Intraseasonal Variations in the Southern Hemisphere Circulation

Abstract

Intraseasonal variations in the Southern Hemisphere atmospheric circulation have been examined through the application of a 10-50-day bandpass filter to daily ECMWF analyses for 1980-88. Variations on this time scale contribute more than 40% of the daily variance in 500 hPa geopotential over much of the middle and high latitudes of the Southern Hemisphere. EOF analysis of the unnormalized variance for all seasons shows that 49% of this variance can be explained by zonal wave trains centered on the South Pacific and southern Atlantic/Indian oceans, a high-latitude mode of global extent, and a wavenumber 3 pattern at midlatitudes. These modes are essentially equivalent barotropic but slope westward with height so that the patterns at 100 hPa typically lag those at 1000 hPa by around 10o longitude. There appears to be little interaction with the low-latitude circulation. All of the leading modes propagate eastward but the most consistent movement is shown by the South Pacific wave train represented by EOFs 1 and 2. This wavenumber 4 pattern moves eastward at 4o-7o longitude per day near 55oS. Spectral analysis indicates the main contribution to the variance of the intraseasonal modes comes from spectral bands peaking at 13-14 and 22-24 days.