African dry air outbreaks

Abstract

Dry air outbreaks from Africa into the tropical Atlantic region are documented using daily layered precipitable water (LPW) data of satellite remote sensing for 1993–1997. A dry air outbreak is defined as an abrupt decrease in middle‐troposphere (700–500 hPa) LPW that is greater than 1 mm d−1 and remains below normal for at least two consecutive days in a reference box off the west coast of Africa. Pathways of individual dry air outbreaks thus identified are tracked both forward and backward in time. Dry air outbreaks intruding into the tropical Atlantic may originate from different source regions: the northern Atlantic Ocean, northern Africa, Sahel, and equatorial Africa. Some of these outbreaks can keep their identities up to 2 weeks, propagate across the Atlantic Ocean, and reach South America and the Caribbean Sea. Boreal summer is the overall peak season, with the majority of the outbreaks occurring north of the Intertropical Convergence Zone (ITCZ). In spring more outbreaks occur south of the ITCZ. Precipitation in the ITCZ tends to decrease during periods of the outbreaks in both spring and summer. Observed large‐scale patterns of African dry air outbreaks are compared to known characteristics of African dust outbreaks. The results from this study suggest that African dry air outbreaks should and can be treated as prominent phenomena independent of African dust outbreaks. This study calls for systematic investigations on the similarities and differences between African dust and dry air outbreaks for a better understanding of their effects on weather and climate in the tropical Atlantic region.