Stock & Land (Australia): Victoria's catastrophic bushfires may have their origins 5000 kilometres away, in the Indian Ocean, and a phenomenon only first identified a decade ago. Last week, researchers from the University of NSW reported that the event known as the Indian Ocean Dipole (IOD) may be the primary driver of drought in south-eastern Australia, overriding even the El Nino/La Nina cycles. A positive outcome of this finding may be better predictions of dry spells, possibly up to six months ahead.The grim news is that all current understanding about the IOD points to an increased number of drought years as the planet warms. Like the El Nino Southern Oscillation (ENSO), which reflects the warming and cooling of surface temperatures across the Pacific Ocean, the IOD refers to temperature patterns over the Indian Ocean.
As in the Pacific, Indian Ocean waters are alternately warmer or cooler in the east and west, although the pattern is uneven and historically less frequent. In what scientists have called the "positive" phase of the IOD, cooler ocean temperatures around Indonesia generate less of the moist, turbulent air that in better years—when Indonesian waters are warmer than those of the western Indian Ocean—streams across the Australian continent to deliver the south-east its spring rain.
The IOD has now been positive for three years running, since 2006. There has been a strong run of positive IOD events since the mid-1990s, when the Murray-Darling Basin first began to encounter water deficits. Dr Caroline Ummenhofer of UNSW’s Climate Change Research Centre analysed droughts since the 1880s and concluded that the Federation drought around 1900, the prolonged WWII drought of the 1940s and the current "Big Dry" have all been associated with either positive IOD phases, or a lack of wetter negative IOD phases….
1 comment:
The way I see it, there is a terrible problem with attributing the current dry weather in Victoria to positive IOD phases or lack of negative IOD phases.
This is that rainfall over central and southeastern Western Australia, which before 1997 was reduced during positive IOD phases, has been 40 percent higher the before then.
A more logical explanation for the dry weather is that it comes from the Southern Ocean, an area that gets a way-to-small share of attention from our scientists. We do know that the Southern Annular Mode (SAM) regulates the westerly winds. In its positive state, the polar westerlies are strengthened but the westerlies are weaker over Australia, whereas in its negative state the reverse occurs. There is little evidence the SAM suddenly changed around 1997, but the last three very dry years suggest to me a shift to unprecedently positive SAM values might well be occurring as we speak. Such a shift is very much in accord with paleoclimate records that show no winter-rainfall-type climates existed before the Pleistocene glaciations. It would also help explain the increases over central and southeastern WA, where easterly winds are moister than the former midlatitude westerlies.
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