High-Altitude Winds During Summers Shifted Northwards, Worsening Heatwaves in North, Study Finds

High-altitude winds during summers have shifted northwards in recent decades, worsening heatwaves over north central India, a study led by the Indian Institute of Technology, Bombay has found.

The northward shift in these winds at 10-20 kilometres above sea level, along with rising temperatures post-1998, accounts for 25 per cent of the changes in frequency, duration and overall heat during heatwaves in the region, researchers found.

A temperature increase in the Pacific Ocean around 1998 likely triggered the change in wind patterns, which was possibly exacerbated by global warming, the researchers suggested.

"This change in high-altitude wind patterns is making heatwaves happen twice more often as compared to pre-1998, with more than double cumulative heat during the heat wave. This is really worrying for the large populations who live in this area," corresponding author Arpita Mondal, Associate Professor at IIT Bombay, said.

Incorporating these wind patterns in climate models can help predict heatwaves more accurately, thereby informing forecasts and long-term projections, and developing heat action plans, the authors said. The findings are published in the journal Geophysical Research Letters.

While while global warming is increasingly making heatwaves more frequent and intense, the study showed that changing patterns in winds at high-altitudes is also playing a role, the authors said.

The India Meteorological Department (IMD) defines a heatwave in the plains as a temperature of 45 degrees Celsius or above, with a lower threshold in other regions.

For the analysis, the researchers used temperature datasets of the IMD and weather-related ones from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis from 1973 to 2022.

Lead author Roshan Jha, from IIT Bombay, said, "We found that since 1998, the temperature over North-Central India has increased by about 0.7 degrees Celsius during the pre-monsoon summer season. This increase appears to be linked to the northward movement of subtropical westerly jet, the band of strong upper tropospheric winds."

"We find that the meridional shift in the subtropical westerly jet explains more than 25 per cent of the variability in heatwave characteristics over North-Central India, implying that the increase in heatwaves post-1998 is associated with a northward shift of the jet," the authors wrote.

"This change (in wind patterns) isn't just affecting India but also the nearby areas like Pakistan and Middle East," said Jha.

The research shows how important it is to understand changes in wind patterns, whether they're natural or caused by climate change, are making heatwaves worse in this region, he said.