Northern and southern water masses in the equatorial Atlantic: distribution of nutrients on the WOCE A6 and A7 lines

Abstract

In the framework of the WOCE Hydrographic Program, two trans-Atlantic CTDO/tracer sections with closely-spaced stations, along 7°30′N and 4°30′S (WHP Lines A6 and A7), and two meridional sections, along 3°50′W and 35°W joining the two zonal sections, were occupied in January–March 1993 (CITHER 1 cruise on board the N/OL′ATALANTE). CTD profiles and nutrient (silicate, phosphate and nitrate) data at 32 depths between surface and bottom were obtained at each station. The distributions on vertical sections, and on isopycnal surfaces, of these three chemical tracers are presented and discussed in the context of large-scale circulation in the equatorial Atlantic Ocean.The nutrient fields are used to show the main components of the circulation on four main levels: near-surface, intermediate, deep and bottom layers. Near the surface the nutrient distribution pattern is dominated 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 1000m depth), at a larger depth than that of the salinity minimum of the Antarctic Intermediate Water, enable a differentiation of the Upper Circumpolar Water (UCPW) from the former. In the deeper layers, the nutrient distribution confirms the bifurcation of the Deep Western Boundary Current (DWBC), carrying the North Atlantic Deep Water, at the equator into an eastward flow and another one continuing southward along the western boundary. The eastward flow of the UNADW along the equator can be traced as far as 3°50′W. The analysis of nutrient distribution on isopycnal surfaces also shows the existence of recirculation components of the DWBC in the western equatorial region. The northward succession of bottom concentrations of silicate and phosphate indicates that the flow of the Antarctic Bottom Water (AABW) exiting the Brazil Basin is topographically constrained along the equator. The higher silicate and phosphate concentrations in the near-bottom waters west of the Mid-Atlantic Ridge (MAR) and north of the equator indicate that the main flow of bottom waters is northwestward rather than through the equatorial fracture zones into the eastern Atlantic. Finally, the distributions of silicate and phosphate on near-bottom and isopycnal surfaces suggests a recirculation of AABW in the northern part of the Brazil Basin.