Momentum flux, flow symmetry, and the nonlinear barotropic governor

Abstract

A number of idealized life-cycle simulations of baroclinically unstable waves are systematically analyzed to study the effects of eddy momentum flux and of zonal mean horizontal shear on the finite-amplitude evolution of the waves. The finite-amplitude baroclinic waves are sensitively influenced by the vertically integrated eddy momentum flux of the normal mode via the large barotropic shear it spins up in the mean flow. This "barotropic governor' mechanism prevents the eddy from attaining all the available potential energy stored in the domain leading to irreversible barotropic decay. Barotropic shear in the basic flow, the earth's spherical geometry, and nonquasigeostrophic motion all introduce spatial asymmetry into the normal mode, whose nonlinear evolution therefore rapidly departs from the purely baroclinic solution. The details of the departure depend sensitively on the shape of the initial asymmetry, however. -from Author