Every summer, communities across northern Australia brace for the tropical cyclone season. Tropical cyclones draw their power from the warm seas, extracting heat and moisture from ocean water.
To improve cyclone forecasting – and better protect lives and property – we enlisted an unlikely ally: deep-diving sea turtles equipped with oceanographic sensors on their shells.
At times these turtles have encountered powerful tropical cyclones, allowing their sensors to gather and can gather critical information on how the temperature in the water changes as the storm passes overhead.
How oceans affect cyclones
Tropical cyclones are fuelled by warm tropical seas. Higher sea surface temperatures usually generate more powerful storms .
As a cyclone passes over the ocean, its powerful winds and waves churn the water around like a blender. This action mixes warm surface water with cooler water from deeper down, leaving behind a “cool wake” – colder ocean water left behind on the sea surface. This cooling reduces the heat available to the storm, which can in turn can limit its strength.
Traditionally, cyclone forecasters have focused on sea surface temperatures, but the ocean depths also matter. If cooler water sits just below the surface, a storm can rapidly cool the sea surface by mixing it upwards. But if warm water extends deeper down, the mixing does not have the same cooling effect. This means forecast accuracy is highly dependent on the ocean temperatures below the waves .
Observing northern seas
Despite the key role northern Australian seas play in shaping tropical cyclones, we have surprisingly few observations below the surface. The region, which includes the Timor and Arafura seas between Australia and Indonesia, is vast, remote, and shallow.
This makes it difficult or expensive to use traditional ocean observation methods. Ships and underwater ocean gliders can only sample small areas, and nobody wants to collect measurements in the middle of a cyclone.
Australia needs an ocean observing system that can work in all weathers in this region. Following a collaboration between the Australian Institute of Marine Science, the Integrated Marine Observing System, CSIRO and Macquarie University, the University of Western Australia, UNSW and traditional owners, we think we’ve found one.
Turtle power: how it works
To address the data desert to Australia’s north, we have enlisted the help of deep-diving olive ridley and flatback sea turtles , which lay their eggs on beaches around tropical Australia.
Biologists have tracked sea turtles for decades using miniaturised GPS tags . Building on experience from overseas , we paired a GPS tracker with an oceanographic probe that measures temperature when the turtle dives, sometimes as deep as 80 metres.
The temperature data is transmitted almost immediately by satellite, making it available to forecasters in near real time. Over several deployments on 46 animals between 2014 to 2024, the turtles took more than 8,000 ocean temperature snapshots across northern Australia, from the Pilbara to the Gulf of Carpentaria .
Into the maelstrom
In April 2023, a tropical storm formed off the Kimberley coast. This storm, named Ilsa, would go on to become a Category 5 cyclone and register the Bureau of Meteorology’s fastest-ever recorded sustained winds: 219 kilometres an hour . Tragically, two fishing vessels were caught in the storm, and eight people died .
As Ilsa was drawing energy from the warm Timor Sea, it passed directly over one of our turtles carrying ocean instruments. This reptile, seemingly unbothered by the maelstrom above, collected sub-surface observations within the eye of the storm – before, during, after the storm’s passage.
From the turtle observations, we found Ilsa’s cold wake cooled the ocean surface by about 2°C. Had the storm stayed still, this cooling would have potentially reduced Ilsa’s strength from category five to category 4.
Excited by this result, we matched turtle observations with the Bureau of Meteorology’s historical storm database. Our turtles had taken real time observations within the core of five tropical cyclones, including the category four Cyclone Rusty. In all but one case, turtles observed strong cooling of the ocean as the storm passed overhead.
When we compared the turtle data with a state-of-the-art ocean model, we found a potential weakness in Australia’s weather forecasts. Our turtle observations indicated the ocean model did not capture the mixing of deeper, cool water with the warm surface, the very process that causes the cold wake. This can lead to overestimates of cyclone intensity.
Sentinel turtles
To improve tropical cyclone forecasts, we need reliable sub-surface ocean observations.
But ocean observation is expensive and developing an all-weather observing system is beyond the capabilities of many of the nations most vulnerable to tropical storms.
The instruments we used can be deployed on land with minimal equipment, and the Olive Ridley turtle lives in tropical seas. Sea turtles could become a key part of observing cyclone weather systems, protecting lives and livelihoods across the tropics.
