On the secondary meridional circulation associated with the quasi-biennial oscillation

Concentrations and distributions of stratospheric aerosol, hydrogen fluoride and ozone from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) are used to investigate features associated with transport by the secondary meridional circulation induced by the quasi-biennial oscillation (QBO). The points of maxima in the divergence and convergence of the QBO-induced meridional velocity at the equator are identified from the meridional gradients of the tracers. Such points can be identified from the tracer fields in the westerly shear zones but not in the easterly shear zones. The temporal variation of tracer concentration at the equator is determined mainly by vertical advection, which is significantly larger during the westerly shear phase of the QBO than during the easterly shear phase, since the QBO-induced equatorial sinking motion amplifies the vertical gradient. Thus, the vertical advection associated with the secondary circulation has a stronger influence on the equatorial tracer variation during the westerly shear phase than during the easterly shear phase.