INTERACTION OF EXTRATROPICAL ROSSBY WAVES WITH WESTERLY QUASI-BIENNIAL OSCILLATION WINDS

The role of laterally radiating stratospheric Rossby waves from the wi nter hemisphere in the momentum balance of the quasi-biennial oscillat ion (QBO) remains an unresolved issue. Observations of extratropical R ossby waves penetrating close to the equator suggest that Rossby wave absorption in the tropics must occur at least occasionally. This paper presents numerical simulations using a global shallow water model to examine the pattern of wa,ve absorption at low latitude during the wes terly wind phase of the QBO. Earlier results indicating that extratrop ical Rossby waves radiating to the equator do not break there when the QBO phase is westerly are reproduced. It is shown that the westerly Q BO jet can persist, undiminished at its core, over seasonal length int egrations, because the laterally radiating Rossby waves are absorbed a long the QBO jet's flanks, where easterly acceleration occurs. The net effect of the Rossby waves on the westerly QBO jet is a reduction in the width of its meridional profile but not its core strength. This be havior is interpreted by considering the filtering of equatorward radi ating Rossby waves by critical; layer absorption, a process that depen ds on the background zonal mean wind and the waves' phase speed and am plitude. The simulations also show that adequate horizontal resolution is needed to capture the suppression of wave breaking at the QBO jet core and persistence of the jet core strength over seasonal timescales . The implications of this result for simulations of the QBO in genera l Circulation models are considered.