Resonant ocean current responses driven by coastal winds near the critical latitude

Abstract

The currents forced by wind stress near the critical latitude (30 degrees N or 30 degrees S) appear enhanced as a result of the combination of natural modes of ocean currents (e.g., near-inertial motions) and diurnal land/sea breeze-driven currents. Here we assess the wind-current transfer function, derived from observations of coastal surface currents and surface winds off the U.S. West Coast (32 degrees N to 47 degrees N), as a function of the latitude (alongshore direction) and frequency. We compare the transfer functions at the diurnal frequency derived from observations and two analytical models (e.g., the Ekman model and a near-surface averaged Ekman model). The amplitude of the data-derived transfer function decreases with increasing distance from the critical latitude, and its argument varies nearly within the range estimated from two analytical models. Our results confirm that the resonant wind-current responses near the critical latitude are 8 to 12 times stronger than the purely diurnal land/sea breeze-forced currents in other latitudes.