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An extratropical cyclone on the coast of southeastern Australia From Wikipedia, the free encyclopedia
Australian east coast lows (known locally as east coast lows, maritime lows, and east coast cyclones[1]) are extratropical cyclones or low-pressure systems on the coast of southeastern Australia that may be caused by both mid-latitude and tropical influences over a variety of levels in the atmosphere.[2][3] These storms should not be confused with Australian region tropical cyclones which typically affect the northern half of the continent.[4]
The most intense of these systems have many of the characteristics of subtropical cyclones.[5] They develop between 25˚ south and 40˚ south and within 5˚ of the eastern Australian coastline,[1] mostly during autumn and early winter with a peak in June.[6][7][8] Prior to the introduction of satellite imagery in the early 1960s, many east coast lows were classified as tropical cyclones. ECLs are responsible for more than half of all days with rainfall above 50 mm (2.0 in) in the southeastern seaboard.[9]
Generally, the low occurs when there exists a temperature contrast between the warmer Tasman Sea and cooler continental landmass (including the cold air in the upper atmosphere), a setup similar to the lake-effect rain. The interaction of such airs create heavy moisture and heightened wind speed.[10] Two thirds of the recorded low-pressure systems are fully cold core storms while the remaining third display hybrid features, which are defined by a warm core at lower levels and a cold core at upper levels. A smaller portion are fully warm core cyclones, where they somehow transition to a tropical cyclone, as well as in part warm seclusion cyclones. Furthermore, 46% of ECLs were hybrid cyclones.[11]
An east coast low can develop in a variety of other weather conditions, including:
Explosive cyclogenesis is seen on average just once per year, but these storms cause significant wind and flood damage when they occur.[7] East Coast Lows by and large have shorter lifespans than Tropical Cyclones, lasting only a few days.[12] Australian east coast cyclones vary in size from mesoscale, around 10–100 km (6–62 mi), to synoptic scale, approximately 100–1,000 km (62–621 mi).[14][15] Cold core cyclones are more predominant in the southern parts of the ECL region during the cool season, while hybrid cyclones are more regular closer to the tropics, particularly in the warm season.[11]
Australian east coast cyclones, although variable in size and intensity, are typically characterised by widespread heavy rainfall.[15] Rain associated damages attributed to east coast cyclones are estimated in millions to tens of million dollars annually and are a major contributor to the total weather-associated insurance losses for all of Australia.[15] Seven per cent of all major Australian disasters since 1967 can be directly attributed to east coast cyclones.[15]
ECLs are the cause of more than 25% of yearly rainfall and more than 60% of days with mass coastal flooding or significant dam influxes.[9] A Tasman Low is an offshore low that occurs around 200 km away from the coast which can cause light to moderate rainfall and gusty winds, although its effect is insignificant compared to that of an east coast low.[10]
East coast lows usually occur along the subtropical east coast of Australia from Gladstone in Queensland to the Victoria/New South Wales border in the South Coast (near eastern Gippsland), and often affect large populated cities such as Sydney and Brisbane, between which over one-third of the Australian population resides.[15] ECLs are generally more intense between Batemans Bay in the south to Coffs Harbour in the north, which include the Mid North Coast, Central Coast, Hunter Valley, Sydney CBD, the Illawarra, and as well as the Blue Mountains and Southern Highlands. South East Queensland, the far southern coast and Northern Rivers of New South Wales, and eastern Victoria are generally less impacted than the areas situated at the centre of the NSW coastline.[16]
More powerful systems may percolate inland into the Central Tablelands, New England, the Central West, South Western Slopes, the Monaro and, albeit rarely, the Orana and the Riverina regions in NSW; also the Australian Capital Territory, the Hume region in Victoria; as well as Darling Downs and the Brigalow Belt in Queensland, although the rainfall in these interior areas will often be insignificant.[9]
In a June 2007 observation by the Bureau of Meteorology, there were about ten significant maritime lows with an "explosive development" usually occurring once per year.[12] Much recently, it was observed that around 22 east coast lows were recorded annually – around 2 to 3 causing daily rainfall above 100 mm (3.9 in) and 7–8 causing daily rain above 25 mm (0.98 in).[3] A Black nor'easter, which occurs in the warm months in eastern New South Wales, can intensify as it heads south and turn into an east coast low.[17]
The incidence of these types of storms can be seen to fluctuate quite widely from one year to the next, with none in some years and the highest incidence being twelve in 1978/79.[4] Another feature of east coast low development is the tendency for clustering of events when conditions remain favourable. For example, near Brisbane, almost one third of events occur within 20 days of a preceding event.[4] Areas south of the low can experience heavy rain, while those at the north of the rotating low would experience sunny skies. The movement of the low pressure system is generally difficult to predict for weather forecasters.[12]
Correlations of east coast cyclones with the interannual differences of the Southern Oscillation Index (SOI) indicate a strong preference for these storms to form just after a large swing from negative to positive Southern Oscillation index values and especially between swings from negative SOI the year before and positive SOI the year after. This suggests a preference for formation of east coast cyclones between extreme events of the Southern Oscillation Index.[15] A positive SAM is linked with a higher frequency of deep cyclones in August–October and of shallow upper cyclones in December–February.[9]
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