Summary

Impact of interannual ozone variations on the downward coupling of the 2002 Southern Hemisphere stratospheric warming

Researchers at the Bureau of Meteorology have shown that including the amount of ozone in the stratosphere improved weather forecasts in the spring of 2002. The late winter and spring of 2002 was the first recorded sudden stratospheric warming (SSW) in the southern hemisphere. The event had greater than expected impacts on surface conditions in Australia and led to a record breaking negative Southern Annular Mode (SAM). The SAM is a large-scale (non-seasonal) climate driver involving the north-south latitudinal movement of the strong westerly winds that circling the southern hemisphere between 40° and 60° latitude. These winds are associated with the east to west movements of cold fronts and storms bringing rainfall to southern Australia. The SAM has three phases (positive, neutral, and negative) with the positive or negative events generally lasting around one to two weeks. A negative SAM indicates the westerly winds are occurring at more northerly latitudes.

Sub-annual changes in ozone have previously been associated with the Southern Annular Mode. When Bureau researchers included the amount of ozone in the seasonal climate model ACCESS-S1, the forecast for 2002 better described the observed weakening of the southern polar vortex and the magnitude of the negative SAM in October. This led to a more accurate forecast for the warm and dry conditions in eastern Australia in October 2002. Future research will determine if including ozone will improve predictions for SAM in other years.