154 | Chapter 8
Talk of Fronts in the Southern Ocean perhaps provokes thoughts of
an ocean barrier the size of the Great Wall of China, thousands of kilo-
metres long. That has some truth, but the Fronts are sometimes inter-
rupted where eddies, a few tens or hundreds of kilometres across, spin
off. Such mesoscale eddies can also occur where the flow of ocean cur-
rents is diverted by underwater topography. Where the eastward- flowing
Antarctic Circumpolar Current* squeezes through Drake Passage, be-
tween Cape Horn and Antarctica, numerous eddies spin off to the east
of Cape Horn, contributing to high regional marine productivity and
hence the fabulous abundance of seabirds in South Georgia and the Falk-
lands. However, such eddies are by no means confined to cool waters.
Off the Great Barrier Reef, the East Australian Current flows south.
At the latitude of the Tropic of Capricorn, the continental shelf has an
inward pocket. Into this pocket, this geological bursa called the Capri-
corn Wedge, spins a clockwise eddy, appropriately named the Capricorn
Eddy. The spinning eddy lifts nutrient- rich water to the surface and
that ultimately is why the eddy is so important as a feeding area to the
100,000 Wedge- tailed Shearwaters that make their home on Heron Is-
land positioned on the fringe of the Wedge.
Underwater topography may not only spin off eddies at modest
depths. It can also divert deep ocean currents towards the surface. Then
the water reaching the sunlit surface will be cool but rich in nutrients;
ideal for plankton to multiply and ultimately for bird food to flourish.
Anyone who has been lucky enough to visit the Galápagos Islands will
know that the two flightless seabirds, Flightless Cormorants and Galápa-
gos Penguins, that necessarily rely on food close at hand are restricted
to the archipelago’s western islands. This is where the Cromwell Cur-
rent, flowing from the west, hits the submarine mountain topped by the
visible islands of the Galápagos. Diverted upwards, the cool current fos-
ters an abundance of marine life. A similar effect explains feeding areas
- (^) Also known as the West Wind Drift. Never absolutely blocked by land, the Current girdles
Antarctica in a continuous flow of about 100 million cubic metres per second, an unimag-
inable quantity. To say it is about 500 times greater than the Amazon flow gives the imagina-
tion scant help.