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Keywords:

  • South East Australia;
  • rainfall;
  • subtropical ridge

Abstract

During the 20th century and early 21st century, the subtropical ridge (STR) in the vicinity of eastern Australia has intensified significantly. While the position of the STR has often been the focus of past studies, its influence on rainfall across southern Australia is less pronounced than the influence of the intensity of the ridge. Most of the Australian continent has experienced above average rainfall from 1997 to 2009, but areas of below average rainfall, including southeastern Australia (SEA), exist and tend to match the pattern of the influence of the STR on Australian rainfall. The spatial extent makes the 1997–2009 rainfall deficit in SEA different from previous dry decades such as 1935–1945. It is also noted that the annual cycle of the rainfall deficiency centred on a continuum from March to October overlaps well with the time of the year when the STR intensity has a strong influence on rainfall. Using simple linear statistics, a rainfall decline for the period 1997–2009 equivalent to nearly two thirds of that observed can be inferred from the intensification of the ridge. The apparent southward shift of the ridge in certain seasons does not appear to have had an additional effect on SEA rainfall. During the 1935–1945 drought, almost a third of the rainfall decline can be attributed to the strengthening of the ridge. Finally, it was observed that the intensification of the STR was not monotonic during the 20th century but happened mostly during two extended periods: from 1900 to the 1940s, culminating at the time of the 1935–1945 dry decade, and from 1970 to 2010 culminating with the 1997–2009 rainfall deficit in SEA. That multidecadal behaviour is reminiscent of the global warming of the planet. Copyright © 2012 Royal Meteorological Society