Question

Explain how thermohaline circulation is influenced by salinity and temperature.

Answer 1

Thermohaline circulation (THC) — often called the global conveyor belt — is the large-scale ocean circulation driven by density differences in seawater. Those density differences come mainly from two properties: temperature (thermo) and salinity (haline). Here’s how each controls the circulation and how they interact.

How temperature affects THC
- Colder water is denser than warmer water. When surface water cools (for example at high latitudes in winter), it becomes heavier and can sink, forming deep water masses (e.g., North Atlantic Deep Water).
- Warming of surface waters reduces their density, increases stratification (a stable light layer over denser deep water), and inhibits sinking. That can weaken the overturning circulation.
- Temperature-driven density changes are often strongest at high latitudes where air-sea heat loss is large.

How salinity affects THC
- Saltier water is denser than fresher water. Processes that increase surface salinity (evaporation, sea ice formation that rejects brine) make surface water heavier and more likely to sink.
- Freshening (from rainfall, river discharge, or melting ice) makes surface water lighter and reduces sinking. Persistent freshening in convection regions can shut down or weaken deep-water formation.
- In some regions the salinity effect is dominant (e.g., formation of dense outflows from the Mediterranean or Red Sea), so haline processes can drive local deep-water formation even where temperatures are relatively high.

Interaction and consequences
- THC results from the combined effects: density = f(temperature, salinity). The same density can result from different T–S combinations.
- Stratification (a strong density gradient between surface and deep water) suppresses vertical mixing and deep-water formation. Strong cooling or salinification of surface waters tends to erode stratification and promote sinking.
- Climate changes that warm surface waters and/or freshen key convection regions (e.g., increased Arctic/Greenland meltwater) reduce the density contrast that drives sinking and can slow the Atlantic Meridional Overturning Circulation (AMOC). That has large climate implications because the THC transports heat and affects regional climates.

Processes that illustrate the mechanisms
- Brine rejection: when sea ice forms, salt is excluded from the ice and stays in the surrounding water, raising its salinity and density and promoting sinking.
- Evaporation in subtropical basins raises salinity; currents can transport that saline water to areas where it contributes to deep-water formation.
- Upwelling and mixing elsewhere return deep water to the surface, where it is heated/freshened and completes the global loop.

In short: temperature and salinity set seawater density. Where surface water becomes sufficiently dense through cooling and/or salinification, it sinks and drives the deep limb of the thermohaline circulation; warming or freshening of surface waters reduces density, strengthens stratification, and can weaken or alter the circulation.