The decay of wind‐forced mixed layer inertial oscillations due to the β effect

Wind generation of mixed layer near‐inertial frequency oscillations has been observed and successfully modeled many times. Modeling of the decay of these currents by linear wave theory has been more difficult because the necessary horizontal scales are much smaller than the typical horizontal scales of the wind. A new, and highly effective, mechanism for generating such scales by the β effect is proposed here. An asymptotic analysis of the linear equations is presented which suppresses high‐frequency gravity waves and thus simplifies the near‐inertial frequency dynamics. The computed residence time for inertial motions in the mixed layer depends both on the scales of the wind and on β, with β imposing an upper limit of 1–2 weeks. The relative importance of the wind and β is estimated using realistic wind stress fields, generated by advecting Seasat scatterometer data over the simulated ocean. The predicted horizontal scales and decay times of mixed layer inertial motions are similar to those observed. The subsynoptic scales of the wind, the advection speed, and β are all important in determining the decay time of the simulated mixed layer inertial currents.