E9 Ocean - Cryosphere Interaction

Topic

The interaction between the ocean and the cryosphere occurs primarily in polar regions, where the formation and melting of sea ice govern the density of surface waters and, consequently, the ventilation of the deep ocean. It is precisely this ability of sea ice to alter the density of the water beneath it that makes polar regions one of the few places on Earth where the ocean surface communicates directly with its deepest layers.

When seawater freezes, the resulting ice crystals consist almost entirely of water—containing virtually no salt—causing the salt previously dissolved in that water to be expelled into the water directly beneath the ice. This salt rejection progressively increases the salinity and density of polar subsurface waters until they eventually sink, forming the densest water masses in the entire ocean: Antarctic Bottom Water in the Antarctic, and North Atlantic Deep Water in the North Atlantic.

Antarctic floating ice shelves interact with the ocean at their bases, where Circumpolar Deep Water—a relatively warm water mass with temperatures ranging from one to two degrees Celsius—penetrates the cavities beneath the shelves. This intrusion of warm water causes the bases of the shelves to melt at rates of one to one hundred meters per year, introducing fresh water into the Southern Ocean and altering its stratification; furthermore, this basal melting poses the primary threat to the stability of West Antarctic ice shelves and their future contribution to sea-level rise. The feedback loop between sea ice and sea surface temperature is a central positive feedback in the polar climate system, and it is self-reinforcing: when sea ice melts, the fraction of solar radiation reflected by the surface drops from the 70–90% reflected by ice to the mere 6% reflected by open ocean; this causes the newly exposed surface to absorb significantly more solar radiation and warm up, melting even more sea ice and thereby closing the loop back to the ice itself. This mechanism is the primary driver of polar amplification in climate warming.

Thus, from the salt rejection that causes water to sink alongside newly formed ice to the feedback loop that accelerates its own melting—and including the basal melting experienced by Antarctic ice shelves from below—the interaction between the ocean and the cryosphere results in sea ice governing both the formation of the planet's densest water and the rate at which polar regions warm and contribute to sea-level rise.

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