Woods Hole (Bernd Laeschke – Mai 2011): Massive, swirling ocean eddies that can measure up to 311 miles (500 kilometers) across at the surface have been discovered by researchers from Woods Hole Oceanographic Institution (WHOI). Eddies, or reverse currents, are created when fluid flows past an obstacle. They can reach all the way to the ocean bottom at mid-ocean ridges, some 8,200 feet (2,500 meters) deep, transporting tiny sea creatures, chemicals, and heat from hydrothermal vents over large distances.
The previously unknown deep-sea phenomenon, reported in the journal Science, helps explain how some larvae travel huge distances from one vent area to another. “We knew these eddies existed,” said Diane K. Adams, lead author at WHOI and now at the National Institutes of Health. “But nobody realized they can affect processes on the bottom of the ocean. Previous studies had looked at the upper ocean.”
Using deep-sea moorings, current meters and sediment traps over a six-month period, along with computer models, Adams and her colleagues studied the eddies at the underwater mountain range known as the East Pacific Rise. That site experienced a well-documented eruption in 2006 that led to a discovery reported last year that larvae from as far away as 217 miles (350 kilometers) somehow traveled that distance to settle in the aftermath of the eruption.
The eddies are generated at the surface by atmospheric events, such as wind jets, which can be strengthened during an El Niño, and “are known to have a strong influence on surface ocean dynamics and production,” say Adams and Dennis J. McGillicuddy from WHOI. But this “atmospheric forcing…is typically not considered in studies of the deep sea,” they report.
Moreover, the eddies appear to form seasonally, suggesting repeated interactions with undersea ridges such as the Eastern Pacific Rise. The models “predict a train of eddies across the ocean,” Adams said. “There may be two to three eddies per year at this location,” Adams said. Each one “could connect the site of the eruption to other sites hundreds of miles away. There are numerous places around the globe where they could be interacting with the deep sea.”
In her 2010 report on larvae traveling great distances to settle at the eruption site, WHOI Senior Scientist Lauren S. Mullineaux, along with Adams and others, suggested the larvae traveled along something like an undersea superhighway, ocean-bottom “jets” travelling up to 10 centimeters a second. But conceding that even those would not be enough to carry the larvae all that distance in such a short time, the researchers speculated that large eddies may be propelling the migrating larvae even faster.
“Transport [of ocean products] could occur wherever…eddies interact with ridges - including the Mid-Atlantic Ridge, the Southwest Indian Ridge, and the East Scotia Ridge - and the surrounding deep ocean,” the researchers say. The eddies appear to form repeatedly, and the high-speed, long-distance transport can last for months. “Although the deep sea and hydrothermal vents in particular are often naively thought of as being isolated from the surface ocean and atmosphere, the interaction of the surface-generated eddies with the deep sea offers a conduit for seasonality and longer-period atmospheric phenomena to influence the ‘seasonless’ deep sea,” the team writes. “Thus, although hydrothermal sources of heat, chemical and larval fluxes do not exhibit seasonality, there is potential for long-distance transport and dispersal to have seasonal to inter-annual variability.”