Seasonal Changes in the Tropical Atlantic Circulation: Observation and Simulation of the Guinea Dome

The Guinea Dome is a permanent, quasi-stationary feature on the eastern side of the thermal ridge extending zonally across the tropical North Atlantic. The dome is a part of the large-scale near-surface flow fields associated with the North Equatorial Current, the North Equatorial Countercurrent and the North Equatorial Undercurrent. In the present study, historical and recently obtained hydrographic data are combined to investigate the thermohaline structure and geostrophic flow field in the vicinity of the dome. It is shown that the Guinea Dome exists throughout the year both in subthermocline and thermocline layers, that it has a corresponding cyclonic geostrophic flow, and that seasonal changes occur with respect to its vertical structure, horizontal extent, and position. The observational results are then compared with simulations from a general circulation model of the tropical Atlantic. A seven-year simulation forced by observed monthly winds is run to compute a monthly climatology. The model adequately simulates the Guinea Dome with respect to its structure, flow field, and seasonal variability

Direct measurements of western boundary currents off Brazil between 20°S and 28°S

Current measurements from three moored arrays on the Brazilian continental slope between 20 degrees S and 28 degrees S are investigated for the existence and strength of western boundary currents from near the surface down to the North Atlantic Deep Water. The Brazil Current is found to deepen southward from 100 m to more than 670 m and to strengthen its volume transport to 16.2 x 10(6) m(3)/s. Antarctic Intermediate Water is transported in a well-developed boundary current southward at 28 degrees S and northward north of Cabo Frio (24 degrees S). This result supports earlier suggestions derived from the analysis of hydrographic data that Antarctic Intermediate Water enters the Brazil Basin from the east and bifurcates as it meets the continental break off Brazil. North Atlantic Deep Water is transported southward in a weakly developed boundary current that leads to lower estimates of volume transport than expected from earlier hydrographic data analysis

Path of the North Atlantic Deep Water in the Brazil Basin

Recent hydrographic sections and high-quality historical data sets are used to determine geostrophic currents at subtropical latitudes in the western basin of the South Atlantic. Levels of no motion are determined from water mass information and a mass balance constraint to obtain the transport field of North Atlantic Deep Water (NADW) in this region. The incoming NADW transport of about 20 Sv from the north at 19 degrees S appears to be balanced by only one third of this transport leaving in the south and two thirds leaving to the east or northeast at the Mid-Atlantic Ridge. A simple model is proposed to determine the cause of the NADW branching. It is shown that potential vorticity preservation in the presence of topographic changes leads to a similar flow pattern as observed, with branching near the Vitoria-Trindade-Ridge and also an eastward turning of the southward western boundary current at about 28 degrees S, the latitude where a balance of planetary vorticity change and stretching can be expected