A Simple Model for Transient Eddy Momentum Fluxes in the Upper Troposphere

A simple model for transient eddy momentum fluxes is constructed based on the hypothesis that planetary waves radiating from low-level baroclinic eddies act effectively as a random forcing of the upper troposphere. The response of the upper troposphere is modeled by the barotropic, nondivergent vorticity equation, linearized about a zonally symmetric flow. Rayleigh friction and scale-selective damping are included to crudely represent the effect of vertical diffusion and nonlinear equilibration mechanisms; the associated damping rates constitute the only tunable parameters of the model. A distinguishing feature of the model is that wave-activity theory is used to specify the statistics of the random forcing as a function of the background flow and low-level eddy heat fluxes. The resulting model simulates the annual cycle of momentum fluxes reasonably well, but tends to underestimate the eddy kinetic energy in midlatitudes. The results are interpreted in light of an analytic solution to the linear response of a barotropic model to random forcing.