The major ocean basins are dominated by large, roughly circular current systems called gyres. Wind patterns and the Coriolis effect together produce subtropical gyres that rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, and smaller subpolar gyres of opposite sense.

Western boundary currents within the gyres are narrow, deep, and fast; eastern boundary currents are broad, shallow, and slow. The subtropical gyres are characterized by convergent surface flow and downwelling in their centers, leading to low nutrient levels and clear blue water. Subpolar gyres are more productive because of upwelling and strong winter mixing.

Gyres organize the distribution of heat, salt, plankton, and floating debris on basin scales. They are visible in satellite data as coherent patterns of sea-surface height and temperature. Changes in wind forcing or in the larger climate system can alter gyre strength and extent, with consequences for regional climate and marine ecosystems.

• Large-scale circular ocean current systems
• Subtropical and subpolar types
• Shape heat transport and productivity patterns
• Visible in satellite sea-surface data