A note on the variability of the mid-tropospheric flow over the Southern Ocean in the Australian region

Variability in the strength of the westerly wind component at 500 hPa over the Australian sector of the Southern Ocean is investigated by means of a zonal index (ZI) obtained from daily analyses of the Australian Bureau of Meteorology. This index reveals a marked seasonal variation, from a maximum in January to a minimum in July. Interannual variability of the westerlies is also demonstrated by the index, and appears to be modulated by the phase of the Southern Oscillation. This response of the ZI is shown to be related most closely to variability of geopotential heights of the 500 hPa surface over southern Australia. During warm El Niño-Southern Oscillation (ENSO) events, above average annual means of geopotential height are normally observed, but the variability, as determined by the standard deviation of the daily heights, is low. In the years prior to the ENSO events of 1982 and 1987, variability of daily geopotential heights was high, but annual mean geopotential heights were near or below average. The high variability in the years prior to an ENSO may provide an indication of a significant circulation change in the Australian region.