Thermohaline Circulation

The thermohaline circulation is the global-scale, density-driven component of ocean circulation powered by gradients in temperature and salinity. It connects the surface ocean to the deep ocean, transporting heat, carbon, and nutrients across thousands of kilometres and over timescales of centuries to millennia.

The circulation is not a single loop but a network of regional cells — the Atlantic Meridional Overturning Circulation, the Antarctic Bottom Water formation, the Pacific and Indian Ocean overturning — each driven by localised deep water formation at high latitudes. The density gradients that power the circulation are maintained by surface heat exchange with the atmosphere and by the hydrological cycle, which adds fresh water through precipitation and removes it through evaporation.

The thermohaline circulation is a tipping element in the Earth system. Its sensitivity to freshwater forcing means that it can reorganise or weaken under climate change, with consequences that propagate across the entire climate system. Paleoclimate evidence from the Younger Dryas and Heinrich events demonstrates that the circulation has shifted abruptly in the past, producing regional temperature changes of 5–10°C within decades.