Diagnosing the QBO’s Influence on Circumpolar Vortex Variability Using MSU Brightness Temperatures and MSU-Derived Winds

Abstract

Using brightness temperatures from channels 3 and 4 of the Microwave Sounding Unit (MSU) as approximations to mean-layer temperatures, the geostrophic winds at 50 mb can be computed through a ?bottom-up? approach. When this method is applied at high latitudes during austral winter and spring, it is found that accurate descriptions of the seasonal evolution and interannual variability of the lower-stratospheric circumpolar vortex are obtained. Variations in early-spring vortex strength from year to year appear to relate well to variations in the timing of the first large late-winter wavenumber one event in the lower stratosphere. Since wave forcing of the mean flow in the lower stratosphere is known to be weak, the variability in vortex strength may result from variations in wave-induced subsidence through the downward control principle. Previous studies have demonstrated a biennial harmonic in both extratropical wave forcing and the mean flow, suggesting a link with the equatorially confined quasi-biennial oscillation (QBO). This study attempts to find a similar signal in the strength of the lower-stratospheric austral circumpolar vortex. It is first found that during the easterly (westerly) phase of the QBO large-amplitude wavenumber one in MSU channel 4, brightness temperature generally occurs earlier (later) in the season than normal. Subsequently, for most years of the study when the QBO is in its easterly (westerly) phase, the circumpolar vortex is observed to be weaker (stronger) than average.