Dynamical Response to the QBO in the Northern Winter Stratosphere: Signatures in Wave Forcing and Eddy Fluxes of Potential Vorticity

Abstract:

The quasi-biennial oscillation (QBO), a quasi-periodic oscillation of the stratospheric equatorial zonal wind between easterlies and westerlies, is known to affect the stratospheric circulation and transfer anomalies downward into the troposphere via a modulation of the winter polar vortex. However, the exact mechanism(s) governing this remain unclear. In this study, wave-mean-flow interactions associated with this effect, the so-called Holton-Tan effect (HTE), are studied using the ERA-Interim reanalysis dataset. Significant evidence of the HTE in isentropic coordinates is found, with a weaker and warmer polar vortex present when the lower stratospheric QBO is in its easterly phase (QBOe). For the first time, we quantify the QBO modulation of wave propagation, wave-mean-flow interaction and wave decay/growth via a calculation of potential vorticity (PV)-based measures, the zonal-mean momentum budget and up/down-gradient eddy PV fluxes. Stratosphere-troposphere coupling is also investigated with particular focus on the effect of the tropospheric subtropical jet on QBO modulation of the wave activity. In the subtropical to midlatitude lower stratosphere, QBOe is associated with an enhanced upward flux of wave activity across the tropopause, and corresponding wave convergence and wave growth, which leads to a stronger zonal-mean Brewer-Dobson Circulation and consequently a warmer polar region. In the middle stratosphere, QBOe is associated with increased poleward wave propagation, leading to enhanced wave convergence and in-situ wave growth at high latitudes and contributing to the weaker polar vortex. In agreement with recent studies, our results suggest that the critical-line effect cannot fully account for the wave anomalies associated with the HTE. Instead, it is suggestive of a new, additional mechanism that hinges on the QBO-induced meridional circulation effect on the latitudinal positioning of the subtropical jet. Under QBOe, the QBO-induced meridional circulation causes a poleward shift of the subtropical jet, encouraging more waves to propagate into the stratosphere at midlatitudes.