MODELING THE QUASI-BIENNIAL OSCILLATIONS INFLUENCE ON ISENTROPIC TRACER TRANSPORT IN THE SUBTROPICS

Tracer transport in the lower stratosphere is investigated using an of f-line semi-Lagrangian transport model and winds from a mechanistic th ree dimensional middle atmosphere model, interpolated to the 650 K ise ntropic surface, over a winter season, By comparing two simulations, t he effect of the quasi-biennial oscillation's (QBO's) easterly or west erly wind phases on low-latitude transport is examined, especially reg arding the formation of an apparent subtropical transport barrier at t he equatorward edge of the surf zone. The tracer field is diffused les s than potential vorticity (PV) and shows sharper gradients and finer structures developing over the winter. In both QBO phases the winter e xtratropical surf zone is bounded by large tracer gradients equatorwar d of about 15 degrees latitude. The inability of quasi-stationary Ross by waves to propagate into QBO easterlies leads to a much sharper edge forming between the rapidly stirred extratropical surf zone and the r elatively unstirred tropics during the easterly QBO phase than occurs during the westerly QBO phase, In the westerly phase some Rossby wave activity can propagate across the equatorial westerlies and break on r eaching the summer hemisphere easterly winds, causing mixing there, Si mulations using a tracer initially confined to the tropics show that m ore tropical air is mixed out to the winter extratropics by Rossby wav e stirring when the QBO phase is easterly, despite the sharper band of stronger tracer (or PV) gradient which separates the tropics and the winter extratropics in this phase. While air was mixed out of the trop ics in both QBO phase simulations, neither QBO phase simulation showed extratropical air being irreversibly mixed into the tropics. These re sults are related to observations of the tropical reservoir of stratos pheric aerosols, especially the dynamics and transport of its winter h emisphere boundary.