O'Sullivan, D., and P. Chen, 1996: Modeling QBO influence on isentropic tracer transport in the tropics. J. Geophys. Res., 101, 6811-6821.
Tracer transport in the lower stratosphere is investigated using an off-line semi-Lagrangian transport model and winds from a mechanistic three dimensional middle atmosphere model, interpolated to the 650 K isentropic surface, over a winter season. By comparing two simulations, the effect of the quasi-biennial oscillation's (QBO's) easterly or westerly wind phases on low-latitude transport is examined, especially regarding the formation of an apparent subtropical transport barrier at the equatorward edge of the surf zone. The tracer field is diffused less than potential vorticity (PV) and shows sharper gradients and finer structures developing over the winter. In both QBO phases the winter extratropical surf zone is bounded by large tracer gradients equatorward of about 15° latitude. The inability of quasi-stationary Rossby waves to propagate into QBO easterlies leads to a much sharper edge forming between the rapidly stirred extratropical surf zone and the relatively 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 reaching the summer hemisphere easterly winds, causing mixing there. Simulations using a tracer initially confined to the tropics show that more tropical air is mixed out to the winter extratropics by Rossby wave 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 tropics in both QBO phase simulations, neither QBO phase simulation showed extratropical air being irreversibly mixed into the tropics. These results are related to observations of the tropical reservoir of stratospheric aerosols, especially the dynamics and transport of its winter hemisphere boundary.