Coupling between the Atlantic cold tongue and the West African monsoon in boreal spring and summer

1 The formation of the Atlantic cold tongue (ACT) is the dominant seasonal sea surface temperature signal in the eastern equatorial Atlantic (EEA). A comprehensive analysis of variability in its spatial extent, temperature, and onset is presented. Then, the physical mechanisms which initiate ACT onset, as well as the feedbacks from the ACT to the maritime boundary layer, and how the ACT influences the onset of the West African monsoon (WAM) are discussed. We argue that in the EEA, the air-sea coupling between the ACT and WAM occurs in two phases. From March to mid-June, the ACT results from the intensification of the southeastern trades associated with the St. Helena anticyclone. Steering of surface winds by the basin shape of the EEA imparts optimal wind stress for generating the maximum upwelling south of the equator. During the second phase (mid-June–August), wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the ACT and the tropical waters circulating in the Gulf of Guinea (GG). It is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the GG and that increased northward winds shift convection over land. Correlations between the ACT and the WAM onset dates over the last 26 years (1982–2007) measure as much as 0.8. This suggests that the ACT plays a key role in the WAM onset.