorological Organisation said the ongoing La Niña is the 21st century’s first ‘triple dip’: spanning three consecutive winters. But in a break from convention, the coldness is deeper in the south. This reveals the second driver.

As a phenomenon, La Niña is the opposite of El Niño, in which equatorial waters off the coast of South America become unusually warmer. One effect is that in winter, the subtropical westerly jet over North India is pushed southward, allowing the western disturbance to create cold winters in the north. But in La Niña years, there is a ‘highway’ of chill wind coming southward from the Siberian High, “a cold, high-pressure block [of air] that is occupying the central Asian region and affecting winds coming into India,” in Prof. Murtugudde’s words.

The Siberian High is responsible for the bitter-cold of the tundra and has been known to affect the weather from Italy to the Philippines. But this time, according to him, it is “anomalously strong”.

Given the panoply of factors, it’s harder to simulate or predict hyperlocal conditions, but by and large, the third La Niña winter in a row plus an unusually strong Siberian High conspired to create a cooler-than-normal winter in South India. The temperature further dropped in Tamil Nadu’s interior areas thanks to the withdrawal of the northeast monsoons from January 12, which allowed the cooler dry-land winds to become stronger.

Taken together, Udhagamandalam — a hill-station — normally has lows of 5-10º C but on Saturday experienced a low of 1.7º C and a ground temperature of less than 0º C in some parts.

Unlike the El Niño-driven cold air that sweeps India between the southeast and the northwest, in La Niña years “the winds mostly tend to come from the north and zip down the pressure trough far into peninsular India,” Prof. Murtugudde said. Thus, they cover more area and affect more people.

Scientists expect climate change to affect El Niño and La Niña, but the precise mechanisms are under study. Predictions of the seasonal weather over India are further complicated, among others, by the temperature of the Indian Ocean, the monsoons, wind systems like the subtropical westerly jet, and the fate of Himalayan glaciers.

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