Expansion of the Hadley Cells : Discovering the anthropogenic fingerprints behind climate change
In 1735, an English lawyer and amateur meteorologist by the name of George Hadley proposed an atmospheric circulation model by which the trade winds are sustained. At the time, his theory was novel and remained relatively unknown. However, by the second half of the 19th century, the theory gradually became known as ‘Hadley’s Principle.’
Today we refer to the global scale tropic atmospheric circulation between the equator and subtropics as a Hadley cell. Solar heating, which is greatest near the equator, results in the buoyancy of warm air that rises and moves poleward and up to the tropopause. As the air cools in the high altitude, it sinks near the 30th parallels and moves back towards the equator as it nears the surface. The Coriolis Effect causes the eastward movement near the tropopause—subtropical jet streams—and the westward movement near the surface—the trade winds. Furthermore, the Hadley cells create not only the jet streams and trade winds, but also tropical rain belts and hurricanes, and subtropical deserts.
It is thus alarming that in the past few decades, the Hadley cells have been visibly expanding towards the poles for reasons unknown to scientists. It is widely believed that this expansion will bring forth potentially catastrophic changes to the ecosystems near the edges of the cells.
Research conducted by Professor Seung-Ki Min’s team from the Division of Environmental Science and Engineering at Pohang University of Science and Technology in collaboration with the research team from Seoul National University has made great contributions to solving this mystery by discovering the anthropogenic fingerprints behind the expansion of the Hadley cell in the Southern Hemisphere. This achievement, published in the Geophysical Research Letters, has been selected as a Journal Highlight by the American Geophysical Union.
The team used a technique called optimal fingerprinting analysis to compare a series of model simulations to examine long-term changes (from 1979 to 2009) in the southernmost edge of the Hadley cell in the Southern Hemisphere. They discovered that during this period, the edge expanded farther in the regions near the Atlantic and Indian oceans. While examining the causes of the uneven expansion, the team discovered a link between the expansion of the cells and the depletion of the ozone layer caused by CFC gas. In other words, the team discovered evidence of ‘anthropogenic forcings’ on the expansion of the Hadley cells.
Since this is the first study to provide formal detection and attribution analysis of Hadley cell expansions, the team intends to perform additional studies to pinpoint the exact mechanism between ozone depletion and Hadley cell expansion, and what the future holds if the ozone layer does not recover.
This study was funded by the Korea Meteorological Administration Research and Development Program under grant KMIPA 2015-2082.