Antarctic sea ice helps maintain the Southern Ocean overturning circulation

Recent trends in sea ice have been studied heavily. A less well-understood problem is how sea ice affects the underlying ocean, particularly the poorly observed Southern Ocean. A new study by Abernathey et al., published in the journal Nature Geoscience, shows how the seasonal drift of Antarctic sea ice may be more important for the global ocean overturning circulation than previously realized.

A team of scientists used the Southern Ocean State Estimate, a data-assimilating numerical model which to synthesizes millions of ocean and ice observations collected over six years, to quantify the processes which influence the density of seawater in the upper ocean. Employing a thermodynamical analysis technique called water-mass transformation, the study estimated the relative contributions of sea ice freezing and melt, glacial melt, open-ocean evaporation / precipitation, surface heat fluxes, and mixing in sustaining the ocean overturning circulation. Overturning circulation brings deep water and nutrients up to the surface, carries surface water down, and distributes heat and helps store carbon dioxide as it flows through the world’s oceans, making it an important force in the global climate system. This circulation requires a continuous transformation of light water to dense, and dense to light. The study found that freshwater played the most powerful role in changing water density in the Southern Ocean, and that melting of wind-blown sea ice contributed 10 times more freshwater than melting of land-based glaciers did.

A vital contributor to the freshwater fluxes, the scientists discovered, was the seasonal drift of the ice, which is largely driven by winds. When sea ice forms around the edges of Antarctica each winter, the salt in the ocean water doesn’t freeze; it stays behind, a process called brine rejection. That makes the water near the coast much saltier and therefore denser than water offshore. Meanwhile, as sea ice melts farther out in the open ocean, it deposits its freshwater. If the sea ice were instead forming and melting in the same place, there would be no net effect. Oceanographers have known for some time that changes in water density, particularly the sinking of cold, saline water, contribute to the ocean’s abyssal circulation, the deepest, coldest branch of the ocean conveyor belt which moves cold Antarctic water northward along the ocean bottom. The study shows that sea ice also plays a central role in the upper branch of the Southern Ocean overturning by providing the buoyancy gain needed for circumpolar deep water to be transformed into lighter mode and intermediate water.

This new central role implies that changes in Antarctic sea ice concentration, thickness, or drift associated with future climate change, or which occurred in past climates, have strong potential to influence the global overturning circulation.