Effects of the quasi‐biennial oscillation on the zonally averaged transport of tracers

Abstract

The influence of the quasi‐biennial oscillation (QBO) on the transport of long‐lived tracers out of the tropics and the mechanism responsible for the QBO in subtropical ozone and its dependence on the seasonal cycle are examined with a two‐dimensional model. The modeled QBO induces a meridional circulation which modulates transport of long‐lived tracers out of the tropics. The induced circulation also produces a QBO in ozone in the subtropics by advection of ozone out of the tropics and down from higher altitudes. In agreement with observations, the subtropical anomalies in ozone are greatest in the winter season. This seasonal 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 analysis 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, transport out of the tropics is enhanced in the middle stratosphere. This modulation of transport out of the tropics significantly influences the stratospheric distribution of long‐lived tracers. Depending on the phase of the QBO, mixing ratio surfaces of long‐lived tracers (such as N2O) in the extratropics can be displaced poleward by more than 10°.