Climate change making summer weather stormier: Study

Washington: Climate change is shifting the energy in the atmosphere that fuels summertime weather, which may lead to stronger thunderstorms and more stagnant conditions for regions of the Northern Hemisphere, including India, an MIT study has found.

The study, published in the journal Proceedings of the National Academy of Sciences, found that rising global temperatures, particularly in the Arctic, are redistributing the energy in the atmosphere. More energy is available to fuel thunderstorms and other local, convective processes, while less energy is going towards summertime extratropical cyclones -- larger, milder weather systems that circulate across thousands of kilometres. These systems are normally associated with winds and fronts that generate rain. 

"Extratropical cyclones ventilate air and air pollution, so with weaker extratropical cyclones in the summer, you are looking at the potential for more poor air-quality days in urban areas," said Charles Gertler, a graduate student at the Massachusetts Institute of Technology (MIT) in the US. "Moving beyond air quality in cities, you have the potential for more destructive thunderstorms and more stagnant days with perhaps longer-lasting heat waves," Gertler said. 

In contrast to more violent tropical cyclones such as hurricanes, extratropical cyclones are large weather systems that occur poleward of the Earth's tropical zone. Extratropical cyclones feed off the atmosphere's horizontal temperature gradient -- the difference in average temperatures between northern and southern latitudes. This temperature gradient and the moisture in the atmosphere produces a certain amount of energy in the atmosphere that can fuel weather events. The greater the gradient between, say, the Arctic and the equator, the stronger an extratropical cyclone is likely to be. 

Researchers looked at the ERA-Interim Reanalysis, a project that has been collecting available satellite and weather balloon measurements of temperature and humidity around the world since the 1970s. From these measurements, the project produces a fine-grained global grid of estimated temperature and humidity, at various altitudes in the atmosphere. From this grid of estimates, the team focused on the Northern Hemisphere, and regions between 20 and 80 degrees latitude. They took the average summertime temperature and humidity in these regions, between June, July, and August for each year from 1979 to 2017. 

The researchers then fed each yearly summertime average of temperature and humidity into an algorithm that estimates the amount of energy that would be available in the atmosphere, given the corresponding temperature and humidity conditions. "We can see how this energy goes up and down over the years, and we can also separate how much energy is available for convection, which would manifest itself as thunderstorms for example, versus larger-scale circulations like extratropical cyclones," said Associate Professor Paul O'Gorman from MIT. 

Since 1979, they found the energy available for large-scale extratropical cyclones has decreased by six per cent, whereas the energy that could fuel smaller, more local thunderstorms has gone up by 13 per cent. Their results mirror some recent evidence in the Northern Hemisphere, suggesting that summer winds associated with extratropical cyclones have decreased with global warming. Observations from Europe and Asia have also shown a strengthening of convective rainfall, such as from thunderstorms.