Exchange of Atmospheric Angular Momentum between the Hemispheres

Abstract

IN regions of surface easterly winds (where the surface is moving in space faster than the air) a transfer of westerly relative angular momentum takes place from the Earth's surface to the atmosphere, while in regions of surface westerlies the transfer is from the atmosphere to Earth. These transfers are necessarily accompanied by transports of angular momentum within the atmosphere from low latitudes, where easterly winds prevail, to middle latitudes where westerly winds prevail. Such transports are accomplished chiefly by large-scale quasi-horizontal eddy processes as first suggested by Jeffreys¹. Lorenz² and Starr³ have recently summarized the observational evidence which bears out Jeffreys's suggestion and have shown how the angular momentum transports act to maintain the angular momentum of the Earth's atmosphere. Concomitantly with the transfer of angular momentum between low and middle latitudes we have recently found evidence of a transfer of momentum between the hemispheres⁴. In the winter hemisphere the low latitude surface easterlies are strong and more angular momentum is communicated to the atmosphere than is transported polewards. The excess is transferred across the equator into the summer hemisphere. The purpose of this letter is to communicate our recent numerical estimates of the interhemispheric transfer and to compare them with the hemispheric contents of angular momentum.