On the effect of Rossby wave breaking on vertically propagating disturbances

The reaction of a non-linear flow to a vertically propagating large amplitude Rossby wave is examined with a multilevel quasi-geostrophic beta plane channel model. Concentrating on the interaction between wave breaking and vertical propagation, the role of wave-wave interactions in affecting the dynamics and the time evolution of the flow is examined. Wave breaking is found to coincide with a rapid reduction of the zonal mean potential vorticity gradients, leading to a strong downward reflection of the propagating wave. Surprisingly however the adjustment of the forced disturbance to the changes in the zonal mean flow is found to occur mostly in a linear fashion, despite the strong wave breaking signature. The direct effect of wave-wave interactions on the propagating wave seems to be insignificant. Moreover, the anomalous eddy fluxes that lead to the abrupt modification of the mean flow are almost exclusively associated with the large scale propagating disturbance. Wave-mean flow interactions thus appear to dominate the dynamics. Wave-wave interactions only affect the flow indirectly, by triggering a strong interaction between the forced wave and the zonal mean flow when a region of closed Eulerian circulation forms. Interestingly, a low resolution model run is found to reproduce the large scale flow evolution quite accurately. As the low resolution run shows no sign of wave breaking it is concluded that the details of the small scale potential vorticity deformation are not important for the large scale dynamics.