Ocean Warming and Late-Twentieth-Century Sahel Drought and Recovery

Abstract

The influences of decadal Indian and Atlantic Ocean SST anomalies on late-twentieth-century Sahel precipitation variability are investigated. The results of this regional modeling study show that the primary causes of the 1980s Sahel drought are divergence and anomalous anticyclonic circulation, which are associated with Indian Ocean warming. The easterly branch of this circulation drives moisture away from the Sahel. By competing for the available moisture, concurrent tropical Atlantic Ocean warming enhanced the areal coverage of the drought. The modeled partial recovery of the precipitation in the 1990s simulations is mainly related to the warming of the northern tropical Atlantic Ocean and an associated cyclonic circulation that supplies the Sahel with moisture. Because of the changes in the scale and distribution of the forcing, the divergence associated with the continued Indian Ocean warming during the 1990s was located over the tropical Atlantic, contributing to the recovery over the Sahel. In general, the influence of SSTs on Sahel precipitation is related to their modulation of the easterly flow and the associated moisture transport. Precipitation anomalies are further enhanced by the circulation patterns associated with local convergence anomalies. These convergence anomalies and circulation patterns are sensitive to the scale and distribution of the SST anomalies and the moisture.

The recent trend of increasing precipitation in Sahel and the associated inter‐hemispheric dipole of global SST

“…This trend shift agrees with previous studies (e.g. Fall et al, 2006;Hoerling et al, 2006;Hagos and Cook, 2008). Our results and their implications can be summarized as follows.…”

“…Fall et al, 2006;Folts and McPhaden, 2008;Hagos and Cook, 2008;Omotosho 2008). Hoerling et al (2006) also pointed out that Sahel might experience a recovery in rainfall from a low point in the 1980s to values after 2000 that consistently exceed the 1950-1999 climatology.…”

“…Hoerling et al, 2006;Folts and McPhaden, 2008;Hagos and Cook, 2008). On the basis of the results of a model simulation, Hagos and Cook (2008) emphasized that the partial recovery of precipitation during the 1990s was related mainly to the warming of the northern tropical Atlantic Ocean and the associated cyclonic circulation, which supplies Sahel with moisture. Hoerling et al (2006) emphasized that drying over Sahel during the boreal summer is a response to warming of the South Atlantic sea surface temperature (SST) relative to the North Atlantic.…”

Assessment of uncertainties in the response of the African monsoon precipitation to land use change simulated by a regional model

“…The African monsoon circulation is forced by the Sahara heat low (Xue et al 2010) and the associated mid-tropospheric high. The former drives cyclonic, near surface moist southwesterly winds from the Gulf of Guinea and the high associated with this heat low in turn drives mid-tropospheric anticyclonic circulation (African Easterly Jet) that transports moisture westward out of the Sahel region into the Atlantic (Hagos and Cook 2008;Hagos and Zhang 2010). This meridional temperature gradient along with the dry Harmattan winds from the northeast determine how far north the moisture is transported by the low-level southwesterly winds, the latitudinal location of the African Easterly Jet (AEJ), and ultimately the Sahel precipitation.…”

“…These variations have been attributed primarily to global sea surface temperature (SST) variations (Giannini et al 2003;Hagos and Cook 2008). However, they also coincide with a period of rapid population growth and associated changes in land use.…”