How do temperature and salinity differences drive thermohaline circulation?

Thermohaline circulation is driven by horizontal differences in temperature and salinity, which determine the density of seawater.12

In specific regions, particularly at high latitudes during winter, surface water becomes dense enough to sink due to intense cooling or increased salinity from evaporation and the rejection of brine during ice formation. This sinking process, known as convection, forces the water to descend until it reaches a depth where its density matches that of the surrounding water.123

As this dense water sinks and spreads along the seafloor, it creates a pressure gradient that necessitates the movement of other water to replace it, establishing a global, sluggish circulation pattern. This system is balanced by the slow upwelling of deep water in other parts of the ocean, which eventually returns to the surface to replenish the sinking regions. Consequently, these density-driven currents transport heat, nutrients, and materials across the global ocean, playing a critical role in regulating the Earth's climate.124