Pierre, It looks as if the quasi-static approximation has a different meaning in fluid dynamics vs electromagnetism. What I want to explore is how a standing wave such as ENSO can be related to a travelling wave such as TIW. The analytic solution is a standing wave, as the temporal and spatial dimensions are seperable. I am thinking about the math involved in casting it back into a travelling wave. There is likely a slow or quasi-static approximation that blends the temporal and spatial and thus creating a travelling wave.