Wednesday, 10 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Numerous lines of observational evidence suggest that Earth’s tropical belt has expanded over the late 20th century and early 21st century, pushing the subtropical dry zones poleward. It is natural to expect that this poleward displacement should be associated with drying on the poleward margins of the dry zones, but it is less clear to what degree the drying should be zonally symmetric. In this project, we adopt the usual convention of quantifying the tropical width itself in terms of zonally averaged variables. We primarily use the latitude at which the mean meridional overturning circulation at 500 hPa transitions from the thermally direct Hadley cell to the thermally indirect Ferrel cell. Using the resulting time series of Hadley cell widths, we test 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 these changes are or are not zonally symmetric. We test both CMIP5 models and reanalysis products, and find that in both cases, the local changes associated with Hadley cell expansion are mostly confined to certain centers of action which lie primarily over the ocean basins. We also find that these conclusions remain valid after ENSO and other effects mediated by sea-surface temperature changes are removed. These results reveal that the tropical expansion measured by zonally averaged variables is not necessarily associated with systematic drying over subtropical land regions, as is often assumed.
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