About 70% of the Earth's surface is sea water and the ocean currents have a major influence on climate and weather. Surface currents are produced by the prevailing winds and follow the same general directions. They play a major role in transferring heat from the tropics to the polar regions. Large surface circulation systems are known as gyres. In addition to surface currents, there is also slow constant upwelling (upwards movement) of cool water rising and mixing with warmer water near the surface.

Land surfaces are poor conductors of heat; their temperature increases rapidly in sunshine and decreases equally quickly at night. The sea is different with little change of surface temperature from day to night except where it is exceptionally shallow. Because of this, the sea warms up and cools down more slowly than the land, creating temperature contrasts at different times of year. These contrasts produce local land and sea breezes along coastlines.

On a larger scale the sea acts as a reservoir of heat from the summer, keeping coastal regions milder in the autumn than regions inland. In summer, it warms up slowly providing cooling sea breezes keeping temperatures near coasts below those inland. On a global scale, temperature contrasts are responsible for the effect known as continentality. The inland areas of continents tend to have a much greater temperature variation than coastal areas, where the influence of the sea produces a much smaller variation.