New research led by CSIRO has described how heavy rainfall caused the top layer of the southeast Indian Ocean to be less salty, creating a barrier layer which trapped the heat during the deadly marine heatwave – La Niña and the ‘Ningaloo Niño’ of 2010-11. The result was a larger volume of warmer water being driven by a stronger current down the WA coastline.
The finding, which is part of WAMSI’s Kimberley Marine Research Program, provides further scientific knowledge to help predict responses to climate change, and adds another layer to consider when forecasting extreme marine heatwave events.
|Surface salinity anomalies (psu) derived from gridded Argo product (relative to 2005–2012 monthly climatology) in February 2011.|
According to CSIRO’s physical oceanographer Dr Ming Feng, the change in salinity levels on the top 100 metres of the ocean also raises more questions, such as how it will affect some marine species.
“In the past we have followed the Pacific Ocean climate closer in evaluating WA marine environment. We haven’t focused much attention on the Indian Ocean variables and the effect on the WA environment,” Dr Feng said. “It seems that the more we find out about the Indian Ocean, the more we realise it operates very differently to any other ocean on earth.”
The Leeuwin Current, the eastern boundary current of the southeast Indian Ocean, carries warm fresh tropical water southward along the west coast of Australia. The current is stronger in Australia’s winter and weaker in summer; it also tends to be stronger during La Niña events.
|Surface currents in the Indian Ocean during austral summer, adapted from Schott et al. [] and Menezes et al. []. The background shading shows the sea surface temperature anomalies (°C) in February 2011 derived from the Argo float data. ITF: Indonesian throughflow; LC: Leeuwin Current; SJC: South Java Current; SEC: South Equatorial Current; SECC: South Equatorial Counter Current; SC: Somali Current; NMC: Northern Monsoon Current; NEMC: North East Madagascar Current; SEMC: South East Madagascar Current; SICC: South Indian Countercurrent.|
The latest research compared salinity observations at the CSIRO/Integrated Marine Observing System (IMOS) Rottnest National Reference Station collected during the past 60 years. Analysis of these observations revealed the unusual, but significant, freshening of the ocean’s top layer during 2010-2011.
Dr Feng suspects this may not be the first or last time we experience such an event.
“Something similar probably happened in 1999/2000, so it might happen in the future, especially as we experience more frequent warmer conditions,” he said.
It’s thought that the change in salinity can take a few years to return to normal but the El Niño forecast for 2015, which was to result in a weakening in the Leeuwin Current and cooler water temperatures along the coast of WA, has not rung true.
“Typically we should be experiencing cooler temperatures off the west coast this year but the temperatures have still been warmer than normal, so this is an unusual year, and it shows we don’t fully understand the impact of the Indian Ocean,” Dr Feng said.
Ming Feng, Jessica Benthuysen, Ningning Zhang and Dirk Slawinski (October 2015) Freshening anomalies in the Indonesian throughflow and impacts on the Leeuwin Current during 2010–2011 Research Letters DOI: 10.1002/2015GL065848
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.