Radiative effects of ozone waves on the Northern Hemisphere polar vortex and its modulation by the QBO

The radiative effects induced by including interactive ozone, in particular, the zonally asymmetric part of the ozone field, have been shown to significantly change the temperature of the NH winter polar cap, and correspondingly the strength of the polar vortex. However, there is still a debate on whether this effect is important enough for climate simulations to justify the numerical cost of including chemistry calculations in long climate integrations. In this paper we aim to understand the physical processes by which the radiative effects of including interactive ozone, and in particular the radiative effects of zonally asymmetric ozone anomalies (ozone waves), amplify to significantly influence the winter polar vortex. Using the NCAR Whole Atmosphere Community Climate Model in the natural configuration, in which ozone depleting substances and green house gases are fixed at 1960's levels, we find a significant effect on the winter polar vortex only when examining the QBO phases separately. Specifically, the seasonal evolution of the midlatitude signal of the QBO – the Holton-Tan effect – is delayed by one to two months when radiative ozone wave effects are removed. Since the ozone waves affect the vortex in an opposite manner during the different QBO phases, when we examine the full time series, besides an early fall direct radiative effect, we find no statistically significant winter effect. We start by quantifying the direct radiative effect of ozone waves on temperature waves, and consequently on the zonal mean zonal wind, and show that this effect is most significant during early fall. We then show how the direct radiative effect amplifies by modifying the evolution of individual upward planetary wave pulses and their induced mean flow deceleration during early winter when stratospheric westerlies just form and waves start propagating up to the stratosphere. The resulting mean-flow differences accumulate during fall and early winter, after which they get amplified through wave-mean flow feedbacks. We find that the evolution of these early-winter upward planetary wave pulses and their induced stratospheric zonal mean flow deceleration are qualitatively different between QBO phases, providing a new mechanistic view of the extratropical QBO signal (the Holton-Tan effect). We further show how these differences result in an opposite effect of the radiative ozone wave perturbations on the mean flow deceleration for east and west QBO phases.