Rain in Australia
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 forecast to continue for another 50 years. In an address 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. A long, severe drought, the worst on record is being experienced in many parts of Australia. As of November 2006, the late-winter to mid-spring rainfalls had failed. The average rainfall in the state of South Australia was the lowest since 1900. Across Victoria and the Murray-Darling Basin the season was the second driest since 1900. New South Wales' rainfall was boosted by above normal falls along the north coast of the state, however the state average rainfall for the season is the third driest since 1900. The situation has been worsened by temperatures being the highest on record since the 1950s.
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," Dr Frederiksen said.
"Our recent work on climate model projections suggests a continuation of these trends over the next 50 years."
"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,"
CSIRO's Dr Jorgen Frederiksen said.
The research, based on observations and climate modelling, 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 1949 to 1968 and 1975 to 1994 south-west WA rainfall reduced by 20 per cent. In south-east Australia, there were reductions of 10 per cent.
"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.
Professor Neville Nicholls meanwhile said such extremes as the heat wave in Victoria that accompanied the Black Saturday bushfires, similar heat in Pakistan and Russia, and the devastating tornadoes that have ripped through parts of the US this year are, in many cases, unprecedented in modern times. But global warming couldn't be blamed, he said.
'"Whenever these things happen people ask 'Was it caused by global warming?','' Professor Nicholls told the International Union of Geodesy and Geophysics conference.
''They were all caused by well-known and reasonably well-understood weather and climate events, even with some predictability.''
''Global warming doesn't produce these events, however, it's pretty hard to avoid the conclusion that global warming has exacerbated the frequency and the intensity of these heat waves,'' the Monash University professorial fellow said.
But this year's floods in Australia do not fit into that picture.
''It is much harder to make the connection to link those floods in Queensland in early 2011 to global warming,'' he said. The 2010 Victorian storms in March did little to help Melbourne's storage levels, but steady winter rains, and the 2010 Victorian floods in September, caused storage levels to remain above about 32.7%, rising to over 46% in September and 51% by late November.
For further information: http://www.csiro.au/news/Fewer-rain-storms-across-southern-Australia.html