4.3 Salinity, temperature, and density relationships

Seawater's properties shape ocean dynamics. Density, driven by salinity and temperature, controls global circulation patterns and determines how stable the water column is. Understanding these relationships is central to grasping how oceans move and mix.

T-S diagrams and salinity changes help identify water masses and trace their origins. These tools reveal how processes like evaporation, precipitation, and freezing impact ocean circulation, nutrient distribution, and marine ecosystems worldwide.

Seawater Properties and Dynamics

Density in seawater

Density is mass per unit volume, and for seawater it typically ranges from about 1020 to 1030 kg/m³. Two variables dominate density changes: salinity (increasing it raises density) and temperature (increasing it lowers density). Pressure also matters, especially at depth.

The Equation of State of Seawater captures all three influences:

$\rho = \rho(S, T, p)$

where $\rho$ is density, $S$ is salinity, $T$ is temperature, and $p$ is pressure. This isn't a simple linear formula; it's an empirically derived relationship that accounts for the complex behavior of dissolved salts in water.

Two coefficients describe how density responds to changes in each variable:

• The thermal expansion coefficient quantifies how much density decreases per degree of warming. Warmer water expands and becomes less dense.
• The haline contraction coefficient quantifies how much density increases per unit of added salinity. Saltier water is more compact and dense.

A key detail: the thermal expansion coefficient itself changes with temperature. Cold water near 2–4°C is relatively insensitive to temperature changes, meaning salinity dominates density at high latitudes. In warm tropical waters, temperature has a much stronger effect.

T-S diagrams for water masses

A T-S diagram plots temperature on the vertical axis against salinity on the horizontal axis. Each point on the diagram represents a unique combination of temperature and salinity, and curved lines called isopycnals connect points of equal density.

These diagrams are powerful because different water masses occupy distinct regions of T-S space:

• North Atlantic Deep Water (NADW) clusters around relatively high salinity and low temperature
• Antarctic Bottom Water (AABW) sits at very cold temperatures with slightly lower salinity than NADW
• Surface waters from the tropics plot in the warm, variable-salinity region

When two water masses blend, their properties fall along a mixing line connecting the two source points on the diagram. By tracing these lines, you can figure out which water masses are mixing and in what proportions.

Core properties are the extreme temperature or salinity values within a water mass. These extremes point back to the source region where that water mass formed. As water moves away from its source, mixing gradually erodes these extremes, so stronger core signals mean you're closer to the origin. Oceanographers use T-S diagrams to trace circulation pathways for currents like the Gulf Stream and Labrador Current.

Salinity changes through processes

Three main surface processes alter salinity, and each one shifts density in predictable ways:

• Evaporation removes freshwater, raising both salinity and density. This is especially strong in subtropical regions with high heat and low rainfall. The Mediterranean Sea (~38 psu) and Red Sea (~40 psu) are classic examples of evaporation-dominated basins.
• Precipitation adds freshwater, lowering salinity and density. Tropical rain belts and polar regions receive heavy precipitation. The Bay of Bengal (~32 psu at the surface) stays notably fresh because of intense monsoon rainfall and river input.
• Freezing forms sea ice, but most of the dissolved salt gets expelled in a process called brine rejection. The cold, salty brine left behind is very dense and sinks. This process is a major driver of deep water formation in polar regions like the Weddell Sea.

These processes create or destroy vertical stratification. Strong stratification (light water sitting on top of dense water) resists vertical mixing, while weak stratification allows water to overturn more easily. Regional differences in these processes explain why the Atlantic is generally more saline than the Pacific: the Atlantic loses more water to evaporation and exports moisture to the Pacific basin via atmospheric circulation.

Density's role in ocean dynamics

Density differences are the engine behind many of the ocean's large-scale movements.

Thermohaline circulation is the global overturning system driven by density contrasts. In the North Atlantic and around Antarctica, surface water becomes cold and salty enough to sink to great depths, forming deep water masses that spread across ocean basins. This "global conveyor belt" takes roughly 1,000 years to complete a full circuit.

Horizontal density gradients drive geostrophic currents. Where density varies across a horizontal distance, pressure gradients develop, and the Coriolis effect deflects the resulting flow. Major currents like the Gulf Stream and Kuroshio Current are sustained partly by these gradients.

Vertical density gradients control stability and mixing:

• Strong vertical gradients (a sharp pycnocline) suppress mixing and trap nutrients below the surface
• Weak gradients allow overturning, bringing nutrient-rich deep water upward
• Upwelling occurs where winds or density differences push deep water toward the surface, fueling high biological productivity in regions like the Peru and Benguela coasts

Several more subtle density-driven processes also shape the ocean:

• Double diffusion arises because heat and salt diffuse at different rates. When warm, salty water sits above cold, fresh water, salt fingering can occur. The reverse arrangement produces diffusive convection. Both create small-scale vertical mixing.
• Cabbeling happens when two water masses with different T-S properties mix and produce water that is denser than either parent. This is a consequence of the nonlinear equation of state and can trigger localized sinking.
• Baroclinic instability develops where density surfaces tilt relative to pressure surfaces, releasing potential energy that feeds mesoscale eddies. Gulf Stream rings and Agulhas rings are examples of eddies generated this way, and they play a significant role in transporting heat and nutrients across ocean basins.