The ocean's impact on our bushfire season
Our island home
Australia is known for being the driest inhabited continent in the world, so it’s not surprising that bushfires are a regular feature of life. But it’s not just the land and its management that impacts the frequency and severity of these fires: the oceans play an important role.
“A lot of our climate variations are actually impacted by oceanic temperature from the Indian Ocean, from the Pacific and the Southern Ocean. They are far away from us, yet they play such a very important role in our climate variability,” says Dr Wenju Cai, a physical oceanographer and the Director of the Centre for Southern Hemisphere Oceans Research.
Elected a Fellow of the Australian Academy of Science in 2020 in recognition of his contributions to Australian science, Cai has always been fascinated by the weather.
“Australia is actually a very tantalising place to study climate variability and impact … It has a real impact on human life, on our livelihood,” says Cai.
He studies climate variability and change, focusing on modes of climate variability, their dynamics, their impact in Australia and around the globe, and their response to greenhouse warming.
Girt by sea
There are three oceans surrounding our island home: the Pacific Ocean, the Indian Ocean and the Southern Ocean. These three oceans are each home to what are known as climate drivers: patterns of wind directions, ocean surface temperatures and air pressure that can influence the climate of south-eastern Australia.
The driver most familiar to us is El Niño-Southern Oscillation (ENSO), which occurs in the Pacific Ocean and has three main states: El Niño, La Niña and neutral. It describes the east–west balance of sea temperatures between Australia and South America: wind blows from areas of cool water to areas of warm water.
The Indian Ocean is home to the Indian Ocean Dipole (IOD), which occurs between Australia and India. It also has three states: positive, negative and neutral. “When the [positive] Indian Ocean Dipole occurs, it leads to drought in southern Australia in winter and spring, […] our main rainfall season,” says Cai.
Together with the Southern Ocean’s Southern Annular Mode (SAM), the north–south variation of the westerly wind circling Antarctica, these drivers can combine to affect rainfall patterns across Australia.
“Sometimes the Pacific impact and the Indian Ocean impact are opposite, and sometimes they are superimposing,” says Cai.
A sunburnt country
The risk of bushfires occurring over summer depends on how much rainfall we have during the preceding seasons: generally speaking, a wet winter reduces the risk, while a dry winter increases it.
Thanks to Cai’s work on the link between these climate drivers and rainfall, firefighters and policymakers can use the state of the IOD and ENSO to predict the severity of the bushfire season sometimes as early as six months in advance.
“If we don't have that rainfall coming into summer, summer is a dry season,” says Cai. “And we often, almost every year, we had bushfires … So when we have the winter drought … and the eastern Indian Ocean is cooler than normal, we know the upcoming summer bushfire is going to be much more severe—everything else being equal.”