The last month has seen a spate of articles linking the decline in rainfall over southern Australia to a succession of positive Indian Ocean Dipole events and an absence of negative ones since 1994.
People now are saying that the IOD is the principle cause of rainfall variations over Australia, yet even apart from the increases over central-western Australia that I have tried desperately hard since 2001 to draw towards the CSIRO’s attention.
However, I recently discovered a map that casts a lot of doubt as to IOD-based explanations for the drying of southern Australia. One might think that the absence of wet negative IOD events between 1880 and 1905 when Melbourne gathered thirty percent more rain than since 1997 is due to poor data. However, when I e-mailed the publisher of this data, Bob Beale of the University of New South Wales, he said that there really were no negative Indian Ocean Dipole events between 1880 and 1905. That contradicts the statement in the article “...the longest period of its kind since records began in the late 19th Century.”
What irritates me no end is the failure of people at the CSIRO and Bureau of Meteorology to take the following statement about rainfall increases in central-western Australia that came to me from an e-mail by Ian Smith:
“My thinking at the moment is that the increase represents a response to the Australian continent as a whole, warming up faster than the oceans over recent time. This would increase the land-ocean temperature gradient and drive a stronger monsoon-like circulation. We have some climate model experiments, which tend to support this hypothesis.”
This may not necessarily seem to relate to the decline in rainfall over southern Australia. However, what I think – and observe from real synoptic charts – is that the “stronger monsoon-like circulation” increases the northerly component of airflow over southeastern Australia. This produces dry winds from the interior, especially in areas like southern Victoria shielded by the Great Divide, and a consequent reduction in rainfall. Why others at the CSIRO and Bureau of Meteorology fail to take this idea serious I do not know, but suspect highly political motives are involved.