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

Abstract

The influence of the quasi‐biennial oscillation (QBO) on isentropic transport and mixing in the tropical and subtropical stratosphere is investigated over a period of 6 years. The transport and mixing is quantified by the equivalent length diagnostic, calculated from tracers simulated in chemical transport models using European Centre for Medium‐Range Weather Forecasting (ECMWF) analyzed winds. A new procedure for calculating equivalent length from tracers, such as nitrous oxide (N 2 O), with a tropical maximum or minimum is devised. Results from different tracers and different chemical transport models demonstrate the robustness of the equivalent length diagnostic. Equivalent length calculated both from an artificial tracer and from simulated N 2 O indicates that, when the QBO winds are easterly, mixing is inhibited in the tropics throughout the broad region occupied by the easterlies and that, when the QBO winds are westerly, mixing is strongly inhibited within the narrow region occupied by the westerlies themselves but is enhanced in the subtropics. Examination of absolute vorticity gradients and horizontal Eliasen‐Palm (EP) fluxes (broken down into contributions from different zonal wavenumbers) suggests that, in the ECMWF analyses, barotropic shear instability of the westerly jet, as well as propagation of planetary waves from the extratropics, drives the subtropical mixing seen in the westerly phase.