The vertical and horizontal distribution of temperature in the oceans, in which a warm sunlit surface layer overlies a zone of rapid temperature decline (the thermocline) and then a uniformly cold deep layer.
- The ocean has three thermal layers: a warm, well-mixed surface layer; the thermocline, where temperature drops sharply with depth; and the cold, near-uniform deep layer.
- Surface temperature is highest near the equator and falls toward the poles; it also falls with depth, so deep ocean water everywhere is cold (a few degrees Celsius).
- Enclosed seas of the dry subtropics (such as the Red Sea) are warmer at the surface than open oceans at the same latitude; ice-covered polar seas are coldest.
- Temperature, with salinity, controls sea-water density and drives the deep thermohaline circulation; warm surface water is also the fuel for tropical cyclones.
- The thermocline is most marked in the tropics and weak or absent in polar waters; it acts as a barrier between surface and deep water.
The three-layer structure, the thermocline as a sharp temperature drop, the equator-to-pole surface gradient, and the link of warm water to cyclones are recurring oceanography points.
Thermocline (rapid temperature change with depth) versus halocline (rapid salinity change); surface water is warmest at the equator, deep water is cold everywhere; the thermocline is strong in the tropics, weak at the poles.
Warm surface layer, a sharp thermocline, then cold deep water; surface temperature falls from equator to poles and with depth.