DEEP CIRCULATION IN THE TROPICAL NORTH-ATLANTIC

A transatlantic CTD/ADCP (Conductivity, Temperature, Depth/Acoustic Doppler Current Profiler) section along 11N, taken in March 1989, has been used to compute geostrophic velocities; geostrophic transport is required to balance in situ values of the Ekman and shallow boundary current transports. The horizontal flow structure is described for eight layers, with particular emphasis on deep and bottom waters (four layers below B = 4.7 degrees C). In the shallow layers, total North Brazil Current (NBC) transport agrees with other observations previously made in the month of March, while net northward flow of these layers across the western basin is also consistent with recent observations to the north. For each of the four deep layers, circulation patterns are illustrated by means of schematic cartoons. Each of these layers flows southward in the Deep Western Boundary Current, which has a magnitude of 26.5 Sv. Roughly half of this flow returns northward to the west of the Mid-Atlantic Ridge, confirming the existence of a hypothesized cyclonic recirculation gyre in the western basin of the tropical Atlantic. To varying degrees the deep and bottom waters also circulate cyclonically in the eastern basin, with net northward flow across this basin. Partly as a result of the unusual appearance of the North Equatorial Countercurrent in March 1989, the in situ values of the meridional overturning cell (5.2 Sv), heat flux (3.0 x 10(14) W), and freshwater flux (-0.65 Sv) computed from the 11N section depart significantly from estimates of these quantities in the literature. By forcing the 11N geostrophic velocities to balance annual average Ekman and NBC transports, annual average values of these fluxes (12 Sv; 11 x 10(14) W; -0.6 Sv) are obtained, and are shown to agree well with historical estimates.