A thermocline is a distinct layer in a body of water, such as an ocean or a lake, where the temperature gradient changes rapidly with depth. It typically occurs in the lower layers of water, separating the warmer upper layer, called the epilimnion (or mixed layer), from the colder deeper layer known as the hypolimnion.
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A water layer across which temperature changes rapidly with depth. In a reduced gravity description of the upper ocean, its displacement is represented by the interface between warm surface water and colder deep water. Deepening the eastern tropical Pacific thermocline makes upwelling less effective at cooling the surface during El Niño.