On the fluctuations and vertical structure of the shelf circulation off Walvis Bay, Namibia

Abstract

Temporal variations of the current field on the shelf of the South East Atlantic off Walvis Bay were studied during austral summer and fall by moored current profiler and temperature-salinity recorder located on the shelf 20 nm off Walvis Bay, Namibia. Spatial and temporal variations of the wind field in the South East Atlantic were investigated by 3-day averaged wind fields measured by the QuikSCAT satellite. The local wind was provided by a time series of hourly wind vectors measured on a moored buoy off Swakopmund. The significant tidal motion on the shelf off Walvis Bay is composed of the anticlockwise rotating barotropic semidiurnal constituents M2 and S2. The tidal currents are described to a first order by a barotropic deep ocean Kelvin wave. The land-sea breeze forces anticlockwise rotating diurnal variations of the current in the surface mixed layer and an opposite directed current below the thermocline, which compensates the mass flux normal to the coast in the surface layer. Most of the energy of the current fluctuations were concentrated around the local inertial period of T i = 30.7 h . These fluctuations consisted of at least four significant anti-clockwise rotating vertical modes with amplitudes ranging between 10 and 20 cm/s. On a large scale the amplitudes of inertial motions correlate with the wind fluctuations but locally the motions consist of a superposition of locally wind forced inertial oscillations and inertial waves generated at larger distances as well near the surface as close to the bottom. The vertical structure of the inertial motion suggests that it could be a possible candidate for mixing in the bottom boundary layer by differential advection. Coastal trapped waves of horizontal mode number less that four were observed at periods of about 7 days. The mean cross shelf circulation consisted of a two cell structure. The upper cell was made of the Ekman offshore current and a compensating onshore current at intermediate depth below the thermocline. The lower cell consisted of an offshore current in the bottom layer below 80 m depth and was closed by the onshore current in the intermediate depth range. The cross shelf circulation controlled the fraction of ESACW on the shelf water mass. The long shelf circulation consisted in the surface mixed layer of an equatorward coastal jet driven by the local meridional wind stress and an almost barotropic poleward counter current appearing partly as free Kelvin wave emanating from the Angola-Benguela front and as coastal counter current forced by the local negative curl of the wind stress. The longshore current controlled the fraction of the SACW on the shelf water mass, which increased continuously between austral summer and fall.