DEEP CIRCULATION IN THE TROPICAL NORTH-ATLANTIC

A transatlantic CTD/ADCP (Conductivity, Temperature, Depth/Acoustic Do ppler Current Profiler) section along 11N, taken in March 1989, has be en used to compute geostrophic velocities; geostrophic transport is re quired to balance in situ values of the Ekman and shallow boundary cur rent transports. The horizontal flow structure is described for eight layers, with particular emphasis on deep and bottom waters (four layer s 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 acros s the western basin is also consistent with recent observations to the north. For each of the four deep layers, circulation patterns are ill ustrated by means of schematic cartoons. Each of these layers flows so uthward 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 t he Mid-Atlantic Ridge, confirming the existence of a hypothesized cycl onic recirculation gyre in the western basin of the tropical Atlantic. To varying degrees the deep and bottom waters also circulate cyclonic ally in the eastern basin, with net northward flow across this basin. Partly as a result of the unusual appearance of the North Equatorial C ountercurrent in March 1989, the in situ values of the meridional over turning 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 es timates of these quantities in the literature. By forcing the 11N geos trophic 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.