Does the Holton–Tan Mechanism Explain How the Quasi-Biennial Oscillation Modulates the Arctic Polar Vortex?

Idealized experiments with the Whole Atmosphere Community Climate Model (WACCM) are used to explore the mechanism(s) whereby the stratospheric quasi-biennial oscillation (QBO) modulates the Northern Hemisphere wintertime stratospheric polar vortex. Overall, the effect of the critical line emphasized in the Holton–Tan mechanism is less important than the effect of the mean meridional circulation associated with QBO winds for the polar response to the QBO. More specifically, the introduction of easterly winds at the equator near 50 hPa 1) causes enhanced synoptic-scale Eliassen–Palm flux (EPF) convergence in the subtropics from 150 to 50 hPa, which leads to the subtropical critical line moving poleward in the lower stratosphere, and 2) creates a barrier to planetary wave propagation from subpolar latitudes to midlatitudes in the middle and upper stratosphere (e.g., less equatorward EPF near 508N), which leads to enhanced planetary wave convergence in the polar vortex region. These two effects are mechanistically distinct; while the former is related to the subtropical critical line, the latter is due to the mean meridional circulation of the QBO. All of these effects are consistent with linear theory, although the evolution of the entire wind distribution is only quasi-linear because induced zonal wind changes cause the wave driving to shift and thereby positively feed back on the zonal wind changes. Finally, downward propagation of the QBO in the equatorial stratosphere, upper stratospheric equatorial zonal wind, and changes in the tropospheric circulation appear to be less important than lower stratospheric easterlies for the polar stratospheric response. Overall, an easterly QBO wind anomaly in the lower stratosphere leads to a weakened stratospheric polar vortex, in agreement with previous studies, although not because of changes in the subtropical critical line.