A vertical temperature gradient, in some layer of a body of water, that is appreciably greater than the gradients above and below it; also a layer in which such a gradient occurs. The permanent thermocline refers to the thermocline not affected by the seasonal and diurnal changes in the surface forcing; it is therefore located below the yearly maximum depth of the mixed layer and the influence of the atmosphere. The seasonal thermocline refers to the thermocline not affected by the diurnal changes in the surface forcing. In general, it is established each year by heating of the surface water in the summer, and is destroyed the following winter by cooling at the surface and wind-driven mixing. The diurnal thermocline refers to the thermocline that, in general, is established each day by heating of the surface water and is destroyed the following night by cooling and/or mixing. See Also transition layer.