Evaluation of directly wind-coherent near-inertial surface currents off Oregon using a statistical parameterization and analytical and numerical models

Directly wind-coherent near-inertial surface currents off the Oregon coast are investigated with a statistical parameterization of observations and outputs of a regional numerical ocean model and three one-dimensional analytical models including the slab layer, Ekman, and near-surface averaged Ekman models. The transfer functions and response functions, statistically estimated from observed wind stress at NDBC buoys and surface currents derived from shored-based high-frequency radars, enable us to isolate the directly wind-forced near-inertial surface currents. Concurrent observations of the wind and currents are crucial to evaluate the directly wind-forced currents. Thus, the wind stress and surface current fields obtained from a regional ocean model, which simulates variability of the wind and surface currents on scales comparable to those in observations, are analyzed with the same statistical parameterization to derive the point-by-point transfer functions and response functions. Model and data comparisons show that the regional ocean model describes near-inertial variability of surface currents qualitatively and quantitatively correctly. The estimated response functions exhibit decay time scales in a range of 3-5 days, and about 40% of the near-inertial motions are explained by local wind stress. Among the one-dimensional analytical models, the near-surface averaged Ekman model explains the statistically derived wind-current relationship better than other analytical models. Key Points * Interpretation of near-inertial currents using slab layer and two Ekman models * Use of numerical model outputs to evaluate the observed near-inertial currents * Scales of directly wind-forced near-inertial surface currents