Melbourne, Sep 27 (The Conversation) Meteorologists are again predicting a possible La Nina this summer, which means Australia may face wetter and cooler conditions than normal.
It would be the fourth La Nina in Australia in five years, and highlights the need for Australians to prepare for what may be an extreme weather season.
Typically, a La Nina or its counterpart, El Nino, signals its arrival earlier in the year. Signs of this potential La Niña are emerging fairly late. That’s where new research by my colleagues and I may help in future.
La Nina and El Nino explained La Nina and its opposite phase, El Nino, are created by changes in ocean temperatures in the Pacific Ocean’s equatorial region. Together, the two phenomena are known as the El Nino Southern Oscillation.
The oscillation is said to be in the positive phase during an El Nino and the negative phase during a La Niña. When sitting between the two, the cycle is in neutral phase.
Earlier this month, the World Meteorological Organisation said there was a 60 per cent chance of La Nina conditions emerging by year’s end.
In the United States, the National Oceanic and Atmospheric Administration put the likelihood at 71 per cent. Australia’s Bureau of Meteorology is in “watch” mode, predicting a 50 per cent chance of a La Nina weather pattern forming later this year.
La Nina occurs when strengthening winds change currents on the ocean surface, pulling cool water up from the deep.
The winds also cause warm surface waters in the western Pacific and north of Australia, bringing increased rainfall and clouds. This usually means above-average rainfall and cooler temperatures for Australia, particularly in the east and north.
Conversely, an El Nino weather pattern generally brings hotter temperatures across Australia, and less rainfall in the east and north.
Paths of destruction La Nina or El Nino events can cause devastation around the world.
The El Nino in 2015–16, for example, caused crops to fail and affected the food security and nutrition of almost 60 million people globally.
In Australia, El Nino events can bring increased risk of drought, bushfires and heatwaves, and water shortages.
Meanwhile, rainfall associated with La Nina conditions can lead to greater crop yield. But particularly heavy rainfall can wash crops away. It also heightens flood risks for some communities.
These far-reaching impacts mean it’s essential to plan ahead when a La Nina or El Nino is on the cards. But predicting these events has always been tricky.
Both types of events usually develop in the Southern Hemisphere autumn, peak in late spring or summer, and weaken by the next autumn. But it’s now late spring without a clear La Nina declaration. Why the delay? Climate change is one factor. The Bureau of Meteorology says as oceans absorb heat from global warming, it’s harder to spot the specific warming patterns linked to La Nina.
The sheer complexity of the ocean-atmosphere system adds to the difficulty. The computer models used to predict El Nino and La Nina are improving all the time. But scientists still need more information on deep ocean processes, and how winds affect the oscillation.
Predictions are hardest during the Southern Hemisphere’s autumn. That’s because the cycle then is very susceptible to change – teetering at a point where either a La Nina or El Nino could develop.
That’s why the earliest an El Nino or La Nina can be predicted is usually around May or June.
But new research offers a way to predict the events much earlier – and start preparing if necessary.
Better, earlier forecasts The study, which I led, assessed the likelihood of La Nina or El Nino events occurring in succession – either in the eastern or central region of the Pacific Ocean.
This distinction is important. For Australia, El Nino and La Nina events peaking in the Central Pacific, close to our continent, have greater impacts here compared to those peaking in the east, closer to South America.
We analysed weather observations, and the sequence of past El Nino and La Nina events, over the past 150 years. We also examined climate models for future changes in transitions between El Nino and La Nina events.
From this, we determined the likelihood of an El Nino or La Nina occurring in two consecutive years.
We found most El Nino events are followed by neutral conditions the next year (with a likelihood of 37–56 per cent).
But La Nina behaves differently. In 40 per cent of cases, a Central Pacific El Nino could follow an Eastern Pacific La Nina. And there is a 28 per cent chance of two consecutive La Nina events in the Central Pacific.
These results allow for more advanced predictions. By identifying patterns in this way, the odds of an El Nino or La Nina can be predicted up to a year in advance.
Looking ahead So, what does our research suggest for Australia? Will a La Nina develop here this year? From September last year, Australia experienced a strong Eastern Pacific El Nino. So our findings suggest there is only a 17 per cent chance of La Nina this year.
If the La Nina arrives, it will likely peak in the Central Pacific, potentially affecting Australia rainfall. But overall, any La Nina that develops this late is likely to be weak and relatively short-lived.
Our research also found that as climate change accelerates, the El Nino Southern Oscillation is likely to shift. For example, the odds of two consecutive El Ninos peaking in the central Pacific region will likely increase. And we can expect fewer calm, neutral years between events.
We hope our research enables more accurate, long-range forecasts, giving communities additional time to plan and prepare. (The Conversation) PY PY
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