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Roaring forties



Tasmania, smack bang in the path of the roaring forties. Image: NASA (Source: NASA /

If you've spent time on the west coast of Tasmania you'll know that the prevailing wind is westerly. And how. Stretching from latitudes 40-44 degrees South, Tasmania lies smack bang in the path of the roaring forties - westerly winds that rip around the Earth in latitudes between 40 and 50 degrees.

While similar wind patterns exist in the northern hemisphere, the lack of land in the south means there's little to stop or slow these gusty winds as they complete their circumpolar race.

"Because Tasmania dangles down from Australia it's more exposed to the westerlies. All of southern Australia experiences them, but more often than not Tasmania is within those westerlies," says Ian Barnes-Keoghan from the Bureau of Meteorology's Tasmania and Antarctica Regional Office.

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When hot air meets a spinning Earth

Global wind currents like the roaring forties are influenced by the Earth's shape and spin, and by basic thermodynamics: hot air rises.

In the southern hemisphere, warm air rises at the equator and moves south, while cooler air moves north to replace it. You might expect that this elevated warm air would continue to travel straight to the South Pole, where having cooled it would sink. But our atmosphere is too thin and the distance too great to sustain a single convection cell like that. Instead, the warm air that rises south of the equator sinks again at around 30 degrees south - (think Coffs Harbour in the east and Kalgoorlie in the west). From here the air travels along the surface of the Earth until latitude 60 south, after which it rises again as it moves towards Antarctica. In simple terms, it's like a double figure 8 conveyor belt of air stretching from the Equator to the South Pole.

But if convection were the only factor affecting the movement of air we wouldn't have global wind systems like the roaring forties or trade winds. At the same time as this air is moving from the Earth's surface up into the atmosphere and back down again, the spin of the Earth forces air that's moving south along the surface (in particular the air between latitudes 30 and 60) to slide below overlying air. This creates a flow of air from west to east - the westerly roaring forties.

The great speed of the roaring forties is produced because of the relative change in rotational speed of the land at the Earth's surface. Because it's a sphere spinning on its axis, the surface of the Earth is traveling at different speeds depending on latitude. The equator - with a circumference of 40,000km - zips around the Earth at speeds of about 1500 km/hr, while Coffs Harbour is only moving through space at 260 km/hr. So as air moves from the equator to the South Pole, it does so over land that is rotating more slowly. To someone on land, this air has sped up, making these westerly winds seem furious in their speed and constancy.

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Shifting winds

The roaring forties don't stay in their exact latitudinal designations but roam north and south across the Earth's surface as seasons change.

During the Australian summer, more direct sunlight falls on the southern hemisphere. The latitudinal band at 23.5 degrees south (about level with Alice Springs) receives sunlight at a 90 degree angle, shifting the zone of uprising air from the equator southwards and pushing the sub-tropical ridge south. So during summer, the roaring forties shift further towards the Pole. In winter, the latitudinal band at 23.5 degrees north gets the 90 degree angle sunlight, and the roaring forties in the Southern Hemisphere move north into mid-latitudes.

The winds also vary in their strength throughout the seasons.

"There's a peak in wind strength in spring and a slightly weaker peak in autumn - that's the time when you're more likely to get lots of wind," says Barnes-Keoghan.

While the roaring forties influence the entire island of Tasmania and southern parts of South Australia, Western Australia and Victoria, it's the mountainous western half of Tassie that feels the full brunt of the winds. Gusts of up to 200 km/hr - as fierce as a strong cyclone - have been measured.

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Wet and wild

The landscape of western Tasmania counters this constant battering of wind and the associated wash of rain (dumped on the mountainous slopes of western Tasmania and leaving the rest of the state relatively dry) by developing two unique ecosystems that thrive in the wet and wild. Here, the climate and topographic inaccessibility has created some of the most spectacular habitat on Earth: World Heritage listed rainforest and buttongrass moorland.

The peat-rich, buttongrass moorland is perhaps the most characteristic and unique habitat dominating southwestern Tasmania, and covers about one-seventh of the state, says zoologist Michael Driessen of the Department of Primary Industries, Water and Environment (DIPWE) Biodiversity Conservation Branch. Buttongrass moorland is unique in that it is the only vegetation system dominated by buttongrass (Gymnoschoenus sphaerocephalus) - a member of the sedge family - and because the underlying peat is derived from sedge and not sphagnum moss. It's also home to a unique set of animals, says Driessen.

Here, camouflaged, green-patterned ground parrots - one of only three species of parrot found in the world that nest on the ground - make their homes. An unlikely neighbour is the native burrowing crayfish, whose numerous burrows are a rare source of water for other moorland inhabitants when the peat dries out in summer. The moorland is also home to the sedge-eating herbivore, the broad-toothed mouse, a mainland species that thrives in Tasmania's remote west.

Many grassland and forest animals have adapted to the cold and wet brought by the roaring forties. Most skinks and all three snake species; the tiger snake, white-lipped snake and the copperhead, bear live young in order to keep their developing young warm. Mountain skinks, some restricted to tiny habitats atop remote southwestern mountains, slow down their reproduction so that they bear young once every two years rather than annually to save much-needed energy.

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Forest life

The mostly unexplored wilderness of southern Tasmania's rainforests is rich in endemic species but relatively poor in species diversity, compared to rainforests in similarly wind-blown latitudes in southern Chile. This is due to Tasmania's long isolation and the ancient lineage of the Gondwana species that inhabit the rainforest.

Here, eco-tourists may walk beneath tall, long lived Myrtle-beech (Nothofagus cunninghamii); Sassafras (Atherosperma moschatum), a unique species whose next closest relative is found in southern Chile; and the fragrant Taswegian honey tree, Leatherwood (Eucryphia lucida). Intrepid tourists may curse the aptly named entwined branches of the low-growing south-western native Horizontal (Anodopetalum biglandulosum) or the riverbank-hugging Native Plum (Cenarrhenes nitida).

On the forest floor, the delicate, long-tailed mouse, endemic to the southwest, holds its tail in a gentle arc as it rummages through the leaf mould for insects. It shares area with Tassie natives such as the eastern quoll, the wallaby-like Tasmanian pademelon, and of course the Tasmanian devil.

Tassie forests are a haven for mosses and lichens, including great swaths of sphagnum moss that hang like hair from woodland trees in central highlands. More than 200 species of lichen have been described in Tasmania. The stringybark Eucalyptus oblique, the dominant tree in southern wet sclerophyll forest, has yielded 30 species of lichens and 25 mosses on its buttresses alone.

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Cool birds

Bird life in western Tasmania ranges from the delightful fairy penguin to the rare Cape Barren goose and includes honey eaters, currawongs and robins. Many bird species are found on the mainland; 13 species, including the ground parrot and the Tasmanian native hen, are endemic to the island.

"We get a lot of birds migrating from Asia and of course the mutton bird does the circum-Pacific route," says Driessen.

The mutton bird or short-tailed shearwater, Puffinis tenuirostris, is one of the world's most impressive fliers. Dipping within centimeters of the sea surface in the face of strong winds, and diving to up to 10 metres, shearwaters inhabit offshore islands around the Tasman with up to 18 million birds arriving in September each year. Here, in burrows about a metre long, they mate and breed. In April the adults abandon their well-fed young, still covered in down, as they set off on a 15,000 km journey to the area around the Bering Strait between North America and Siberia. The young, despite starving for several weeks, develop their flight feathers and after several practice runs set off to join their parents in early May, despite a complete lack of expertise and guidance.

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Special thanks to Michael Driessen and Ian Barnes-Keoghan.

Tags: environment, weather

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Published 20 September 2007