A regime shift around 1998 changed the pre-monsoon temperature pattern over North-Central India. Researchers say a northward-moving subtropical westerly jet is associated with more persistent heatwaves, while global warming adds another layer of risk.

But a study published in Geophysical Research Letters points to another piece of the puzzle one that is several kilometres above the ground.

Researchers found that the atmospheric circulation over North-Central India underwent a significant shift around 1998. Since then, the subtropical westerly jet, a fast-moving band of winds in the upper atmosphere, has migrated northward. The researchers link this change to a rise in pre-monsoon temperatures and changes in heatwave behaviour across the region.

The finding matters because heatwaves are not produced by surface temperature alone. They are also shaped by the large-scale movement of air, pressure systems, soil moisture and atmospheric circulation.

 

A warmer baseline after 1998

The study by R. Jha, A. Mondal, S. Ghosh and R. Murtugudde examined temperature and atmospheric data from 1973 to 2022, focusing on March and April, the pre-monsoon period over North-Central India.

The researchers identified a change point around 1998. Their analysis found that the average pre-monsoon maximum temperature in the region was approximately 0.7°C higher after 1998 than in the earlier regime.

The difference can be seen not simply as a gradual climb but as a change between two statistical regimes.

Before the shift, the spatially averaged pre-monsoon temperature was around 36.7°C. In the later period, it rose to approximately 37.4°C, according to the study.

That may sound like a relatively small change. But when temperatures are already close to dangerous thresholds, even a modest increase in the baseline can make extreme heat more likely.

 

The jet stream connection

The subtropical westerly jet is a powerful belt of winds high in the atmosphere. Its position influences weather patterns across the subtropics.

The researchers found evidence that the jet shifted towards higher latitudes after 1998. They argue that this northward displacement created atmospheric conditions favourable for persistent heat over North-Central India.

One consequence is an increase in anticyclonic conditions — areas associated with descending air, clearer skies and stronger heating near the surface.

The study found that the meridional shift of the jet could explain more than 25% of the variability in heatwave characteristics over North-Central India. Importantly, this does not mean that the jet caused 25% of all warming or that global warming is responsible for only the remainder. It means the observed jet displacement was statistically associated with more than a quarter of the variability in the heatwave characteristics examined by the researchers.

 

Natural variability or global warming?

This is where the story becomes more complicated.

The researchers describe the late-1990s change as a regime shift and note that it is consistent with the broader climate shift around 1997–98. They also discuss possible links to major changes in the Pacific Ocean around that period.

But the paper does not establish a single cause for the regime shift. The authors explicitly say they have not demonstrated what triggered the shift in temperature and winds. They note that anthropogenic warming could have contributed, alongside natural climate variability.

That distinction is important.

The study is not saying that heatwaves in India are simply a product of natural variability. Nor does it suggest that global warming can be dismissed.

Instead, it points towards an interaction: a warming world may be operating alongside changes in atmospheric circulation that determine where and when extreme heat becomes concentrated.

 

The heat is already being observed

According to the India Meteorological Department, 2024 was India's warmest year since nationwide records began in 1901, with the annual mean land-surface air temperature 0.65°C above the 1991–2020 average. Heatwave conditions occurred over much of northwest India in May and parts of northern and central India in June.

During May 2024, persistent heatwave to severe heatwave conditions affected northwest and adjoining central India and Gujarat before extending into parts of Chhattisgarh, Bihar and Vidarbha.

The extremes are particularly significant because North-Central India is densely populated and includes large agricultural areas. Heat affects not only outdoor workers but also electricity demand, water availability, crop productivity and public health.

 

Why accumulated heat matters

A heatwave is not only about the day's maximum temperature.

A sequence of moderately extreme days can produce substantial cumulative heat stress. The body gets less opportunity to recover when nights remain warm and daytime temperatures repeatedly climb into dangerous territory.

The researchers found changes in heatwave frequency, duration and cumulative heat between the two atmospheric regimes.

This creates a challenge for early-warning systems.

Forecasting a single exceptionally hot day is different from anticipating a circulation pattern that could keep a region unusually hot for days.

Understanding the behaviour of the jet could therefore have implications for seasonal and subseasonal heatwave forecasting, the researchers suggest.

 

What happens next?

The important question is whether the post-1998 atmospheric pattern will persist, intensify or change again.

Climate adaptation cannot rely only on tracking the long-term rise in average temperatures. It also needs to account for the atmospheric circulation patterns that determine where extreme heat is concentrated.

For cities, this means heat action plans, cooling centres, water access and electricity planning need to consider prolonged heat rather than isolated temperature records.

For agriculture, accumulated heat can affect crops even when individual daily temperatures do not set national records.

And for public health, the combination of a higher temperature baseline and circulation patterns that favour persistent heat could make seemingly familiar pre-monsoon conditions increasingly hazardous.

The central message from the research is therefore more nuanced than “global warming is causing India's heatwaves.”

The atmosphere itself is changing its circulation patterns.

Over North-Central India, researchers have found evidence that a shift in those winds around the late 1990s helped create a warmer, more heatwave-prone regime, now superimposed on a planet that continues to warm.