An international team of scientists led by The University of Western Australia has analysed coral cores from three reefs in the eastern Indian Ocean to understand how marine heatwaves unfold among the unique coral reefs of Western Australia.
The research, which also involved researchers from Australian Institute of Marine Science (AIMS), Curtin University, CSIRO and the University of Santa Barbara California, was published in the international journal Nature Communications.
Professor Malcolm McCulloch, ARC Laureate Fellow with UWA’s Oceans Institute and research team leader, said the findings provided new insights into how the change in air temperature between the Maritime Continent and the central Pacific affected the wind and ocean circulation leading up to heat waves in the far away south-eastern Indian Ocean.
The Maritime Continent is a term commonly used by scientists to describe the region between the Indian and Pacific Oceans including the archipelagos of Indonesia, Borneo, New Guinea, the Philippine Islands, the Malay Peninsula and the surrounding seas.
“Due to the lack of long-term observations of marine climate we used long coral cores, with annual growth bands similar to tree rings, to provide a record of the past,” Professor McCulloch said.
“By measuring the chemical composition of the coral skeleton from year to year we could see how changing winds and ocean currents in the eastern Indian Ocean were impacted by climate variability in the western tropical Pacific Ocean.”
Dr Jens Zinke, Senior Research Fellow at Curtin University and lead author of the research paper, said the long coral records allowed the scientists to look at the occurrence of marine heat waves as far back as 1795.
When the Maritime Continent is warmer than the central Pacific, a pattern amplified during strong La Niña events in the tropical Pacific, it creates an ocean temperature gradient which reinforces warming in the far western Pacific and south-eastern Indian Ocean.
Dr Zinke said this happened through a series of ocean-atmosphere interactions that resulted in a strengthened Leeuwin Current and unusually warm water temperatures and higher sea levels off south-west Western Australia.
“A prominent example is the 2011 heat wave along WA’s reefs which led to coral bleaching and fish kills,” he said.
The international team found that the temperature gradient in the western Pacific was particularly strong after the late 1990s. The coral cores also reveal that this temperature gradient was intensified in the early and late 1800s, yet against a much lower background ocean temperature off WA.
The authors concluded that strong warming over the past 215 years made it easier for natural climate events, such as La Niña and West Pacific temperature gradient events, to exceed the critical temperature threshold for marine heatwaves and mass coral bleaching to occur off Western Australia.
This will likely diminish the future ability of the coral reefs of Western Australia to serve as a climatically stable area for coral under future ocean warming.
Dr Janice Lough, AIMS Senior Principal Research Scientist, said it was likely that, given ongoing global climate change, future La Niña events coupled with a strong West Pacific temperature gradient would result in more extreme warming and high sea-level events with potentially significant consequences for the maintenance of WA’s unique marine ecosystems.
The researchers used core samples of massive Porites colonies from the Rowley Shoals, Ningaloo Reef and the Houtman-Abrolhos Islands in the south-eastern Indian Ocean which are directly in the path of warm water transport from the Western Pacific to the coast of Western Australia.
The researchers measured the chemical composition of the annual coral growth bands to reconstruct sea surface temperature of the West Australian shelf for 215 years, from 1795 to 2010.