The Global Conveyor Belt
The ocean's thermohaline circulation is a planet-spanning conveyor belt driven by density contrasts. In the North Atlantic, warm Gulf Stream water travels northward, losing heat to the atmosphere and gaining salinity through evaporation. By the time it reaches the Norwegian and Labrador seas, it is cold and salty enough to sink to the abyss, forming North Atlantic Deep Water (NADW). This sinking drives the Atlantic Meridional Overturning Circulation — moving ~18 Sv of water and transporting ~1.3 PW of heat that keeps Europe anomalously warm for its latitude.
Stommel's Bistability
In 1961, Henry Stommel showed with a simple two-box model that the thermohaline circulation has two stable states. Temperature differences drive the circulation in one direction, while salinity differences oppose it. When freshwater input dilutes the North Atlantic enough, salinity-driven effects dominate and the circulation collapses — and crucially, it stays collapsed even if the freshwater input is removed. This hysteresis makes the AMOC a potential climate tipping point.
Abrupt Climate Events
The paleoclimate record is punctuated by abrupt climate shifts linked to AMOC disruption. Heinrich events — massive iceberg discharges that flooded the North Atlantic with freshwater — caused AMOC shutdowns and rapid European cooling. The Younger Dryas (~12,900 years ago) was likely triggered by a glacial meltwater pulse that collapsed the AMOC, plunging the Northern Hemisphere back into near-glacial conditions for over a millennium. These events demonstrate that the climate system can shift abruptly when circulation tipping points are crossed.
A Modern Tipping Point?
Today's AMOC appears to be weakening. Greenland ice-sheet melting injects freshwater into the critical deep-water formation regions, and climate models project further AMOC weakening — some predict collapse under high-emission scenarios. This simulation uses Stommel's framework to show how freshwater forcing pushes the system toward its tipping point. Adjust the freshwater flux to see the nonlinear transition from strong overturning to a collapsed state — and the hysteresis that makes recovery so difficult.