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A circular or spiral motion of water, the term usually being applied to a semi-closed current system. A major gyre exists in each of the main ocean basins, centred at about 30° from the equator and displaced towards the western sides of the ocean (‘western intensification’). Gyres are generated mainly by surface winds and move clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. Gyres form part of the global system of oceanic circulation (see great conveyor), which has an important function in the redistribution of energy around the world, especially from the tropics towards the poles. In the North Atlantic, the gyre was strongly affected around 13 000 years ago when fresh water draining from what is now the St Lawrence River formed a layer floating above the denser sea water. This suppressed the northward movement of the Gulf Stream (the North Atlantic Current or North Atlantic Drift). The outcome was a sudden cooling of northern Europe leading to the Younger Dryas stadial.

Subjects: Ecology and Conservation.

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