Variability of Near-Surface Currents in the Atlantic North Equatorial Countercurrent during GATE

Abstract

Measurements of wind at 5 m height, currents at 7.6, 15.4. 21.4 and 27.5 m depths, and temperature at 19 depths between 0.4 and 43.3 m depths were made from 19 August to 12 September 1974 at 8°41?N, 23°10?W (Site E3 of the GATE C-scale Oceanographic Experiment). In the uppermost 30 m where the temperature was isothermal to within 0.1°C m?1, the 24-day mean wind-driven trans-port represented ?20% of the total transport and the geostrophic transport was 80%. Time variations of the current records were dominated by near-inertial period and semidiurnal tidal period motions with rms amplitudes of 0.05-0.08 m s?1 and 0.05 m s?1, respectively. Throughout the frequency range the currents were coherent with zero phase difference between 7.6 and 27.5 M. Several squalls occurred in which the wind stress was greater than 0.1 N m?2 and the temperature records did not indicate entrainment of cold water into the mixed layer from the upper thermo-cline. The upper pycnocline was very strongly stratified. The vigorous wind-generated turbulence in the mixed layer, as represented by a vertical eddy viscosity coefficient of about 6 ? 10?3 m2 s?1, and the measured current shears of ?10?2 s?1 were not large enough to induce entrainment.