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Six great circuits are found in the world ocean, four in the southern hemisphere (South Atlantic, South Pacific, Indian, and Antarctic Circumpolar) and two in the northern (North Atlantic, North Pacific). The gyres correspond fairly well to the biogeographic distribution of oceanic species.

With the exception of the Antarctic Circumpolar Current (ACC), which is not technically a gyre since it flows uninterrupted around the Antarctic continent, all gyres flow around the continental margins (Figure 1.5). The circuits are bounded on the east and west by eastern and western boundary currents and on the north and south by transverse currents. The Gulf Stream in the North Atlantic is an excellent example of a western boundary current and the Canary Current, along west Africa, of an eastern boundary current. Partly because of the convergence of the westerly winds, western boundary currents are far stronger and more clearly defined than their counterparts in the east. Additional contributors to the greater strength of western boundary currents are the fact that the “hill of water” at the heart of the geostrophic circulation is displaced to the west by the eastward rotation of the Earth, creating a steeper pressure gradient and a stronger poleward flow of water. The phenomenon is termed “westward intensification.” Table 1.2 gives a comparison of the characteristics of eastern and western boundary currents.