Featured Image Credit: Adventure to Africa
By Eva Gruber
The Agulhas Current is one of the world’s greatest ocean currents, and marks the western boundary of the Indian Ocean. It sweeps south off the Cape of Africa, creating what is known as a convergence zone as it meets the Benguela Current. This movement creates one of our planet’s great areas of intense marine productivity, and the waters off South Africa are filled with sardines, seabirds, marine mammals, and all kinds of fish – including the famed Great White Sharks.
Despite its fame and importance in the global ocean system, there is still much to be discovered about how it works. With that goal in mind, researchers headed there to drop off a pair of one of marine sciences’ latest technological innovations: remotely operated underwater vehicles known as Seagliders. These UAVs can stay in the water at depths of 100 – 1000 meters for longer periods of time, relying on the movement of water rather than on an electric motor with a limited-life battery. This is a novel propulsion method that will revolutionize how oceanographic data is collected.
By small changes in its fins and buoyancy, causing the glider to move in a sawtooth pattern, the UAV can stay underwater for up to months at a time, relaying data it collects, and receiving instructions via satellite when it surfaces periodically. This was the first time that a Seaglider was deployed in the Agulhas Current. As it traveled at depth, it would be carried along by the current and measure speed.
However, not long into its deployment, one of the Seagliders began to travel opposite to the direction of the current. Baffled at first, the scientists realized that it had become trapped in a swirling eddy of water – the aquatic equivalent of a cyclone or tornado. This patch of water was roughly 15 kilometers in diameter, and moved at the speed of a walking person – that’s quite swiftly for a medium as dense as water. While this is not the first ocean cyclone we’ve seen – much larger oceanic cyclones have been spotted from space through satellites – it is the first one of such small size.
This phenomenon very much affects life on our planet, acting in unison with other small cyclones and fitting in to the larger cycles of the currents as a whole. Sardines and anchovies, the base of the food chain in the Southern Ocean, likely rely on these cyclones to help them migrate to their spawning and feeding grounds. Their presence in turn is why the ocean around South Africa is so productive with a huge diversity of marine life.
This discovery is a perfect example of how science somehow works: researchers are doing their thing and then suddenly stumble upon something they’ve never encountered before. It presents an excellent opportunity for discovery. This discovery resulted in a published paper in Geophysical Research Letters, and one more small expansion in our understanding of how the oceans, and our planet, works.