Description: In this study, we examine the simulation results from the paleoclimate version of the National Center of Atmospheric Research coupled Climate System Model (CSM 1.4) for the Last Glacial Maximum (LGM) in order to understand changes in the South Atlantic (SA) circulation relative to the Present Day (PD). The LGM simulation is validated with the available proxy data in the region. The results show good agreement, except in the eastern equatorial and eastern SA region, where the model is not able to reproduce the correct cloud cover and the associated air–sea interactions. Ocean transport in the PD simulation is in good agreement with observational estimates. Results show that at subsurface levels there are two distinct patterns: (i) strengthening of the transport for the LGM in the southern SA (35°S to 25°S); and (ii) weakening of the mass transport in the northern SA (25°S to the Equator). In intermediate layers, there is an intensification of the subtropical gyre and a northward shift of the South Equatorial Current (SEC) bifurcation for the LGM. This leads to the intensification of the southward transport by the Brazil Current (BC) and the associated BC recirculation cell in the southern basin for the LGM. This shift in the position of the SEC bifurcation leads to a weakening in the northward transport and the western recirculation of the central SEC in the northern basin. This northward shift of the SEC (upper limit of the subtropical gyre) is consistent with the northward shift observed in the subtropical convergence zone and suggests a displacement of the sub-tropical gyre 3°–5° towards the Equator. In deeper layers, a shallower and weaker North Atlantic Deep Water (NADW) circulation in the LGM contributes to the reduction of the southward transport in the northern part of the basin and is associated with a greater northward intrusion of Antarctic Bottom Water. This intrusion plus the increase of the Indian Water inflow is responsible for the northward transport intensification in the southern basin.

Description: Deep Chlorophyll Maximum (DCM) modifies the upper ocean heat capture distribution and thus impacts water column temperature and stratification, as well as biogeochemical processes. This energetical role of the DCM is assessed using a 1 m-resolution 1D physical-biogeochemical model of the upper ocean, using climatological forcing conditions of the Guinea Dome (GD). This zone has been chosen among others because a strong and shallow DCM is present all year round. The results show that the DCM warms the seasonal thermocline by +2°C in September/October and causes an increase of heat transfer from below into the mixed layer (ML) by vertical diffusion and entrainment, leading to a ML warming of about 0.3°C in October. In the permanent thermocline, temperature decreases by up to 2°C. The result is a stratification increase of the water column by 0.3°C m−1 which improves the thermocline realism when compared with observations. At the same time, the heating associated with the DCM is responsible for an increase of nitrate (+300%, 0.024 μM), chlorophyll (+50%, 0.02 μg l−1) and primary production (+45%: 10 mg C m−2 day−1) in the ML during the entrainment period of October. The considered concentrations are small but this mechanism could be potentially important to give a better explanation of why there is a significant amount of nitrate in the ML. The mechanisms associated with the DCM presence, no matter which temperature or biogeochemical tracers are concerned, are likely to occur in a wide range of tropical or subpolar regions; in these zones a pronounced DCM is present at least episodically at shallow or moderate depths. These results can be generalized to other thermal dome regions where relatively similar physical and biogeochemical structures are encountered. After testing different vertical resolutions (10 m, 5 m, 2.5 m, 1 m and 0.5 m), we show that using at least a 1 m vertical resolution model is mandatory to assess the energetical importance of the DCM.