Thermohaline Circulation: The Ocean's Global Conveyor Belt Driven by Salt & Temperature

The currents driven by sinking/rising saltwater are called thermohaline circulation (THC) or meridional overturning circulation (MOC)—the planet's deep-ocean conveyor belt.

The Engine: Density Differences

• Thermo = temperature (cold = dense).
• Haline = salinity (salty = dense).
• Cold + salty = sinks.

The Cycle (Simplified)

1. North Atlantic: Gulf Stream cools, evaporates → cold, salty water sinks (North Atlantic Deep Water formation).
2. Southward flow: Deep current hugs bottom along Americas, across Southern Ocean.
3. Upwelling: Rises in Southern Ocean (Antarctic divergence) and North Pacific.
4. Return flow: Warm, fresher surface water flows north (Gulf Stream, Kuroshio) to restart.

Timescale & Impact

• Full cycle: ~1,000 years.
• Transports: Heat (poleward), carbon (sequestration), nutrients (upwelling fuels fisheries), oxygen (ventilates deep sea).
• Climate regulator: Keeps Europe mild; monsoons alive; CO₂ buffered.

Climate Change Threat

Greenland melt → freshwater cap → weakens sinking. THC slowing observed (15% since 1950s). Potential tipping point: abrupt cooling in North Atlantic, shifted rain belts, sea-level rise on US East Coast.

THC isn't just currents—it's Earth's climate thermostat. And we're turning the dial.