The dynamics of nearshore surface currents generated by tides, wind and horizontal density gradients

Abstract Nearshore surface currents measured over 23 days by HF Radar are analysed. These data are equivalent to 100 current meter deployments spaced approximately 1.5 km apart over an area 15 km square. The semidiurnal tidal constituents predominate and exhibit an even coherent propagation through the region. The higher harmonic tides are small, indicating little non-linear interaction—a consequence of the relatively deep water (20–50 m). This low level of interaction also enables the wind-driven component to be clearly separated, thereby revealing dynamics never previously identified with such clarity. The time-varying (non-tidal) residual currents are shown to be primarily due to wind forcing, with the surface currents veering up to 45° to the right of the wind at a magnitude of between 1 and 2% of the wind speed. To reproduce these features with a simple steady-state analytical model requires a vertical eddy viscosity of approximately 0.01 m 2 s −1 . Using an empirical orthogonal function analysis, these wind-driven residuals are shown to move in a uniform slab-like manner with the direction rotating clockwise at a rate close to the inertial frequency (as suggested by the analytical model). The persistence of winds towards the northeast over the survey period generated significant net residual surface currents. An additional (non-wind-driven) net residual was also evident and this was shown to be consistent with forcing by (depth and time-averaged) horizontal density gradients with a vertical eddy viscosity varying linearly from 0.01 m 2 s −1 at the bed to zero at the surface. Since this eddy viscosity formulation is close to that derived for the wind-driven currents, the possibility exists of using HF Radar measurements of surface currents to determine appropriate vertical eddy viscosity coefficients for 3-D models.