Global Angular Momentum Balance: Earth Torques and Atmospheric Fluxes

Abstract

Computations of various kinds are synthesized to give an estimate of the angular momentum balance for the principal latitude zones. Contributions considered are frictional and mountain torques, atmospheric fluxes due to correlation of velocity components, seasonal changes of momentum, and minor effects such as momentum carried by water mass fluxes. Large imbalances result (except in the northern winter) if calculated oceanic stresses are used to represent the total frictional torques. In the southern temperate belt, stresses based on a strong increase of drag coefficient with wind speed are shown to produce torques around one-third too great. In spring and summer in the northern tropics, excessive eastward torques result if surface resistance over land is not considered. New estimates of the frictional torques are derived for each season, taking into account the other momentum sources, sinks and meridional fluxes, to achieve a balance within each hemisphere. Principal physical features, generally confirming earlier results, are that: Frictional torques in both the tropical and temperate-latitude belts vary much more with season in the Northern Hemisphere (NH) than the Southern Hemisphere (SH), being strongest in winter. Mountain torques vary during the year, being of greatest relative importance in NH temperate latitudes in spring and summer; in both tropical belts, their seasonal variation augments that of the frictional torques. Poleward momentum fluxes across subtropical latitudes vary mostly in harmony with surface sources and sinks in the same hemisphere, but the balance is significantly modified by seasonal spinup or spindown of momentum (largest in the tropics) and by trans-equatorial flux. The trans-equatorial flux, toward the hemisphere in which lies the rising branch of the Hadley circulation (and dominated by the monsoons), is appreciable especially in NH summer when it nearly equals the flux across 3ON. Both polar caps are generally momentum sources; in the NH, where the southward flux across 6ON is 10% of the northward flux across 3ON in each season, the source is primarily mountain torque. Among the minor contributions, the momentum carried by water mass flux is appreciable, but the effect is nearly cancelled by that of water mass exchange across the earth's surface.