Abstract
The influence of vertical advection on the descent rate of the zero-wind line in both phases of the equatorial quasi-biennial oscillation (QBO) is investigated with the help of the “THIN AIR” stratosphere two-and-a-half-dimensional model. The model QBO is forced by two symmetric easterly and westerly waves, and yet the model reproduces qualitatively the observed asymmetry in the descent rates of the two shear zones due to the enhanced heating during easterly descent combined with the equatorial heating induced by the extratropical planetary waves. Observations show that the maximum easterly accelerations occur predominantly from May until July, which is when the modeled equatorial planetary-wave-induced heating rates are weakest. Hence, model results are consistent with the theory that vertical advection induced by extratropical planetary waves slows significantly the descent of the easterly shear zone. The model also shows the observed increase in vertical wind shear during stalling of the easterly descent (which increases the impact of vertical advection). In the model, the effect of cross-equatorial advection of momentum by the mean flow is negligible compared to the vertical advection. Changes in the propagation of planetary waves depending on the sign of the equatorial zonal wind have a small effect on the modeled equatorial heating rates and therefore do not play a large part in producing the modeled asymmetry in descent rates.
