NORTHERN AND SOUTHERN WATER MASSES IN THE EQUATORIAL ATLANTIC - DISTRIBUTION OF NUTRIENTS ON THE WOCE A6 AND A7 LINES

In the framework of the WOCE Hydrographic Program, two trans-Atlantic CTDO/tracer sections with closely-spaced stations, along 7 degrees 30' N and 4 degrees 30'S (WHP Lines A6 and A7), and two meridional section s, along 3 degrees 50'W and 35 degrees W joining the two zonal section s, were occupied in January-March 1993 (CITHER 1 cruise on board the N /O L'ATALANTE). CTD profiles and nutrient (silicate, phosphate and nit rate) data at 32 depths between surface and bottom were obtained at ea ch station. The distributions on vertical sections, and on isopycnal s urfaces, of these three chemical tracers are presented and discussed i n the context of large-scale circulation in the equatorial Atlantic Oc ean. The nutrient fields are used to show the main components of the c irculation on four main levels: near-surface, intermediate, deep and b ottom layers. Near the surface the nutrient distribution pattern is do minated by westward advective flows on either side of the equator from the eastern regions enriched by coastal upwellings, Beneath the lower thermocline water, high silicate concentrations (at about 1000 m dept h), at a larger depth than that of the salinity minimum of the Antarct ic Intermediate Water, enable a differentiation of the Upper Circumpol ar Water (UCPW) from the former. In the deeper layers, the nutrient di stribution confirms the bifurcation of the Deep Western Boundary Curre nt (DWBC), carrying the North Atlantic Deep Water, at the equator into an eastward flow and another one continuing southward along the weste rn boundary. The eastward flow of the UNADW along the equator can be t raced as far as 3 degrees 50'W, The analysis of nutrient distribution on isopycnal surfaces also shows the existence of recirculation compon ents of the DWBC in the western equatorial region. The northward succe ssion of bottom concentrations of silicate and phosphate indicates tha t the flow of the Antarctic Bottom Water (AABW) exiting the Brazil Bas in is topographically constrained along the equator. The higher silica te and phosphate concentrations in the near-bottom waters west of the Mid-Atlantic Ridge (MAR) and north of the equator indicate that the ma in flow of bottom waters is northwestward rather than through the equa torial fracture zones into the eastern Atlantic. Finally, the distribu tions of silicate and phosphate on near-bottom and isopycnal surfaces suggests a recirculation of AABW in the northern part of the Brazil Ba sin. (C) 1998 Elsevier Science Ltd. All rights reserved.