Detection and Attribution of 20th Century Northern and Southern African Monsoon Change

Abstract

In concert with wide-spread African surface warming since 1950, a drying trend has been observed, most infamously over the Sahel during July-September, but also following the march of monsoon rains into southern Africa during December-April. The nature and causes for these 1950-1999 downward trends in both northern and southern African summer monsoon rainfall are diagnosed. They are found to be attributable to the atmosphere's response to observed global sea surface temperature variations of the last half-century. Each and every member of 80 atmospheric climate simulations, forced by the observed ocean history since 1950, yields an African drying trend. Analysis of coupled ocean-atmosphere climate simulations suggests that these drying trends are detectable and distinguisable from natural coupled ocean-atmospheric variations. Yet, neither northern nor southern African drying trends during 1950-1999 are found to occur in greenhouse gas forced coupled ocean-atmosphere simulations. An explanation for the the observed 20th Century drying trends is offered, and the role of regional oceanic changes including their relation to greenhouse gas influences, is assessed.