The seasonal variability of the northern Benguela undercurrent and its relation to the oxygen budget on the shelf

The seasonal variations in the advection and mixing of water masses in the northern Benguela were studied in relation to the oxygen minimum zone over the Namibian shelf. The used data set consists of hydrographic and current measurements from an oceanographic mooring 20 nm off Walvis Bay, monthly CTD transects from the Namibian 23°S monitoring line and recent large-scale hydrographic surveys. The current time series showed an intermittent southward continuation of the Angola Current (AC) through the Angola–Benguela frontal zone (ABFZ) into the northern Benguela, commonly known as poleward undercurrent. In austral summer hypoxic, nutrient rich South Atlantic central water (SACW) from the Angola Gyre is transported into the northern Benguela, whereas during the winter season the oxygen rich Eastern SACW (ESACW) spreads northward. The water mass analysis reveals a mixing between both water masses in the northern Benguela between the ABFZ and the Lüderitz upwelling cell (27°S). The oxygen balance over the Namibian shelf depends to a high extent on the water mass composition of the upper central water layer, controlled by the large scale and local circulation. The deviation of the measured oxygen concentration from its mixing concentration, calculated with the source water mass properties, was used to quantify the oxygen consumption. A new local definition SACW was derived to exclude biogeochemical processes, taking place in the Angola Gyre. The oxygen deficit in the northern Benguela central water amounted to about 60–80 μmol l−1 at the shelf edge and increased up to 150 μmol l−1 on the shelf, due to local oxygen consumption. In the austral summer anoxic bottom waters are observed at the central Namibian shelf, which correlate to an SACW fraction >55%. Periods with high SACW fraction in the water mass composition were congruent with hydrogen sulphide events detected by remote sensing.