Klenz, Thilo (2016) Seasonal variability of the boundary circulation, water masses and oxygen content in the coastal upwelling region of the eastern tropical North Atlantic. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 62 pp.

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Abstract

The seasonal variability of the boundary circulation, water masses and oxygen content in the climatically and socio-economically important coastal upwelling region of the eastern tropical North Atlantic ocean is analysed. Main basis for analysis is an extensive data set consisting of nine research cruises conducted off the coast of Mauritania between the years 2005-2016. Observations show enhanced northward flow in the upper central water layer within the poleward undercurrent and surface Mauritania current after upwelling-favorable wind stress has seized, while during the upwelling season flow over the shelf is predominantly southward and the undercurrent is weakened. Northward transports during the nonupwelling season are enhanced by a factor of three, with maximum transports during one summer cruise of 2.1Sv. The lower central water layer is characterised by weak, southward flow year-round. The variability of the boundary circulation in the upper layers can to a large extent be explained by the seasonally varying cyclonic wind stress curl that dominates the near-coastal region year-round. Mesoscale variability in the form of eddies drives variability on shorter timescales. Water mass analysis within the central water stratum shows that waters of South Atlantic origin transported in the boundary currents act to renew the waters of the upper central water layer during times of enhanced flow. A water mass transition at the boundary between upper and lower central waters was observed during all cruises and a mixture consisting of North and South Atlantic central waters dominates the lower layer. Seasonal variability in this layer is less pronounced, but a possible northerly ventilation pathway for this layer is apparent in the observations. Despite the seasonal renewal of southern water masses a pronounced shallow oxygen minimum was observed around 100m water depth during the summer cruises. Lowest oxygen concentrations below 40μmolkg-1 were observed over large parts of the study region during one summer cruise, caused by prolonged primary productivity in the surface layer and remineralization below. Enhanced diapycnal and isopycnal oxygen supply are likely the reason for higher oxygen concentrations in the range of the shallow oxygen minimum observed during the winter cruises.

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