Australia La Niña drives cool temperatures, high rainfall during Australian summer

00:29  03 march  2021
00:29  03 march  2021 Source:   abc.net.au

Australia is facing a wetter than usual autumn

  Australia is facing a wetter than usual autumn Australians should brace themselves for wet months ahead with El Nina conditions to continue well into the autumn months after the wettest and coolest summer Australia has seen in five years.The country has shivered through its coolest and wettest summer in five years but experts said there will be no sign of a reprieve in the coming months.

During a La Niña period, the sea surface temperature across the eastern equatorial part of the central Pacific Ocean will be lower than normal by 3 to 5 °C (5.4 to 9 °F). An appearance of La Niña persists for at least five months. There is a strong correlation between the strength of La Niña and rainfall : The greater the sea surface temperature and Southern Oscillation difference from normal, the larger the rainfall change.[16] There are also cooler daytime temperatures south of the tropics and fewer extreme highs , and warmer overnight temperatures in the tropics.

In eastern Australia , the average December-March rainfall during La Niña years is 20% higher than the long-term average, with eight of the ten wettest such periods occurring during La Niña years. This is particularly notable for the east coast, which tends to be less affected by La Niña during the winter months but can experience severe flooding during La Niña summers . La Niña years tend to see cooler than average maximum temperatures across most of mainland Australia south of the tropics, particularly during the second half of the year. This is due to increased cloud cover and rainfall .

clouds in the sky: Australia experienced almost a third more rain than average this summer, driving temperatures down. (ABC My Photo: @Nature.By.jj) © Provided by ABC NEWS Australia experienced almost a third more rain than average this summer, driving temperatures down. (ABC My Photo: @Nature.By.jj)

Australia recorded its wettest summer in half a decade and its coolest in nine years as La Niña snapped an eight-year hot streak in 2020/21.

Rainfall across the country was 29 per cent above the long-term average, making it the soggiest summer in four years.

Victoria and South Australia received 14 per cent more rain than the long-term average.

The Murray-Darling Basin received 13 per cent above average rain — the most since the La Niña summer of 2011/12.

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The tropics also experienced some highly unseasonable rain events during late autumn and winter 2010, in what is typically the dry season. Abundant vegetation growth following high rainfall in the usually arid interior fuelled widespread grassfires in central and northern Australia between August and Several notable tropical cyclones occurred during the summers of 2010–11 and 2011–12. Severe tropical cyclone Yasi was the most significant – Yasi was the strongest cyclone to make landfall in Queensland since at least 1918 (also a La Niña year), crossing the coast near Mission Beach

Generally speaking, in relative terms La Niña 's impact on Australian summer rainfall is less than the winter-spring impact. This is especially the case over Tasmania, SA, the NT and WA which have large areas of deciles 5 and 6 in the image above. As in the June-November period, the impact from La Niña is more widespread and intense during summer than the corresponding impact from El Niño. There is a contraction and weakening of the effect in western Queensland, but in the east of that state, the La Niña -induced tendency towards wetter than average conditions continues to be moderate to strong.

Tasmania's rain was 18 per cent above average.

Western Australia received 54 per cent more rain after some inland areas received a good deal more rainfall than average courtesy of some tropical lows.

Rain in the Northern Territory was up 39 per cent this summer, in line with a long-term trend of wetter wet seasons in the Top End.

Queensland only recorded falls eight per cent above the average, making it the driest of the bunch.

Parts of eastern Queensland missed out on the cooler, wetter summer, with above average temperatures and below average rainfall south of Mackay.

The above average rainfall was a key factor behind farmers celebrating Australia's .

"La Niña cycles are really important in Australia for breaking droughts by bringing heavy, soaking rain," Australian National University climatologist Nerilie Abram said.

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U.S. La Niña Impacts The first three months of the year during a La Niña typically feature below normal precipitation in the Southwest, the central and southern sections of the Rockies and Great Plains, and Florida. Wetter than normal conditions are also observed over southeastern Africa and northern Brazil, during the northern hemisphere winter season. During the northern hemisphere summer season, the Indian monsoon rainfall tends to be greater than normal, especially in northwest India.

Average La Niña rainfall patterns over a dozen moderate to strong events. Included is a graph of the winter-spring SOI, composite diagrams, commentary and maps showing the evolution of Australian rainfall patterns during a "classic" La Niña event.

Skip in record run

Australia's mean temperature this summer was .06 degrees Celsius warmer than average – and yet was cooler than the past eight years – after climate change drove the nation's four .

"It was it was 1.8 degrees Celsius colder this summer compared to last summer," senior climatologist Bureau of Meteorology climatologist Simon Granger said.

"That's quite a substantial change."

Dr Abram said cooler temperatures were driven by the high rainfall delivered by La Niña, Dr Abram said.

"When we've got a lot of rainfall and we've got lots of vegetation growth, lots of soil moisture, what that means is firstly there's a lot of clouds, so we don't get as much solar energy," she said.

"Energy that does arrive to the surface is able to cause evaporation which prevents the temperatures from getting as warm as they otherwise would."

It was a particularly cool summer for the southern states — Victoria recorded its lowest mean temperatures since 2004/05.

SA saw its coolest mean temperatures since 2001/02.

Not what it used to be

Dr Abram said El Niño and La Niña cycles were powerful enough to affect temperatures around the world because oceans absorbed extra heat.

"In a La Niña year, globally, the temperature will be a bit cooler compared to an El Niño year," she said.

Despite La Niña, January was the since records began, according to the Berkeley Earth environmental data science and climate monitoring service.

In Australia, mean temperatures were still slightly higher than the long-term average even though it was the coolest summer in nine years.

"The effect of La Niña, in terms of being able to counteract the warming of the climate system, is becoming less and less," said Dr Abram.

North Queensland braces for more flooding as potential cyclone builds off coast .
North Queensland is set to receive another drenching as a tropical low off the coast of Cairns threatens to develop into a cyclone. A flood warning has been issued for a 180-kilometre stretch of coastline between Mission Beach, south of Cairns, and Rollingstone.The Bureau of Meteorology's Jonathan How said the warning could be extended to Townsville and its surrounds in the coming days.https://twitter.

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