Why does the wake of boats leave a long lasting print on the sea surface?

A picture is better than a long explanation. The boat is long gone but its wake is visible for a very long time and forms a pattern looking like a trail on the sea surface. The two lines are not even divergent like the envelope of the wake. My first guess is that it is created by a change in refractive index of the water due to a non uniform mixing of the water temperature in the rear of the boat. But I can't explain why the perturbation is stable. It should diffuse or be conic as the wake of the boat.