Climate Change

What is behind deadly summer heatwaves in north-central India? 

Pacific Meridional Mode may be a factor

 
By Rohini Krishnamurthy
Published: Thursday 15 September 2022

Deadly summer heatwaves in north-central India can be linked to a climate pattern in the Pacific called the Pacific Meridional Mode (PMM), according to a new study.

PMM evolves via interactions between the ocean and the atmosphere. A positive PMM phase is characterised by warm sea surface temperatures, according to experts.

Heatwave intensity and duration over north-central India significantly increased when PMM was in a strong positive phase, the study published in Nature Communications showed

“In general, we notice that during the positive phase of PMM, the temperature over north and central Indian region is higher than its mean temperature,” Vittal Hari, assistant professor, Indian Institute of Technology (Indian School of Mines), Dhanbad, told Down To Earth.

During the negative phase, he added, the region may witness less-than-average temperature.

“The main finding of our study is the far-reaching impact of PMM and its novel association to extreme temperatures over the northern Indian region,” the paper read.

In recent decades, heatwaves have caused more deaths than any other natural hazard, the study stated.

“The recent deadly heatwaves of 2015 and 2019, claiming more than 1,200 lives, occurred during late May and extended till June,” the study highlighted. North-central India, in particular, is a hotspot for heatwave events.

Though the impacts of Indian summer heatwaves on human health have become well-known in recent years, there is little understanding of what factors drive the heatwave events in the country, the researchers wrote in the study.

So, experts from India, Europe and the United States collected daily maximum temperature and rainfall data from the India Meteorological Department from 1951–2019 to investigate the factors responsible for heatwaves.

They also performed climate model experiments to understand the links between the heatwave and PMM, focusing on the last 500 years.

The team suspected El Niño / Southern Oscillation (ENSO), a climate pattern emerging from the interaction between the atmosphere and ocean in the tropical Pacific, could be responsible. 

But nearly 66 per cent of the total heatwave events had occurred during non-ENSO years, the study stated.

Next, they looked at PMM. Their analysis showed that the PMM was exerting significant control on the May-June heatwaves over north-central India.

The PMM, the researchers explained, weakens a zonal Walker circulation over the Pacific. 

A Walker circulation is like a loop, with airflow in the lower part blowing from east to west across much of the tropics and the flow in the upper part from west to east at higher altitudes, according to experts.

A weakened Walker circulation, in turn, changes lower-level westerlies (winds blowing from the west in mid-latitudes) into easterlies (dry, cold prevailing winds blowing from the east over the Indian ocean).

This, according to the study, creates conditions ripe for hot and dry summers over north and central India.

It could impact the monsoon, he said. “A positive PMM phase is characterised by the weakening of westerlies over the Indian region during June, and these processes delay the monsoon onset.”

When sufficient moisture is not entering the Indian landmass, there is a decline in rainy days over India, specifically during June. “This kind of exacerbates the heatwaves,” he added.

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