Friday, 14 March 2008

A lesson from Marble Bar

The so-called "heatwave" at the tiny Pilbara town of Marble Bar is well-known. I have always been suspicious it is a world record due to poor data in hot parts of the Sahara and Arabia.

The summer of 1923/1924 was one of extraordinary drought in the Pilbara. Some stations in the de Grey district which includes Marble Bar did not record measurable rain during 1924, and the average for the de Grey district was only around 33 millimetres. This drought caused the persistent heat since only a cyclone can lower temperatures below 100˚F.

In Melbourne, however, that very summer was the fifth coolest on record, and the year 1924 easily holds the record for the fewest 30˚C days with only twelve. Lower greenhouse gas levels and the absence of a monsoon in the northwest permitted highs to establish over Northern Territory longitudes and drive cool air persistently over southeastern Australia. As many as a third of the days during summer were below 20˚C in Melbourne, and on the west coast fewer than a third of the days in February 1924 reached 20˚C. Moreover, with all those moist southerly winds and cold fronts, Melbourne's dams actually filled during that summer - something they do not do in winter today.

What's worse is that as cold fronts and southern lows become a thing of the past in Australia's weather and climate, Melbourne will begin experiencing the sort of heatwaves Marble Bar did in that era of weaker monsoons. Without cold fronts or southern lows, the situation that used to apply only in the tropics that only influxes of moisture can keep temperatures down will apply everywhere over the continent except the eastern coast. With no rain to speak of not only in Melbourne, but as far north as Alice Springs, one is tempted to think that the inflows of moist air from the tropics that caused rainfall over pastoral districts of SA to increase from 1968 (and delay declines in Victoria and settled areas of SA by over a quarter of a century) may not be so much related to land surface heating as to stratospheric ozone depletion, which the phase-out of CFCs is just beginning to reverse. If I am right, then southeastern Australia could be in for much worse heatwaves than were moist air inflows and heavy (if variable) rain likely during persistent blocking in the Tasman.

No doubt we will get longer and longer heatwaves in southeastern Australia in the future. If my theory that rainfall increases over South Australia are related to stratospheric ozone loss pulling moist tropical air southward, then as global warming allows natural halogens to thin the ozone layer heatwaves will be broken by heavy rain more often inland and north of Mount Lofty.

In southern Victoria, however, with cold fronts and soon cool changes a thing of the past at present greenhouse gas emissions, summer heatwaves will by 2020 be unrelenting in a way Melbourne has never known. Sixty successive days over 35˚C or thirty successive over 40˚C will be normal in summer, and with the monsoonal winds that might relieve the north producing a rain shadow, Melbourne could by 2050 be the most arid place in Australia, with annual rainfall too low even for extensive grazing. In the absence of increased monsoonal weather, aridity beyond anything historically known (except perhaps during the dry era of 1922 to 1938) will be general over southern Australia right up to the Thomson and maybe anywhere west of the Snowy.

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