Temperature and salinity affect seawater density. Changes in deep-water formation alter ocean heat transport and can interact with climate feedbacks.
Thermohaline circulation is the density-related component of large-scale ocean overturning. Thermo refers to temperature and haline to salt. Cooling and higher salinity generally increase seawater density. At high latitudes, sufficiently dense surface water sinks and contributes to deep currents.
In the Atlantic, warm upper-ocean water carries heat northwards. Some water loses heat, becomes dense and sinks; deep water returns southwards. The Atlantic Meridional Overturning Circulation, or AMOC, is part of the larger overturning system. Winds and mixing also matter, so a single conveyor-belt line is a simplified model.
A large freshwater input from melting land ice or increased precipitation can lower surface salinity and density, making sinking harder. A weaker overturning circulation can reduce northward ocean heat transport and cool parts of the North Atlantic region relative to conditions without that change.
Feedbacks can amplify or oppose the response. Reduced northward salt transport can help maintain fresher northern surface water and weaker sinking: a positive salt-advection feedback. Regional cooling can also expand reflective sea ice, reinforcing local cooling through albedo. These links depend on the wider ocean-atmosphere state.
Ocean-core evidence links past circulation shifts with abrupt regional Quaternary climate changes. This does not mean every local cooling marks a global cooling, nor that the entire ocean circulation suddenly stops. The timing and magnitude of future changes require evidence and carry uncertainty.
Step by step
Change density
Cooling, salinity or freshwater input changes the buoyancy of surface water.
Change overturning
Deep-water formation and connected flows adjust.
Change heat transport
Regional temperatures respond and may trigger further feedbacks.
Worked example: Freshening in a warming world
If meltwater freshens the North Atlantic, weakened overturning can reduce warming there relative to the global trend. A relatively cool region therefore does not by itself disprove global warming: heat is redistributed unevenly.
Watch out for this
Thermohaline circulation is driven only by surface winds.
Density differences associated with temperature and salinity are central to thermohaline circulation; winds and mixing also contribute to the broader overturning system.
Check your understanding
Why can added freshwater inhibit high-latitude sinking?
- It warms the surface, and only temperature controls seawater density.
- It lowers surface salinity and can reduce water density.
- It lowers surface salinity, making the water denser.