2.3 Density & Stratification
The Driver of Ocean Circulation
Density differences drive the thermohaline circulation. Seawater density depends on temperature, salinity, and pressure. Stratification—the layering of water by density—controls vertical mixing and the distribution of heat, nutrients, and dissolved gases.
Equation of State
$$\rho = \rho(T, S, p)$$
Temperature Effect
Warmer = less dense (usually). Dominates in tropics.
Salinity Effect
Saltier = more dense. Dominates at high latitudes.
Pressure Effect
Higher pressure = slightly more dense. ~4.5% increase at 10km.
Sigma-t (σₜ)
$$\sigma_t = \rho(T, S, 0) - 1000 \text{ kg/m}^3$$
Potential density referenced to surface pressure. Typical range: 23-28 kg/m³.
Ocean Stratification
Surface Mixed Layer
Top 20-200m. Uniform T, S, density. Mixed by wind and waves. Seasonal variation.
Pycnocline
Zone of rapid density increase. Contains thermocline (T) and halocline (S). Barrier to vertical mixing.
Deep Layer
Below ~1000m. Cold, uniform. Slow circulation. Water age: 100s-1000s years.
Stability & Mixing
Brunt-Väisälä Frequency
$$N^2 = -\frac{g}{\rho}\frac{d\rho}{dz}$$
N greater than 0: Stable (stratified). N² less than 0: Unstable (convection).
Stable Stratification
Light water over dense. Resists vertical mixing. Typical of tropics.
Convective Overturning
Dense water forms at surface (cooling/evap). Sinks. Creates deep water.