EFFECTS OF THE QUASI-BIENNIAL OSCILLATION ON THE ZONALLY AVERAGED TRANSPORT OF TRACERS

The influence of the quasi-biennial oscillation (QBO) on the transport of long-lived tracers out of the tropics and the mechanism responsibl e for the QBO in subtropical ozone and its dependence on the seasonal cycle are examined with a two-dimensional model. The modelled QBO indu ces a meridional circulation which modulates transport of long-lived t racers out of the tropics. The induced circulation also produces a QBO in ozone in the subtropics by advection of ozone out of the tropics a nd down from higher altitudes. In agreement with observations, the sub tropical anomalies in ozone are greatest in the winter season. This se asonal synchronization of the subtropical anomalies occurs because the induced circulation is stronger always in the winter hemisphere as a result of nonlinear momentum advection in the tropics and subtropics. Meridional transport in the model is enhanced by the QBO through an '' upper'' and a ''lower'' transport regime, in agreement with the analys is by Hitchman et al. [1994]. When there are descending westerly winds in the tropics in the model, transport out of the tropics is enhanced in the lower stratosphere. When there are descending easterlies, tran sport out of the tropics is enhanced in the middle stratosphere. This modulation of transport out of the tropics significantly influences th e stratospheric distribution of long-lived tracers. Depending on the p hase of the QBO, mixing ratio surfaces of long-lived tracers (such as N2O) in the extratropics can be displaced poleward by more than 10 deg rees.