Tracer distributions and deep circulation in the western tropical Atlanticduring CITHER 1 and ETAMBOT cruises, 1993-1996

This paper presents CFC and nontransient tracer observations in the western equatorial Atlantic Ocean on repeated sections along 730'N, the 35 degrees W meridian, and a transect crossing the Ceara Rise. Three World Ocean Circ ulation Experiment cruises have been carried out in this area, in February- March 1993 (CITHER 1) and September-October 1995 and April-May 1996 (ETAMBO T 1 and 2). Together with the tracer data, the direct current measurements are used to deduce the circulation pathways. The data confirm the principal circulation features of the Upper North Atlantic Deep Water within the are a. The Deep Western Boundary Current flows southward along the continental slope, while, adjacent to the DWBC, there is a northward flow corresponding to the DWBC recirculation whose origin appears variable. The largest varia bility is observed along the 35 degrees W meridional section, where the DWB C bifurcates eastward north of 3 degrees S but the eastward equatorial flow does not appear to be permanent. At the Middle North Atlantic Deep Water l evel, CFC distributions exhibit the most important contrast between waters of northern and southern origin. The circulation at the Lower North Atlanti c Deep Water level, mainly controlled by the topography, looks more permane nt because the DWBC is regularly observed during the three cruises, and the re is no evidence for a northwestward recirculation along the Mid-Atlantic Ridge. The transient behavior of CFC distributions, which is superimposed o n local circulation effects, can be seen clearly through their temporal evo lution within the DWBC. The variability of the deep circulation observed du ring the 1993-1996 interval rules out a dominant variability in response to semi annual forcing.