Interannual Variability of the Southem Hemisphere Circulation: Representativeness of the Year of the Global Weather Experiment

Abstract

An analysis has been made of the interseasonal and interannual variability of mean circulation and eddy statistics for both summer and winter in the Southern Hemisphere. Total variance fields of geopotential height, the noith-wuth and east-west wind components and poleward transient eddy momentum fluxes at 500 mb are analyzed along with their contributions from two broad frequency bands covering 2-8 day and 8-64 day period fluctuations. Largest interannual variability occurs between 40-60°S in association with the main jet stream in summer or the polar jet stream in winter and the main belt of eddy activity within each season. The circulation and eddy statistics during the year of the Global Weather Experiment (GWE) are compared with the means and standard deviations over all years from 1972?80, and contrasted with individual years The GWE summer of 1978?79 is contrasted with 1976?77, and the 1979 winter is contrasted with 1980. The year of the GWE was charactrized by an exceptionally deep circumpolar trough, an increase in westerlies between 45?70°S and a decrease to the north, with a southward shift in the main westerly jet during summer 1978?79 and a considerably enhanced and southward shifted polar jet but weaker subtropical jet in winter 1979. Associated with these changes was a southward shift in storm tracks and high frequency eddy activity throughout the year. In both seasons anomalous convergence of momentum by the eddies into the jets was such that it would have helped sustain the abnormal distribution of westerlies against surface friction. Many of the anomalies in the circulation statistics during the GWE are statistically significant. most notably in winter, and their reality is supported by station data and the dynamical consistency of the relationships between the anomalous mean flow and storm tracks. In addition, the deficit of mass over the Southern Hemisphere revealed by sea level pressures in April-July 1979 is compensated by the surfeit that occurred in the North Hemisphere. Although the vastly improved observations during the GWE may have contributed to the size of the anomaly, they cannot account for the systematic change in location of the features of the flow. The circulation during the GWE appears to have been at one extreme of the large natural interannual variability that is so much a feature of the Southern Hemisphere flow. The atypical nature of the circulation should be borne in mind in analyses based solely on the GWE over the Southern Hemisphere.