AIMS scientists together with a team from The University of Western Australia, CSIRO and the University of San Diego have analysed coral cores from the eastern Indian Ocean to understand how the unique coral reefs of Western Australia are affected by changing ocean currents and water temperatures. The research was published today in the international journal Nature Communications. The findings give new insights into how La Niña, a climate swing in the tropical Pacific, affects the Leeuwin current and how our oceans are changing.

“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. We obtained records of past sea temperatures by measuring the chemical composition of the coral skeleton from year to year. This showed how changing winds and ocean currents in the eastern Indian Ocean are driven by climate variability in the western tropical Pacific Ocean,” said Dr Jens Zinke (Assistant Professor at the UWA Oceans Institute and AIMS-UWA scientist). The long coral records allowed the scientists to look at these patterns of climate variability back to 1795 AD.

La Niña events in the tropical Pacific result in a strengthened Leeuwin Current and unusually warm water temperatures and higher sea levels off southwest Western Australia.

“A prominent example is the 2011 heat wave along WA’s reefs which led to coral bleaching and fish kills,” said Dr Ming Feng CSIRO Principal Research Scientist.

The international team found that in addition to warming sea surface temperatures, sea-level variability and Leeuwin Current strength have increased since 1980. The coral cores also reveal that the strong winds and extreme weather of 2011 off Western Australia are highly unusual in the context of the past 215 years. The authors conclude that this is clear evidence that global warming and sea-level rise is increasing the severity of these extreme events which impact the highly diverse coral reefs of Western Australia, including the Ningaloo Reef World Heritage site.

“Given ongoing global climate change, It is likely that future La Niña events will result in more extreme warming and high sea-level events with potentially significant consequences for the maintenance of Western Australia's unique marine ecosystems,” said Dr Janice Lough, AIMS Senior Principal Research Scientist.

The researchers used core samples of massive Porites colonies from the Houtman-Abrolhos Islands, the most southerly reefs in the Indian Ocean which are directly in the path of the Leeuwin Current. Using the chemical composition of the annual coral growth bands they were able to reconstruct sea surface temperature and Leeuwin Current for 215 years, from 1795 to 2010.