Influence of the quasi-biennial oscillation on isentropic transport and mixing in the tropics and subtropics

The influence of the quasi-biennial oscillation (QBO) on isentropic transpo rt and mixing in the tropical and subtropical stratosphere is investigated over a period of 6 years. The transport and mixing is quantified by the equ ivalent length diagnostic, calculated from tracers simulated in chemical tr ansport models using European Centre for Medium-Range Weather Forecasting ( ECMWF) analyzed winds. A new procedure for calculating equivalent length fr om tracers, such as nitrous oxide (N2O), with a tropical maximum or minimum is devised. Results from different tracers and different chemical transpor t models demonstrate the robustness of the equivalent length diagnostic. Eq uivalent length calculated both from an artificial tracer and from simulate d N2O indicates that, when the QBO winds are easterly, mixing is inhibited in the tropics throughout the broad region occupied by the easterlies and t hat, when the QBO winds are westerly, mixing is strongly inhibited within t he narrow region occupied by the westerlies themselves but is enhanced in t he subtropics. Examination of absolute vorticity gradients and horizontal E liasen-Palm (EP) fluxes (broken down into contributions from different zona l wavenumbers) suggests that, in the ECMWF analyses, barotropic shear insta bility of the westerly jet, as well as propagation of planetary waves from the extratropics, drives the subtropical mixing seen in the westerly phase.