Thermocline

The thermocline is the vertical zone of rapid temperature decrease separating the warm surface mixed layer from the cold deep ocean. A seasonal thermocline forms and erodes each year in mid-latitudes, while the permanent (main) thermocline sits at roughly 200 to 1,000 m. Because temperature dominates density in most of the ocean, the thermocline usually coincides with the pycnocline and acts as a barrier to vertical mixing and nutrient supply.