Interannual Variations in the Southern Hemisphere Winter Circumpolar Vortex: Relationships with the Semiannual Oscillation

Several studies of Southern Hemisphere circulation report changes in the late 1970s and 1980s through the mid- and high latitudes that are associated with a weakening in the semiannual oscillation (SAO) of sea level pressure and geopotential height. This weakening is thought to reflect changes in the amplitude of zonal wavenumber 3, a deepening of the subantarctic trough, intensification of subtropical ridging, and an increase in the strength of the westerlies. These conclusions suggest an equatorial expansion of the circumpolar vortex in high latitudes and a poleward contraction of the vortex in the midlatitudes. The goal of this study is to examine the size and spatial variation of the Southern Hemisphere circumpolar vortex in winter for evidence of such change. The size of the 500-hPa vortex was calculated for the mid- and high latitudes in 5° longitude sectors from both the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset (1958–96) and Australian Bureau of Meteorology operational archives (1972–92). A comparison of the vortex data derived from these datasets reveals similar patterns of interannual variability with some systematic differences, especially in the high latitudes. The longer NCEP–NCAR reanalysis record shows vortex contraction in the midlatitudes from 1958 through the early 1980s followed by gradual expansion. By contrast, the vortex in the polar regions exhibits a general pattern of expansion from the beginning of the record through the 1980s. The midlatitude vortex contraction and polar expansion through the 1980s are associated with geopotential height increases over the midlatitude oceans near 90°E, 180°, and 50°W; a deepening of polar geopotential heights within the intervening longitudes; and an enhancement of the meridional height gradient. These locations are similar to those associated with zonal wavenumber 3. The findings of this study are consistent with the hypotheses proposed in earlier studies of SAO variability.