The influence of mean wind shear on the propagation of Kelvin waves

The long Kelvin waves observed in the equatorial lower stratosphere transport sufficient westerly momentum upwards to account for the westerly acceleration of the quasi-biennial oscillation, provided that this momentum is absorbed in the westerly shear zone of the mean wind. The behavior of linearized Kelvin waves as they propagate through such a shear zone is deduced by scaling the wave equations with the ratio of the latitudinal scale to the zonal scale (the aspect ratio) which is a small parameter for the observed waves. The resulting system is reduced to a single elliptic equation which is solved numerically for various configurations of the mean zonal wind.

It is found that passage through a westerly shear zone with no critical level modifies the waves as follows:

(1) Both the latitudinal and vertical scales of the waves decrease. (2) The upward momentum transport becomes concentrated towards the equator. (3) The latitudinally averaged vertical momentum transport remains nearly constant.

If the zonal wind profile contains a critical level the waves are nearly totally absorbed at that level. Furthermore, the wave amplitude has a pronounced maximum at the equator just below the critical level. For cases in which the zonal wind has lateral as well as vertical shear it was found that the Kelvin waves are effectively absorbed at the height of the critical level at the equator even when the critical level height rises rapidly away from the equator.