Thermohaline circulation is the large-scale movement of ocean water driven by differences in water density caused by temperature and salinity. It redistributes heat around the planet and therefore influences regional and global climate.
Level: HL only: Subtopic 4.1 Water systems
How Does Thermohaline Circulation Work?
The term combines thermo, meaning temperature, and haline, meaning salinity. Both variables affect seawater density:
| Change in seawater | Effect on density and movement |
|---|---|
| Cooling | Increases density, causing water to sink |
| Increasing salinity | Increases density, causing water to sink |
| Warming | Decreases density, encouraging water to remain near the surface |
| Decreasing salinity | Decreases density, reducing sinking |
Warm surface currents transport heat from low latitudes toward higher latitudes. In parts of the North Atlantic, this water cools and becomes saltier as evaporation or sea-ice formation removes freshwater. The denser water sinks and forms a deep current that moves through the global ocean.
Elsewhere, deep water eventually rises through upwelling and returns through surface currents. Together, these connected surface and deep flows are often described as the global ocean conveyor belt, although this is a simplified model of a complex system.
The Atlantic Meridional Overturning Circulation (AMOC) is a named example. It transfers heat northward through the Atlantic, contributing to western Europe's relatively mild climate. Increased freshwater input from melting ice can lower North Atlantic salinity, reduce water density, and potentially weaken sinking and circulation.
A common misconception is that thermohaline circulation is driven directly by wind. Wind strongly affects surface currents, but thermohaline circulation specifically refers to density-driven movement caused by temperature and salinity differences.
Exam Technique
For an IB ESS explanation, give a clear causal sequence: temperature or salinity changes density; density differences cause sinking or rising; connected currents redistribute heat and affect climate. If evaluating change, link global warming and ice melt to possible circulation disruption, while acknowledging uncertainty in predictions and consequences for ecosystems and societies.