ROLE OF THE QUASI-BIENNIAL OSCILLATION IN THE TRANSPORT OF AEROSOLS FROM THE TROPICAL STRATOSPHERIC RESERVOIR TO MIDLATITUDES

The temporal evolution of the stratospheric aerosol distribution in th e tropical stratospheric reservoir after the eruption of Mount Pinatub o was observed from 1992 to 1995 by the HALOE instrument on the UARS s atellite. Since the spatial gradient of aerosol loading is large at th e boundaries of the tropical stratospheric reservoir due to the volcan ic aerosols, the effect of the meridional circulation on the distribut ion is seen clearly. The mechanism for dispersal of aerosol in the low er stratosphere from the tropics into midlatitudes strongly depends on the phase of the equatorial zonal wind. The time-latitude crosssectio ns of the normalized distribution of aerosol on isentropic surfaces ar e used to observe the equatorial variation as well as change in meridi onal dispersal during the quasibiennial period. Observed tropical stra tospheric winds are used with a simple analytical dynamical model to e xamine transport processes of tracers from the tropics during several phases of the quasibiennial oscillation (QBO) from 1992 to 1995. The L agrangian meridional circulation in the tropics is consistent with the vertical and meridional velocities correlated with the QBO in the zon al wind. We find that vertical motion plays a crucial role in vertical and subsequent meridional transport. The pattern of meridional diverg ence derived from the vertical velocity is closely related to the obse rved HALOE aerosol distributions and their temporal development in the equatorial region. The westerly (easterly) shear phase of the QBO is associated with sinking (rising) motions at the equator and subsequent poleward (equatorward) transport in the lower stratosphere.