The Atlantic Meridional Mode and Its Coupled Variability with the Guinea Dome

Abstract

Abstract Using an ocean–atmosphere coupled general circulation model, air–sea interaction processes associated with the Atlantic meridional mode are investigated from a new viewpoint of its link with the Guinea Dome in the northern tropical Atlantic. The subsurface thermal oceanic dome develops off Dakar from late spring to late fall owing to wind-induced Ekman upwelling. Its seasonal evolution is due to surface wind variations associated with the northward migration of the intertropical convergence zone (ITCZ). Since the upwelling cools the mixed layer in the Guinea Dome region during summer, it is very important to reproduce its variability in order to simulate the sea surface temperature (SST) there. During the preconditioning phase of the positive (negative) Atlantic meridional mode, the dome is anomalously weak (strong) and the mixed layer is anomalously deep (shallow) there in late fall. This condition reduces (enhances) the sensitivity of the mixed layer temperature to the climatological atmospheric cooling. As a result, the positive (negative) SST anomaly appears there in early winter. Then, it develops in the following spring through the wind–evaporation–SST (WES) positive feedback associated with the anomalous northward (southward) migration of the ITCZ. This, in turn, leads to the stronger (weaker) Ekman upwelling and colder (warmer) subsurface temperature in the dome region during summer. It plays an important role on the decay of the warm (cold) SST anomaly through entrainment as a negative feedback. Therefore, simulating this interesting air–sea interaction in the Guinea Dome region is critical in improving prediction skill for the Atlantic meridional mode.