Model-inferred upper ocean circulation in the eastern tropics of the North Atlantic

Abstract

The seasonal regime associated with the eastern part of the cyclonic tropical gyre in the North Atlantic is studied using a high-resolution model. The model reproduces a vertical water mass transition (between South and North Atlantic Central Water) previously observed near σ 0 = 26.8 , which approximately coincides with the base of the North Equatorial Undercurrent (NEUC). The same density may be taken as the lower limit of the large-scale cyclonic flow. The gyre southern flank is formed of the North Equatorial Countercurrent (NECC) in the near-surface layer, and of the NEUC and deep NECC (when this current is developed) in the Central Water. Its poleward limb is made up from the Ekman drift of the trade winds, a northeastward extension of the NEUC/deep NECC, and reinforced by an extension of the Guinea undercurrent along the African continental slope. Model experiments with two different wind forcings were carried out to estimate the transports across 12°N, an approximate gyre central latitude. The time-averaged transport for densities lower than σ 1 = 32.3 and to the east of 35°W is around 4 Sv northward, about one-third of the basin-wide net value which measures the Meridional Overturning Circulation. The near-surface layer accounts for about 3 Sv, and the Central Water for more than 1 Sv, in keeping with the presence of South Atlantic Central Water in the tropical eastern basin. The eastern basin near-surface flow is lowest in summer, when the trade winds nearly vanish at this latitude. In this season the NECC, though having entered its phase of intensification, leaves the region of positive wind stress curl, and temporarily does not act as the tropical gyre southern limb. At the subsurface, most of the NEUC folds up westward at ∼10°N in winter, thus forming an inner circulation of the basin-wide gyre, previously reported from observations. The cyclonic flow associated with the Guinea Dome is another inner circulation, which the model found to be permanent below the surface down to 400–500 m, but masked in the Ekman layer, except in summer.