The Impact of Mediterranean SSTs on the Sahelian Rainfall Season

Abstract

A variety of regional and global sea surface temperature (SST) patterns are known to affect interannual to decadal variations of summer rainfall over the Sahel, and for most of these patterns considerable progress has been made towards understanding their influence. However, a possible link between Mediterranean SSTs and Sahelian rainfall has yet to be studied, and so the aim of this paper is to use observational and atmospheric general circulation model (AGCM) data to confirm and understand this relationship. In years when the Mediterranean is warmer than average, it is shown that the Sahel tends to be wetter than normal, whereas in cool Mediterranean years it tends to be drier. The observed data also demonstrate that during the last five decades (1947?96) the strength of this impact has been roughly equal to that of Pacific SSTs, and a little less than that of the tropical Atlantic. Moreover it is most apparent on timescales of a decade or more, although it does also exert some influence at shorter timescales. It is also speculated that the Mediterranean may partly explain the impact of an interhemispheric pattern of SST anomalies found in earlier studies. Analysis of the AGCM data provides the most convincing evidence that the observed relationship is indeed due to an influence of the Mediterranean on the Sahel. In particular, a pair of idealized experiments forced by warmer (colder) than average SSTs in the Mediterranean, and climatological SSTs elsewhere, produce a clear and significant summer rainfall response over the Sahel. Data from these experiments are then used to explain this impact. If SSTs in the Mediterranean are warmer than average, then local evaporation is enhanced, and the moisture content of the lower troposphere increases. This additional moisture is advected southward across the eastern Sahara by the mean flow, leading to enhanced low-level moisture convergence over the Sahel, which feeds enhanced rainfall. This is then amplified by four positive feedback mechanisms: a more rapid influx of moisture from the tropical Atlantic triggered by enhanced convective heating, a reduced outflow of moisture from the midlevel African easterly jet, an enhanced hydrological cycle, and a larger rainfall contribution from African easterly waves.