Thermohaline circulation (THC) is driven by density differences caused by temperature (thermo) and salinity (haline). In the North Atlantic, warm surface water from the tropics cools as it flows northward, becoming dense enough to sink — a process called North Atlantic Deep Water (NADW) formation. This sinking pulls the surface flow northward, sustaining the conveyor.
The Atlantic Meridional Overturning Circulation (AMOC) transports roughly 1.3 petawatts of heat northward, moderating European climate. Current strength ~18 Sverdrups (1 Sv = 10⁶ m³/s). Paleoclimate records show AMOC collapsed during the Younger Dryas (~12,900 ya) following glacial meltwater floods — triggering a 1,200-year cold period.
Modern concern: Greenland ice sheet melt and Arctic freshening reduce North Atlantic salinity, inhibiting deep water formation. Caesar et al. (2018) found AMOC has slowed ~15% since 1950 — potentially approaching a tipping point. A collapse would dramatically cool Northern Europe, intensify African/Asian monsoons, and raise sea levels on the US East Coast.