In 1735, meteorologist George Hadley shook up his field by proposing a novel model of global atmospheric circulation, since named the Hadley cell, in which warm air rises at the equator and heads toward the poles before cooling and sinking back toward Earth’s surface at midlatitudes. Over time, especially within the past few decades, these large-scale circulation patterns have been visibly widening for reasons unknown to scientists.
The southernmost edges of the Hadley cell in the Southern Hemisphere are defined by the set of points where sea level pressure is highest. Using a technique called optimal fingerprinting analysis, Kim et al. compared a series of model simulations to examine long-term observed changes in these edges during the austral summers (December through February) of 1979–2009. They found that the southernmost edges of the Hadley cell over the Atlantic and Indian oceans expanded farther poleward during this period.
While examining causes of these trends, the researchers detected within the models strong signals of anthropogenic forcing, or human activities such as industry and agriculture, which has ultimately led to increases in greenhouse gases. Specifically, the models showed a link between the expansion of Hadley cells and the depletion of stratospheric ozone. Ozone depletion can lead to “holes” in the ozone, like the one detected over Antarctica in 1985. This would induce cooler conditions over Antarctica, shifting the lower-latitude circulation system poleward, including Hadley cells.
This newfound knowledge provides an important link in the chain for scientists seeking to understand Earth’s evolving climate. The authors note that the correlation they uncovered needs to be fleshed out with causes that pinpoint exactly how ozone depletion leads to Hadley cell expansion and what the future holds for Hadley cells if ozone does or does not recover. (Geophysical Research Letters, https://doi.org/10.1002/2016GL072353, 2017)
—Sarah Witman, Freelance Writer