How does sea ice formation affect the salinity of water beneath the ice?

When seawater freezes to form sea ice, the salt content of the water is largely excluded from the ice crystals. As this salt is expelled as brine into the surrounding water, the salinity of the water directly beneath the ice increases.123

This process of brine rejection has significant implications for ocean dynamics:

• Convection: The increase in salinity, combined with the cold temperatures of the water, increases the density of the water beneath the ice. This triggers salt- and temperature-driven convective overturn, where the dense, saline water sinks.23
• Deep Water Formation: This mechanism is essential for the formation of dense water masses, such as Antarctic Bottom Water, which sink to the deepest parts of the ocean and help drive the global thermohaline circulation.24
• Mixing: The rejection of brine from ice growing in areas of open water, such as leads and polynyas, further promotes deep mixing of the ocean.4

Conversely, when sea ice melts during warmer seasons, it releases fresh water back into the surface layer, which decreases salinity and stabilizes the water column.23