On the Control of the Stratosphere-Troposphere Dynamical Coupling by Finite-amplitude Rossby Waves

A diagnostic relationship between finite-amplitude wave activity and the associated adiabatic adjustments to the zonal flow is used to investigate the downward influence of stratospheric polar vortex weakening events. Unlike most previous theories based on the Eulerian zonal mean, a new formalism of wave activity density (negative pseudomomentum) that works for small and large-amplitude eddies has been applied. The net adjustment to the mean flow by eddies is quantified by calculated eddy-free reference state flow, which is constructed by zonalizing potential vorticity (PV) contours conservatively and by inverting the resultant PV gradient. The result suggests that the downward migration of wind anomalies from the stratosphere to the surface is largely explained by conservative dynamics due to direct (local) effect of eddy activity with nonnegligible contribution by nonconservative effects. The net adiabatic adjustment to the mean flow is consistent with enhanced finite-amplitude wave activity during the events. After the onset of the events, the contribution by nonconservative effects that stem from eddy-induced mixing and non-adiabatic forcing become important and act to dampen the effect of advective eddy-mean flow interaction in the lower-to-upper stratosphere. It is also found that the assumed no-slip surface boundary condition for the reference state is more suitable to quantify the adiabatic adjustments by eddy to the zonal flow. The relative roles of finite-amplitude wave activity in the stratosphere and troposphere for the above-mentioned mechanism will also be discussed.