AsianScientist (Jul. 4, 2011) – Decreasing autumn and winter rainfall over southern Australia has been attributed to a 50-year decrease in the average intensity of storms in the region – a trend which is forecasted to continue for another 50 years.
In an address today to the International Union of Geodesy and Geophysics conference in Melbourne, CSIRO climate scientist, Dr. Jorgen Frederiksen, said these changes are due to reductions in the strength of the mid-latitude jet stream and changes in atmospheric temperatures. The jet stream comprises fast moving westerly winds in the upper atmosphere.
“The drop in winter and autumn rainfall observed across southern Australia is due to a large downturn in the intensity of storm formations over at least the last three decades compared with the previous three decades, and these effects have become more pronounced with time,” Frederiksen said.
Research on climate model projections also suggest a continuation of these trends over the next 50 years.
The new publication in the International Journal of Climate Change: Impacts and Responses is the result of a collaboration between the Bureau of Meteorology’s Dr. Carsten Frederiksen and Janice Sisson, and CSIRO’s Dr. Jorgen Frederiksen and Stacey Osbrough.
The research, based on observations and climate modeling, centers on the changes in southern Australian winter rainfall linked to atmospheric circulation changes that are directly associated with storm formation, and particularly rain bearing lows and frontal systems crossing southern Australia.
The most important circulation feature associated with winter storm formation is the strength of the sub-tropical jet stream. For example, winter storms give south-west Western Australia much of its rain. Between the 20-year periods from 1949 to 1968 and 1975 to 1994, south-west WA rainfall reduced by 20 percent. In south-east Australia, there were reductions of 10 percent.
“Our research has identified the historic relationship between the reduction in the intensity of storms, the southward shift in storm tracks, changing atmospheric temperatures and reductions in mid-latitude vertical wind shear affecting rainfall,” said Frederiksen.
The researchers expect a continuation of these trends as atmospheric temperatures rise, based on projections from climate models forced by increasing carbon dioxide concentrations.
“Trends during the 21st century are likely to be similar to those observed during the second half of the 20th century, when we saw substantial declines in seasonal rainfall across parts of southern Australia,” Frederiksen said.
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Source: CSIRO.
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