Equatorial upper‐ocean dynamics and their interaction with the West African monsoon

Abstract

Zonal wind anomalies in the western equatorial Atlantic during late boreal winter to early summer precondition boreal summer cold/warm events in the eastern equatorial Atlantic (EEA) that manifest in a strong interannual Atlantic cold tongue (ACT) variability. Local intraseasonal wind fluctuations, linked to the St. Helena anticyclone, contribute to the variability of cold tongue onset and strength, particularly during years with preconditioned shallow thermoclines. The impact of cold tongue sea surface temperature (SST) anomalies on the wind field in the Gulf of Guinea is assessed. It contributes to the northward migration of humidity and convection and possibly the West African monsoon (WAM) jump.

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

“…This reduces the likelihood of shear instabilities to occur in the Gulf of Guinea and indeed turbulent parameters such as dissipation rates as well as diapycnal ML heat losses were observed to decrease in magnitude from the western equatorial region towards the east. Despite stronger zonal subsurface velocities at 23°W compared to 10°W (Brandt et al 2011a), shear levels are reduced at 23°W compared to 10°W (Jouanno et al 2011b). This can be explained by the shoaling of the EUC towards the east, which limits the depth range of opposite flowing currents, westwards at the surface and eastwards in the subsurface.…”

“…In particular, inter-annual SST variability within the Atlantic cold tongue (ACT) is known to influence the onset and strength of the West African Monsoon (Marin et al 2009;Brandt et al 2011a;Caniaux et al 2011). Hence, in order to improve predictions of rainfall variability, SST variability and its causes need to be understood.…”

Symmetry of the Atlantic Niño mode

“…While differences in the character of these two phenomena exist, as, for example, in the strength and seasonality of the SST anomalies [e.g., Keenlyside and Latif, 2007;Burls et al, 2012], they are both related to equatorial wind stress and equatorial thermocline variations via the Bjerknes feedback [Bjerknes, 1969]. Understanding the Atlantic Niño mode variability is of socioeconomic importance as SST anomalies in the eastern equatorial Atlantic are related to precipitation over Northeast Brazil [Ruiz-Barradas et al, 2000] and the onset of the West African monsoon [Brandt et al, 2011a]. The equatorial Atlantic also affects the equatorial Pacific Ocean [e.g., Rodriguez-Fonseca et al, 2009], so that accounting for equatorial Atlantic SST variability may improve the prediction of El Niño events [Keenlyside et al, 2013].…”

The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models

“…Over the tropical Atlantic, the oceanic and atmospheric processes are strongly coupled and this coupling is a major determinant of climate variability over the adjacent tropical continents. For example, the year-toyear variability of the seasonal development of Atlantic cold tongue has strong impacts on rainfall variability over West Africa in general, particularly on the onset of the West African monsoon (Kushnir et al 2006;Brandt et al 2011). Thus, the warm SST biases in coupled models severely inhibits the ability of these models to correctly reproduce both the observed SST variability in the equatorial Atlantic and the dynamics governing that variability (Ding et al 2015).…”