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Tasmanian farmers face drought Blocking highs help drench the Apple isle Sail: Weather links: Stormwatch: La
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Tasmanian farmers facing drought are worried
that conditions will never again return to ‘normal’. Perhaps a warm pool of
Southern Ocean water will make all the difference ...
A curious climate wobble Blobs of warm and cold water circling Antarctica might have a lot to do with how much it rains along southern coastlines July, 2000What’s the difference between warm water in the wild Southern Ocean and cold? Just two degrees Celsius. Not much. In fact, try to tell the difference by sticking your big toe in the water and the only thing you will get is a cold toe. But two degrees appears to make a surprisingly big difference to rainfall. Now, researchers are hoping to discover if such temperature fluctuations have a real bearing on climate in coastal areas affected by the Southern Ocean’s cold, wet kiss. This illustration shows how the circumpolar wave is made up of two blobs of warm water and two blobs of cold water. It takes about eight years for the ocean to travel right ’round Antarctica and every two years there is a change in temperature at any one spot. Image courtesy CSIRO, Australia. Scientists at Scripps University in San Diego, California , the Antarctic Co-operative Research Centre in Hobart, Tasmania, and the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) have discovered a pattern in the Southern Ocean in which alternating blobs of warm and cold water rotate around Antarctica with the circumpolar current. The arrival of warm water appears to bring more rain than cold water, and this might - in part - explain the current drought in the Central Highlands of Tasmania. There are two blobs of warm water and two blobs of cold water. It takes about eight years for the ocean to travel right ’round Antarctica and every two years there is a change in temperature at any one spot. Right now in the Southern Ocean below Tasmania, a cold blob of water has moved away to the east and a warm blob is approaching from the west. Will this have an impact on climate in Tasmania and southern Victoria Sea ice affected According to the CSIRO’s Steve Rintoul, who is studying the effect of Southern Ocean currents on long term climate trends, this circumpolar wave certainly affects sea ice - with a visible bulge showing up in satellite images where cold blobs of water lie. He also says the change in ocean temperatures affects wind in the atmosphere. "This ocean pattern was discovered just a few years ago and people wondered if it affects weather patterns in high latitudes," Rintoul explains. Warren White, at Scripps University, found that when the effect of the circumpolar wave was looked at in the right way, there appeared to be a strong link between sea surface temperatures in the Southern Ocean and rainfall in southern and western Australia. "When you look at variations in rainfall and ‘smooth them’ - not monthly or annually but changes over two to four years - changes in rainfall over that time are correlated with changes in Southern Ocean temperature," explains Rintoul. "It is surprisingly to see a change in rainfall so strong for such small changes in ocean temperature." "A number of people are looking at what causes the Antarctic circumpolar wave in the first place. Then maybe we can make better long term rainfall predictions," he says. "Warren White shows such a connection between the Antarctic circumpolar wave and Southern Australian climate does exist," says Rintoul. "It explains lot of variation in rainfall in those areas not dominated by changes in equatorial Pacific Ocean temperatures - El Nino and La Nina. Tasmanian drought It is too early to tell if the Antarctic circumpolar wave is behind the excruciating drought facing central Tasmanian graziers, but evidence exists that warmer waters down south signal more rain. And lately, waters to the south of the Apple Isle have been cold. However, Rintoul points out that the Antarctic circumpolar wave appears to fluctuate from decade to decade. "Some decades are stronger than others," he says. "At the moment we are in a relatively weak phase. "There is a stronger connection between the circumpolar wave and Tasmanian rainfall during some periods than others. We still don’t know the whole story. "We tend to get more rain as we come into a warm phase. And at the moment we are coming out of a cold phase. But I really don’t want to give an impression that we can expect the drought will break." New science Only in last year or so have scientists bothered to pursue the connection between rainfall and the circumpolar wave. "Satellite measurements let us look at sea surface temperatures and the extent of sea ice," explains Rintoul. "However, satellite measurements have only been around for 15 years. That’s not even two cycles of the polar wave. So this science really is in early days. "At least there is some evidence that this climate wobble may prove to be a useful tool for predicting what rainfall will be like a year or two ahead." © weather-wise.com 2000 |
Did you know? Tasmanian weather varies a lot over such a small state. Frontal systems bring rains to the west coast but not to the east, because of mountains in the middle. Heavy rainfall along the east coast is associated with ex-tropical systems and east coast lows which migrate south with the east Australian current. Western dry In Western Australia there has been a downward trend in rainfall over the last 20 to 30 years. This decline in rainfall has something to do with a shift in westerly winds to the south. Why? No one is sure. On top of this long-term climate profile, there are variations in rainfall over a four year cycle that seem to be associated with Antarctic circumpolar wave.
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