Other Processes

Many air–sea processes and quantities are written in terms of a relative wind—the difference between the wind velocity and some representation of the surface current. In our framework the relevant surface current is the Lagrangian mean flow \(\overline{\mathbf{u}}^L\); Stokes drift would importantly modify the effective relative wind. In particular, the following merit further investigation (some of which is already being undertaken):

• Wind–stress parameterizations. Bulk formulas often take the stress to depend on the difference between \(\overline{\mathbf{u}}_E\), the Eulerian mean velocity, in the air and the ocean. If the atmosphere instead “feels” \(\overline{\mathbf{u}}^L\), then the wave-induced component \(u_S\) changes the relative wind and, with it, the surface momentum flux.
• Processes based on relative wind, such as eddy killing. Mechanisms where the wind extracts energy from mesoscale eddies or other currents depend on both the sign and magnitude of the relative wind. A wave-induced contribution to \(\overline{\mathbf{u}}^L\) alters that relative wind and can therefore modify the strength and pattern of these feedbacks.
• Spatial variability in observations. Observed spatial variability in the wave field implies a spatially varying Stokes drift and thus a spatially varying Lagrangian mean and relative wind. Doppler-based surface–current measurements already capture this variability, providing a pathway to incorporate the wave-induced feedback into wind–stress parameterizations.

In this sense, the same Lagrangian drift that significantly modifies Miles’ instability could also significantly modify any process that depends on relative wind, from idealized growth rates to the large-scale air–sea energy exchanges associated with eddies and other currents.