Density stratification
Also called: Stratification · Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Class 11, Ch 12 "Water (Oceans)"; Class 11, Ch 13 "Movements of Ocean Water"
Meaning
Density stratification is the layering of ocean water by density, which is how heavy water is for its volume. Saltier water is denser, and warmer water is lighter. So cold, salty water sinks below warm, fresh water. These layers resist mixing. The layer where density changes sharply between them is called the pycnocline. Because little mixing happens, tropical surface water runs short of nutrients, so these seas are "blue deserts". Global warming is strengthening the layering, which reduces mixing and oxygen in the water.
Example
At the Strait of Gibraltar, which links the Mediterranean to the Atlantic, the Mediterranean's dense salty water flows out along the bottom. At the same time, lighter Atlantic water flows in at the surface above it.
Don't confuse with
- Upwelling: upwelling breaks the layering by bringing cold, nutrient-rich deep water to the surface. Stratification keeps the layers apart.