Climate change is expected to increase Earth’s average surface temperature as well as boost the frequency of summer heat waves and winter cold snaps, which can have devastating consequences for human life, ecosystems, and the economy. Scientists have hypothesized that shifts in the location and degree of narrow, meandering bands of wind called jet streams could reduce the speed of eddies that drive upper atmosphere circulation. Such changes may play an important role in generating extreme temperature events, but the effects of these proposed mechanisms have been difficult to isolate.
Now Linz et al. have developed a simple model to disentangle how some mechanisms could affect the statistical distribution of future surface temperatures. They used a two-dimensional model stirred by natural, large-scale waves in the atmosphere and the ocean, which are associated with high- and low- pressure systems that drive local weather. With this tool, they explored how variations in the background (equator-to-pole) temperature gradient, the location of the waves that mix the upper atmosphere, and other factors could influence temperatures above the Southern Hemisphere.
A researcher demonstrates the behavior of baroclinic eddies with a simplified DIYnamics model, setting up the gradient by putting a can of ice in the middle and then spinning up the system to show the mixing of a temperature gradient in a rotating fluid system. The instabilities that develop and turn into the eddies are similar to those in the simple computer model used by Linz et al. Credit: TEXTSpencer Hill/DIYnamics
They found that when they varied the location at which midlatitude winds are stirred, it strongly affected the range of resulting temperatures. When they shifted the latitude at which the wind is stirred toward the South Pole, the cold extremes became colder throughout the midlatitudes. By contrast, when they decreased the background temperature gradient, it reduced the range of temperature values across the Southern Hemisphere. This finding agrees with a number of comprehensive climate model predictions.
By offering a physical basis for understanding temperature observations in a warming climate, this model will serve as a powerful new tool for investigating how potential changes in atmospheric circulation may affect the distribution of temperature—and thus the frequency of extreme temperature events—in the Southern Hemisphere. As a next step, the researchers plan to incorporate additional features into the model to enable comparisons with data from the Northern Hemisphere, which is dynamically quite different because of the configuration of the continents and a greater proportion of land compared with the Southern Hemisphere. (Geophysical Research Letters, https://doi.org/10.1029/2018GL079324, 2018)
—Terri Cook, Freelance Writer
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