Planetary boundary layer of equatorial waves and critical latitude

The boundary-layer solutions for oscillatory interior flow in a neutral fluid show that, away from the critical latitude, the planetary boundary-layer flow is composed of two vertically propagating waves; one is of shorter vertical wavelength and attenuates faster with height while the other is of longer wavelength and attenuates slowly with height. At the critical latitude, the longer wave is of the nature of an inertial oscillation and penetrates the entire depth of the fluid and gives rise to a rather stronger (but finite) vertical motion than in the surroundings at great height, but below 2 km this difference is not very great. In general, the vertical velocity of the boundary-layer flow depends on both the vorticity and the divergence of the interior flow, and the influence of the former is to intensify while that of the latter is to diminish the existing vertical motion. The influence of the stable stratification above a mixed, neutral surface layer on the boundary-layer flow of the equatorial waves is analyzed in detail. It is shown that the boundary-layer flow is greatly impeded by the stable layer and the horizontal velocities of the boundary-layer flows are made to change their directions in the upper part of the mixed layer and above, so that the vertical velocity is greatly reduced. The effect of the critical latitude is almost absent in the antisymmetric Rossby waves under stable stratification.