THE NEAR-SURFACE TROPICAL ATLANTIC IN 1982-1984 - RESULTS FROM A NUMERICAL-SIMULATION AND A DATA-ANALYSIS

Isotherm vertical displacements within the thermocline and surface currents were investigated in the tropical Atlantic Ocean from 12-degrees-N to 12-degrees-S in 1982-84, the period of the FOCAL-SEQUAL experiment. The study is based on a numerical simulation of an oceanic general circulation model tuned for the study of the equatorial regions, and on the analysis of the large scale thermocline displacements and currents using observed temperature profiles. Ground truth is provided by temperature and currents from moorings, records from inverted echo sounders and tide gauges as well as from drifting buoys. Comparison of the analysis with the ground truth shows that some important aspects of the low frequency variability are "captured" by the analysis when the data base is large enough. On large scales, the simulation generally resembles the analysis. Along the equator, the upwelling signal propagates eastward. The seasonal set-up of the westerly winds is associated with large westward currents, and a following overshoot of the zonal dynamic topography. Otherwise, the zonal dynamic topography is in near-equilibrium with the winds. The North Equatorial Countercurrent is portrayed comparably in the analysis and the simulation, where, after starting as a narrow eastward flow near 5-degrees-N, it extends northward through the northern summer. International variations are found both in the analysis and the simulation. In particular, the thermocline flattened early in 1984. However, the simulation differs in significant respects from the real world: the equatorial undercurrent is too weak in the east and the model produces too much variability south of the equator. The 20-degrees-C isotherm is too shallow above the core of the thermocline, and the surface layer is too stratified. Because the surface layer is where the wind stress, main forcing of the model is applied, major effort will have to be devoted to parametrizing the near-surface downward mixing of momentum, heat and fresh water.