Description: A primitive equation model of an idealized ocean basin, driven by simple, study wind and buoyancy forcing at the surface, is used to study the dynamics of mesoscale eddies. Model statistics of a six-year integration using a fine grid (1/6° × 0.2°), with reduced coefficients of horizontal friction, are compared to those using a coarser grid (1/3° × 0.4°), but otherwise identical configuration. Eddy generation in both model cases is primarily due to the release of mean potential energy by baroclinic instability. Horizontal Reynolds stresses become significant near the midlatitude jet of the fine-grid case, with a tendency for preferred energy transfers from the eddies to the mean flow. Using the finer resolution, eddy kinetic energy nearly doubles at the surface of the subtropical gyre, and increases by factors of 3–4 over the jet region and in higher latitudes. The spatial characteristics of the mesoscale fluctuations are examined by calculating zonal wavenumber spectra and velocity autocorrelation functions. With the higher resolution, the dominant eddy scale remains approximately the same in the subtropical gyre but decreases by a factor of 2 in the subpolar areas. The wavenumber spectra indicate a strong influence of the model friction in the coarse-grid case, especially in higher latitudes. Using the coarse grid, there is almost no separation between the energetic eddy scale and the scale where friction begins to dominate, leading to steep spectra beyond the cutoff wavenumber. Using the finer resolution an inertial subrange with a k−3 power law begins to emerge in all model regions outside the equatorial belt. Despite the large increase of eddy intensity in the fine-grid model, effects on the mean northward transport of heat are negligible. Strong eddy fluxes of heat across the midlatitude jet are almost exactly compensated by changes of the heat transport due to the mean flow.

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: Characteristic of the mesoscale variability in the Atlantic Ocean are investigated by analyzing the Geosat altimeter signal between 60°S and 60°N. The rms sea-surface variability for various frequency bands is studied, including the high-frequency eddy-containing band with periods 〈150 days. Wavenumber spectra and spatial eddy characteristics are analyzed over 10° by 10° boxes covering both hemispheres of the Atlantic Ocean. A comparison, with solutions of a high-resolution numerical experiment, developed as the Community Modeling Effort of the World Ocean Circulation Experiment, aids interpretation of the Geosat results in the tropical and subtropical Atlantic and provides a test of the model fluctuating eddy field. Results from Geosat altimetry show a wavenumber dependence close to k1−5 (k1 being the alongtrack wave-number) over almost the entire Atlantic Ocean except for areas in the tropical and subtropical Atlantic where the rms variability in the eddy-containing band is less than 5 cm, that is, not significantly different from the altimeter noise level. Characteristic eddy length scales inferred from Geosat data are linearly related with the deformation radius of the first baroclinic mode over the whole Atlantic Ocean, except for the equatorial regime (10°S to 10°N). The data-model comparison indicates that the high-resolution model with horizontal grid size of ⅓° and ° in latitude and longitude is quite capable of simulating observed eddy characteristics in the tropics and subtropics. In mid- and high latitudes, however, the model fails to simulate the pronounced poleward decrease in eddy scales. This leads to systematic discrepancies between the model and Geosat observation, with model scales being up to 50% larger than deduced from altimetry.