M1 Thermohaline Circulation (Thc)

Topic

Thermohaline circulation is the system of large-scale ocean currents driven by density gradients resulting from temperature and salinity differences across various regions of the global ocean. It is the so-called "ocean conveyor belt" that connects all the planet's oceans: cold, salty water sinks in the North Atlantic and the Weddell Sea (near Antarctica), forming North Atlantic Deep Water and Antarctic Bottom Water, respectively. From there, this water flows slowly through the ocean's deepest basins and eventually returns as warm surface currents, completing the circuit over a period of one to two thousand years.

The driving force behind this thermohaline circulation is the heat loss from the ocean to the atmosphere, particularly in the subpolar North Atlantic and the marginal seas of Antarctica. When surface water cools—or becomes saltier due to evaporation or salt rejection during sea-ice formation—its density increases until it exceeds that of the deep water beneath it. It is this excess density that causes the surface water to sink in deep convection columns, thereby initiating the conveyor belt mechanism described earlier.

Stommel's theory provides the conceptual model for understanding the bistability of this thermohaline circulation: there is an active state—characterized by vigorous circulation like that observed today—and an inactive, nearly stalled state. It is buoyancy forcing—the combination of heat and salt entering or leaving the ocean—that causes the system to flip from one state to the other once that forcing crosses a specific critical threshold. While in its active state, the thermohaline circulation transports heat toward the pole across the Atlantic—at a rate of nearly 1.3 petawatts, as measured at 26° North latitude—and the very sinking of water in deep convection columns ventilates the ocean's deep waters, preventing them from stagnating and becoming oxygen-depleted; furthermore, this circulation is a key regulator of Europe's climate.

Global warming is weakening this Atlantic Meridional Overturning Circulation, primarily due to the influx of fresh water resulting from melting Greenland ice: this additional fresh water lowers the density of subpolar surface water, pushing the system closer to the critical threshold described by Stommel's theory—making this weakening one of the most studied tipping points in the entire climate system.

Thus, from the sinking of cold, salty water—with which this document began—to the fresh-water-induced weakening with which it concludes, and encompassing the bistability described by Stommel's theory, the thermohaline circulation ensures that the fate of heat transport, deep-water ventilation, and the European climate itself hinges on a density balance that is now drawing ever closer to its breaking point.

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