Spiral Eddies

General Discussion

Of all the ocean features discovered by observations from manned spacecraft, the mesoscale, rotating, "spiral" eddy is probably the most important. The sea-surface slicks that are incorporated in the eddies, permitting them to be observed, have widths too narrow to be resolved by the visual sensors on operational environmental satellites. There is a thermal contrast between the slicks and the surrounding rotating water, but it, too, is not detectable with present day satellite imagery techniques.

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Southwest
Indian Ocean
Equatorial
Indian Ocean
Philippine Sea
west of Guam
Southern
Philippine Sea

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Pacific, east
of Marianas
Pacific, northeast
of Hawaii
Caribbean SeaPlankton
sun glint
North Atlantic

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Caspian SeaSanta Catalina
Island, Gulf
of California
Southern
Hemisphere
Indian Ocean
Indian Ocean
front

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Spin-off eddy
Mayotte Island
Mozambique
Channel
Gulf of Oman
(1)
Gulf of Oman
(2)
In-line spirals
eastern
Mediterranean

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Atlantic coast
New Jersey to
North Carolina

      The manifestations of the eddies at the sea surface have been imaged by the SARs on SEASAT and the space shuttle. These systems are not operational, however, so it has remained for the astronauts aboard the space shuttle to acquire information on spiral eddies (Stevenson, 1985).
      It is well known that ocean currents, such as the Gulf Stream and the Kuroshio, meander through the ocean at wavelengths of 300-400 kilometers. The meanders can separate from the main current to form closed circles of distinct water, called rings. Gulf Stream rings up to 200 kilometers in diameter form as cold, cyclonic eddies south of the current, and warm, anticyclonic eddies routinely occur on the north side of this North Atlantic current (Lai and Richardson, 1977). Similar sets of rings form on either side of the Kuroshio Current northeast of Japan in the Pacific Ocean. Cyclonic Gulf Stream rings have been tracked by ships, aircraft, satellites, and drifting buoys for as long as three years. Most, though, have a much shorter life cycle.
      Eddies larger than 30 kilometers in diameter are said to be synoptic. There are four types of synoptic eddies, based on their modes of formation: (1) frontal eddies, or rings, (2) free eddies in the open ocean, (3) topographic, or island/headland, eddies, and (4) atmospheric-induced eddies (Kamenkovich et al., 1982).
      The eddies and vortices in which oceanographers have the most interest are those that form in the open ocean, apparently from horizontal shearing in the upper layers of the sea. Observations from the space shuttle, beginning with STS- 1, have shown that eddies less than synoptic in scale—smaller than 30 kilometers in diameter—exist throughout the ocean. These eddies are typically spiral in form and always rotate cyclonically.
      Until shuttle mission 41G, U.S. space oceanographers thought spiral eddies were a singular and rare oceanic phenomenon. Their presence in the Mediterranean Sea and the western Atlantic Ocean, as observed and photographed during that flight, seemed to make it clear, however, that they are major dynamic features formed along horizontal stress boundaries (ScullyPower, 1986).
      Since October 1984, spiral eddies have been observed in many regions of the world's oceans where ocean currents produce little or no horizontal stress. They seem to be ubiquitous no matter what the flow of the upper ocean. The one area of the ocean where no spiral eddies have been observed or caught in photographs is in the waters immediately either side of the equator, that is, within six degrees north and south latitude.
      There is as yet no useful explanation for the origin of oceanic spiral eddies, nor is there information on their life histories and persistence. It is known that they are always cyclonic, do not occur in counter- rotating pairs, have rotational speeds of from a half to more than three kilometers an hour, and extend 50 to 300 meters below the surface of the sea.
      Observations and photography of spiral eddies are best where biologically produced surfactant films are spread widely over the sea surface. These oily films may be formed by hydrocarbons, fatty acids, ali- phatic alcohols, or any combination of such products. Chemically pure surfactants tend to form monomolecular films in the laboratory. In real conditions on the ocean surface, however, the films are mixtures of different surfactants. As a result, their thicknesses are usually on the order of several effective molecular diameters (Monin and Krasitskiy, 1985).
      Surfactant films are quite different from those created by petroleum products, in which the thickness is many orders of magnitude greater than the more widespread natural films. In contrast to the surfactants, films of petroleum origin do not spread across the sea surface but tend to remain in clumps and patches.
      One of the clearest manifestations of surfactant films on the ocean's surface is their role in the extinction of capillary-gravity waves. In reducing (or eliminating) these waves, they are responsible for the formation of slicks on the surface, especially where there are convergences in the upper layers of the ocean, no matter how minor. Reduction of the capillaries in the slick causes light-reflection anomalies, the slick appearing either lighter or darker than the surrounding water, depending on the angle of vision, the height of the sun, the cloud cover, and the development of wind waves in the area being observed.
      Surfactant films provide a sea surface on which the kinematics of spiral eddies can be readily observed. Not all of the ocean's surface is covered by surfactants, yet even in their absence spiral eddies have been observed. As a fallout from the study of spiral eddies, the recognition of surfactant fllms from the shuttle indicates a far greater amount of biological activity in the surface layers of the sea than had previously been known.

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