The mass coral bleaching that devastated Australia's iconic Great Barrier Reef last year was caused by a 'perfect storm' of factors produced by unprecedented oceanographic conditions, scientists say.
Even in very warm years with a summer El Nino event, such as 1998, there was no massive coral bleaching in the Torres Strait and only small to moderate bleaching in the northern Great Barrier Reef, said Eric Wolanski, professor at James Cook University in Australia.
"So, the extensive coral bleaching in these areas during the summer of 2016 was an unwelcome surprise," Wolanski said.
A 2016 aerial survey of the northern Great Barrier Reef showed that 90 per cent of reefs in some of these areas were severely bleached.
Wolanski said satellite data showed the 2016 El Nino heating started in the Gulf of Carpentaria, with patches of water reaching an exceptionally high 34 degrees Celsius.
The water then flowed east onto the Torres Strait reefs and south to the Great Barrier Reef.
The 'residence time' of the very warm water in the Torres Strait and the Northern Great Barrier Reef was exceptionally long, which increased the thermal stress on the coral.
All of these factors enabled local solar heating to proceed unrestricted.
"Examining surface currents suggests that the North Queensland Coastal Current in the Coral Sea, which would normally flush and cool the Northern Great Barrier Reef, actually did the opposite. It reversed course and brought very warm water to the Northern Great Barrier Reef," said Wolanski.
These processes together made it the perfect thermal storm, he said.
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