Dynamical response to the QBO in the northern winter stratosphere: signatures in wave forcing and eddy fluxes of potential vorticity

Wave-mean-flow interactions associated with the Holton-Tan effect (HTE), whereby the tropical quasi-biennial oscillation (QBO) modulates the Northern-Hemisphere wintertime stratospheric polar vortex, 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. The effect of the tropospheric subtropical jet on QBO modulation of the wave activity is also investigated. In the subtropical to midlatitude lower stratosphere, QBOe is associated with an enhanced upward flux of wave activity, and corresponding wave convergence and wave growth, which leads to a stronger poleward zonal-mean meridional 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 these 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.