Why the Ocean Forms Layers: Stratification Explained

Ocean layering (stratification) arises from density differences driven by temperature (thermocline) and salinity (halocline).

The Mechanism

• Surface Layer (Mixed Layer): Warmed by sun, mixed by wind/waves. Lower density.
• Thermocline: Rapid temperature drop with depth. Density increases sharply.
• Deep Layer: Cold, dense, relatively uniform. Salinity also increases density (halocline).

Consequences

• Barrier to Mixing: Stratification inhibits vertical exchange of heat, oxygen, nutrients, and carbon.
• Biological Impact: Nutrients trapped below thermocline limit surface productivity; upwelling zones break stratification, creating fisheries.
• Climate Role: Ocean stores 90% of excess heat; stratification affects heat uptake and circulation (e.g., AMOC).
• Seasonal Cycle: Winter cooling/storms erode stratification (deep mixing); summer heating restores it.

Climate change is strengthening stratification globally, reducing oxygen, altering ecosystems, and potentially weakening carbon sequestration.