THE INFLUENCE OF PERSISTENT ANOMALIES ON SOUTHERN-HEMISPHERE STORM TRACKS

Empirical orthogonal function (EOF) analysis has been applied to nine years of daily 500-hPa data for the Southern Hemisphere from which syn optic-scale variations had been removed by application of a 10-day low pass filter. This resulted in 11 principal modes, accounting for 66% o f the variance, which were then regionalized through application of a Varimax rotation. These rotated patterns included a high-latitude mode covering the Antarctic, eight others defining regional perturbations in the principal storm tracks at high latitudes, and two residual patt erns not used for this investigation. Persistent anomalies were define d as spells where the time coefficient of the rotated EOF was greater than 1 standard deviation or less than -1 standard deviation for a per iod of 10 days or more, and 220 of these spells were obtained for the nine EOFs used in the analysis. Storm tracks were identified in two wa ys: 1) from high-pass filtering of the daily 500-hPa geopotential heig ht anomalies to include periods of less than six days, and 2) from the tracks of surface geostrophic vorticity centers obtained from twice-d aily analyses over the period 1980-86. The results show that the jet d eviates to the north of cyclonic anomalies and south of anticyclonic a nomalies, provided that the anticyclonic anomaly is not located too fa r south. The high-pass filtered 500-hPa geopotential height variance c losely follows the location of the polar jet but there is no maximum i n the 2-6-day variance associated with the subtropical jet. The surfac e cyclone frequency is greatest a few degrees south of the polar jet a xis with local maxima under and east of upper-level cyclonic anomalies and minima beneath and downstream of anticyclonic anomalies. The high -latitude mode is associated with a split between single and double je t regimes and a meridional displacement of the peak poleward momentum flux, which assists in maintaining the polar jet. The double jet regim e over the western Pacific is reinforced by the weak contribution of t he eddy fluxes to the acceleration of the zonal flow between them.