Andrea Simpson - andrea.simpson@physics.ox.ac.uk
Abstract: The quasi-biennial oscillation (QBO) plays an important role in affecting the interannual variability of boreal-winter stratospheric polar vortex. Multiple mechanisms have been proposed for this so-called “Holton–Tan effect”, i.e. the stratospheric polar vortex tends to be stronger when the lower-stratospheric QBO is in its westerly phase but weaker when the QBO is easterly. So far, most of the existing mechanisms focus on changes in the waveguide and the planetary-scale Rossby waves. These mechanisms are often approached mainly from the viewpoint of wave mean-flow interaction, i.e. the polar vortex is weakened by the wave drag via enhanced Eliassen‐Palm flux convergence. However, it remains puzzling as why the Holton–Tan effect is noticeably weaker in climate models though these large-scale processes should supposedly be resolvable.
In this talk, we reveal some new, additional processes from the viewpoints of potential vorticity (PV). We call attention to a pronounced and significant region where nonlinear wave-wave interaction and finite-amplitude effects prevail. Evidence will be provided to show that wave breaking in the lowermost stratosphere can lead to both irreversible PV mixing and internal wave generation, i.e. wave decay is closely related to wave growth following wave-breaking events. Evidence will also be provided to show that these processes are systematically modulated by the QBO. The nonlinear processes will lead to enhanced net wave forcing on the stratospheric polar vortex and more frequent stratospheric sudden warmings during easterly QBO. Based on these results, we would conclude that the “Holton–Tan effect” is mainly associated with the QBO-induced secondary circulation, instead of a change in the stratospheric waveguide originally proposed by Holton and Tan (1980).