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A new MIT study reveals that climate change is altering the atmospheric energy that feeds the summer weather, which could lead to more severe thunderstorms and more stagnant conditions for mid-altitude regions of the northern hemisphere , including North America, Europe and Asia.
Scientists report that rising global temperatures, especially in the Arctic, redistributes energy into the atmosphere: more energy is available to fuel thunderstorms and other local convective processes, while less energy is used for extratropical cyclones in summer – larger and milder weather systems that run for thousands of kilometers. These systems are normally associated with winds and fronts that generate rain.
"Extratropical cyclones ventilate air and air pollution. Therefore, with weaker extratropical cyclones in the summer, you are examining the potential for poorer air quality days in urban areas, "said Charles Gertler, author of the study, Science of Earth Sciences. Earth, Atmosphere and Planets (EAPS). "By going beyond the air quality in cities, you have the potential for more destructive thunderstorms and stagnant days, with perhaps longer heat waves. "
Gertler and his co-author, EAPS Associate Professor Paul O'Gorman, publish their results this week in the Proceedings of the National Academy of Sciences.
A shrinking gradient
Unlike more violent tropical cyclones such as hurricanes, extratropical cyclones are large-scale meteorological systems that occur downstream of the tropical zone of the Earth. These storm systems cause rapid changes in temperature and humidity along the fronts that extend over vast areas of the United States. In winter, extratropical cyclones can reach Nor'Easters; in the summer, they can bring everything from general cloudiness to light showers to heavy gusts and thunderstorms.
Extratropical cyclones feed on the horizontal temperature gradient of the atmosphere – the difference between average temperatures between north and south latitudes. This temperature gradient and the humidity in the atmosphere produce a certain amount of energy in the atmosphere that can fuel weather events. The more the gradient between the Arctic and the equator is important, the more extratropical cyclone will be strong.
In recent decades, the Arctic has warmed more rapidly than the rest of the Earth, reducing the horizontal temperature gradient of the atmosphere. Gertler and O'Gorman wondered whether and how this warming trend had affected the energy available in the atmosphere for extratropical cyclones and other summer weather events.
They began by reviewing a global reanalysis of recorded climate observations, known as ERA-Interim Reanalysis, a project that collects temperature and moisture measurements from satellite and weather balloon data from around the world. 1970s. From these measurements, the project produces a fine 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 of latitude. They took the average summer temperature and humidity in these areas, between June, July and August, each year from 1979 to 2017. They then introduced each annual summer mean of temperature and humidity into an algorithm developed at MIT, which estimates available energy 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 the amount of energy available for convection, which would manifest itself for example by thunderstorms, as opposed to large-scale circulations like extratropical cyclones, "said O & # 39; Gorman. said.
See the changes now
Since 1979, they have found that the energy available for large-scale extratropical cyclones has decreased by 6%, while the energy available for smaller, more local storms has increased by 13%.
Their results reflect some recent evidence in the northern hemisphere, suggesting that summer winds associated with extratropical cyclones have decreased with global warming. Observations made in Europe and Asia have also shown a rise in convective precipitation, such as that from thunderstorms.
"Researchers are finding that these wind and precipitation trends are probably related to climate change," says Gertler. "But this is the first time that we have robustly connected the average change in the atmosphere to these subdimensional events. We therefore present a unified framework that links climate change to the changing climate we observe. "
The researchers estimated the average impact of global warming on summer atmospheric energy in summer in the northern hemisphere. In the future, they hope to be able to solve this problem further, in order to see how climate change could affect the weather in more specific regions of the world.
"We would like to know what is happening with the energy available in the atmosphere and put trends on a map to see if it is rising in North America, as opposed to Asia and ocean regions," said O ' Gorman. "It's something that deserves to be studied further."
This research was funded by the National Science Foundation.
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