Hadley cells, first theorized by the English physicist and meteorologist George Hadley in 1735, are large-scale atmospheric circulations caused by warm air rising in the tropics and flowing toward the poles and then cooling off, descending, and flowing back toward the equator. These wind cells have a major effect on weather patterns in subtropical regions, including portions of the southern United States.
Numerous studies have shown that the Hadley cells have been expanding farther toward the poles over the past 30–40 years, in part because of natural climate variations and in part because of climate change. Most climate models predict that the Hadley cells will continue to expand poleward in the future as a result of climate change, affecting water availability in the subtropics by enlarging existing dry, desert regions.
Although past studies have generally assumed that this expansion of dryness will be symmetrical across the globe, Schmidt and Grise tested this theory by comparing localized changes in rainfall and sea level pressure. To conduct their study, the researchers used 15 different climate models along with observed wind, sea level pressure, and rainfall data collected by the European Centre for Medium-Range Weather Forecasts, the Global Precipitation Climatology Project, and the National Weather Service’s Climate Prediction Center (among several others) from about 1980 to 2015.
The team’s calculations revealed that the effects of Hadley cell expansion are more localized than is commonly assumed. They found that most of the affected regions are located over oceans and the western coasts of continents, posing less of a direct threat to other land regions.
This study disproves a popular assumption about climate change, the idea that as the Hadley cells expand poleward, they are systematically drying out entire regions of the subtropics. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075380, 2017)
—Sarah Witman, Freelance Writer