Why does cold, salty seawater sink to the bottom of the ocean?

Cold, salty seawater sinks to the bottom of the ocean because it is denser than the surrounding water. In oceanography, the density of seawater is primarily controlled by its temperature and salinity.12

Specifically, seawater density increases as temperature decreases and salinity increases. When surface water in polar regions—such as the Weddell Sea near Antarctica—is cooled, it becomes significantly denser. Furthermore, as sea ice forms, salt is largely excluded from the ice, which further increases the salinity of the remaining water beneath it.13

This combination of extreme cold and high salinity creates very dense water masses, such as Antarctic Bottom Water, which sink to the deepest parts of the ocean. This sinking process is a fundamental driver of thermohaline circulation, the global system of ocean currents that replaces deep water with surface water and transports heat and nutrients around the planet.132