The effect of orography on the global atmospheric angular momentum and the general circulation

Abstract

Orographic features affect the atmospheric circulation not only locally but also globally. In this paper, the latter is quantified in terms of the global relative angular momentum in a series of numerical experiments using the Portable University Model of the Atmosphere (PUMA). Two effects of an orographic barrier on the atmospheric relative angular momentum are identified. On the one hand, an orographic barrier acts to break the zonal symmetry of the circulation. Because of the symmetry breaking, the energy inputed is stored not only in the time-mean part, but also in the time-varying part of the zonal flow, leading to a reduction in the mean and an enhancement in the variance of the relative angular momentum. On the other hand, orographic features lead also to changes in the eddy momentum fluxes, which are necessary to balance the mountain torque. These eddy fluxes induce an increase of global relative angular momentum through a strengthening and an equatorward shift of the zonal flow. The second effect prevents a further decrease in the global relative angular momentum with an increase in the height of the barrier. Zusammenfassung Die Orographie der Erdoberfl¨