Water masses and currents in the upper tropical northeast Atlantic off northwest Africa

Abstract

Recent current measurements in the tropical eastern North Atlantic reproduce the components of the large scale flow field. However, the observations as well as the 1/12°‐FLAME model computations indicate that a lot of eddy scale variability is superimposed on the mean flow field. Despite of the disturbance by variability the signature of the Guinea Dome is well present. In November 2002 the Guinea Dome transport from direct observations was about 2.8 Sv above σθ = 25.8 kg/m3 and 4 Sv between σθ = 25.8 and 27.1 kg/m3. The oxygen minimum in the shadow zone comprises the central water and the Antarctic Intermediate Water (AAIW) layers and is located between the equatorial current system and the North Equatorial Current. The North Equatorial Counter‐ and Undercurrents at 3° to 6°N are major oxygen sources for the central water layer of the low‐oxygen regions in the northeastern tropical Atlantic. A second, northern North Equatorial Countercurrent (nNECC) band exists at 8° to 10°N. The nNECC carries oxygen rich water from the southern hemisphere eastward but with an admixture of water from the northern hemisphere. A float at 200 m depth was spreading eastward in the North Equatorial Undercurrent (NEUC), at 28°W it shifted northward into the nNECC, and then was trapped in the Guinea Dome region for more than 3 years. The model indicates the region 22° to 32°W as the area of exchange between the NECC/NEUC and the nNECC bands. In the AAIW layer the northern Intermediate Countercurrent acts as oxygen source for the oxygen minimum zone.

A model study of the seasonality of sea surface temperature and circulation in the Atlantic North-eastern Tropical Upwelling System

“…A similar seasonal cycle is found in the observations but an important difference should be noted: the observation derived CC appears quite relatively patchy, as appear the total currents. The model westward circulation is comparatively less complex, in particular during CP where the westward flow is quite uniform north of 16 • N. In the latitude range [7][8][9][10][11][12][13][14][15][16] • N, the offshore flow is more seasonally variable. The MC and GD recirculation are both very clear in observed and simulated geostrophic currents.…”

“…On both sides of approximately latitude 18 • N, the Ekman pumping peaks occur out of phase, in agreement with Lathuilière et al [3], due to the seasonal shift of the inter-tropical convergence. Low latitudes are marked by an intense winter regime, followed by a relaxation phase in summer, in particular off Senegal (10)(11)(12)(13)(14)(15)(16) • N). North-Mauritania and south-Morocco latitudes (18)(19)(20)(21)(22) • N) experience a peak in summer.…”

“…However, observations are lacking and its latitudinal extension to the south is uncertain, with some model studies showing it down to the Cape Verde peninsula (see dashed green arrow in Figure 1). South of the region, the circulation is dominated by the North Equatorial Counter-Current (NECC), which has a large seasonal cycle [13,14]. It is located near 5 • N in winter and reaches 10 • N in summer.…”

“…

The climatological seasonal cycle of the sea surface temperature (SST) in the north-eastern tropical Atlantic (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) • N, 26-12 • W) is studied using a mixed layer heat budget in a regional ocean general circulation model. The region, which experiences one of the larger SST cycle in the tropics, forms the main part of the Guinea Gyre.

…”

Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

“…1), separating the OMZ regime from the wind-driven and well-ventilated North Atlantic subtropical gyre. In the south, towards the Equator, oxygen is supplied via zonal current bands (Stramma et al, 2005;Brandt et al, 2015). The vertical oxygen distribution shows two distinct oxygen minima, an upper one at about 75 m depth and a deep OMZ core at about 400 m Karstensen et al, 2008;Stramma et al, 2008b).…”