The tropical Atlantic surface wind divergence belt and its effect on clouds

Abstract

A well-defined surface wind divergence (SWD) belt with distinct cloud properties forms over the equatorial Atlantic during the boreal summer months. This belt separates the deep convective clouds of the Intertropical Convergence Zone (ITCZ) from the shallow marine stratocumulus cloud decks forming over the cold-water subtropical region of the southern branch of the Hadley cell in the Atlantic. Using the QuikSCAT-SeaWinds and Aqua-MODIS instruments, we examined the large-scale spatiotemporal variability in the SWD belt during a 6-year period (2003-2008) and the related links to cloud properties over the Atlantic Ocean. The Atlantic SWD belt was found to be most pronounced from May to August, between the Equator and 2°N latitude. A positive correlation and a strong link were observed between formation of the SWD belt and a sharp sea-surface temperature gradient on the northern border of the cold tongue, supporting Wallace's vertical-mixing mechanism. The dominant cloud type over this region was shallow cumulus. Cloud properties were shown to be strongly linked to the formation and strength of the SWD zone. The findings will help to understand the link between ocean-atmosphere dynamics and cloud properties over this region, and suggest that the SWD zone be considered a unique cloud belt of the southern branch of the Atlantic Hadley cell.