Pycnocline is a zone in the oceans where water density increases rapidly with depth in response to changes in temperature and salinity.
Except at high latitudes, the ocean is divided into three horizontal depth zones based on density: the mixed layer, pycnocline, and deep layer. At high latitudes, the pycnocline and mixed layer are absent.
The pycnocline, situated between the mixed layer and the deep layer, is where water density increases rapidly with depth because of changes in temperature and/or salinity.
About 90% of the Earth’s oceans exist below the pycnocline.The ocean’s pycnocline is very stable thus suppressing mixing between the mixed layer and deep layer; that is, the pycnocline acts as a barrier to vertical motion within the ocean.
Typically, the pycnocline extends to a depth of 500 to 1000 m . The dark, cold deep layer below the pycnocline accounts for most of the ocean’s mass. Within the deep layer, density increases gradually with depth and water moves slowly; in only a few locations (usually near the bottom) are water movements fast enough to be considered currents.
The pycnocline encompasses both the halocline (salinity gradients) and the thermocline (temperature gradients) .Because density is a function of temperature and salinity, the pycnocline is a function of the thermocline and halocline.
Where a decline in temperature with depth is responsible for the increase in density with depth, the pycnocline is also a thermocline. On the other hand, if an increase in salinity is responsible for the increase in density with depth, the pycnocline is also a halocline.
Because temperature tends to be the dominant factor influencing seawater density, the depth range and base of both the pycnocline and thermocline often tend to be similar. Therefore, below the pycnocline or thermocline, temperature and salinity are relatively constant.