A Ubiquitous Wavenumber-5 Anomaly in the Southern Hemisphere During FGGE

Abstract

Predominant wavenumber-5 patterns frequent the temperature fields of the lower stratosphere of the Southern Hemisphere during the summer months of FGGE. These pentagonal features, of broad latitudinal extent, appear to remain quasi-stationary or propagate eastward with periods on the order or 10 days. Ensemble statistics over the summer season confirm the presence of large-amplitude wavenumber-5 anomalies in the geopotential fields. Magnified height amplitudes appear in the zonal spectra at wave 5, near the tropospheric jet core: 50°S and 300 mb. The enhanced temperature anomalies in the lower stratosphere arise not only from the magnified geopotential amplitude of wavenumber 5, but also from its sharp evanescence above the jet. Time series analysis reveals that the peak in rms amplitude results primarily from the fluctuating contribution. The transient component of wavenumber 5 shows a regular eastward phase progression. Geopotential power spectra exhibit a pronounced peak, corresponding to eastward propagation with a period of approximately 11 days and half-power points at 8 and 15 days. Hence, the disturbances appear concentrated in both wavenumber and frequency. A nearly barotropic phase structure is characteristic of the bandpassed wave field over most of the amplified region, becoming more propagating near the extremities of the disturbance, where the amplitude is weak. Such behavior is suggestive of a partially trapped, or leaky normal feature, probably excited by baroclinic energy conversion. The eastward phase progression and height structure maximizing in the jet are not inconsistent with features of a baroclinically unstable mode. However, the variance peak at relatively low wavenumber, and perhaps more importantly, its discrete character in both space and time, are inconsistent with conventional views of baroclinic instability. Phase structures suggest that the refractive character of the basic flow and perhaps the temperature gradient at the Antarctic escarpment may be involved in the regularity of the disturbance.