Density gradients in the oceans
Distinguish surface zone, pycnocline zone, & deep zone
Near equator, strong thermocline & strong halocline lead to well-developed pycnocline
In subtropics, strong thermocline leads to well-developed pycnocline even though surface water has higher salinity than water beneath it
In mid-latitudes, halocline & thermocline form in summer, leading to a seasonal pycnoclines
Near poles, find cold, highly saline water at surface - pycnoclines are rare