Seasonal evolution of the QBO‐induced wave forcing and circulation anomalies in the northern winter stratosphere

Diagnostics of wave‐mean‐flow and wave‐wave interactions in isentropic coordinates are analyzed regarding the mechanisms that govern the Holton‐Tan effect (HTE), whereby the tropical quasi‐biennial oscillation (QBO) modulates the northern winter stratospheric polar vortex. We find a clear seasonal evolution of a QBO modulation of Rossby waves and linear and nonlinear wave interactions, along with multiple mechanisms at work. In early winter, when the lower stratospheric QBO is easterly, there is enhanced upward stationary planetary‐wave activity in the subtropical to midlatitude lower stratosphere. These upward‐propagating waves are first refracted toward the zero‐wind line. Approximately 1 month later, poleward‐propagating wave anomalies appear at midlatitudes as a result of nonlinear wave interactions. The enhanced upward wave propagation appears to be due to a combination of the QBO influence on the latitudinal location of the zero‐wind line and the increased midlatitude baroclinicity associated with the QBO‐induced meridional circulation. This is further aided by a gradual poleward shift over the winter of the middle‐stratospheric waveguide, refracting waves poleward. The associated wave breaking weakens the polar vortex and drives a stronger meridional circulation throughout the lower to middle stratosphere, contributing to the high‐latitude warming. It is additionally found that in late winter, the modified high‐latitude waveguide associated with the weaker polar vortex, consequently reduces upward‐propagating transient planetary waves at high latitudes while simultaneously enhancing upward‐propagating synoptic waves at midlatitudes. This highlights the need to view the HTE and its mechanisms as an evolving phenomenon throughout the winter.