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Understanding the impact of the Indian Ocean on WA climate
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 full extent of the spatial and temporal evolution of the freshening anomalies of the Indonesian Throughflow waters are captured in the Indian Ocean for the first time, based on the Argo float data stream supported by IMOS. This is important to track water mass pathways and the large scale marine connectivity.
The finding, which is part of The Western Australian Marine Science Institution (WAMSI) 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.
According to CSIRO’s Dr Ming Feng, who also leads the IMOS Western Australia moorings sub-facility, 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,” said Dr Feng.
“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,” he said.
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.
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.
“It is important to maintain the IMOS national reference stations around Australia so that we are better informed about the impacts of the extreme climate events on coastal environments,” 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.
IMOS Director, Tim Moltmann, believes publication of this work is very timely.
“It is fifty years since the first international Indian Ocean expedition, and while much has been learnt over that period, many new questions have also emerged.
“The international marine science community is gearing up for a large, coordinated effort over the next five years and Australia needs to be involved.
“We have much to contribute, and will benefit tremendously as a country with a significant presence on the Indian Ocean Rim,” said Mr Moltmann.
Tim Moltmann, Ming Feng and a number of other Australian researchers are presenting at an international conference on ‘Dynamics of the Indian Ocean’ being held in Goa, India from 30 November to 4 December.
This article has been adapted from the WAMSI Bulletin, ‘Indian Ocean creates its own flow-on effect’, 30 November 2015
*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