Description: A high-resolution general circulation model of the North Atlantic, first developed at the National Center for Atmospheric Research and then run at the Institut für Meereskunde in Kiel for two different wind climatologies and reduced vertical friction, is evaluated in the upper 500 m for the western tropical Atlantic, 5°S to 15°N. Although the general features of the vigorous seasonal circulation changes documented in previous studies and in the earlier high-resolution model of Philander and Pacanowski (1986a) are reproduced, there are some interesting differences. Lack of eastward penetration of the Equatorial Undercurrent (EUC) and a thermocline that is too diffuse are model deficiencies due to the constant vertical eddy diffusion coefficient. In the lower friction case the undercurrent partially surfaces in the west, causing an eastward surface current on the equator, which is not apparent in the earlier model studies. Further, the zonal currents, in the low-friction version, have high-velocity bands, resulting, e.g., in two separate current cores in the North Equatorial Countercurrent (NECC) region; and an eastward surface core just south of the equator, connected to the EUC. Particularly interesting are equatorward undercurrents along the western boundary, one of which has already been confirmed in recent measurements off French Guyana. In winter it connects with the EUC in the model, in summer with the NECC. A northward undercurrent in the model exists off Brazil, between 5° and 10°S, but that is already close to the southern boundary of the model domain. The annual mean throughflow from the southern hemisphere into the Caribbean along the western boundary is small in the model, and in particular, there is no enhanced throughflow in winter, when the cross-equatorial North Brazil Current transport is not taken up by the NECC.

Description: The high-resolution model of the wind-driven and thermohaline circulation in the Atlantic Ocean developed in recent years as a “community modeling effort” for the World Ocean Circulation Experiment is examined for the temporal and spatial structure of the deep equatorial current field and its effect on the spreading of North Atlantic Deep Water (NADW). Under seasonally varying wind forcing, the model reveals a system of basin-wide zonal currents of O(5 cm s−1), alternating east-west, and oscillating at an annual period. The current fluctuations are induced by the seasonal cycle of the wind stress in the equatorial Atlantic and show characteristics of long equatorial Rossby waves with westward phase propagation of about 15 cm s−1. The mean flow in the deep western tropical Atlantic is governed by a deep western boundary current (DWBC) with core velocities of more than 10 cm s−1. Only a small fraction of the DWBC branches off at the equator, with correspondingly low mean eastward currents of only about 1 cm s−1. Despite this weak advection along the equator, a well-developed salinity tongue is observed in the model, which is reminiscent of observed property distributions at the upper NADW level. The model evaluation indicates the salinity pattern to be a result of a balance between mean zonal advection and meridional diffusion of salt. The presence of the zonal current oscillations appears to have no significance for the existence of the salinity tongue.

Description: Sources of near-surface oceanic variability in the central North Atlantic are identified from a combined analysis of climatology, surface drifter, and Geosat altimeter data as well as eddy-resolving math formula and math formula Community Modeling Effort North Atlantic model results. Both observational and numerical methods give a consistent picture of the concentration of mesoscale variability along the mean zonal flow bands. Three areas of high eddy energy can be found in all observational data sets: the North Equatorial Current, the North Atlantic Current, and the Azores Current. With increasing horizontal resolution the numerical models give a more realistic representation of the variability in the first two regimes, while no improvement is found with respect to the Azores Current Frontal Zone. Examination of the upper ocean hydrographic structure indicates baroclinic instability to be the main mechanism of eddy generation and suggests that the model deficiencies in the Azores Current area are related to deficiencies in the mean hydrographic fields. A linear instability analysis of the numerical model output reveals that instability based on the velocity shear between the mixed layer and the interior is also important for the generation of the mid-ocean variability, indicating a potential role of the mixed layer representation for the model. The math formula model successfully simulates the northward decrease of eddy length scales observed in the altimeter data, which follow a linear relationship with the first baroclinic Rossby radius. An analysis of the eddy-mean flow interaction terms and the energy budget indicates a release of mean potential energy by downgradient fluxes of heat in the main frontal zones. At the same time the North Atlantic Current is found to be supported by convergent eddy fluxes of zonal momentum.