Oceans Aswirl: Massive Eddies Influence Earth's Climate, Marine Ecosystems, Even Big Business
Perkins, Sid, Science News
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. …