Australia BOM declares a La Nina, signalling wet spring and summer likely for northern, eastern Australia

11:45  29 september  2020
11:45  29 september  2020 Source:   abc.net.au

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La Nina events are also associated with heightened risk of cyclones as well as cool daytime Weather nerds with good memories will recall a La Nina was declared in 2017 and, rather than bringing Many will remember that summer for widespread flooding along the east coast, including the worst floods in

The six wettest winter– spring periods on record for eastern Australia occurred during La Niña years. Australian winter spring mean rainfall deciles averaged for thirteen strong La Niña events Wetter and dryer areas are described within the article text.

Conditions are now officially at La Nina levels. (Supplied: Bureau of Meteorology) © Provided by ABC NEWS Conditions are now officially at La Nina levels. (Supplied: Bureau of Meteorology)

The Bureau of Meteorology has declared a La Nina is officially underway, signalling we could be in for a wet spring and summer.

La Nina is a phase of the El Nino Southern Oscillation (ENSO), which describes ocean and atmospheric circulations over the Pacific Ocean.

During a La Nina phase, Australia's northern waters are warm with increased convection.

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" La Nina basically means, for Australia , an increased risk of rainfall — particularly in central " La Nina seasons are just a lot wetter on average than either El Nino or neutral seasons," Dr King said. "There haven't been many signals at all indicating above-average rainfall for anywhere in particular

A La Nina means wetter days are more likely across much of eastern Australia . A La Nina forms when sea surface temperatures in the Pacific cool bringing wetter conditions to Australia ’s east . “ A La Nina will normally peak in late spring and early summer then weaken by autumn although many

This allows more moisture to be lifted into the air than normal, typically resulting in increased rain for eastern and northern Australia — but, historically, the south-east misses out.

That moisture can lead to cooler daytime temperatures according to Andrew Watkins, the BOM's head of climate operations.

"We tend to have more cloud and a bit more moisture around to evaporate to keep the air a bit cooler," he said.

"Conversely at night, when you've got the cloud acting like a lid trapping in that heat, it can be a bit warmer."

But every La Nina is different.

Will we get a repeat of 2011?

The last major La Nina events were in the summers of 2010-11 and 2011-12. They resulted in 2010 to 2012 being Australia's wettest two-year period on record.

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Values above +7 typically indicate La Nina conditions while below -7 indicates El Nino. (Supplied: Bureau of Meteorology). But to tell what is likely to happen The following diagram shows that the Australian Bureau of Meteorology's model has the lowest likelihood of reaching El Nino sea surface

Potential for dry start to northern wet season. What El Nino and La Nina actually mean for weather. "The dry conditions are very severe in large parts of eastern Australia and it's been quite El Nino not acting alone. Ms Duell said there was cool water to the north - east of Australia that indicated the

Flooding was widespread and devastating.

In early 2011, large parts of South-East Queensland were under water; the Lockyer Valley was hit by a cascade of water coming off the Toowoomba range and Brisbane saw its worst flooding since 1974.

But Dr Watkins said this year's event was looking weaker than 2010-11 based on the current forecast.

"Also the fact that by this time in 2010, we were already well into a La Nina event and had been pretty much since mid July."

So 2010-11 was well ahead of where we are now. In fact, it was one of the four strongest La Ninas ever observed, according to Dr Watkins.

What does this mean for cyclone season?

The warm waters and increased convection also lead to La Ninas being associated with increased numbers of cyclones.

In a normal year we see roughly between nine and 11 tropical cyclones in Australian waters.

But according to Dr Watkins, we could be in the upper end or even above that range this year.

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Outlooks indicated a wetter than average spring and early summer for much of northern and eastern Australia , but were not as dramatic as they were The 2011–12 La Niña gradually declined over late summer , with some atmospheric indicators continuing to show a La Niña signal during March 2012

Outlooks indicated a wetter than average spring and early summer for much of northern and eastern Australia , but were not as dramatic as they were The 2011–12 La Niña gradually declined over late summer , with some atmospheric indicators continuing to show a La Niña signal during March 2012

"That also increases the risk of tropical cyclones crossing the coast, so lots of impacts of La Nina, particularly as we get into summer."

Will farmers be able to get crops out?

After the past few years of drought on the east coast, recent months of good rains for the south-east have been welcome.

The forecast of more rain will be welcome for those who have so far missed out in 2020.

But during late spring and summer, farmers generally need dry conditions to get a crop out — too much rain in late spring and early summer can be a disaster.

Dr Watkins said the odds were easing back on a wet harvest, but it was still on the above-average-rainfall side of things.

"Hopefully nice finishing rains but that it won't be too wet for the actual harvest," he said.

Fire outlook better than recent years

Meanwhile, the fire risk is at least looking better than this time last year.

"Last year we went into the spring and summer period with a really strong positive Indian Ocean Dipole event; that was keeping things very dry, particularly in eastern Australia and it also had been very hot."

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La Niña . Trade winds strengthen, increasing the temperature of the warm water north of Australia . Cloudiness and rainfall north of Australia are enhanced, typically leading to above average winter– spring rainfall for eastern and central parts of the country, and a wetter start to the northern wet

"Many people in eastern Australia know we were in the grip of a drought in many areas and of course, droughts do contribute to above average temperatures, particularly It was a La Nina for a large part of this financial year! La Nina usually means wet and cold conditions for the east coast, doesn't it?

Dr Watkins said he hoped the La Nina would mean there was a bit more rain before summer that would help to keep the bushfire risk lower than the past couple of years.

"But we can never forget southern Australia will always get dry and hot over summer," he said.

"There always will be some bushfires, but hopefully this year we have the conditions in place that we won't see those widespread, long-campaign fires that we did see unfortunately last year."

Video: NSW lashed with wild weather and snowfall (ABC NEWS)

How the La Nina weather event could affect Australia for the rest of 2020 .
A La Nina weather pattern could finally put an end to the drought in parts of Australia, but for some it has raised concerns about potential cyclones and flooding. The last La Nina event stretched from 2010 to 2012 and resulted in one of Australia's wettest two-year periods, but conditions this year are forecast to be less extreme.Bureau of Meteorology senior forecaster Lauren Pattie said while flooding and cyclones were associated with La Nina, it was too early to predict the severity of the weather pattern. "Each one is different, but the last significant La Nina we saw, we saw very intense flooding," Ms Pattie said.

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