Mean circulation and variability of the tropical Atlantic during 1952-2001 in the GECCO assimilation fields

The shallow subtropical-tropical cells (STC) of the Atlantic Ocean have been studied from the output fields of a 50-yr run of the German partner of the Estimating the Circulation and Climate of the Ocean (GECCO) consortium assimilation model. Comparison of GECCO with time-mean observational estimates of density and meridional currents at 10 degrees S and 10 degrees N, which represent the boundaries between the tropics and subtropics in GECCO, shows good agreement in transports of major currents. The variability of the GECCO wind stress in the interior at 10 degrees S and 10 degrees N remains consistent with the NCEP forcing, although temporary changes can be large. On pentadal and longer time scales, an STC loop response is found between the poleward Ekman divergence and STC-layer convergence at 10 degrees S and 10 degrees N via the Equatorial Undercurrent (EUC) at 23 degrees W, where the divergence leads the EUC and the convergence, suggesting a pulling mechanism via equatorial upwelling. The divergence is also associated with changes in the eastern equatorial upper-ocean heat content. Within the STC layer, partial compensation of the western boundary current (WBC) and the interior occurs at 10 degrees S and 10 degrees N. For the meridional overturning circulation (MOC) at 10 degrees S it is found that more than one-half of the variability in the upper limb can be explained by the WBC. The explained MOC variance can be increased, to 85% by including the geostrophic (Sverdrup) part of the wind-driven. transports.