Interaction of extratropical Rossby waves with westerly quasi‐biennial oscillation winds

Abstract

The role of laterally radiating stratospheric Rossby waves from the winter hemisphere in the momentum balance of the quasi‐biennial oscillation (QBO) remains an unresolved issue. Observations of extratropical Rossby 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 wave absorption at low latitude during the westerly wind phase of the QBO. Earlier results indicating that extratropical Rossby waves radiating to the equator do not break there when the QBO phase is westerly are reproduced. It is shown that the westerly QBO jet can persist, undiminished at its core, over seasonal length integrations, because the laterally radiating Rossby waves are absorbed along 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 behavior is interpreted by considering the filtering of equatorward radiating Rossby waves by critical layer absorption, a process that depends on the background zonal mean wind and the waves' phase speed and amplitude. 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 general circulation models are considered.