Glider 543 goes overboard

Fred Fourie

Underwater probes detect ‘sea cyclones’ off coast of Africa

The Agulhas Current off the southern coast of Africa is one of the world's fastest moving ocean flows and likely responsible for enormous rogue waves that sink supertankers without a trace. To better understand this treacherous stream of water—and its role in the Southern Hemisphere’s marine biodiversity—researchers guided autonomous underwater vehicles known as Seagliders (one shown here entering the water) through the Agulhas Current for the first time. The two roughly human–sized probes measured current speed using onboard sensors as they traveled along the coast of South Africa. When one of the Seagliders unexpectedly started moving against the Agulhas Current, the scientists realized that it had become trapped in an eddy of swirling water. The researchers had stumbled upon the ocean’s equivalent of a cyclone, a patch of water roughly 15 kilometers in diameter circling at the speed of a walking person. Much larger eddies have been observed using satellites, but small cyclones have been very rarely measured, the researchers will report in an upcoming issue of Geophysical Research Letters. The combined motion of many of these small cyclones in the Agulhas Current creates a countercurrent running northeast that fish like sardines and anchovies likely rely on to travel to their spawning grounds. It’s thanks to all these small fish that top predators like sharks are so plentiful near South Africa.