UPPER LAYER TEMPERATURE STRUCTURE OF THE WESTERN TROPICAL ATLANTIC

Mean monthly topographies of the 20-degrees-C and 10-degrees-C isother mal surfaces are used to describe the vertical displacements of the up per and lower thermocline in the western tropical Atlantic. The isothe rm topographies are generated from expendable bathythermograph data co llected between 1966 and 1993. The topographies confirm, and extend cl oser to the coast, earlier findings that demonstrate large spatial and temporal variability in the region. For example, the ridge and trough systems observed previously in the interior are shown, and their exte nsion to the western boundary is described. In particular, it is shown that the ridge associated with the North Equatorial Countercurrent (N ECC) extends from the interior northwestward along the western boundar y, reaching farther north along the boundary in the upper thermocline than in the lower thermocline. South of the equator the northwestern c orner of the countercurrent trough is apparent on the lower surface bu t not on the upper. The annual and semiannual harmonics of the vertica l isotherm displacements account on the average for about 60% of total variance on both surfaces. The horizontal structure of the first harm onic amplitude is similar for both surfaces, showing maximum amplitude along the axis of the NECC ridge. Minimum amplitudes are observed to the north along the axis of the countercurrent trough. These distribut ions are similar to the pattern of the first-harmonic amplitude of the wind stress curl, supporting earlier studies of curl forcing of near- surface current features.