The variability and forcing of currents within a frontal region off the northeast coast of England

Abstract

During the summer of 1988 a collaborative experiment between the Ministry of Agriculture, Fisheries and Food (MAFF), the Proudman Oceanographic Laboratory (POL) and University College North Wales (UCNW) was undertaken to study the dynamics of a near-shore frontal region off the northeast coast of England. The experiment is one of the most intensive studies of the current structure of a shelf sea front undertaken. Currents were measured using an ocean surface current RADAR (OSCR), ship-borne ADCP, Lagrangian drifters and moored current meters. The current meter moorings held conventional meters at mid-depth and near the bed and S4 electromagnetic current meters near the surface. The mean, low frequency (<0.04cph) and tidal currents measured by the moored current meters will be discussed in this paper. A persistent southeastward along-shore mean flow was observed at the surface and mid-depth throughout the array. The results from simple correlation models and a multivariate spectral model suggest that the wind plays a secondary rôle to the cross-frontal density gradients in forcing this mean flow. The percentage of current variance coherent with the local wind stress varied from 50 to 60% at the surface to approximately 40% near the bed, except at a shallow inshore site where three-quarters of the variance was wind related. Inshore surface currents displayed a directionally dependent response to wind forcing while offshore surface currents demonstrated a directionally independent response. The offshore surface response resembles that predicted by classical Ekman theory but the insignificant change in angle between wind and current with depth, above the thermocline, suggests a secondary barotropic response is dominant. Below the thermocline a strong baroclinic response, forced by along-shore winds, is evident. The vertical structure of diurnal and semi-diurnal tides are qualitatively in agreement with the theory of Prandle (1982, Continental Shelf Research, 1, 191–207) with some deviations due to nearshore effects.