The Atlantic North Equatorial Countercurrent: Models and observations


  • Silvia L. Garzoli


An analysis of the dynamic height and volume transport variability of the North Equatorial Countercurrent (NECC) in the western tropical Atlantic (west of 38°W) is presented. The study is based upon a subset of the data (16 month-long series of dynamic height obtained with an array of inverted echo sounders) collected in the tropical Atlantic during 1987 and 1988 as part of the National Science Foundation/Tropical Ocean-Global Atmosphere array. Results are compared with those from a previous work in the center of the basin (28°W); a wind product for 1987–1988 is jointly analyzed for further interpretation of the results. In order to obtain transports from the echo sounder data, which do not provide information on the vertical structure of the geostrophic velocities, the product from the Community Model Experiment (CME) is analyzed. In this way the present work follows the objectives: to provide a description of the dynamics of the NECC in a region where maximum reversal is predicted and compare the results with those in the center of the basin and, to validate the product of the CME model. A method for data compression is utilized to validate the model against the observations. Dynamic height and geostrophic velocities are calculated from observations and the CME product. The time-latitude variability of the dynamic height in the area of the NECC, for longitudes 38°W and 28°W, is studied. The comparison is quantified by means of the empirical orthogonal functions method. The analysis shows that the first mode of variability, due to the annual cycle of the currents (more than 60% of the total variance), is more important at 28°W than at 38°W. The second mode (22 to 25% of the total variance), more important at 38°W than at 28°W, represents, in addition to the semiannual signal, 30- to 60-day oscillations that have higher amplitudes west of 28°W and that are not a direct response to the wind stress. The model accurately reproduces these results. Observations show that the meridional displacement of the core of the NECC, observed in the center of the basin following the migration of the intertropical convergence zone, is less pronounced in the western basin. Instead, dipoles of negative and positive flow are observed at the time when the curl of the wind stress changes sign across the basin. The analysis of a wind stress product obtained for the period of time that the data were collected indicates that differences in time dependence are due to differences between the climatological forcing used to force the model and the 1987–1988 wind product. After validation, the model product is used to obtain a better approximation for calculating the geostrophic transport from the inverted echo sounder's data. Maximum values are observed during 1987 at 4.5°N, 38.5°W (44.7 Sv). These values are larger that the one observed at the same latitude at 44°W (35.3 Sv). While the mean geostrophic transports are larger at 38°W (17.8 Sv at 4.5°N, 38.5°W), due to the pulse observed at the end of 1987, the mean modeled transport decreases from east to west. For latitudes in the center of the analyzed area (9°N to the equator), the mean transport is 22.1 Sv at 44°W, 13.3 Sv at 38°W, and 8.3 Sv at 28°W.