Rings and Eddies

The flow of the Gulf Stream parallels the coast of the United States from the Straits of Florida to Cape Hatteras, North Carolina. Beyond Cape Hatteras, the Gulf Stream begins to flow away from the coast. As the Stream flows away from the eastern coast of the United States, it separates the cold Slope water to the north from the warm Sargasso Sea to the south. The Gulf Stream then flows into deeper water where it is free from bathymetric and topographic constraints. It is now capable of meandering over distances as great as 350 km. These meanders give rise to what is known as "Gulf Stream rings" or eddies.

The eddies which form from the Gulf Stream are called mesoscale eddies ('meso' is a prefix meaning intermediate). They have diameters of 50 to 200 km and can last anywhere from a month to several years. These eddies travel slowly, perhaps only a few kilometers per day. Mesoscale eddies usually move westward and/or toward the equator. Their duration is largely dependent on the path they follow. The fastest currents in an eddy occur at the surface, but they persist down to a depth of at least 1500 meters.

Eddies are most commonly produced in regions with horizontal density differences. These regions include boundaries between water masses. For example, a distinct boundary exists between the warm water of the Gulf Stream and the water around it. These boundaries are unstable. When a vertical velocity shear exists, waves form on the boundaries and may evolve into eddies.

Eddies play an important role in the transport and redistribution of heat and salinity from one water mass to another. The stirring motion associated with eddy flow acts to homogenize the water characteristics within the water column. During the life span of an eddy, it constantly exchanges energy, heat, water, nutrients, and organisms with its surroundings. Eddies also play an important role in exchanging heat and water with the atmosphere. On a larger scale, eddies can transfer water characteristics between oceans. Eddies similar to Gulf Stream rings have been observed forming from the Agulhas Current, the southward flowing current on the eastern side of Africa. These eddies may be important agents in the transfer of water between the Indian and Atlantic Oceans.

In the Gulf Stream, eddies form from meanders in the current. They then pinch off to form their own independent circulatory system, known as a "ring." Large southward meanders can separate from the Stream, trapping cold water in the Sargasso Sea to form a cold core ring. Conversely, large meanders to the north can trap warm water in the Slope water to form a warm core ring. Once formed, the rings drift westward until reabsorbed into the Gulf Stream.

Cold core eddies may extend as deep as the sea floor, about 4000 to 5000 meters. Warm core eddies usually do not extend as deep, although they do extend deep enough to intrude on the continental slope and rise as they drift to the west. Warm core eddies can last anywhere from a few months to a year. Cold core eddies tend to last longer, sometimes as long as a few years. Cold core eddies usually range in diameter from 150 to 300 km. Warm core eddies are slightly smaller, having typical diameters of 100 to 200 km. At any one time, as much as 15% of the Sargasso Sea and 40% of the Slope water may be covered by cold and warm core eddies, respectfully.