The Ocean's Global Conveyor Belt
The thermohaline circulation is a planet-spanning system of ocean currents driven by density differences created by temperature and salinity variations. Warm, salty surface water flows poleward, cools, and becomes dense enough to sink to the ocean floor in key 'deep water formation' regions — primarily the North Atlantic near Greenland and around Antarctica. This deep water then flows along the ocean bottom for thousands of kilometers before eventually upwelling back to the surface, completing a cycle that takes roughly 1,000 years.
The Engine of Deep Water Formation
Deep water formation is the engine that drives thermohaline circulation. In the North Atlantic, the Gulf Stream carries warm, salty water northward where it cools through contact with cold Arctic air. The combination of high salinity (from evaporation during its tropical journey) and low temperature makes this water extremely dense. It sinks to depths of 2,000-4,000 meters, forming North Atlantic Deep Water (NADW) at a rate of approximately 15-20 Sverdrups (million cubic meters per second). This sinking motion pulls more warm surface water northward, sustaining the circulation.
Heat Transport and Climate Regulation
The thermohaline circulation transports approximately 1.3 petawatts of heat from the tropics toward the poles — roughly 25% of the total poleward heat transport (the atmosphere carries the rest). This heat delivery is why London at 51°N is temperate while Labrador at the same latitude is subarctic. The heat transport parameter in this simulation shows how changes in overturning rate directly affect the energy delivered to high latitudes, with dramatic consequences for regional and global climate.
AMOC Weakening and Climate Risk
The Atlantic Meridional Overturning Circulation (AMOC) has weakened by approximately 15% since the mid-20th century, likely due to increased freshwater input from Greenland ice melt and Arctic sea ice loss. Fresh water is less dense than salty water, inhibiting the sinking that drives the conveyor belt. Paleoclimate data shows that AMOC shutdowns have caused abrupt climate shifts of 5-10°C within decades. The freshwater input slider lets you explore how increasing ice melt progressively weakens the overturning circulation toward potential tipping points.