|Title||Water vapor-forced greenhouse warming over the Sahara Desert and the recent recovery from the Sahelian drought|
|Publication Type||Journal Article|
|Year of Publication||2015|
|Authors||Evan AT, Flamant C., Lavaysse C., Kocha C., Saci A.|
|Journal||Journal of Climate|
|Type of Article||Article|
|Keywords||CMIP5; dust; dynamics; Equatorial Atlantic; heat low; impact; intraseasonal variability; rainfall; summer; variability; west-african monsoon|
The Sahel region of West Africa experiences decadal swings between periods of drought and abundant rainfall, and a large body of work asserts that these variations in the West African monsoon are a response to changes in the temperatures of the tropical Atlantic and Indian Oceans. However, here it is shown that when forced by SST alone, most state-of-the-art climate models do not reproduce a statistically significant upward trend in Sahelian precipitation over the last 30 years and that those models with a significant upward trend in rainfall seem to achieve this result for disparate reasons. Here the role of the Saharan heat low (SHL) in the recovery from the Sahelian drought of the 1980s is examined. Using observations and reanalyses, it is demonstrated that there has been an upward trend in SHL temperature that is coincident with the drought recovery. A heat and moisture budget analysis of the SHL suggests that the rise in temperature is due to greenhouse warming by water vapor, but that changes in water vapor are strongly dependent upon the temperature of the SHL: a process termed the Saharan water vapor-temperature (SWAT) feedback. It is shown that the structure of the drought recovery is consistent with a warming SHL and is evidence of a fundamental, but not exclusive, role for the SHL in the recent increase in Sahelian monsoon rainfall.
In this paper, we have attempted to elucidate processes contributing to the recovery from the Sahelian drought of the 1980s, a period of time we focus on primarily because of the availability of consistent reanalysis products and quality-controlled in situ observations. Although a large body of work suggests that Sahel rainfall changes in step with tropical SSTs, we show that when forced by observed SSTs, most state-of-the-art atmospheric GCMs do not reproduce a statistically significant increase in Sahel rainfall, and the models reproducing a realistic upward trend in rainfall seem to do so for disparate reasons. Based on the multimodel mean trend of Sahelian rainfall, which is one-half the magnitude of the observed trend, one could argue that one-half of the Sahelian rainfall trends are directly forced by SST changes.