The animated movie Finding Nemo is responsible for most children’s (and parents') image of the East Australian Current.
Marlin: “I need to get to the East Australian Current – E A C.”
Crush: “You’re ridin' it dude. Check it out!”
And so Marlin, the father of little clownfish Nemo, rides on the back
of a turtle as they hitch a ride on the East Australian Current all the
way to Sydney. ‘Surfing’ the East Australian Current.
But did Hollywood tell us the truth about what happens in our own backyard?
Well, the movie may have added just a little poetic license to the
real ways of the ocean. The East Australian Current is not the
fast-flowing warp-tube as it’s portrayed in the movie – it’s an even
better ride than that.
Let the current flow
The East Australian Current plays a crucial role in our east-coast climate and ecosystems. As it is World Oceans Day this weekend (June 8), it’s a good time to have a closer look at the big current in our own backyard and what it does for us.
Let’s start with what’s correct in the movie. The East Australian
Current does flow along the East coast of Australia, obviously. And it
does flow southward from the Great Barrier Reef. Speeds in the core of
the EAC are among the strongest in the South Pacific, up to 7km an hour. The East Australian Current – more a ribbon than a tube.Eric Oliver
The movie would have you think that the East Australian Current is a
narrow jet that you can jump in and out of as your thrill-seeking self
desires. But actually the real East Australian Current is much bigger
and much wilder than in the movie – just not so conducive to surfing.
It transports a staggering 40 million cubic metres of water southward
each second. That is the equivalent of 16,000 Olympic swimming pools
flowing along our coastline, every second. The current is almost 100km
wide, and more than 1.5km deep – in fact, more like a ribbon than a
tube.
Why does the East Australian Current exist?
The wind systems over the ocean and the spin of the Earth cause water
in both hemispheres to slowly flow toward the equator in what’s called
the subtropical gyres (also where all our plastics ends up in the infamous garbage patches).
Obviously, the water that flows towards the equator has to go
somewhere. It does so in strong currents trapped tightly against the
eastern coasts of landmasses.
Our East Australian Current is part of a family of five. All
subtropical ocean basins have one of these western boundary currents:
the Gulf Stream in the North Atlantic Ocean
the Brazil Current in the South Atlantic Ocean
the Agulhas Current in the Indian Ocean
the Kuroshio Current in the North Pacific Ocean
the East Australian Current in the South Pacific Ocean.
Each of these is set up by the same wind patterns. The theory that
explains western boundary currents was developed by Norwegian
oceanographer Harald Sverdrup and was one of the major accomplishments
of physical oceanography in the 1940s.
Here come the eddies
Once the East Australian Current reaches New South Wales, the current breaks up into a train of giant, 100km-wide vortices.
These so-called eddies are giant droplets of warm tropical water, and
they slowly move south while rotating anti-clockwise at 5 to 10km per
hour. The amount of eddies passing along the NSW coast is so large that
some have termed the region “Eddy Avenue”.
For many people, the eddies and strength of the East Australian
Current become worth millions of dollars as the Sydney to Hobart yacht
race kicks off on Boxing Day each year. If you’re on the wrong side of
the eddy, the current is against you in your race south. Choosing the
right path around the eddies is big business.
Apart from yachts, the EAC also transports marine species southward
with tropical fish – including Marlin and Dory in their search to find
Nemo – finding themselves in subtropical latitudes.
A change in the EAC
Climate change is already having an impact on the East Australian Current. The part south of Sydney has warmed up very fast, much faster
than most of the rest of the ocean. And this warming is expected to
continue in the future, with the current probably getting faster and
stronger as well.
The warmer water being carried south to Tasmania already has an
impact on marine life there. With warmer water, new species such as the
spiny sea urchin arrive.
These new species, as well as the warmer water itself, slowly destroy Tasmania’s kelp forests, which support unique marine ecosystems, transforming them into rocky barrens.
One of the most iconic kelp species, Macrosystis pyrifera or “giant kelp”, forms underwater forests up to 30m tall and is rapidly disappearing along Tasmania’s shores. The giant kelp forests off Tasmania are quickly disappearing.
Such large relocations of marine ecosystems can occur more often in a
warming ocean. Species will be forced to move southward as their water
temperature “at home” increases.
While this might be bad for all ecosystems, it is particularly dire
for species now living in Tasmania. Unlike the tropical species in
Queensland or the temperate species in New South Wales, which can move southwards (albeit not without problems), Tasmanian species have nowhere to go.
If they try to move southward they encounter the edge of the
continental shelf. With the next piece of habitable shelf more than
3,000km to the south in Antarctica, that’s the end of the line.
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