Wind- and wave induced drift in the uppermost part of the ocean

Trajectories of Lagrangian surface drifters are investigated in terms of wind and wave forcing on surface currents. While the behavior of the interior part of the Ekman layer has been extensively studied using drogued drifters and current meters, this study uses drifters at the direct surface that include a contribution from the Stokes drift. The wind- and wave response of surface currents on time scales from 1 hour to 10 days is investigated using two types of satellite-tracked drifters: (i) iSpheres, a spherical type floating on the surface, and (ii) CODE drifters with a cross-shaped drogue centered at 70 cm depth. Significant differences in their drift behavior is attributed to differences in wind- and wave exposure. The iSpheres respond symmetrically to cyclonic and anticyclonic wind forcing and are wind-coherent for sub-inertial frequencies. In contrast, the CODE drifters respond primarily to anticyclonic wind forcing, being particularly responsive to forcing that resonates with the intrinsic ocean dynamics – mostly near-inertial oscillations and tides. The wind-coherence of surface currents vanishes at the inertial frequency and above, indicating that the upper meter of the ocean does not reach a steady state with respect to the forcing at these frequencies. Values for leeway and drift velocity deflection angles are mostly in agreement with previous findings, with some exceptions that may be attributed to regional and seasonal variations in the near-surface stratification. The spectral response of drift velocities to waves (i.e. Stokes drift) differ to the wind response in terms of the drift deflection angle for frequencies above the inertial, possibly because the Coriolis-Stokes force induces an asymmetric response for cyclonic and anticyclonic forcing regimes.