The thermocline (gr: thermo = hot and clino = slope) is generally a steep gradient of rapid temperature change, but applied more particularly to a layer, in a body of water, in which the temperature gradient is greater than in the overlying or deeper layers. A thermocline is stable and prevents vertical mixing of the water.

In the oceans at low and middle latitudes permanent thermoclines start at 50-150 m below the surface and can extend down to more than 1000 m. In polar regions the thermocline is generally absent since the ocean surface is covered with ice in winter and solar radiation is small in summer.

Changes in the depth of the thermocline in the Pacific are an important factor in El Nino Southern Oscillation events. Seasonal thermoclines may even occur in shallow bodies of water, such as a lake, when solar heating produces warm surface water.

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