Within the large-scale circulation described in Section 1.3.3, are more localized water movements known generally as eddies. These may be likened to our atmospheric weather systems of cyclones, anticyclones and fronts. Some of the best-known eddies form when the boundary of a strong current such as the Gulf Stream starts to meander rather like a river. As the amplitude of the meanders increases, the current flow may pinch off a meander (like an ox-bow lake along a river) forming isolated rotating eddies of water either side of the current boundary. In the case of the western boundary of the Gulf Stream, warm water eddies pinch off into the cold water outside the Gulf Stream and cold water eddies pinch off into the warm Gulf Stream. Eddies range in size from tens to hundreds of kilometres in diameter and may maintain their identity for some considerable time. A relatively new discovery is the existence of Meddies which are eddies of Mediterranean outflow water which track across the Atlantic but do not come to the surface. Only a few kilometres across, they may last two years or more and move considerable distances (Richards and Gould, 1996; Robinson, 1983).
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