Solar heating warms the surface layer, creating a density gradient that resists vertical mixing. The thermocline — where temperature drops sharply — acts as a barrier between the warm, nutrient-depleted epipelagic zone and cold, nutrient-rich deep water.
Where surface waters diverge (coastal upwelling zones, equatorial upwelling), cold, nutrient-rich deep water rises to replace them. These regions — California Current, Peru Current, Benguela — are among the most biologically productive on Earth.
Hurricanes and winter storms can mix the thermocline, deepening or temporarily destroying it. The resulting nutrient injection fuels phytoplankton blooms detectable by satellite chlorophyll sensors weeks after a storm passes.
A warming ocean increases stratification, making the thermocline shallower and stronger. This shoaling suppresses nutrient upwelling, potentially reducing global ocean primary productivity by 2–16% by 2100.