E10 Cryosphere-Ocean Interactions

Topic

The interaction between the cryosphere and the ocean occurs in both directions simultaneously and is fundamental to sustaining global ocean circulation.

The most critical process within this interaction is the formation of dense water through salt rejection as seawater freezes: as sea ice forms, it expels its salt content into the liquid water directly beneath it, making that receiving water denser and saltier than the surrounding water. This dense, salty water sinks into the ocean depths through deep convection; this sinking motion drives global thermohaline circulation—including both the Atlantic Meridional Overturning Circulation and its counterpart in the waters surrounding Antarctica. When the cryosphere loses sea ice, this dense-water formation process weakens, which in turn can weaken global thermohaline circulation itself.

The intrusion of warm Circumpolar Deep Water beneath Antarctic ice shelves is the most active oceanic mechanism destabilizing glaciers that rest on the seabed. This Circumpolar Deep Water arrives at temperatures between one and three degrees Celsius—well above the ice's melting point at that depth (approximately minus two degrees)—melting the base of the ice shelves at rates of ten to fifty meters per year in some sectors of the Pacific Antarctic. This basal melting reduces the buttressing effect these ice shelves exert on the tributary glaciers feeding them, and this reduction in buttressing accelerates the discharge of those tributary glaciers into the ocean. Within the cavity formed between the base of an ice shelf and the seafloor, warm water initiates a heat-driven circulation cell: the warm water enters beneath the ice shelf, melts some of the ice—thereby becoming cooler and fresher—rises along the shelf's base because it is now less dense than the surrounding water, and finally exits the cavity at an intermediate ocean depth. This same circulation cell transports oceanic heat to the ice-ocean interface and ultimately controls the melting rate of the ice shelf base—the very feature that began this description of the interaction between the cryosphere and the ocean.

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