Seasonal Asymmetry of the Low- and Middle-Latitude QBO Circulation Anomaly

Abstract

It is usually assumed that the circulation anomaly induced directly by the quasi-biennial oscillation (QBO) in the equatorial zonal wind is, approximately, a seasonally independent, two-cell structure symmetric about the equator and confined to the Tropics. It is shown here using a simple two-dimensional model that although the two-cell structure exists at equinox, at solstice the summer cell disappears and the winter cell is greatly strengthened (about three times stronger than at equinox) and expanded. Strong cross-equatorial flow is induced near the shear zone where the QBO winds are easterly. This result may explain why the observed anomalies in trace gases are small in the summer hemisphere and it also reduces the need for a modulation of the planetary wave fluxes at low and middle latitudes in order to explain the modulation of the circulation there. At low latitudes, the shape of the isopleths of a modeled tracer closely resemble those observed in both easterly and westerly phases of the QBO. The difference between the hemispheres stems from the nonlinear advection of the equatorial zonal wind anomaly into the winter hemisphere, where it leads to a large temperature anomaly due to the explicit latitudinal dependence of the thermal wind equation. An asymmetric circulation anomaly is induced both at steady state and in the transition to steady state: during transition, an asymmetric circulation works to produce the asymmetric temperature anomaly required by thermal wind balance, while at steady state the same circulation balances the Newtonian cooling induced by the (now established) temperature anomaly. Thus, in the real stratosphere, an asymmetric circulation may exist even in the absence of a large asymmetric temperature anomaly, though the circulation anomaly will eventually produce such a temperature anomaly. The poleward extent of the circulation anomaly is increased by moderate Rayleigh friction on the zonal wind poleward of about 15° in the winter hemisphere. However, low-latitude friction reduces the low-latitude zonal wind anomaly and hence the circulation anomaly.