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Numerous lines of observational evidence suggest that Earth's tropical belt has expanded over the past 30-40 years. It is natural to expect that this poleward displacement should be associated with drying on the poleward margins of the subtropics, but it is less clear to what degree the drying should be zonally symmetric. This study tests the degree to which poleward motion of the Hadley cell boundary is associated with changes in local precipitation or sea level pressure and the degree to which those changes are zonally symmetric. Evidence from both reanalysis data and global climate models reveals that the local changes associated with Hadley cell expansion are mostly confined to certain centers of action which lie primarily over oceans. Consequently, the tropical expansion measured by zonally averaged variables is not associated with systematic drying over subtropical land regions, as is often assumed.

Plain Language Summary The climate of the earth's tropical regions is dominated by atmospheric circulation patterns known as the Hadley cells. In these circulations, heated air rises at the equator, moves poleward, and sinks around 30 degrees latitude in each hemisphere. The rising air is responsible for the heavy precipitation of the equatorial regions such as the Amazon, central Africa, and Indonesia. The sinking air is responsible for the so-called "subtropical dry zones" which roughly form two bands around the Earth around 30 degrees N and 30 degrees S. These dry zones include many of the world's desert regions, including North Africa, the Middle East, and parts of Mexico and Australia, among others. Computer models of Earth's climate suggest that global warming will cause the Hadley cells to expand poleward, shifting the dry zones poleward in the process. Observations confirm that Hadley cell expansion has already started. However, this study finds that the drying associated with Hadley cell expansion is stronger over oceans than over land. While this drying does still affect some land regions, it does not occur in unbroken bands around the Earth. This work emphasizes the fact that while climate change may be global, its effects often play out differently depending on location.