Australia’s weather maps are changing, and a clear trend is emerging — high pressure is becoming more dominant while the number of low-pressure systems, including tropical cyclones and east coast lows, is declining.
This climate change-driven shift is not surprisingly having a profound impact on our weather, as low-pressure systems are a key source of rainfall and extreme weather.
One of the most concerning consequences is a relatively rapid decrease in vital cool season rains over key agricultural regions of south-west and south-east Australia, and a greater rainfall reliance on short duration events and thunderstorms.
The vanishing lows have been outlined in multiple recent studies and unanimously linked to a warming world, with the decreasing trend projected by most modelling to continue for the remainder of the 21st century.
A classic east coast low in June 2007 which brought severe flooding and destructive winds to Newcastle. (Supplied: BOM)
Australia’s winter rainfall under threat
Low pressure systems are associated with air moving vertically away from the surface, and since clouds form when air is rising, they are one of Earth’s primary rain makers.
Lows tend to bring soaking falls over a large area, and are therefore responsible for the majority of Australia’s widespread heavy rainfall events, as opposed to weather systems like troughs, which often generate erratic, short-lived and isolated bursts of rain.
Due to the high rain producing ability of low-pressure systems, they account for about half of southern Australia’s total precipitation, while also playing a major role in delivering tropical rain, particularly over northern parts of Western Australia.
Any shift in their frequency will therefore have a significant impact on both agriculture and water security, and new research published in the Journal of Southern Hemisphere Earth Systems Science has outlined a clear reduction in low pressure systems over southern Australia across all seasons since the 1960s.
The study, authored by climate scientist Acacia Pepler, was confident in the trend despite large year-by-year variability and the recent triple La Niña when lows were plentiful.
“The trends … are negative across almost all time periods … despite above average frequencies of lows during 2020–2022. This indicates a robust declining trend,” Dr Pepler stated.
JRA55 re-analysis shows a marked decline in surface low pressure systems over southern Australia during the cooler months. (Source: Journal of Southern Hemisphere Earth Systems Science)
According to the research, since the middle of last century there has been up to a 35 per cent reduction in low pressure systems forming near south-west WA and 30 per cent in the Great Australian Bight from May to October.
And since weather systems move from west to east, Dr Pepler has found this absence is also having an impact on eastern states.
“The overall decrease in cyclone [low] frequency in southern Australia can be completely explained by the decrease in cyclogenesis [low formation] in these two regions.”
The decline is unsurprisingly having a major impact on precipitation through the cooler months with the most severe deficits developing in south-west WA and near the Eastern Seaboard where rainfall has dropped by an average of 1.1 millimetres and 1.2mm per year, respectively over the past six decades.
“Trends from lows are largest on the east coast, where they explain more than 70 per cent of observed rainfall changes since the 1960s.” Dr Pepler said.
Perth’s rapidly drying climate is a prime illustration – so far this century the city’s May to October rain has averaged 575mm, well below the 751mm recorded in the 20th century.
So what’s the relationship between climate change and less frequent low pressure systems around Australia in winter? It’s theorised the link is due to a broad expansion of the Hadley Cell – a circulation of air which transports heat from the tropics to the mid latitudes.
Essentially, a wider Hadley Cell represents an expansion of the tropics, which is pushing non-tropical lows further towards the poles.
Despite a similar reduction in November to April lows, the study found changes in warm season rain over southern Australia is far more variable with decreases observed in Tasmania but increases over much of the south-east inland.
“During the warmer half of the year, a large proportion of rainfall in south-east Australia is linked to thunderstorms. While it is difficult to assess long-term trends in thunderstorm activity, some studies are suggesting thunderstorm-related rainfall might be increasing” Dr Pepler said.
JRA55 re-analysis shows a marked decline in surface low pressure systems over southern Australia during the warmer months. (Source: Journal of Southern Hemisphere Earth Systems Science)
Cyclone and east coast low numbers failing fast
The decline in low pressure cells is not just limited to your standard daily “L” depicted on the nightly weather map, but also extends to Australia’s two most destructive weather systems, tropical cyclones and east coast lows.
Both are capable of generating widespread rain along with extreme weather including flooding, strong winds, storm surges and coastal erosion, and both have shown a clear downward trend since last century.
While tropical cyclones have decreased 23 per cent worldwide since 1950 according to a 2022 study in Nature, the speed of decline has been faster in the Australian region, falling around 30 per cent from an average of 12.6 per year from 1970 to 2000 to just 8.8 per year this century.
This reduction is occurring despite warmer sea surface temperatures (SST) as the increased heat to fuel cyclones is being offset by a more stable tropical environment – essentially because warmer air can hold more moisture, there is less opportunity for the necessary condensation step in cyclone formation.
Future predictions indicate this decline will continue through the 21st century, including up to a 40 per cent total reduction in cyclone numbers around Australia under global warming of 2 degrees Celsius.
Model projections for east coast low frequency indicate a substantial drop this century with both moderate and high emission scenarios. (Supplied: Climate Dynamics Journal)
While total numbers are falling, the decrease is mostly from weaker systems, leading to a higher proportion of category 4 or 5 cyclones which cause the majority of damage.
Several recent studies have also indicated with relatively high confidence that global warming will increase cyclone intensity and average rain rates, while higher sea levels also raise the severity of storm surges.
East coast lows, which bring severe weather from south-east Queensland to Tasmania, are also becoming less frequent according to a 2019 study published in the Climate Dynamics Journal.
The paper projects fewer east coast lows in the future, particularly during the cooler months, with a total decline of around 25 per cent by the end of this century under a moderate emissions scenario and around 42 per cent under a high emissions scenario.
However, as with tropical cyclones, rising sea levels are likely to increase the impacts of large waves on coastal regions, while increases in rainfall intensity raises the risk of flooding.
Despite the dwindling activity of numerous low-pressure types around Australia, many parts of the country are actually becoming wetter, a trend which Dr Pepler attributes to the global shift towards high intensity rainfall in a warmer world.
“This increase in rainfall on days with no low pressure system are consistent with observed increases in rainfall from thunderstorms and a shift towards rainfall events of shorter duration.”
