Low pressure, not low rainfall
Percentage of total rainfall due to deep low pressure systems, 1980–2009. Darker colours on the map indicate areas where deep low pressure systems cause a large proportion of annual rainfall.
In southern parts of Australia, low pressure systems such as east coast lows and extratropical cyclones cause more than a third of all rainfall and at least half of all heavy rain days, especially during the cool half of the year. Rainfall from these systems is associated with highly impactful weather events such as the Pasha Bulker storm which impacted coastal areas around Newcastle in June 2007 and the System Black event which caused major power outages in South Australia September 2016. Low pressure systems also played a role in the severe east coast floods in early 2022.
This is particularly true of low pressure systems where the low extends 5 km or more above the surface, which can be called “deep lows”. These are often linked with a “cut off low”. While the frequency of lows varies a lot from season to season, research indicates that the total rainfall from low pressure systems has decreased in parts of southern Australia in recent decades.
What does this mean for our future climate?
In a warming climate, global model projections have long indicated a southward shift in the storm tracks, and a decrease in lows including east coast lows in southern Australia. New research from the Climate Systems Hub, using higher resolution regional models over Australia, is giving us new insights into what that means for Australia’s future rainfall.
Consistent with previous work, they have found that there will be fewer deep lows in southern Australia in the late 21st century, particularly in the cool half of the year. Under a high emissions scenario, this would mean at least 25% fewer low pressure systems each year in 2070-2099 than in 1980-2009. With fewer lows, total rainfall from lows also decreases, and this missing rainfall explains 80% of projected declines in cool season rainfall in southern Australia.
Widespread heavy rainfall from low pressure systems plays an important role in inflows to dams and rivers, and a decrease in this rainfall could cause pressures on future water security. In contrast, future changes in other types of rainfall such as from thunderstorms remain an active area of research, with variation between different models, and is a key area of focus for the Hub over coming years.
Change in total rainfall from deep lows between 1980–2009 and 2070–2099 in May–October and November–April, shown as mm per degree of global warming. The darker colours show areas where the total rainfall decreases the most. Dots indicate where the models disagree on the direction of change.
Fewer lows, but could they be wetter?
A warmer atmosphere can hold more water vapour than a cooler one, and this relationship alone can increase moisture in the atmosphere by 7 per cent per degree of global warming. This has an impact on the distribution of rainfall from lows. While the frequency of lows with moderate rainfall decreases, the number of lows with very heavy rain is likely to have little change and may even increase. Combined with the effect of rising sea levels on waves and erosion, this points to a potential increase in the frequency and intensity of lows that cause substantial impacts in coastal areas, even in a world where there are fewer lows each year on average.
Read more on how low pressure systems and other extreme events will change in a changing climate.