Depth at which the rate of decrease of temperature with increase of depth is the largest. In general the sea water temperature decreases from the surface to the deepest levels, except in high latitudes where the configuration can be more complex. There exists in most ocean areas (apart from polar and sub-polar oceans) a zone where the rate of decrease of temperature is much larger compared with that above and below, hence the definition. Depending on the geographical location, the thermocline depth ranges from about 50m to 1000m. A simplified view is to consider the thermocline as the separation zone between the mixed-layer above, much influenced by atmospheric fluxes, and the deep ocean. In the tropics, the thermocline can be quite shallow on average, as in the eastern Pacific (50m), or deeper as in the western part (160-200m). In extra-tropical regions a permanent (or main) thermocline is found between 200m and 1000m. However the thermocline depth varies seasonally, especially in the mid-latitude regions where a secondary and much shallower thermocline (above 50m) occurs in summer. In high latitudes, a thermocline may appear only seasonally. Thermocline can also vary from one year to the next, as in the tropical Pacific where thermocline vertical displacements play a fundamental role during ENSO. As the pycnocline, the thermocline is a prominent feature of the ocean which conditions many physical, chemical and biological processes occurring in the oceanic upper layers. In many situations, the thermocline can be identified with the pycnocline when the vertical contrasts of salinity are small.