The role of transient eddies in low-frequency zonal variations of the Southern Hemisphere circulation

Abstract

The dominant modes of low-frequency variation of the Southern Hemisphere (SH) circulation in winter have been identified using 15 years of monthly mean analyses of the SH troposphere. The two leading modes are primarily zonally-symmetric, representing out-of-phase variations of geopotential height between middle and high latitudes in one case and between the tropics and middle latitudes in the other. There are accompanying variations of zonal wind and temperature associated with these modes, which have an equivalent barotropic vertical structure in the extratropics. The two modes represent 25% of the variance of the monthly mean height in the SH. The high-latitude mode is also the dominant pattern of variation of 5-day average height anomalies, indicating that this is an important mode on time scales from about a week to a season. Using composites of monthly mean fields and transient eddy statistics for opposite extremes of this high latitude mode, the interaction between transient eddies and the anomalous mean flow is described. During one extreme of the variation, there is an increased height gradient, a stronger zonal jet and more baroclinicity at high latitudes. Although there may be some doubt about the reliability of the transient eddy statistics, they show an enhanced storm track at high latitudes, with transient eddy fluxes which help to maintain the anomalous mean flow. DOI: 10.1034/j.1600-0870.1990.00005.x