The Role of the Seasonal Cycle in the Quasi-biennial Oscillation Of Ozone

Abstract

Satellite and station data have shown that the quasi-biennial oscillation (QBO) in total column ozone is asymmetric about the equator, unlike the zonal wind oscillation. There is an asymmetry in phase, as subtropical ozone anomalies maximize in the winter?spring season in both hemispheres, showing strong synchronization with the seasonal cycle irrespective of the phase of the equatorial QBO. There is also an asymmetry in amplitude, which we suggest is due to the timing of the equatorial QBO relative to the seasonal cycle and possible seasonal variation of the Hadley circulation. These asymmetries change with time as the phase relationship between the equatorial QBO and seasonal cycle changes, producing a slow modulation of the subtropical ozone QBO. Numerical simulations of the ozone QBO with a two-dimensional radiative?dynamical?photochemical model successfully reproduce these features of the ozone QBO and show that mean motions near the base of the equatorial stratosphere are largely responsible for the asymmetry of the oscillation. The column oscillation is a complex superposition of number densities at various levels due to phase descent of the dynamical QBO and strong spatial gradients in the strength and direction of the Hadley circulation. The role of ozone photochemistry is also discussed, and comparison is made to the simulated quasi-biennial oscillation of NOy.