Wave Forcing of Zonal Mean Angular Momentum in Various Coordinate Systems

Abstract

The wave forcing of the atmospheric mean flow in isentropic coordinates has been investigated intensively in the past with the divergence of the Eliassen–Palm flux playing a dominating role. These concepts are reviewed briefly and it is pointed out that angular momentum is attractive in this context because the wave driving can be written in the form of a flux divergence. This helps to evaluate the wave forcing in other coordinate systems with a different separation of waves and mean flow. The following coordinates are chosen: (lambda, phi, z), (lambda, phi, theta), and (lambda, theta, z). To be consistent, only one type of zonal averaging should be used. Massweighted averaging is applied in the isentropic standard case and simple averaging is applied in the others. The wave driving is presented for all three systems. It has to balance essentially the mean-flow part of the ‘‘Coriolis term’’ in the angular momentum budget in (phi, z) and (theta, z) coordinates but not in the (phi, theta) system where the form drag is a mean-flow term and, therefore, the forcing pattern differs from what has been published so far.