Unprecedented oceanographic conditions in 2016 produced the “perfect thermal storm” that led to a massive mass coral bleaching, say researchers. While warmer water temperature is generally associated with coral bleaching, the new study, published in the journal Estuarine, Coastal and Shelf Science, suggests that a combination of factors made the 2016 Great Barrier Reef coral bleaching particularly severe.
When water is too warm, corals expel the algae living in their tissues causing the coral to turn completely white. This is called coral bleaching. Corals can survive a bleaching event, but they are under more stress and are subject to mortality, according to National Ocean Service, US National Oceanic and Atmospheric Administration.
But even in very warm years with a summer el Nino event, such as in 1998, there was no massive coral bleaching in the Torres Strait and only small to moderate bleaching in the northern Great Barrier Reef, Professor Eric Wolanski from James Cook University in Australia said.
“So, the extensive coral bleaching in these areas during the summer of 2016 was an unwelcome surprise,” he said.
A 2016 aerial survey of the northern Great Barrier Reef led by Professor Terry Hughes from James Cook University showed that 90 per cent of reefs in some of these areas were severely bleached.
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, Wolanski said. 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 and all of these factors enabled local solar heating to proceed unrestricted, the researchers explained.
“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,” Wolanski said, adding that these processes together made it the perfect thermal storm.
The study employed oceanography models used extensively to study water flow in the region, which were then calibrated with real oceanographic data, he said.
“What we presented is our best-informed attempt to reveal the mechanisms involved in causing the event, based on the available oceanographic data combined with the existing body of knowledge on the water circulation in and around the Torres Strait/Northern Great Barrier Reef region,” he said.