The vertical structure of coastal currents

Abstract

A vertical array of four equidistant current meters was used to measure horizontal currents in 18 m of water. The instruments resolved frequencies up to 15 cph for a period of 33 days in late summer 1974. Onshore (EW) and longshore (NS) currents were essentially uncorrelated at all depths. Longshore currents exhibit significant coherence with the surface tide but not at frequencies higher than the tidal frequencies. The effect of a southerly wind lasting over 3 days was evident as a northbound current that was most intense near the surface. The spectrum of onshore currents exhibits a peak at the semidiurnal frequency corresponding to internal tides, and there is a second lower, but broader, peak at frequencies between 1 cph and the buoyancy frequency. Onshore current amplitudes corresponding to the internal tides are on the order of 20 cm s⁻¹, while the higher frequency fluctuations have amplitudes on the order of 3 cm s⁻¹. There is significant coherence between all measurements of onshore current with a phase shift of π between surface and bottom currents. The onshore current measurements are consistent with the mode 1 oscillations of a three-layer model consisting of homogeneous surface and bottom layers separated by a layer having constant buoyancy frequency.