A corresponding physical explanation of the mechanism behind the generation of waves due to wind was given five years after Miles' work, by Lightill (1962). We consider the same setup, but here the streamlines are drawn in the reference frame of the wave, i.e., corresponding to $U-c$.

This flow configuration yields a pressure field that is higher over troughs and lower over crests.

The pressure is $90^\circ$ out of phase with the wave (or surface elevation $\eta$).

The flow configuration also creates regions of upflow and downflow.

Consider a parcel starting at the blue circle.

It will propagate rightward, meet the region of high pressure, and become deflected downward and leftward.

It will take its lower vorticity with it, resulting in a region of reduced vorticity.

A parallel argument for a parcel starting below the critical layer instead shows there will also be a region of enhanced vorticity.