Oceans aswirl: massive eddies influence Earth's climate, marine ecosystems, even big business

Science News, June 14, 2003 by Sid Perkins

The hallmarks of rotation are written all over the ocean. Huge currents flowing past islands and peninsulas generate enormous swirls in their wake, occasionally casting off giant whirlpools. Currents meandering across the open ocean can also shed massive, long-lasting eddies. Just as the atmosphere's large- and small-scale motions mix the air, the ocean's hierarchy of eddies blends cold waters with warm, the nutrient-rich with the nutrient-poor, and the salt-laden with fresher waters.

In the process, these massive swirls--many of them hundreds of kilometers across--transport some of the ocean's heat from tropical climes to higher latitudes and create biological oases vast enough to be visible from space. What's more, eddies can influence weather across a wide region, and their currents can have effects ranging from disrupting operations at deep-sea oil platforms to influencing the outcomes of long-distance yacht races.

Too large for earthbound scientists to recognize directly, researchers wielding ever more powerful computer models, informed by data collected from ships, are beginning to account for the effects of eddies on Earth's oceans and climate.

CHURN IT UP When the marine microorganisms that form the base of the ocean's food chain die, they often sink to the seafloor, carrying nitrates, organic carbon, iron, and other nutrients with them. Therefore, large portions of the ocean's surface--especially those that lie over deep waters--can lack basic chemical ingredients required for life to flourish.

In some areas, particularly along the western edges of continents, strong currents usher nutrient-rich waters to the surface. In the open seas, however, it's typically ocean eddies that scoop those vital substances back from the abyss.

Cyclonic eddies--those that rotate in the same direction as a cyclone, which is counterclockwise in the Northern Hemisphere and in the opposite direction in the Southern Hemisphere--usually bring relatively cold, nutrient-filled waters to the ocean's upper, sunlit layers, where phytoplankton can proliferate. These so-called cold-core eddies fuel a population explosion among zooplankton, shrimp, fish, squid, and other aquatic species higher up the food chain, says Douglas C. Biggs, an oceanographer at Texas A&M University in College Station. "A cold-core eddy can be a real biological hotspot," Biggs notes.

The centers of anticyclonic, warm-core ocean eddies typically are zones of downwelling and therefore are nutrient-deficient. However, fluid friction along the edges of these whorls can create counter-rotating eddies that bring cool, nutrient-rich waters to the surface. Also, the swirling action of warm-core eddies can entrain and concentrate cooler waters--and their biological inhabitants--from surrounding ocean regions.

These phenomena are well known among the crews of fishing trawlers, who often seek out eddies to maximize their catch. Some of the ocean's top nonhuman predators do the same thing, says Bruce Mate, an oceanographer at Oregon State University's Hatfield Marine Science Center in Newport.

He and his colleagues tagged and tracked several right whales in the shallow waters off Nova Scotia during the early 1990s. At one point during a feeding season, one of the whales left the group, swam offshore to a warm-core eddy 360 kilo meters southeast of Cape Cod, and fed along its edge for 6 days. The eddy was pulling relatively cool waters south from the Gulf of Maine, and the right whale gorged on the abundant copepods that were being funneled into a narrow zone at the eddy's edge, says Mate.

Now, Mate is part of a team that since last July has been tracking 18 radio-tagged sperm whales--a small part of the estimated 1,000 or so sperm whales that live in the northern portions of the Gulf of Mexico. One of the team's hypotheses is that these whales often forage within eddies or along their edges. A third of the tags, which cost about $5,000 each, are still active. When the last tag falls silent later this summer, says Mate, the researchers will analyze the whales' movements with respect to the position and movement of eddies, among other ocean phenomena.

Most large eddies in the Gulf of Mexico are shed from the Loop Current, which gets its name from the path it takes across southeastern portions of the Gulf. After this current passes through the Yucatan Strait between Mexico and Cuba, it heads north toward Louisiana and then loops back to the east and passes south of Florida. At irregular intervals of 3 to 17 months, great eddies with warm-water cores spin off the Loop Current and drift westward across the Gulf. These eddies, which range up to 400 km across and have a clockwise rotation, are some of the largest in the world, says Robert R. Leben, an oceanographer at the University of Colorado in Boulder. The rotating currents generated within the eddies can flow at up to 4 knots, or 2 meters per second.

Slow as that sounds, it's more than strong enough to hamper operations at deep-sea oil-drilling platforms that dot the northern portions of the Gulf, says Leben. Some procedures, such as laying pipeline on the seafloor or positioning a ship that's drilling for oil, can be performed in steady currents of up to 2 knots. However, Leben notes, faster currents or chaotic flows can damage equipment or lead to accidents. Because the eddies progress across the Gulf at speeds of only 2 to 5 kilometers per day, a single eddy that stretches hundreds of kilometers may shut down operations at a particular location for weeks at a time.