Tropical waves

On a flat equatorial beta plane the equations of fluid motion and continuity have low‐frequency wavelike solutions that are separable into vertical and horizontal modes. The horizontal modal structure is the same in the case of a compressible fluid in hydrostatic equilibrium (the usual meteorological case) and in the case of an incompressible fluid under the Boussinesq approximation (the usual oceanographic case), but the vertical modal structure, normally expressed in terms of an equivalent depth, is different in the two cases. The theory of low‐frequency tropical waves (excluding tides) is used to highlight the similarities and the disparities in the oceanographic and meteorological approaches to them. This is followed by a brief review of the observational evidence for the existence of these low‐frequency waves in the oceans, atmosphere, and ionosphere of the tropics.