The ocean's impact on our bushfire season

The ocean can increase our bushfire risk, which can have devastating effects on communities, like Mallacoota, Victoria. Image adapted from: licensed under

How does an ocean start a fire?

The ocean's impact on our bushfire season

Our island home

Australia is known for being the , so it鈥檚 not surprising that bushfires are a regular feature of life. But it鈥檚 not just the land and its management that impacts the frequency and severity of these fires: the oceans play an important role.

鈥淎 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,鈥� says Dr Wenju Cai, a physical oceanographer and the Director of the .

Elected a Fellow of the 精东视频 in 2020 in recognition of his contributions to 精东视频, Cai has always been fascinated by the weather.

鈥淎ustralia is actually a very tantalising place to study climate variability and impact 鈥� It has a real impact on human life, on our livelihood,鈥� 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 : 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 , which occurs in the Pacific Ocean and has three main states: El Ni帽o, La Ni帽a and neutral. It describes the east鈥搘est balance of sea temperatures between Australia and South America: wind blows from areas of cool water to areas of warm water.

During an El Ni帽o period, warm water in the central Pacific causes moist air to rise, which then pushes dry air over eastern Australia, contributing to drought conditions. *Image: public domain, adapted from NOAA/PMEL/TAO*.

The Indian Ocean is home to the , which occurs between Australia and India. It also has three states: positive, negative and neutral. 鈥淲hen the [positive] Indian Ocean Dipole occurs, it leads to drought in southern Australia in winter and spring, [鈥 our main rainfall season,鈥� says Cai.

When the positive Indian Ocean Dipole occurs, it leads to drought in southern Australia in winter and spring. *Image adapted from BOM.*

Together with the Southern Ocean鈥檚 (SAM), the north鈥搒outh variation of the westerly wind circling Antarctica, these drivers can combine to affect rainfall patterns across Australia.

鈥淪ometimes the Pacific impact and the Indian Ocean impact are opposite, and sometimes they are superimposing,鈥� 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鈥檚 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.

鈥淚f we don't have that rainfall coming into summer, summer is a dry season,鈥� says Cai. 鈥淎nd we often, almost every year, we had bushfires 鈥� So when we have the winter drought 鈥� and the eastern Indian Ocean is cooler than normal, we know the upcoming summer bushfire is going to be much more severe鈥攅verything else being equal.鈥�