On the Interaction Between Kelvin Waves and the Mean Zonal Flow

Abstract

The descent of westerly wind regimes of the quasi-biennial oscillation in the equatorial stratosphere is marked by two regions of strong zonal wind fluctuations in the 8?20 day period range: one in the easterly regime below the westerly shear zone and another in the upper level westerlies. There is a distinct minimum wave activity within the shear zone. The lower waves are identified as Kelvin waves, while the upper waves are shown to require the existence of a meridional wind component. The vertical transport of zonal momentum by the waves is computed and compared to the balance requirements. It is shown that the distribution of momentum flux divergence due to Kelvin waves is in good agreement with the observed zonal accelerations during the descent of westerly regimes of the quasi-biennial oscillation. The Kelvin waves give up much of their momentum to the zonal flow in the shear zone which marks the leading edge of the descending westerly regime, even though they apparently do not encounter a ?critical layer? within this zone. The occurrence of strong Kelvin waves appears to be confined to periods of descending westerly regimes.