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pycnocline
A pycnocline is the cline or layer where the density gradient () is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, terms of temperature and salinity differences, wind, Coriolis effect, and tides caused by the gravitational pull of the Moon and the Sun. In addition, the physical properties in a pycnocline driven by density gradients also affect the flows and vertical profiles in the ocean. These changes can be connected to the transport of heat, salt, and nutrients through the ocean, and the pycnocline diffusion controls upwelling.〔1. Anand Gnanadesikan. 1999. A simple predictive model for the structure of the oceanic pycnocline. ''Science'' 283 (5410): 2077–2079.〕
Below the mixed layer, a stable density gradient (or pycnocline) separates the upper and lower water which hinders vertical transport.〔2 Mann and Lazier (2006). Dynamics of marine ecosystems. 3rd edition. Blackwell Publishing. Chapter 3.〕 This separation has important biological effects on the ocean and the marine living organisms. However, vertical mixing across pycnocline is a regular phenomenon in oceans, and occurs through shear-produced turbulence.〔(Turbulent Mixing in Stratified Fluids, Annual Review of Fluid Mechanics (1991) )〕 Such mixing plays a key role in the transport of nutrients.〔(Vertical Mixing and Transports through a Stratified Shear Layer, Journal of Physical Oceanography (2001) )〕
== 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.〔3. Knauss, John A. (1997). Introduction to Physical Oceanography. 2nd edition, Prentice-Hall. Chapter 1〕 The temperature difference through this layer may be as large as 20℃, depending on latitude. 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.〔4. Lalli and Parson (1993). Biological oceanography: an introduction. Pergamon press. Chapter 2.〕
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