The result of alternative gain of negative and positive vorticity in the water column when the forcing from the wind stress is relaxed. The rotation induced by a change in vorticity drives the water particles away from the coast in some places and towards the coast in others. As a result, the location of the water particles changes from the red line to a new position on the yellow line.
The net effect of the change in particle location is a northward shift of the wave pattern from the red to the yellow location. For an observer in a fixed location this appears as a northward propagating wave.