Physical Function
Turbulent mixing produced by winds and waves transfers heat downward from the surface. In low and mid-latitudes, this creates a surface-mixed layer of water of almost uniform temperature which may be a few meters deep to several hundred meters deep. Below this mixed layer, at depths of 200–300 m in the open ocean, the temperature begins to decrease rapidly down to about 1000 m. The water layer within which the temperature gradient is steepest is known as the permanent thermocline. The temperature difference through this layer may be as large as 20℃. The permanent thermocline coincides with a change in water density between the warmer, low-density surface waters and the underlying cold dense bottom waters. The region of rapid density change is known as the pycnocline, and it acts as a barrier to vertical water circulation; thus it also affects the vertical distribution of certain chemicals which play a role in the biology of the seas. The sharp gradients in temperature and density also may act as a restriction to vertical movements of animals.
Read more about this topic: Pycnocline
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