The East Australian Current (EAC) is wide and powerful and plays a crucial role in our east-coast climate and ocean ecosystems. But there are big gaps in our knowledge about this dynamic ocean current that can move from hugging the coast one day to being hundreds of kilometres out to sea the next.
IMOS has been observing this major boundary current since 2012 with an array of six full-depth moorings that measures the speed, salinity and temperature of the EAC.
Ocean observing at this scale is only possible by the major Australian marine national research infrastructure. The RV Investigator voyages that enable the maintenance of multi-year monitoring of the EAC are a collaboration between IMOS, the Marine National Facility who operate the vessel, and the scientists and engineers that operate the IMOS Deep Water Moorings Facility at CSIRO.
The East Australian Current carries up to 40 million cubic metres of water south each second, the equivalent of 16,000 Olympic swimming pools. It's almost 100 kilometres wide in parts and more than one and half kilometres deep in some areas.
The EAC is a complex and highly energetic western boundary system of the South Pacific Ocean off eastern Australia. It closes the South Pacific subtropical gyre, transporting heat, salt and other nutrients southward and onto the continental shelf.
Chief Scientist for this voyage, Dr Bernadette Sloyan, a CSIRO scientist and a leader of the IMOS Deep Water Mooring Facility, explains the importance of gathering the data about the EAC.
“We're all very comfortable with going to Brisbane in July and knowing the mean temperature will be this so I'll pack my clothes to do that. “
“But if we think about the mean conditions of the EAC in June or July or any other month of the year we really haven’t had the ocean observations to do that.”
“So, by combining the ocean observations from the moorings with models of different resolutions along our coast, we will be able to provide an understanding of what the mean conditions of the current are.”
“Then when we see bigger changes, we can quantify how big that change is, and the variability, allowing us to start thinking about what's driving the changes that we see.
The current doesn't just control weather and seasonal forecasting. Bernadette Sloyan explains the more scientists know about the EAC the better equipped they'll be to protect the marine animals living in and around it.
“The EAC does control not just our climate, so our weather and our seasonal forecasting, but it also controls very strongly the marine biodiversity along the coast.”
“We've seen many records in recent times of species being found much further south than they normally are. Some of this is controlled by the current strength, so advecting those species south. But also change in the temperature and the properties of the ocean that allow species to actually extend their range.”
“However, for vulnerable species this range extension may put them at their tolerance level, say of water temperature, that's actually a cause for concern.”
The new data from the retrieved moorings will be added to the IMOS data already in the Australian National Data Network (AODN) Portal. Bernadette Sloyan outlines how she will use the latest data.
“We're collecting the temperature and the salinity of the ocean, and the velocity of the ocean every hour. So now we have 18 months’ worth of hourly data and we can put that together to present a movie of what the current does,” says Dr Sloyan.
“The EAC rapidly moves inshore and offshore, it doesn't take a week it can do it in a day. It's a dynamic system and understanding what drives why it's moving so rapidly onshore or offshore, or then why is it stable for a long period of time, is what really interests me.”
Bernadette Sloyan says long time-series observations are essential to learn how to protect the ocean.
“There are key places in the ocean and the ocean surrounding Australia that I think long term monitoring is going to be the thing we have to do. That will then enable us to work with the modelling community and sophisticated circulation models of the ocean to actually then project change in a much wider domain than just in the EAC at Brisbane.”