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News / Comment

25MAR
2015
NEWS / Forecast of less stormy weather may not always be good news...
Category: Agribusiness & Forestry, Latest News, Water & Sanitation

Image: Two people out in a canoe on Round Lake with a receeding summer storm in the background: CC by 3.0

By Kieran Cooke

Storms on fine summer days might be unwelcome to many, but at least the rain and winds does act like a big brush on the weather system − bringing fresh air and relief from oppressive heat.

And scientists now warn that a decrease in the frequency of such storms across much of the US, Europe and Russia in recent decades − with climate change the probable cause – could mean that summer heat waves and droughts are likely to become ever more persistent and intense.

Scientists from the Potsdam Institute for Climate Impact Research (PIK) in Germany report in Science journal that storm activity data they collected from weather stations and satellites shows a clear reduction in the frequency and intensity of summer storms in the mid-latitudes of the northern hemisphere over recent decades.

 

Heat extremes

This makes heat extremes – such as the period of intense heat that hit Russia in 2010, causing widespread crop failure and multiple wildfires – ever more likely.

“While you might expect reduced storm activity to be something good, it turns out that this reduction leads to a greater persistence of weather systems in the northern hemisphere mid-latitudes,” says Dim Coumou, an Earth systems analyst at PIK and lead author of the study.

“In summer, storms transport moist and cool air from the oceans to the continents, bringing relief after periods of oppressive heat. Slack periods, in contrast, make warm weather conditions endure, resulting in the build-up of heat and drought.”

The PIK study looks at a particular set of turbulences − called synoptic eddy − in weather systems over the summer months, and calculates the total energy of their wind speeds.

It shows that the level of this energy, which measures the interplay between the intensity and frequency of high and low pressure systems in the atmosphere, has dropped by approximately 10% over the past 35 years.

Previous studies have focused mostly on winter storms, which tend to do more damage than those in summer. The PIK study found that average storm activity in the winter months in many regions is largely unchanged.

The Arctic region probably holds the key to the drop in summer storm activity, say the scientists.

Temperatures around the globe are rising due to greenhouse gas emissions caused by the burning of fossil fuels, but the rate of warming is faster in the Arctic.

As the sea ice cover in the Arctic shrinks, the surface reflects less sunlight and absorbs more heat. The warmer waters then warm the air, setting in motion a process through which the relative difference in temperature is reduced between the cold polar region and the rest of the northern hemisphere.

 

Air circulation

Temperature differences drive air circulation. As the difference in temperatures between the two regions decreases, so does the rate of summer storm activity.

The study also found that this reduction in the temperature differential weakens the polar jet stream, which − often travelling at speeds of more than 200 miles per hour high up in the troposphere − acts as a boundary between the cold polar air and warmer air further south.

“From whichever angle we look at the heat extremes, the evidence we find points in the same direction,” Dim Coumou says.

“The heat extremes do not just increase because we’re warming the planet, but because climate change disturbs airstreams that are important for shaping our weather.

“The reduced day-to-day variability that we observed makes weather more persistent, resulting in heat extremes on monthly timescales. So the risk of high-impact heat waves is likely to increase.”

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