After a tumultuous 18 months that saw much of Australia’s east-coast underwater and some communities experiencing multiple incidences of flooding, the announcement the weather is about to get even wetter is not exactly welcome news.
On 13 September the Australian Bureau of Meteorology confirmed that we’re up for a ‘triple dip’ La Niña.
With catchments already full, rivers high and dams at capacity from the last two years’ La Niñas, there’s nowhere for the new wave of rain to travel except over land, which itself is already saturated from two years of intense rainfall. Large parts of Australia’s east coast now face the risk, once again, of catastrophic flooding.
What causes a La Niña, and how are they different to the other climate pattern we hear about, El Niño? How worried should we be? And is the climate crisis making them worse?
What is La Niña – and should we be worried?
La Niña and its opposite El Niño are phases of a natural climate pattern across the tropical Pacific Ocean that swings back and forth every three to seven years. Together, they are called the El Niño-Southern Oscillation (ENSO), and make up the most dramatic year-to-year variation of Earth’s climate system. They can affect public health, fresh water availability, agriculture, wildlife, power generation and economic activity across the planet.
La Niña, says Dr Agus Santoso from UNSW Climate Change Research Centre, may be thought of as an enhancement of the tropical Pacific climatology, where stronger than normal trade winds (which blow steadily towards the equator from the north-east in the northern hemisphere, or the south-east in the southern hemisphere) push warm water toward the maritime continent to the north of Australia.
The maritime continent is the huge patch of water almost the size of Australia located between the Indian and Pacific Oceans and encompassing Indonesia, Borneo, New Guinea, the Philippine Islands, Malay Peninsula and the surrounding seas. The warmer this area gets, the more water evaporates around eastern Australia and turns into rain.
“On the other side of the Pacific, close to South America, there is less moisture and the surface ocean is cool as the trade winds suck up cool water from deep within the ocean to the surface,” Santoso explains.
“During La Niña, these equatorial winds get stronger, the water close to Australia becomes warmer than normal while the water in the central to eastern equatorial Pacific is cooler than normal.”
Weather-wise, La Niña means greyer, overcast conditions on the eastern seaboard, cooler temperatures and, of course heavier rains, largely from Queensland through to New South Wales.
“The chances are stacked much higher for floods,” says University of New South Wales Scientia Professor Matthew England. “The last three to four La Niñas have brought catastrophic flooding events such as the flooding last year and the 2011 Brisbane floods.”
Three forces at work
To make things a little more complicated, it’s not just La Niña that is affecting Australia’s climate at the moment. Two other natural climate forces are also in play: the Indian Ocean Dipole (IOD), which is characterised by variable sea surface temperatures in the Indian Ocean; and the Southern Annular Mode, distinguished by the positioning of winds and weather systems to Australia’s south.
Associate Professor Andrea Taschetto, an oceanographer from UNSW, says the Indian Ocean, which washes the coast of Western Australia, is extremely influential on Australian weather.
“At the moment, we are in the Indian Ocean Dipole negative phase, which works similarly to La Niña but in the Indian Ocean,” she says. “It means that water close to Australia is warmer than usual, and in Africa it’s cooler than usual, which, when working together with La Niña, tends to amplify the rainfall conditions in Australia.”
Does climate change impact the frequency of La Nina events?
While we may understand the mechanisms that cause La Niña and El Niño, we know very little of the likely impact of anthropogenic climate change on these events, says Emeritus Professor in Climate Science at the University of Southern Queensland, Dr Roger Stone.
“Will they become more severe? Will they become more frequent? We don’t know, and this is despite the fact that Australia – and especially eastern Australia – is one of the most vulnerable regions on the globe in regards to the direct impacts of La Niña or El Niño or its subtle variations,” he says.
We know that severe, drought-inducing El Niños are very often followed by intense or protracted La Niñas, Stone says, so Australia should have been prepared for this period of back-to-back severe seasonal climate patterns of extreme drought and extreme flood.
And yet, when it comes to our understanding of how double and triple La Niñas may change as global warming progresses, more research needs to be done.
Santoso – who co-wrote an influential scientific paper on the topic with CSIRO’s director of Southern Hemisphere Oceans Research Dr Wenju Cai – says that based on climate models that contribute to the IPCC assessment reports, we should expect increased frequency of extreme La Niña events under greenhouse warming.
In 2021, an international team of scientists (including Santoso and Cai) carried out another review by synthesising a range of different models to make predictions about the El Niño Southern Oscillation. They again found that the “variability of ENSO is increasing in response to anthropogenic greenhouse emissions”, meaning that the El Niño and La Niña fluctuations are getting more intense, and becoming more frequent.
More La Niñas also means more bushfires
It’s not just floods that we need to prepare for – more triple La Niña events also exacerbate the risk of bushfires, adds England. While usually in a decade we would expect seven out of 10 years to be “neutral”, under global warming we’re more likely to see more La Niña/El Niño years instead.
“We’re so much more hit by climate extremes during both La Nina and El Nino events,” he says. “Bushfires in particular are a real worry. Part of what made the 2019-2020 fires so bad is that there was so much fuel after heavy rainfalls.”
La Niñas tend to regrow vegetation, thus creating a perfect storm for its opposite twin, El Niño, which tends to brings drought, rains, floods, heatwaves and other extreme weather events to many parts of the world (including Australia). The worst thing that can happen in Australia is having more frequent changes between the two – extremely wet to extremely dry – which creates the perfect conditions for catastrophic bushfires, England says.
This scenario was tragically borne out in the 1998-99 extreme La Niña event that followed the 1997-98 extreme El Niño event, which swapped El Niño-induced severe droughts to devastating floods in western Pacific countries, and vice versa in the southwestern United States.
What can meteorologists learn?
By understanding how El Niño, La Niña and the Indian Ocean Dipole work and affect rainfall, meteorologists can improve seasonal forecasting, which can have important implications for agriculture and other economic sectors in Australia that rely on accurate information on weather and climate, says Taschetto.
“One of the things scientists are really trying to understand is how El Niño, La Niña and Indian Ocean Dipole interact with synoptic systems and cause extreme events in Australia,” she explains. “It is a matter of ongoing research here at the ARC to understand how these large phenomena like La Niña interact with the smaller weather systems to culminate in extreme rainfall and floods.”
While generally El Niño and La Niña last six months to a year, Taschetto says we still don’t know why sometimes the events hang around for longer.
“While La Niña can stay in the tropical Pacific for months, the impact for east Australia can be severe in certain locations and quite specific in time,” she says. “This was the case of the February-March floods when a series of low-pressure systems stalled for a week or so off the east coast of Australia, and caused heavy rainfall and floods in parts of NSW.”
Changes: On Oct 2, 2022, we removed the following sentence from Matthew England: “I don’t know of any severe bushfire years that didn’t follow La Niña,” because we inadvertently misquoted him.