Definition - What does Thermocline mean?
As the depth of water increases, temperatures decrease, but there is a distinct layer in which temperatures drop much more quickly with increasing depth than they do in the layers of water above or below it, which is known as a thermocline. Thermoclines may vary depending on seasons, tides, currents, latitude and water movement as a result of wind, tides and currents.
Safeopedia explains Thermocline
Thermoclines occur in both oceans and lakes or reservoirs. The upper layer of water is relatively warm and water heated by the sun mixes with cooler water. When the thermocline is reached (in oceans this is usually at a depth of 660 – 3,000 feet) the temperature of the water decreases rapidly with depth. Below the thermocline, temperatures drop much more slowly. In warm seasons, the thermocline is at its shallowest and in winter, reduced sunshine and increased mixing of surface water through storms causes the thermocline to drop to lower levels.