Decomposing observations of high-frequency radar-derived surface currents by their forcing mechanisms: Locally wind-driven surface currents

Abstract

The wind impulse response function and transfer function for high-frequency radar-derived surface currents off southern San Diego are calculated using several local wind observations. The spatial map of the transfer function reflects the influence of the coast on wind-current dynamics. Near the coast (within 20 km from the shoreline), the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry. Meanwhile, the amplitude of low-frequency currents increases near the coast, which is attributed to the local geostrophic balance between cross-shore pressure gradients against the coast and currents. Locally wind-driven surface currents are estimated from the data-derived response function, and their power spectrum shows a strong diurnal peak superposed on a red spectrum, similar to the spectra of observed winds. Current magnitudes and veering angles to a quasi-steady wind are typically 2-5% of the wind speed and vary 50 degrees-90 degrees to the right of the wind, respectively. A wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance. Moreover, the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change.