"Temporal and Spatial Variability in the Southern Hemisphere 500-hPa Circumpolar Vortex and its Relationship with the Semiannual Oscillation"

Adam W. Burnett Department of Geography Colgate University Hamilton, NY, 13346

In recent years, considerable attention has been given to the spatial structure and temporal variability of the semiannual oscillation (SAO). This half-yearly cycle dominates the long-term sea level pressure and geopotential height climatology over southern latitudes poleward of 35°S and is driven by seasonal differences in energy exchange processes and heat storage between the mostly water-covered middle latitudes and polar continental region of Antarctica. The strength of the SAO varies over time scales ranging from interannual to decadal (at least as seen in the historic meteorological record) and is reflected in Antarctic surface temperature records. Several studies report circulation changes in the late 1970s and 1980s that are associated with a weakening of semiannual oscillation. These changes are thought to reflect variations in the structure of southern hemisphere zonal circulation (zonal wave three), a deepening of the subantarctic trough, intensification of subtropical ridging, and an increase in the overall strength of the westerlies. This study examines the SAO through an analysis of the size and spatial structure of the Southern Hemisphere 500-hPa circumpolar vortex. Vortex size was calculated for the middle and high latitudes in 5º longitude sectors from the NCEP/NCAR Reanalysis dataset (1948-2000). The record shows vortex contraction in the middle latitudes from the early part of the record through the beginning of the 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 middle latitude vortex contraction and polar expansion through the 1980s are associated with geopotential height increases over the middle latitude oceans near 90ºE, 180º, and 50ºW, 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 three.