The thermocline is the layer of sea water in which the temperature changes rapidly (≥1°C/100 m) with distance from the surface. The surface water temperature is controlled by radiative heating (from the sun) or cooling (to the night sky) and convective heating or cooling from/to the atmosphere; currents and wave action keep this layer well mixed to a depth of 50 to 100 m. The cold deep water originates in the Arctic and Antarctic regions and flows slowly in the general direction of the Equator, though there are many complex interactions with other currents and land masses. The thermocline lies between the warm surface water and the cold deep water and may have a thickness of 300 to 1,000 m.
The thermocline structure in the major ocean basins can be broadly thought of as a "permanent" or "long-term mean" temperature profile with a superimposed seasonal variation that mainly affects the upper mixed layer, including its thickness. The actual profile varies greatly from location to location. In shallower waters, the seasonal variation is more pronounced and may dominate the profile. At the higher latitudes, the temperature difference is much smaller and the thermocline disappears altogether above about 60° latitude.
Another phenomenon that affects the water density and therefore the stability of the thermocline structure is the dissolved salt concentration: Warmer surface waters may have a sufficiently higher salt concentration that they have a higher density, with vertical mixing occurring.
Pond, S. and Pickard, G. L. (1983) Introductory Dynamical Oceanography, 2nd edn., Pergamon Press, Oxford.