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 kilometers 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 Yucatn 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.
Leben operates a Web site that monitors the positions of Gulf eddies. Because thermal expansion of the water makes a warm area mound higher than the cool waters surrounding it, satellites that use radar and other methods to measure sea level with an accuracy of a few centimeters can spot the warm-core eddies as broad bumps in the ocean. Such overhead observations also enable scientists to estimate the speeds of the currents associated with the eddy. Higher mounds tend to generate stronger currents.
Information on eddies has come in handy for sports enthusiasts. A team competing in a recent cross-Gulf yacht race consulted Leben about how to use the data on his Web site to predict the currents that boats would encounter. It might have helped, says Leben, because the savvy crew won the competition.
When to spin
Although scientists don’t know what phenomena trigger the Loop Current to shed eddies, computer models are identifying possible players.
Hyun-Chul Lee and Lie-Yauw Oey, oceanographers at Princeton University, have developed a computer simulation of the Loop Current and those currents that flow along the northern coast of South America. The model looks at the effects of the currents’ temperature and salinity, the wind patterns across the region, and the presence of eddies already in the Gulf.
Under average conditions, with no eddies or winds present, the simulated Loop Current sheds an eddy about once every 9 months, says Lee. Seasonal changes in water temperature and salinity don’t seem to affect this rate. However, month-to-month variations in wind patterns cause the time between eddy shedding to fluctuate between 4 and 12 months. Lee and Oey reported results of their simulations at the fall meeting of the American Geophysical Union (AGU) in San Francisco last December.
The researchers’ model also showed that when winds aren’t taken into consideration, eddies already swirling in the Gulf tend to extend the duration between eddy sheddings by several months, so intervals then range from 9 to 15 months.
The results correspond qualitatively with those of a computer model developed by Jorge Zavala-Hidalgo, an oceanographer at Florida State University in Tallahassee. His simulations show that large cyclonic eddies occasionally form in the northern portions of the Gulf and block the northward intrusion of the Loop Current. This, in turn, prevents the current from shedding a warm-core eddy, says Zavala-Hidalgo. In such circumstances, according to the model, the current instead spins off a series of small, cold-core eddies that move along the western coast of Florida. Zavala-Hidalgo also presented his findings at last fall’s AGU meeting.
Both Zavala-Hidalgo’s and Lee and Oey’s models seem to closely mimic ocean observations. The longest known period between eddy sheddings by the Loop Current, the 19 months from February 1998 to August 1999, occurred when a large eddy remained in the northern Gulf for several months.
The chemistry of the Gulf’s water, not just its motions, is affected by these eddy dynamics. For example, the interaction between eddies intermittently shed by the Loop Current and the seasonal variation of fresh water dumped into the Gulf of Mexico by the Mississippi and other rivers affects the overall salinity of water in the Gulf, says Zavala-Hidalgo. In the summer, winds predominantly blow toward the east. If an eddy then forms just off the Mississippi delta, it can entrain low-salinity water and transport it intact along the Florida’s western coast as far south as the Florida Keys.
Weather or not
Because the temperature of the water trapped in an eddy often differs substantially from that of the surrounding water, the big swirl can often significantly influence local weather. When a coastal eddy stalls off Los Angeles, for example, it can reduce temperatures and increase rainfall in the metropolitan area. Elsewhere, moisture evaporating from warm-core eddies that are shed into the North Atlantic by the Gulf Stream can produce a thick ocean fog if the moisture drifts back over cold water and condenses.
The heat-carrying capacity of eddies can transport thermal energy from southern oceans to Antarctica. Today, the surface waters surrounding that ice-covered continent are almost completely isolated from other oceans by a circumpolar current that races forever eastward along a latitude unblocked by any landmasses.
Each year, warm-core eddies shed from this so-called Antarctic Divergence Zone transfer southward the equivalent of about 0.3 petawatt of power, which is approximately half the amount of power delivered annually to northern Europe by the Gulf Stream, says John Marshall, a planetary scientist at the Massachusetts Institute of Technology. Even though this power is spread out along the entire 22,000-km distance traveled by the circumpolar current, it’s still enough to moderate Antarctica’s frigid climate somewhat, perhaps raising the temperature by as much as 1C in coastal regions, Marshall notes.
Like their counter-rotating cohorts in the Northern Hemisphere, the cold-core cyclonic eddies surrounding Antarctica can concentrate biological activity. In one example, satellite images of the ocean near the Antarctic coast south of Australia in early December 2001 showed chunks of sea ice being swept into eddylike swirls about 150 km across, says Stephen R. Rintoul, an oceanographer at the Commonwealth Scientific and Industrial Research Organization in Hobart, Australia. By late December, the ice had disappeared, but on-site measurements by Rintoul and his ship-faring colleagues showed that the amount of chlorophyll in the surface waters of the eddies had jumped to as much as 15 times what it had been 2 months earlier. Satellite and shipboard observations indicated that the phytoplankton continued to proliferate until late January 2002 but then died out by early March. Rintoul and his colleagues report their analyses in the May 1 Geophysical Research Letters.
The ships didn’t take enough data to determine whether the eddy-related biological bounty resulted from upwelling of nutrients or influx of nutrients from coastal waters, Rintoul says. Supporting the second scenario, satellite images suggest that small quantities of phytoplankton–and dissolved iron–were swept out to sea from shallows and transported northward by eddies. The blooms might have been later fueled by increasing sunlight as the summer days lengthened.
Future cruises through eddies in the region may determine the factors that stimulate the plankton blooms. This is just the sort of real-world data gathering that researchers need to refine their computer models of eddies and the swirling waters’ multiple effects on phenomena ranging from local plankton populations to the planet’s climate.
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