Hadley Cell Expansion and Extratropical Drying Over Land

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 subtropical dry zones. However, such drying may be heavily modified by other processes, and may be zonally asymmetric. 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.