Temperature of ocean waters

Ocean water is heated by the sun, like land, but heats and cools more slowly.

Factors affecting ocean temperature:

• Latitude: surface water cools from the equator to the poles as insolation decreases.
• Unequal land and water: northern-hemisphere oceans touch more land and receive more heat than southern ones.
• Prevailing winds: offshore winds (from land to sea) push warm surface water away, causing cold upwelling; onshore winds pile warm water against the coast.
• Ocean currents: warm currents raise temperatures in cold areas, cold currents lower them in warm areas. The Gulf Stream (warm) warms the east coast of North America and the west coast of Europe; the Labrador current (cold) cools the north-east coast of North America.
• Enclosed seas in low latitudes are warmer than open seas; in high latitudes, colder.

Vertical distribution. Temperature falls with depth, rapidly to about 200 m, then more slowly. The ocean's maximum temperature is always at the surface, which receives the sun's heat directly.

• In middle and low latitudes there are three layers: a warm top layer about 500 m thick at 20°-25°C (all year in the tropics, only in summer in mid-latitudes); the thermocline, 500-1,000 m thick, where temperature falls rapidly with depth (the boundary usually begins 100-400 m down); and a very cold deep layer to the ocean floor. About 90 per cent of ocean water lies below the thermocline, near 0°C.
• In the Arctic and Antarctic, surface water is near 0°C, so there is only one cold layer from top to bottom.

Horizontal distribution. Average surface temperature is about 27°C, falling about 0.5°C per degree of latitude: about 22°C at 20°, 14°C at 40°, 0°C near the poles. Northern-hemisphere oceans average about 19°C, southern about 16°C. The highest temperatures are slightly north of the equator, not on it.

Example (new): a diver going down off a tropical reef feels the water stay pleasantly warm for the first part of the dive, then turn noticeably colder within a short distance as she enters the thermocline.