how a cyclone, humid air and atmospheric waves drove brutal heat in the southeast
- Written by Tess Parker, Research Scientist in Climate Variability and Hazards, CSIRO
Australia has always had heatwaves. But this week’s heatwave in southeastern Australia is something else. Temperatures in some inland towns in South Australia, New South Wales and Victoria were up to 20°C above average for the time of year, which meteorologists described as “incredibly abnormal”. Victoria’s heat record toppled after Walpeup and Hopetoun hit 48.9°C. The heat is set to continue until Saturday in some areas.
As global temperatures inch upwards, summer heatwaves form against a background of higher temperatures. Heatwaves (commonly defined as three or more days of unusual heat) and extreme heat (shorter periods of intense heat) are becoming more common and more intense.
Heatwaves kill more Australians than all other natural hazards combined. In the four years to 2019, more than 1,000 people died due to heatwaves.
Heatwaves emerge from atmospheric processes which largely take place in the upper atmosphere, around 5–10 kilometres above sea level. Perhaps counterintuitively, the southeast’s record-breaking heatwave is linked to masses of moisture-laden air in Western Australia and a tropical cyclone off the northwest coast.
That’s not all. Slow-moving high pressure systems in the southeast and the southern drought have likely also made this heatwave worse. Heatwaves are complex. As Australia gets hotter, understanding these dangerous phenomena is essential.
Meanders in the jetstream
Our research has shown heatwaves hitting Australia’s southeast have a clear link to the atmospheric phenomenon known as Rossby waves. These waves manifest as disturbances in the narrow, fast-flowing jetstream that flows from west to east in the atmosphere kilometres above the surface. The waves play a major role in the weather in the southern half of Australia.
As the jetstream wobbles, it pushes aside winds to the north and south to form regions of alternating high and low pressure. As the undulations grow and deepen, they amplify the weather we experience on the ground.
Air inside a high-pressure system (or ridge) descends towards the surface and is compressed by the increasing atmospheric pressure, heating it. The end result is higher temperatures near the ground. The slower these systems move, the more delayed the welcome cool change from a summertime cold front. If these systems move slowly enough, they can turn one hot day into several, or even a heatwave.
What makes these systems bigger or slower moving?
Several things can intensify or slow down these high pressure systems.
The first is tied to Rossby waves. Cold fronts form on the western edge of high pressure systems embedded in the atmospheric wave. Moist air flows along the cold front, moving humid air polewards and upwards between the two systems. This is known as a warm conveyor belt.
Processes within the conveyor belt of warm, moist air can make high pressure ridges even stronger and more likely to persist.
This week’s heatwave was given an extra boost by Tropical Cyclone Luana, which made landfall on January 24 over the northwest coast near Derby. Luana moved inland and added even more moisture to the warm conveyor belt, bringing some rain to the Nullarbor.
Luana may have influenced the southeastern heatwave in a second way. Anticyclonic outflows from cyclones higher in the atmosphere can disturb the jet stream, amplifying the Rossby waves and strengthening the upper-level ridge, potentially lengthening the heatwave.
2009 redux?
There are clear similarities between this week’s heat and the intense, long-lasting heatwave starting in late January 2009. That heatwave broke records across the southeast of Australia and paved the way for Victoria’s devastating bushfires on February 7, now known as Black Saturday.
The 2009 heatwave has strong parallels with the current heatwave, such as a cyclone, Rossby waves and the warm conveyor belt.
ECMWF Integrated Forecasting System, CC BY-NC-ND
Both heatwaves are linked to a large-amplitude Rossby wave moving across the continent, with a persistent slow-moving high pressure ridge situated over the southeast. Both took place as a tropical cyclone hit the northwest and both have active moist airstreams intensifying the ridge.
These cyclone-heatwave events are known as compound events, where several types of extreme weather overlap in time, space or both, and can act to intensify each other. In both 2009 and 2026, ongoing drought might have been yet another compounding factor.
In 2009, southern Australia was still in the grip of the 1996-2010 Millennium Drought, which made the heatwave worse. That’s because when soil is moist, extra heat evaporates moisture in the soil first, slowing heating of the air. But if heat hits dry soil, it quickly boosts air temperatures.
Swathes of southern Australia have had periods of drought for several years. This may have intensified the current heatwave.
What’s next?
Heatwaves are often well forecast using current weather prediction models.
At least a week before the heatwave began, forecasts clearly showed extreme, prolonged heat was likely. The main culprits were visible on the charts: Rossby waves, the warm conveyor belt, the tropical cyclone and the high pressure ridge.
As people in inland towns prepare for more days over 40°C, many will wonder what the future holds.
What we know is that a hotter climate will mean heatwaves hit more often, more intensely and last longer. It will also make extremely hot days more likely. But some of these processes – particularly around how moisture and tropical cyclones affect heatwaves – are less well understood and modelled in climate projections.
In extreme weather, the details are critical. As we grapple with unprecedented heat, there’s much work to do to increase our understanding of extreme heat in the 21st century.
Authors: Tess Parker, Research Scientist in Climate Variability and Hazards, CSIRO
