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 synoptic-scale variations had been removed by application of a 10-day low-pass fitter. This resulted in 11 principal modes, accounting for 66% of 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 perturbation in the principal storm tracks at high latitudes, and two residual patterns not used for this investigation. Persistent anomalies were defined as spells where the time coefficient of the rotated EOF was greater than 1 standard deviation or less than −1 standard deviation for a period 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 ways: 1) from high-pass filtering of the daily 5OO-hPa geopotential height anomalies to include periods of less than six days, and 2) from the tracks of surface geostrophic vorticity centers obtained from twice-day analyses over the period 1980–86. The results show that the jet deviates to the north of cyclonic anomalies and south of anticyclonic anomalies, provided that the anticyclonic anomaly is not located too far south. The high-pass filtered 5OO-hPa geopotential height variance closely follows the location of the polar jet but there is no maximum in the 2–6-day variance associated with the subtropical jet. The surface cyclone frequency is greatest a few degrees south of the polar jet axis with local maxima under and cast 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 jet regimes and a meridional displacement of the peak poleward momentum flux, which assist's in maintaining the polar jet. The double jet regime over the western Pacific is reinforced by the weak contribution of the eddy fluxes to the acceleration of the zonal flow between them.