Nonlinear latitudinal transfer of wave activity in the winter stratosphere

The transfer of wave activity between wave guides separated in latitude by a surf zone region of zero potential vorticity gradient is investigated in a simple model of the winter stratosphere and its extension into the Tropics. The distinction from classical wave propagation on a slowly varying background state is illustrated in a conceptual model in which latitudinal separation is reduced locally by wave disturbances of finite amplitude, implying a nonlinear dependence of the transfer on wave amplitude and a nonlinear transfer of wave activity to higher zonal wavenumbers. The response of a topographically forced vortex in the presence of different forms of tropical circulation, comprising either uniform or vertically localized anomalies, is examined numerically, using an exact pseudomomentum‐based wave activity to diagnose separately the vortex and tropical wave responses. Changes to the zonal mean flow in the Tropics are related in a natural way to the potential vorticity structure of the background state and the associated wave guides, which also control the general character of tracer mixing across and out of the Tropics.