Tropical cyclones are considered one of the most devastating weather events in Australia. But they’re erratic — where, when and how many tropical cyclones form each year is highly variable, which makes them difficult to predict.
In our new research published today, we created a statistical model that predicts the number of tropical cyclones up to four months before the start of the tropical cyclone season from November to April.
The model, the Long-Range Tropical Cyclone Outlook for Australia (TCO-AU), indicates normal to above normal tropical cyclone activity with 11 cyclones expected in total, Australia-wide. Though not all make landfall.
This is above Australia’s average of ten tropical cyclones per season, thanks to a climate phenomenon brewing in the Pacific that brings conditions favourable for tropical cyclone activity closer to Australia.
La Niña and tropical cyclones
As we’ve seen most recently with Tropical Storm Sally in the US, tropical cyclones can cause massive damage over vast areas. This includes extreme and damaging winds, intense rainfall and flooding, storm surges, large waves and coastal erosion.
“La Niña” is one phase of ENSO. It’s typically associated with higher than normal tropical cyclone numbers in the Australian region. And the Bureau of Meteorology’s weather and climate model indicates there’s a 95% chance a La Niña will be established by October this year.
Explainer: El Niño and La Niña
Around ten tropical cyclones occur in the Australian region every season, and about four of those usually make landfall.
Historically, La Niña has resulted in double the number of landfalling tropical cyclones in Australia, compared to El Niño phases. An “El Niño” event is associated with warmer and drier conditions for eastern Australia.
During La Niña events, the first tropical cyclone to make landfall also tends to occur earlier in the season. In fact, in Queensland, the only tropical cyclone seasons with multiple severe tropical cyclone landfalls have been during La Niña events.
Severe Tropical Cyclone Yasi, one of the most intense tropical cyclones to have hit Queensland, occurred during a La Niña in 2011. So did the infamous Severe Tropical Cyclone Tracy, which made landfall around Darwin in 1974, killing 71 people and leaving more than 80% of all buildings destroyed or damaged.
While naturally occurring climate drivers, such as La Niña, influence the characteristics of tropical cyclone activity, climate change is also expected to cause changes to future tropical cyclone risk, including frequency and intensity.
Australian tropical cyclone outlooks
Tropical cyclone outlooks provide important information about how many tropical cyclones may pass within the Australian region and subregions, before the start of the cyclone season. Decision-makers, government, industry and people living in tropical cyclone regions use them to prepare for the coming cyclone season.
The Australian Bureau of Meteorology has led the way in producing tropical cyclone outlooks for Australia, usually a couple of weeks before the official start of the tropical cyclone season.
But with monthly guidance up to four months before the start of the season, our new model, TCO-AU, is unmatched in lead time. It considers the most recent changes in ENSO and other climate drivers to predict how many tropical cyclones may occur in Australia and its sub-regions.
As a statistical model, TCO-AU is trained on historical relationships between ocean-atmosphere processes and the number of tropical cyclones per season.
For each region, hundreds of potential model combinations are tested, and the one that performs best in predicting historical tropical cyclone counts is selected to make the prediction for the coming season.
So what can we expect this season?
September’s TCO-AU guidance suggests normal to above normal risk for Australia for the coming tropical cyclone season (November 2020 – April 2021).
With an emerging La Niña and warmer than normal sea surface temperatures in the eastern Indian Ocean, 11 tropical cyclones are expected for Australia. There’s a 47% chance of 12 or more cyclones, and a probable range of between nine and 15.
For the Australian sub-regions, TCO-AU suggests the following:
above normal activity is expected for the Eastern region (eastern Australia) with four cyclones expected. Probable range between three and six cyclones; with a 55% chance of four or more cyclones
normal activity is expected for the Western region (west/northwest Western Australia) with six cyclones expected. Probable range between five and eight cyclones; 39% chance of seven or more cyclones
below normal activity is expected for the Northern region (northwest Queensland and Northern Territory) with three cyclones expected. Probable range between two and five cyclones; 37% chance of four cyclones or more
below normal activity is also expected for the Northwestern region (northwest Western Australia) with four cyclones expected. Probable range between three and six cyclones; 45% chance of five cyclones or more.
Guidance from TCO-AU does not and should not replace advice provided by the Australian Bureau of Meteorology. Instead, it should be used to provide a complementary perspective to regional outlooks and provide a “heads-up” in the months leading up to the start of and within the cyclone season.
Regardless of what’s expected for the coming cyclone season, people living in tropical cyclone regions should always prepare for the cyclone season and follow the advice provided by emergency services.
Tropical cyclones are among the most destructive weather systems on Earth, and the Southwest Pacific region is very exposed and vulnerable to these extreme events.
Our latest research, published today in Scientific Reports, presents a new way of predicting the number of tropical cyclones up to four months ahead of the cyclone season, with outlooks tailored for individual island nations and territories.
Tropical cyclones produce extreme winds, large waves and storm surges, intense rainfall and flooding — and account for almost three in four natural disasters across the Southwest Pacific region.
Currently, Southwest Pacific forecasting agencies release a regional tropical cyclone outlook in October, one month ahead of the official start of the cyclone season in November. Our new model offers a long-range warning, issued monthly from July, to give local authorities more time to prepare.
Most importantly, this improvement on existing extreme weather warning systems may save more lives and mitigate damage by providing information up to four months ahead of the cyclone season.
Tropical cyclones and climate variability
In 2016, Cyclone Winston, a record-breaking severe category 5 event, was the strongest cyclone to make landfall across Fiji. It killed 44 people, injured 130 and seriously damaged around 40,000 homes. Damages totalled US$1.4 billion — making it the costliest cyclone in Southwest Pacific history.
Tropical cyclones are erratic in their severity and the path they travel. Every cyclone season is different. Exactly where and when a tropical cyclone forms is driven by complex interactions between the ocean and the atmosphere, including the El Niño-Southern Oscillation, sea surface temperatures in the Indian Ocean, and many other climate influences.
Capturing changes in all of these climate influences simultaneously is key to producing more accurate tropical cyclone outlooks. Our new tool, the Long-Range Tropical Cyclone Outlook for the Southwest Pacific (TCO-SP), will assist forecasters and help local authorities to prepare for the coming season’s cyclone activity.
According to the latest long-range sea surface temperature outlook, there is a 79% chance that La Niña conditions could develop before the start of the 2020-21 Southwest Pacific cyclone season. La Niña conditions typically mean the risk of tropical cyclone activity is elevated for island nations in the western part of the region (New Caledonia, Solomon Islands and Vanuatu) and reduced for nations in the east (French Polynesia and the Cook Islands). But there are exceptions, particularly when certain climate influences like the Indian Ocean Dipole occur with La Niña events.
Improving existing tropical cyclone guidance
Current guidance on tropical cyclones in the Southwest Pacific region is produced by the National Institute of Water and Atmospheric Research, the Australian Bureau of Meteorology and the Fiji Meteorological Service. Each of these organisations uses a different method and considers different indices to capture ocean-atmosphere variability associated with the El Niño-Southern Oscillation.
Our research adds to the existing methods used by those agencies, but also considers other climate drivers known to influence tropical cyclone activity. In total, 12 separate outlooks are produced for individual nations and territories including Fiji, Solomon Islands, New Caledonia, Vanuatu, Papua New Guinea and Tonga.
Other locations are grouped into sub-regional models, and we also provide outlooks for New Zealand because of the important impacts there from ex-tropical cyclones.
Our long-range outlook is a statistical model, trained on historical relationships between ocean-atmosphere processes and the number of tropical cyclones per season. For each target location, hundreds of unique model combinations are tested. The one that performs best in capturing historical tropical cyclone counts is selected to make the prediction for the coming season.
At the start of each monthly outlook, the model retrains itself, taking the most recent changes in ocean temperature and atmospheric variability and attributes of tropical cyclones from the previous season into account.
Both deterministic (tropical cyclone numbers) and probabilistic (the chance of below, normal or above average tropical cyclone activity) outlooks are updated every month between July and January and are freely available.
Andrew Magee, Postdoctoral Researcher, University of Newcastle; Andrew Lorrey, Principal Scientist & Programme Leader of Climate Observations and Processes, National Institute of Water and Atmospheric Research, and Anthony Kiem, Associate Professor – Hydroclimatology, University of Newcastle
Public attention on the disastrous bushfire crisis in Australia will rightly continue for weeks to come. But as we direct resources to coping and recovery, we should not forget other weather and climate challenges looming this summer.
The peak time for heatwaves in southern Australia has not yet arrived. Many parts of Australia can expect heavy rains and flooding. And northern Australia’s cyclone season is just gearing up.
The events will stretch the ability of emergency services and the broader community to cope. The best way to prepare for these events is to keep an eye on Bureau of Meteorology forecasts.
Let it rain
But relief may be coming. The latest bureau outlooks suggest more normal summer conditions from February to April. If it eventuates, this would include more rain.
The arrival of drought-breaking rains is notoriously hard to predict – in the past, they have come any time between January and May. Global warming is also complicating seasonal climate predictions.
We all hope the rain arrives sooner rather than later, and eases the fire situation. But rain will bring other risks.
Continental-scale droughts such as that experienced over the past few years are often broken by widespread heavy rains, leading to an increased risk of flooding including potentially lethal flash floods. The decade-long Millenium drought that ended in 2009 was followed by two extremely wet years with serious flooding.
A similar situation was seen in Indonesia in recent days when very heavy rains after a prolonged drought produced disastrous floods and landslides.
The flood risk is exacerbated by the bare soil and lack of vegetation caused by drought, and by bushfires that destroy forest and grassland.
Australia’s north may be particularly hard hit. The onset of the tropical wet season has been very much delayed, as the bureau predicted. Over the last three months, some parts of the Australian tropics had their lowest ever October-December rainfall. But there are some suggestions widespread rain may be on its way.
Further south, drought-breaking rains can also be heavy and widespread, leading to increased flood risk. So even when the drought breaks and rains quell the fires, there will likely still be bouts of extreme weather, and high demand for emergency services.
Cyclones and heatwaves
Cyclones often bring welcome rains to drought-affected communities. But we should not overlook the serious damage these systems may bring such as coastal flooding and wind damage – again requiring intervention from emergency services.
And we are still a month away from the riskiest time for heatwaves in southern Australia. We’ve already had some severe heatwaves this summer. However they usually peak in the middle and end of summer, so the worst may be yet to come.
Lives have undoubtedly been saved this summer by improved forecasting of high temperatures and better dissemination of heatwave information by state and local governments. But after an already devastating early summer of fires and heat, warning fatigue may set in amongst both warning providers and the public. We must ensure heatwave warnings continue to be disseminated to populations at risk, and are acted on.
Be thankful for weather forecasters
The recent experience of farmers, fire fighters, water resource managers and communities illustrate the value of the service provided by the Bureau of Meteorology. Greatly improved weather and climate forecasting developed over the past few decades means communities can plan for and deal with our highly variable weather and climate far better than in the past.
Recent drought, fires and heatwaves – exacerbated by global warming – have been devastating. But imagine if we only had the limited weather forecast capabilities of even a few decades ago, without today’s high-speed computers to run weather forecast models, and satellites to feed in enormous amounts of data. How much worse would the impacts have been?
These forecasts have allowed heat alerts to be disseminated to vulnerable communities. Detailed information on weather conducive to fire spread has helped fire agencies provide more targeted warnings and direct resources appropriately.
Never before have weather forecasts been so readily available to the public. Here are ways you can use them to reduce risks to life and property during an extreme event:
- Listen to ABC local radio for emergency updates and detailed Bureau of Meteorology forecasts
- load your state fire service emergency app onto your phone and check it regularly. Or check out the information online, such as at the NSW Rural Fire Service’s Fires Near Me website
- check the bureau’s website for climate and weather forecasts
- download a short-range rainfall forecast app such as Rain Parrot onto your phone. These apps use the bureau’s radar data to make short-range forecasts of rainfall for your location, and notify you if rain is coming.
Global warming is already lengthening the fire season and making heatwaves more intense, more frequent, and longer. It is also increasing the likelihood of heavy rains, and making droughts worse.
We must keep adapting to these changing threats, and further improve our ability to forecast them. And the community must stay aware of the many weather and climate extremes that threaten lives and property.
Jonathan Pollock, Australian Bureau of Meteorology; Andrew B. Watkins, Australian Bureau of Meteorology; Catherine Ganter, Australian Bureau of Meteorology, and Paul Gregory, Australian Bureau of Meteorology
Northern Australia is likely to see fewer cyclones than usual this season, but hot, dry weather will increase the risk of fire and heatwaves across eastern and southern Australia.
The Bureau of Meteorology today released its forecast for the tropical cyclone season, which officially runs from November 1 to April 30.
Also published today is the October to April Severe Weather Outlook, which examines the risk of other weather extremes like flooding, heatwaves and bushfires.
Warmer oceans means more cyclones
On average, 11 tropical cyclones form each season in the Australian region. Around four of those cross the coast. The total number each season is roughly related to how much cooler or warmer than average the tropical oceans near Australia are during the cyclone season.
One of the biggest drivers of change in ocean temperatures is the El Niño–Southern Oscillation, or ENSO. During La Niña phases of ENSO, the warmest waters in the equatorial Pacific build up in the western Pacific and to the north of Australia. That region then becomes the focus for more cloud, rainfall and tropical cyclones.
But during El Niño, the warmest water shifts towards the central Pacific and away from northern Australia. This decreases the likelihood of cyclones in our region.
Explainer: El Niño and La Niña
And when ENSO is neutral, there is little push towards above or below average numbers of cyclones.
Temperatures in the tropical Pacific Ocean have been ENSO-neutral since April and are likely to stay neutral until at least February 2020. However, some tropical patterns are El Niño-like, including higher-than-average air pressure at Darwin. This may be related to the current record-strong positive Indian Ocean Dipole – another of Australia’s major climate drivers – and the cooler waters surrounding northern Australia.
The neutral ENSO phase alongside higher-than-average air pressure over northern Australia means we expect fewer-than-average tropical cyclones in the Australian region this season. The bureau’s Tropical Cyclone Season Outlook model predicts a 65% chance of fewer-than-average cyclones.
At least one tropical cyclone has crossed the Australian coast every season since reliable records began in the 1970s, so people across northern Australia need to be prepared every year. In ENSO-neutral cyclone seasons, this first cyclone crossing typically occurs in late December.
Other severe weather
While cyclones are one of the key concerns during the coming months, the summer months also bring the threat of several other forms of severe weather, including bushfires, heatwaves and flooding rain.
With dry soils inland, and hence little moisture available to cool the air, and a forecast for clear skies and warmer days, there is an increased chance that heat will build up over central Australia during the spring and summer months. This increases the chance of heatwaves across eastern and southern Australia when that hot air is drawn towards the coast by passing weather systems.
Likewise, the dry landscape and the chance of extreme heat also raise the risk of more bushfires throughout similar parts of Australia, especially on windy days. And with fewer natural firebreaks such as full rivers and streams, even greater care is needed in some areas.
Widespread floods are less likely this season. This is because of forecast below-average rainfall and also because dry soils mean the first rains will soak into the ground rather than run across the landscape.
However, as we saw in northern Queensland in January and February this year, when up to 2 metres of rainfall fell in less than 10 days, localised flooding can occur in any wet season if a tropical low parks itself in one location for any length of time.
Most of all, it’s always important to follow advice from emergency services on what to do before, during and after severe weather. Know your weather, know your risk and be prepared. You can stay up to date with the latest forecast and warnings on the bureau’s website and subscribe to receive climate information emails.
Jonathan Pollock, Climatologist, Australian Bureau of Meteorology; Andrew B. Watkins, Head of Long-range Forecasts, Australian Bureau of Meteorology; Catherine Ganter, Senior Climatologist, Australian Bureau of Meteorology, and Paul Gregory, BOM, Australian Bureau of Meteorology