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

The tropical Atlantic ocean plays a key role for climate variability in sensitive regions of the surrounding continents: Previous studies suggest that inter-annual variability of sea surface temperature (SST) in the equatorial Atlantic impacts rainfall variability over northeast Brazil and the coastal regions surrounding the Gulf of Guinea (Chang et al. 2006; Kushnir et al. 2006; Brandt et al. 2011b). 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. Sea surface temperature variability in the tropical Atlantic is dominated by an annual signal formed by the seasonal appearance of the ACT (Fig. 1). During the development of the ACT in boreal spring/summer SSTs decrease by about 6 °C. Towards the end of the year SSTs steadily increase completing the annual cycle (Fig. 2a). On the other hand, the net surface heat fluxes within this region are dominated by a semi-annual signal due to the meridional migration of the Inter Tropical Convergence Zone (ITCZ). The discrepancy between the semi-annual atmospheric forcing and the annual cycle of SSTs suggests ocean dynamics to be of fundamental importance within this region. Furthermore, the Abstract Sea surface temperatures (SSTs) in the eastern tropical Atlantic are crucial for climate variability within the tropical belt. Despite this importance, state-of-the-art climate models show a large SST warm bias in this region. Knowledge about the seasonal mixed layer (ML) heat budget is a prerequisite for understanding SST mean state and its variability. Within this study all contributions to the seasonal ML heat budget are estimated at four locations within the Atlantic cold tongue (ACT) that are representative for the western (0°N, 23°W), central (0°N, 10°W) and eastern (0°N, 0°E) equatorial as well as the southern (10°S, 10°W) ACT. To estimate the contribution of the diapycnal heat flux due to turbulence an extensive data set of microstructure observations collected during ten research cruises between 2005 and 2012 is analyzed. The results for the equatorial ACT indicate that with the inclusion of the diapycnal heat flux the seasonal ML heat budget is balanced. Within the equatorial region, the diapycnal heat flux is essential for the development of the ACT. It dominates over all other cooling terms in the central and eastern equatorial ACT, while it is of similar size as the zonal advection in the western equatorial ACT. In contrast, the