Near-inertial and diurnal motions in the trajectories of mixed layer drifters

We analyze the near-inertial/diurnal motions in the trajectories of surface mixed layer drifters in the California Current System between 19N and 36N. The observed near-inertial or diurnal oscillations are very intermittent in time and have a time scale of about 10 inertial periods. Using a simple slab model of wind-driven inertial currents, we show that their temporal variations are related to the fluctuations in the local wind stress field. Three events of strong (20 cm s⁻¹) near-inertial/diurnal motions are studied in detail. Two events at the diurnal frequency occur on the continental shelf. For the first, the observed subinertial oscillations are interpreted as continental shelf waves generated by the diurnal tide currents and the local winds. For the second, observations are consistent with wind-driven internal waves. A third event of near-inertial oscillations appears for most of the drifters in the wake of a tropical storm. The vorticity of the background mesoscale circulation shifts the frequency of the wind-generated oscillations by as much as ±0.05 cpd.