Abstract
The atmospheric angular momentum balance is studied by analyzing the data during a simulated January of the Oregon State University two-level atmospheric general circulation model. Monthly zonal means of the Coriolis torques, the vertical transfer of relative angular momentum by penetrating convection, midlevel convection, background viscosity, pressure force and vertical motion, the meridional transfer of relative angular momentum by the standing zonal mean motion, standing eddies and transient motion, and the surface exchange of relative angular momentum by oceanic and continental friction and mountain torque are computed.
It is found that midlevel convection is a negligible component in the momentum balance, and the midlevel pressure torque is a significant downward transfer only in the Northern Hemisphere middle latitudes where the major mountains of the model exist. The momentum transfers by penetrating convection and background viscosity are comparable and generally downward, except for the upward momentum transfer by the penetrating convection at the latitudes where the Hadley circulations of both hemispheres join. The standing mean motion transports momentum downward in the subtropics and upward in higher latitudes, whereas the standing eddy and transient motions are negligible components of the momentum balance except for the significant upward transfers by the transient motion in the roaring forties of the Southern Hemisphere.
The mountain torques are downward momentum transfers with a maximum at 30°N and another at 20°S except for minor upward transfers in the northern equatorial latitudes and north of 60°N. The frictional torques are upward momentum transfers in the subtropics and downward transfers in the middle latitudes, and generally dominate the mountain torques except in the Northern Hemisphere middle latitudes.
Although errors in the description of the momentum balance due to the 6 h sampling are indicated, the overall picture given by the model is realistic. The global atmospheric angular momentum is in general drained by the mountain torques and created by the surface friction. The global relative angular momentum in the upper layer is a balance between the downward transfer and the transformation of Ω-momentum into relative angular momentum, whereas the global relative angular momentum in the lower layer is a balance between the downward transfer from the upper layer and the transformation of relative angular momentum into Ω-momentum. The subtropical jets are maintained by a balance between the Coriolis torques on the poleward branches of the Hadley circulations and the downward transfers and divergent poleward fluxes, whereas the upper westerlies in higher latitudes are maintained by a balance between the convergent poleward fluxes and the downward transfer and Coriolis torque on the equatorward branches of the Ferrel circulation.
