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Giant corals reveal WA’s heatwave history

29 October 2015

Analyzing coral cores (UWA)

WAMSI oceanographic research has contributed to new findings documenting historic weather maps contained within WA's coral reefs.

The research, led by The University of Western Australia (UWA), which involved researchers from Australian Institute of Marine Science (AIMS), Curtin University, CSIRO, and the University of California Santa Barbara, drilled into giant porite coral cores from Ningaloo Reef, the Abrolhos Islands and Rowley Shoals to find the marine climate data.

Each of the cores contains seasonal growth bands, similar to tree rings, and provides information about past climate conditions. 

The study concentrated on the relationship between the climate conditions in the central-western Pacific and the southeast Indian Ocean referred to as the Maritime Continent, which includes Indonesia, Borneo, New Guinea, the Phillipine Islands, the Malay peninsula and the surrounding areas.

WAMSI researcher CSIRO’s Dr Ming Feng said analysis of the coral core growth bands enabled the team to successfully reconstruct ocean conditions of the West Australian shelf for 215 years and determine what conditions cause marine heat waves among WA's unique coral reefs.

 “What we found is that 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  Feng said.

Dr Jens Zinke, Senior Research Fellow at UWA and lead author of the research paper published in the journal Nature Communications, said the long coral records allowed the scientists to look at the occurrence of marine heatwaves as far back as 1795.

Dr Zinke said the marine heat waves 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 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, which can be linked to the series of marine heatwaves off the WA coast in the recent decade. 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 heat waves and mass coral bleaching to occur off Western Australia.

Dr Feng, said it was likely that under a warming climate, future La Niña events coupled with a strong West Pacific temperature gradient would result in more extreme ocean warming and high sea-level events with potentially significant consequences for the maintenance of WA’s unique marine ecosystems.

This will likely reduce the future ability of the reefs of Western Australia to serve as a climatically stable area for coral under future ocean warming.

The research builds on previous work and contributes to more accurate thermal reconstructions necessary for better informed resource management.

 

Figure 1: Southeast Indian Ocean reefs and tropical Indo-Pacific SST variability.
From the paper, Coral record of southeast Indian Ocean marine heatwaves with intensified Western Pacific temperature gradient, by Jens Zinke, A. Hoell, J.M. Lough, M. Feng, A.J. Kuret, H. CLarke, V. Ricca, K. Rankenburg, & M.T. McCulloch, Nature Communications (DOI doi:10.1038/ncomms9562)

(a) Locations of the three reef areas (1–3) sampled for long coral cores, (b) rotated empirical orthogonal function 2 (REOF2) covariance of ERSSTv3b15 anomalies, and (c) REOF2 time series, 1960–2013, which explains 21% of the variance. The WPG11 is defined as the standardized difference between average SST over the Niño4 domain14 (black box) and the Western Pacific (WP; blue box), while the Western Australian region is highlighted in grey with coral sampling locations indicated, 1, Houtman Abrolhos, 2, Ningaloo Reef and 3, Rowley Shoals. The black-dashed box marks the Indonesian warm pool region (IWP06 (ref. 26)).

The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government's Kimberley Science and Conservation Strategy co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley. 

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