The thermocline is the zone of steep thermal transition between surface and deep waters. In the seas and oceans, almost all the sunlight striking the surface is absorbed by the surface layer of water which then heats. The wind and waves move the water, distributing heat so nearly uniform on the first tens of meters deep. If convection or mixing are low, the upper layer, less dense and warmer, floats on the cold underlying layer. Through this thin transition zone that is the thermocline, the temperature drops very quickly, by 20°C in tropical oceans. Below the thermocline, the temperature continues to drop with depth but much slower.
Solar forcing tends to swing the thermocline to the rhythm of different cycles: it deepens during a phase of high solar activity when surface water warms. In contrast it rises when solar activity wanes because the thickness of the warm surface layer is reduced as resulting from a balance between incoming and outgoing fluxes.