4.1.12 (HL)—Ocean stratification increase

Warmer or fresher surface water is less dense, so a stronger density contrast can suppress vertical mixing.

Observed stratification increases are strongest in the upper 200 m. Surface warming lowers density globally, while Antarctic ice melt can freshen surface water and lower its density further; reduced mixing can limit nutrient supply upward and oxygen renewal at depth.

Meltwater freshens a polar surface layer; if it becomes lighter than deep water, sinking weakens and deep oxygen renewal may fall.

Evidence for stronger stratification combines a larger density contrast with reduced vertical mixing or nutrient and oxygen exchange; warming alone is not the complete test.

Temperature is not the only control of seawater density; salinity can reinforce or reverse the effect.