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The weather event that caused floods across much of the country is breaking down, but it may soon be replaced by an El Niño pattern of intense heatwaves and drought. By Joëlle Gergis.

What weather disaster will follow La Niña?

A helicopter delivers hay to stranded cattle in the Kimberley region of Western Australia.
A helicopter delivers hay to stranded cattle in the Kimberley region of Western Australia.
Credit: Frontier Helicopters

The Bureau of Meteorology’s seasonal forecast is now an anxiously awaited event; we have become accustomed to bracing ourselves for news of what type of natural disaster is up next.

As the end of the 2022-23 summer approaches, the sustained La Niña event in the Pacific that has drenched much of the country since spring 2020 is finally starting to weaken, with neutral conditions predicted to return by autumn. But given that the northern monsoon is currently under way, we aren’t in the clear just yet. The tropical cyclone season officially runs until the end of April, so the likelihood of heavy rainfall events and associated flooding is still high. Historically many of the most destructive floods in Australian history have occurred in February and March as ocean temperatures reach their summer peak. This was graphically illustrated by the east coast floods of 2022, which saw the disastrous inundation of the northern New South Wales town of Lismore. A study of past flooding along the east coast since 1860 reported that the highest death tolls from freshwater flooding historically occur in February, so emergency services have a nerve-racking month ahead.

The 2022-23 wet season has already seen above-average rainfall across much of northern Australia. So far, we have witnessed this in the Kimberley region of north-western Australia and northern Queensland in January, with record-breaking flooding in some regions already having major impacts on communities and critical infrastructure such as roads and housing. Given that the northern monsoon still has another three months to run, recovery operations can’t begin before then, leaving communities in yet another region of Australia displaced and traumatised by a seemingly unending sequence of natural disasters that are increasingly becoming the new normal in our country.

During the wet season, we typically expect a heightened risk of flooding, damaging winds and destructive storm surges across Queensland, the Northern Territory and Western Australia. When La Niña events occur, our region experiences an above-average number of tropical cyclones and other severe low-pressure systems. Right now, warmer-than-average ocean temperatures are present across northern Australia, especially in the Coral Sea, where conditions recorded in December 2022 were the second hottest since records began in 1900. The warmer the ocean is, the more moisture is available to intensify cyclonic systems that naturally occur during the wet season. Because there is more heat trapped in the ocean from the burning of fossil fuels, normal seasonal storms are becoming more amplified.

While tropical cyclones are predominantly confined to the northern parts of the country, powerful systems can also affect areas further south as decaying low-pressure systems. For example, on March 28, 2017, Cyclone Debbie made landfall at Airlie Beach in the Whitsunday region of north Queensland, weakening as it travelled south as a tropical low, causing severe flooding and infrastructural damage all the way down to south-east Queensland and northern NSW. The storm resulted in an estimated $1.7 billion in insurance claims, with the Insurance Council of Australia ranking it as the second-most expensive cyclone in the nation’s history, after Cyclone Tracy in 1974.

The decaying tropical cyclone hit the Lismore region hard, with the Wilsons River rising to near-record heights. In contrast to the historic floods of 1954 and 1974 that arose from sustained La Niña conditions, almost all of the extreme rainfall from ex-tropical Cyclone Debbie fell within 24 hours. The heaviest rainfall in the Wilsons River catchment was registered at Terania Creek, which received a torrential downpour of 619 millimetres in just 24 hours.

If an extreme rainfall event of a similar magnitude were to occur this year, the consequences for a town such as Lismore, which is still trying to recover from repeated flooding in 2022, would be catastrophic.

 

Human-caused global warming is resulting in changes to historically defined seasonal patterns and natural climate cycles – they are now operating under conditions 1.2 degrees warmer than they were during pre-industrial times. About 90 per cent of heat accumulated from the burning of fossil fuels has been absorbed by the world’s oceans, resulting in profound changes in the Earth’s climate. Ocean heat content – as measured by the energy absorbed by the upper 2000 metres – reached a new all-time high last year, extending the run of record heat observed since 2019.

This build-up of excess heat is influencing the behaviour of natural patterns of weather and climate variability in complex ways that scientists are trying to describe as new conditions unfold. In 2021 the United Nations’ Intergovernmental Panel on Climate Change’s “Sixth Assessment Report” concluded that it is now an “established fact” that human activity has led to an increase in the frequency and intensity of weather extremes such as severe heatwaves and heavy rainfall events in many parts of the world, including Australia. Understanding the interactions between human-caused global warming and naturally occurring cycles operating in the Pacific, Indian and Southern oceans is an area of active research that influences our ability to accurately predict the conditions we experience from season to season in our region.

Over the past three years, an intensification of easterly trade winds pushed warm surface waters into the western Pacific, resulting in prolonged La Niña conditions. While protracted La Niña episodes lasting two or more years are less common than standard events that usually last about nine to 18 months, these longer sequences have occurred before. For example, the most notable wet periods in Australian history took place during the protracted La Niña episodes of 1954-1957, 1973-1976 and 1998-2001. It’s worth knowing that there is evidence from geologic records to suggest that protracted La Niña and El Niño events can last as long as three to seven years and, in rare instances, as long as a decade. While these indirect measures of past events, derived from coral and tree-ring records, may be less accurate than direct ocean temperature measurements, they provide our best estimates of events in the pre-industrial period, when instrumental weather observations were not possible.

 

Currently there is a 50-50 chance that an El Niño will develop by spring. When La Niña conditions decay, it is possible that a relaxation or reversal of the easterly trade winds could release the accumulated heat trapped in deeper layers of the western Pacific Ocean, triggering an El Niño event. When this happens, the ocean surface releases more heat into the atmosphere, with the associated rainbands shifting east. This causes hot and dry weather to prevail over Australia as our rainfall is displaced towards the Pacific Islands. During these events, there is a higher risk of intense heatwaves, bushfires and drought conditions over much of the country.

While it is still too early to predict ocean temperatures in the second half of this year, historical records indicate that it is not uncommon for El Niño events to develop following La Niña events, and vice versa. This “phase flipping” has happened several times in the past. For example, the protracted La Niñas of 1954-1957 and 1973-1976 were followed by El Niño events in 1957-1958 and 1977-1978, respectively. After sustained wet conditions that stimulated prolific growth of vegetation, both of these El Niños resulted in major bushfires in places such as NSW during the years 1957 and 1977.

With the impacts of the Black Summer bushfires of 2019-2020 still relatively fresh in our collective memory, the possibility of another severe bushfire season – in a climate that has warmed 1.47 degrees since 1910 – is a terrifying prospect. Given we have just been through three very wet years, these conditions won’t develop overnight, but prolonged heat will eventually dry out the landscape, priming it to burn.

Even if an El Niño doesn’t establish this year, it is only a matter of time until one does. As these naturally occurring events tend to develop every two to seven years, and our last major one was in 2015-2016, the pendulum will eventually swing back towards hot and dry conditions. Recently released analysis of global temperatures shows that 2022 was the sixth-warmest year since global observations began in 1880 – even during La Niña conditions, which typically cool surface temperatures. In fact, the trend in global warming now means that La Niñas are hotter today than El Niños were 30 years ago.

It’s a confronting reality to take in. Since global temperatures are steadily rising – the 10 warmest years on record have all occurred since 2010 – the presence of an El Niño will almost certainly set new records as the planet’s relentless warming trend continues.

For communities still struggling to get back on their feet after repeated natural disasters, many Australians no longer need a weather forecast to know which way the wind is blowing.

This article was first published in the print edition of The Saturday Paper on January 28, 2023 as "La Niña’s endgame".

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