A dive profile

Lower a thermometer from a ship in the tropics and the reading does not fall smoothly. It follows three stages:

1. The mixed layer. Wind and waves stir the top tens of metres, so temperature and salinity are almost uniform. It is typically about 20 to 100 m thick, thicker in stormy places and in winter.
2. The thermocline. Below the mixed layer the temperature drops quickly with depth. In the tropics and mid-latitudes it runs from roughly 100 to 200 m down to about 1,000 m.
3. The deep ocean. Below the thermocline the water is uniformly cold, mostly between about 1 and 4 °C, all the way to the bottom.

The same idea applies to salinity and density. A halocline is a layer where salinity changes quickly, and a pycnocline is a layer where density changes quickly. Because warm water is lighter than cold water, a strong thermocline makes a very stable stack: light water on top, dense water underneath, and little mixing across the boundary. That keeps nutrients from the deep away from the sunlit layer, which is one reason much of the tropical ocean is clear and blue but poor in plankton.

Near the poles the picture changes. The surface is already cold, so there is little thermocline and deep water can rise and mix much more easily.

Can deep water freeze? Not in the open ocean. Salt lowers the freezing point: sea water with a salinity of 35 g/kg freezes at about −1.9 °C, not at 0 °C. Deep water at 0 to 4 °C is liquid, and under great pressure it stays liquid even more easily. Sea ice forms only at the surface, when the top layer is cooled below its freezing point.