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A gyre in oceanography is any large system of rotating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl (torque).
Because these huge circular currents are blocked by continents, smaller (though still giant) ocean current circles called gyres are formed. Differences in water density affect vertical ocean currents (movement of surface ocean water to the bottom of the ocean and movement of deep ocean water to the surface). Variations in water density are caused by variations in salinity (the amount of salt in water) and temperature.