Diagnosing the annual cycle of the equatorial undercurrent in the Atlantic Ocean from a general circulation model

Ten-year-long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the Equatorial Undercurrent (EUC) in the Atlantic Ocean. Two well-defined transport maxima are found: One, present during boreal summer and autumn in the central part of the basin, is generally recognized and regarded as a near-equilibrium response to the equatorial easterly trades that culminate in this period. Another one, most pronounced near the western boundary, occurs in April May when the trades relax. This second maximum is less patent in the observations, but concomitant signals in previously published analyses of the North Brazil Current and surface velocity seasonal variations might be indirect manifestations of its reality. Because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator, the authors relate its existence to wind stress curl variations at subequatorial latitudes. A link between the interannual variability of the spring transport maximum and that of the low-latitude wind stress curl is, indeed, found in the model. This diagnostic approach suggests that two different dynamical regimes shape up the EUC seasonal cycle: in summer and autumn, local forcing by the equatorial zonal wind component and main supply from the ocean interior; in winter and spring, remote forcing by the low-latitude rotational wind component and supply from the western boundary currents.