How does sea-ice formation make water below the ice saltier and denser?

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When seawater freezes to form sea ice, the ice crystals that develop are composed primarily of pure water, while the salt content of the original seawater is largely excluded from the ice structure. This process, known as brine rejection, expels the salt into the surrounding liquid water beneath the forming ice.123

As a result of this exclusion, the salinity of the water immediately beneath the ice increases. Because the density of seawater increases with higher salinity and lower temperatures, this newly formed cold, salty water becomes significantly denser than the surrounding ocean water. This dense, saline water then sinks, contributing to the formation of bottom water and driving thermohaline circulation within the world's oceans.2456