THE MOTION OF ATMOSPHERIC VORTICES AND THE GENERAL CIRCULATION

Abstract

Atmospheric vortices are treated as masses of air with certain distributions of vorticity, with or without solid boundaries. It is shown that in the northern hemisphere the effect of the vorticity distribution in the surrounding fluid is such as to exert a horizontal force driving cyclonic vortices toward regions of higher absolute vorticity and anticyclonic vortices toward regions of lower absolute vorticity. Thus, cyclonic vortices are in general driven toward the north and anticyclonic vortices toward the south, because of the general northward increase of the vertical component of absolute vorticity. However, in the case of a very strong jet of westerlies, the absolute vorticity has a strong maximum toward the north and minimum to the south. The force then acts in the reverse direction in some regions, driving a cyclone situated to the north of the maximum of absolute vorticity southward, and an anticyclone situated to the south of the minimum of absolute vorticity northward. Thus, cut-off lows and highs may be formed in the upper troposphere. In the lower troposphere, where the westerly current is weak, this reverse effect does not occur. Since a large upper cut-off cyclone is generally associated with a low-level anticyclone, the combined system can move as a single entity without separation. It is found that the influence of divergence is small compared to the effects already mentioned. It is suggested that these effects, together with the principle of conservation of total angular momentum, offer an explanation for the existence of zones of westerlies and easterlies in the atmosphere.