Get all your news in one place.
100’s of premium titles.
One app.
Start reading
ABC News
ABC News
National

La Niña is back. How long and strong will it be and should we prepare for more flooding?

It looks as though this La Niña could be weak and short-lived. But those with long memories will recall the last La Niña summer was also expected to be short — yet the flooding came regardless.  

So how long and strong will this La Niña be and should we expect more flooding? 

"At the moment, this La Niña isn't looking particularly strong and it's looking like it will peak probably fairly early in the summer or late in the spring," according to Andrew Watkins, head of long-range forecasting at the Bureau of Meteorology.

"Which is a little bit unusual, a little bit different to the La Niñas that we've been seeing in recent years." 

La Niña and El Niño events usually start in winter end in autumn or thereabouts.

But Dr Watkins said a number of models from around the world were in fairly close agreement that we can expect to see a peak in the La Niña before the end of the year.

"Quite often we see a range of the different forecasts, but the consistency between the models makes us fairly confident that we'll see an earlier peak than in recent years with this event," he said. 

Andrea Taschetto, associate professor at the University of NSW Climate Change Research Centre and ARC Centre of Excellence for Climate Extremes, has also been looking at the international climate model outlooks.

"In November, all of the seven models cross this threshold for La Niña," she said. 

"In December, those models already start to show a retreat of these temperature anomalies and only four of the seven models indicate La Niña conditions."

By January, it would be down to three and by February the temperatures were basically within the neutral range, with the exception of just one model. 

Not only is it is expected to be short, the La Niña is also expected to be relatively weak.

But before you go resting on your laurels, there is a bit more to it. 

How accurate can we be? 

The La Niña and El Niño declarations can make it seem as though they are black and white, but, in reality they, are messy natural processes that play out a bit differently every time. 

"The forecasts aren't perfect. It's not that easy to forecast the progression of La Niña and El Niño events," according to climate scientist Andrew King from the University of Melbourne.

"We don't have a really complete understanding of the dynamics of them, which limits our ability to really forecast them very well," he said. 

La Niña and  El Niño result from a complicated interaction between the deep ocean, the ocean surface and the atmosphere. 

The limited historical data available makes it even more difficult to decipher trends and how it all works. 

"It's often said that we know less about the ocean than we do about the surface of planets," Dr King said.

Dr Taschetto points out that there are many factors that influence the onset and duration of El Niños and La Niñas.

There are some patterns, but even then there are quirks. For example, she says predicting when a La Niña will end is more difficult than with an El Niño.

"The reason for that is because the El Niño southern oscillation is an asymmetric phenomenon, meaning that El Niño and La Niña years are not exactly the mirror image of each other," she said.

"With El Niño, because there is a reversal or weakening of the cycle when it gets closer to our monsoon season, what we see is a reversal of the winds and that can help to reverse the anomalies.

"But as a La Niña is basically a strengthening of the normal conditions it's slightly harder to predict the termination." 

Is there a difference within La Niñas? 

Generally, stronger La Niñas bring wetter weather to eastern Australia. 

According to Dr Taschetto, the relationship between La Niña and rain in eastern Australia is actually stronger than with El Niño.

"A large La Niña event is generally linked to a larger Australian rainfall response.

"While the strength of an El Niño event sometimes it's not as good an indicator to tell us how dry Australia will be."

But Dr King points out it is an indication, not a guarantee. 

"Hopefully, if we have a mild La Niña, we don't have the super wet conditions but it's not a perfect correlation.

"There are other things going on in the climate that affect us in Australia," he said. 

This spring, those other things are also pushing us towards rain. 

Even if the La Niña is weak, both the currently negative phase Indian Ocean Dipole to the west and the positive phase of the Southern Annular Mode in the south, are ramping up the moisture.

When and if they decline as we get to summer there are still other drivers and weather events such as cyclones that could pump in the moisture. 

Can we rule out flooding this summer? 

In a word; no. 

"We can't rule out flooding, especially as catchments are still pretty full and the soils are still pretty wet, especially below the surface," Dr King said. 

"Even fairly typical summer rainfall patterns could result in some flooding.

"The forecasts indicate a probable mild La Niña but if they're slightly off and actually it turns into a stronger La Niña or it lasts longer, that does heighten the risk of more severe flooding. So we should be prepared for that possibility," he said. 

Dr Taschetto agreed that the past two years of rain and floods have set up a precarious situation this year. 

"We are in a very delicate condition even with a weaker La Niña," she said. 

"It has set the scene for increased rainfall this spring, and we need to keep an eye on what the weather systems now are doing to determine where the risk of floods are higher." 

Dr Watkins reiterated that even if things were to ease off in terms of rainfall, conditions could still be dangerous.

"Remembering that in any northern wet season, which is traditionally the higher risk weather season for Australia, you do have tropical lows, you do have tropical cyclones, you can get the monsoon coming further south at times," he said.

"So, even in the normal summer, we're going to have wet periods, particularly in parts of northern and eastern Australia.

"If they coincide with areas that are already very wet, the flood risk is elevated."

Sign up to read this article
Read news from 100’s of titles, curated specifically for you.
Already a member? Sign in here
Related Stories
Top stories on inkl right now
One subscription that gives you access to news from hundreds of sites
Already a member? Sign in here
Our Picks
Fourteen days free
Download the app
One app. One membership.
100+ trusted global sources.