A team of Japanese and Australian researchers has found that a current along the Kerguelen Plateau and off the coast of Antarctica is far from ordinary. It's 10 times faster than expected and carries a volume of water equivalent to forty Amazon Rivers. Since first visiting the area fifteen years ago, Steve Rintoul, an oceanographer with Australia's Cooperative Research Centres (CRC) and Commonwealth Scientific and Industrial Research Organization (CSIRO), knew that the current was special. Its location meant that it was transporting dense, oxygen-rich water, and that type of water influences how much heat and carbon dioxide oceans around the world can absorb. Rintoul wanted to know just how much water the current was moving, but to do that he needed to gather months of detailed data several miles beneath the ocean's surface.
This is no easy task. The instruments they use to measure data such as velocity and temperature are delicate and must be deployed in one of the most tumultuous regions of the ocean. In fact, says Rintoul, the trickiest part of the whole experiment is just getting the measuring equipment in and out of the water before 30-foot waves and strong winds can destroy the hundred-thousand-dollar instruments.
In 2003, Rintoul and team returned to the current to initiate a two-year study 3 miles below the ocean's surface. They released eight moorings that each included a concrete anchor, a mooring "string" made of wire on the upper half and rope on the lower portion, and forty instruments distributed along the string every 1600 to 3200 feet.
"When we dropped the first mooring in the water," says Rintoul, "it was suddenly gone! When that happened we knew we were dealing with an unusually strong current."
Over the 24 months, the 2-foot-tall instruments recorded the current's velocity, direction, temperature and salinity. The instruments are not high-tech. Housed in cases to withstand intense pressure are rotors to measure velocity, the equivalent of weather vanes to measure current direction, thermistors to measure temperature, and sensors to measure electrical conductivity that can be used to calculate the salinity of water flowing past.
At the end of the experiment, Rintoul and team returned to gather the instruments. To retrieve them, the ship transmits a signal that is picked up by acoustic receivers on the anchors. This activates a mechanism that releases the strings from the anchor. With the help of heavy-duty glass balls attached, the wires float to the surface. Although many things can go wrong while the instruments are underwater, such as leaky cases and dead batteries, only two instruments stopped recording during the study.
The team's data showed that this current exerts a huge influence on waters in the Indian and Pacific Oceans. Not only was it a direct source of dense water for these regions but it was moving extremely large volumes of it. Still, there are many more deep-ocean currents that remain to be studied before scientists fully understand the relationship between ocean circulation and its influence on global climate systems.
In the future, underwater instruments that can gather data for longer periods of time and communicate with researchers throughout a study would save time and money.
"A big advance would be the ability to gather data from instruments by satellite," Rintoul says, "For example, instruments could release glass 'pods' containing electronic recordings of their data. When the pods reach the surface, they could send that data to us in the lab."