Hydrographic sections across the Atlantic at 7°30N and 4°30S

Abstract

Transatlantic hydrographic sections along 7°30N and 4°30S, and shorter meridional ones along 35°W and 4°W in the intervening latitudinal range, provide a basin-wide description of the Atlantic water masses at their crossing of the equator. The water masses belonging to either the cold or warm segment of the global thermohaline cell enter the equatorial region mostly in the form of western boundary currents. The ways they leave it are more varied.The Ekman drift and a geostrophic western boundary current cause the export of near-surface water to the North Atlantic. A part of the southern Salinity Maximum Water, regarded as the shallowest warm water component, is thought to follow this route after experiencing strong property modification in the equatorial upwelling. The underlying South Atlantic Central Water divides into two northward paths, a direct one along the south American continental slope, hardly observed in the data because of an intense variability in the western half of the 7°30N line, and a longer one through the eastern basin, taken by water of the equatorial thermostad. There is no trace of such as eastern northward route for the Antarctic Intermediate Water, which is apparently forced northward from the equatorial region through the highly variable circulation of the western basin.The deep western boundary currents carrying southward the upper and middle components of the North Atlantic Deep Water experience a first partial shift to the eastern boundary on crossing the equator. At deeper levels, a part of the lower North Atlantic Deep Water also bifurcates eastward at the equator, but loses its identity through vertical mixing with the Antarctic Bottom Water in the equatorial fracture zones. The newly formed homogeneous bottom water proceeds eastward in the Guinea Basin, with further indication of an overflow into the Angola Basin.Beside the North Atlantic Deep Water, the deep layer of the equatorial region contains a lower-oxygen component, most clearly present between the middle and lower cores of the North Atlantic Deep Water. Previous results on this water are substantiated, namely, an arrival from the southeast, and northwestward crossing of the equator offshore from the deep western boundary current of northern water. A further northward progression of the southern water requires that the equatorial branching of the southward deep boundary current be only intermittent.A comparison of the temperatures along 7°30N in 1993 with those obtained at 8°N during the International Geophysical Year, 36 years before, reveals a net warming of the intermediate and upper deep waters, and cooling of the bottom water. This result is similar to that obtained at 24°N by other authors, yet there are signs of a southward propagation of a deep cold anomaly in the western basin, which had reached 24°N in 1992, but not yet 7°30N in 1993.