Seasonality in Southern Hemisphere Eddy Statistics at 500 mb

Abstract

An analysis has been made of the spatial and frequency dependence of transient eddy statistics in the Southern Hemisphere at 500 mb. This study emphasizes summer versus winter differences in order to complement previous results of Trenberth (1981), which are shown to nearly correspond to the mean of the summer plus winter statistics. Variance fields of geopotential height, the north–south and east–west geostrophic velocity components, the transient kinetic energy and the poleward transient eddy momentum flux have been analyzed for nine winter and eight summer 128-day seasons from 1972–80. The fields are examined in the frequency domain using Lorenz' (1979) “poor man's spectral analysis” technique. The total fields and the contributions from two broad frequency bands covering periods of 2–8 and 8–64 days are geographically mapped and their zonal means are presented. The spatial distribution of the eddy statistics is quite similar in summer and winter, although the variances in winter are larger and of broader latitudinal extent, thereby indicating a more vigorous circulation overall than in summer. The seasonal changes in eddy statistics closely follow corresponding changes in the mean westerly wind field and are very small over the Indian Ocean, while the largest changes occur in the Pacific Ocean region. A storm track, as indicated by the high frequency fluctuations, exists in the Indian Ocean along 50°S in both summer and winter. The main low frequency variations including blocking-type phenomena occur south of New Zealand and southeast of South America in both seasons, but with small changes in location. Storms in the Pacific Ocean region have a somewhat longer time scale than over the Indian Ocean. A comparison has been made with results from previous studies and, in particular, with statistics based upon station data analyzed at GFDL. The GFDL analyses produce relatively weaker wind speeds over the data sparse oceans. The variance fields are comparable in magnitude but differ in detail, and the GFDL analyses fail to capture the characteristic patterns in the storm track and blocking regions of the hemisphere.