A Slowly Varying Model of the Lower Stratospheric Zonal Wind Minimum Induced by Mesoscale Mountain Wave Breakdown

Abstract

A model proposed recently by Tanaka and Yamanaka indicated that the breaking of mesoscale mountain waves produces enough drag to predict the weak mean-wind region in the lower stratosphere through every season, especially the zonal wind minimum in winter. Critical levels, which were introduced into the model to stop propagation of gravity waves into the mesosphere, are rather artificial and not as concretely founded. Here we used a two-time and two-scale model that is valid under a slowly varying assumption. The slowly varying model includes effects of wave saturation, wave transience and wave self-acceleration based on a consistency relation and leads to formation of a realistic zonal wind minimum in the lower stratosphere when plausible gravity wave drags are specified. At the same time, considerably stronger meridional mean flows than have been estimated so far are also induced in thin layers. Saturation and subsequent breaking of mountain waves are principally responsible for inducing these circulations and also for increasing the poleward angular momentum transport and its sink. An interesting result is that sharp vertical gradients of phase velocities, in other words phase velocity shocks, are formed mainly in the winter sewn. Pseudomomentum shocks also appear at the same altitudes. Therefore, any mountain wave, whose amplitude has already been suppressed by wave saturation, disappears completely at the shock altitudes in the stratosphere. Alleviation of a systematic westerly bias in high resolution general circulation models seems to be achieved by incorporation of mesoscale mountain wave drag.