ON THE ASYMMETRY OF THE MIDDLE-LATITUDE CIRCUMPOLAR CURRENT

Abstract

Evidence is presented that the circumpolar current of temperature latitudes often is not symmetric about the earth's axis of rotation. This asymmetry, which varies with time, is frequently so large that the ?zonal index? no longer represents the true features of the current. Simple methods are developed to measure the asymmetry in terms of a circulation pole about which the current is symmetric, and to compute a representative circulation index. The detailed discussion of a typical example, supplemented by a summary of several other cases, establishes the following: periods of large asymmetry average about a week in duration and occur in the colder season, preferably during the ?high-index? stage of a poleward index trend ; at such times the circumpolar current attains maximum strength, concentrated in a narrow zone, and resembles the classical high-index pattern ; the intensity of disturbances superimposed on the current falls to minimum values; for all large displacements of the circulation pole, its longitude is such that the concentrated current flows at much lower latitudes over the Pacific Ocean than over Europe; and at least over the United States, characteristic broad-scale temperature and precipitation patterns accompany large asymmetry. The fixed orientation and correlation with circulation strength suggest that the asymmetry may be produced by the influence of external forces, such as surface friction and orographic barrier effects.