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 move the thermocline up and down 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 an unbalance between incoming and outgoing fluxes. A positive feedback is exerted on the polar current around the gyre, whose velocity is proportional and in phase with the oscillation of the thermocline: the excess heat transported to the pole resulting from the acceleration of the western boundary current favors the deepening of the thermocline, which further increases the speed of the polar current (therefore the western boundary current).