THE DESCENT RATES OF THE SHEAR ZONES OF THE EQUATORIAL QBO

The influence of vertical advection on the descent rate of the zero-wi nd line in both phases of the equatorial quasi-biennial oscillation (Q BO) is investigated with the help of the ''THIN AIR'' stratospheric tw o-and-a-half-dimensional model. The model QBO is forced by two symmetr ic easterly and westerly waves, and yet the model reproduces qualitati vely the observed asymmetry in the descent rates of the two shear zone s due to the enhanced heating during easterly descent combined with th e equatorial heating induced by the extratropical planetary waves. Obs ervations show that the maximum easterly accelerations occur predomina ntly from May until July, which is when the modeled equatorial planeta ry-wave-induced heating rates are weakest. Hence, model results are co nsistent with the theory that vertical advection induced by extratropi cal planetary waves slows significantly the descent of the easterly sh ear zone. The model also shows the observed increase in vertical wind shear during stalling of the easterly descent (which increases the imp act of vertical advection). In the model, the effect of cross-equatori al advection of momentum by the mean flow is negligible compared to th e vertical advection. Changes in the propagation of planetary waves de pending on the sign of the equatorial zonal wind have a small effect o n the modeled equatorial heating rates and therefore do not play a lar ge part in producing the modeled asymmetry in descent rates.