Decadal Variability of Marine Atmospheric Boundary Layer over the Equatorial Atlantic and Its Impact on West African Rainfall

Abstract

This study analyses the decadal variability of the marine atmospheric boundary layer (MABL) in the equatorial Atlantic and its impact on the atmospheric circulation over West Africa. The study is based on data from the Simple Ocean Data Assimilation Ocean reanalysis, NCEP reanalyses and observations from the Climate Research Unit covering the period 1948-2008. We calculated the MABL index from potential temperature and analysed the mechanisms governing its variability using correlation and linear regression. The results reveal that the MABL is primarily governed by surface dynamic and thermodynamic forcings, in particular surface winds, sea surface temperature (SST) and its meridional gradient. The seasonal cycle shows MABL peaks in July-August-September, whilst the oceanic mixing layer and the SST gradient in September-October-November indicate a lag between atmospheric and oceanic anomalies associated with the cold tongue. Analysis of moving correlations and regression shows that the relationship between the MABL and oceanic and atmospheric parameters is not stationary. Before the early 1970s, variability in the MABL was primarily driven by local processes in the equatorial Atlantic, linked to SST gradients and thermocline dynamics. After this period, a shift in the climate regime becomes apparent, characterised by a weakening of equatorial temperature gradients and a growing influence of inter-basin teleconnections, particularly those associated with El Ni?o-Southern Oscillation. Variability in the MABL also appears to be linked to low-level atmospheric circulation and interactions with the West African monsoon. Before 1970, there was a strong positive precipitation anomaly across the entire Sahel, with maximum anomalies in the western part and negative anomalies in the regions around the Gulf of Guinea. In contrast, after 1970, negative anomalies began to appear in the western/coastal Sahel region. These results highlight the important role of the MABL in ocean-atmosphere coupling and the importance of improving the representation of lower-atmosphere processes in order to enhance future climate projections for West Africa.