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Tasmanian farmers face drought Southern Ocean wobble may offer climate clues
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Blocking highs provide wet wealth for Tasmanian graziers July, 2000 Wet weather on the east coast of Tasmania generally comes from East Coast Lows. These are intense storms that develop out of upper level troughs and lower level cloud bands when these systems combine over the warm south-flowing waters of the East Australian Current. However, in order for these rain-bearing depressions to hang around long enough to give Derwent Valley farmers a good soaking, they must have an effective blocking high pressure system further east in the Tasman Sea towards New Zealand.
This animated image (courtesy of the Australian Bureau of Meteorology) shows daily surface pressure maps from July 5-10, 2000. Note the persistent high (1036hPa) south east of New Zealand. An approaching front west of Tasmania splits over the island with the main trunk of the front and its associated low slipping to the south as it comes up against that large anti-clockwise spinning high. Meanwhile, the northern extremity of the front combines with an upper level trough in the central Tasman Sea and reforms as a low off the east coast of New South Wales. The system then slides south-west towards Tasmania. Again, the Kiwi high is responsible for this depression's backward movement. Unfortunately, this low did not water drought affected areas in central Tasmania, although east coast communities got a fair drenching.
Rain equation Much of Dr Mike Pook’s research with the Antarctic Co-operative Research Centre looks at the role blocking highs play in this Tasmanian rain equation. "When you get periods of effective blocking in winter the cool westerly winds that bring regular rain to western Tasmania are interrupted," he explains. "This results in a decrease in rain over the catchments that supply Tasmania's hydro-electric scheme. But it can be good news for eastern and northern Tasmania because the blocking highs re-organise the circulation of the atmosphere so that mild and moist northeast to northerly airstreams develop. "Often, these conditions only result in drizzle but occasionally, rain
depressions form and move down the east coast drenching the pastoral Re-directing the westerly flow What happens with blocking highs, is that fronts approaching south east Australia from the west are split so that the northern portion breaks off into a region of cold air, and the southern portion shears off and moves well to the south. That is why you tend to get a lack of rainfall in these situations. But an interesting aspect as far as East Coast Lows are concerned, is that the region of cut-off cold air tends to travel over Victoria and New South Wales to redevelop over the warm waters of the East Australian Current. With the blocking high in place, these newly formed lows stall in the Tasman Sea and can often move southwards, towards the Apple Isle. Location, location, location ... However, the birthplace of the storm is important, because if the low forms over the north coast of NSW, its wet weather will never reach Tasmania. The storm will be too far east by the time it reaches Tasmania’s latitude to have any effect on its rolling hills. For optimum rain, the low must develop over the south coast of NSW or east of Bass Strait. And with the effect of the blocking high, the storm will stay put. Pook notes that the situation of a blocked high in the southern Tasman sea and a cyclonic situation to the north is topsy-turvey to the norm. However, it is a very stable pattern that encourages the weather to stay put long enough to give the paddocks of farmers in the central midlands and upper Derwent valley a good soaking. More great reading! Tasmanian farmers face their worst drought in living memory. A curious Southern Ocean wobble may offer climate clues as to why Tasmania faces a big dry.
© 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. |