E5 Sea Ice

Topic

Sea ice forms when the ocean surface temperature drops below the freezing point of seawater—approximately minus 1.8 degrees Celsius for a salinity of 34 grams of salt per kilogram of water. The formation process begins with the nucleation of tiny frazil crystals—small, hexagonal ice needles that form in turbulent water; these crystals clump together to create a layer of "frazil ice" (or slush) on the sea surface, which eventually consolidates into thin sheets known as *nilas* and, subsequently, into recognizable sea ice floes.

As the sea ice continues to grow, it expels its salt content into the liquid water directly beneath it; this expelled saline water becomes denser than the surrounding water, sinks toward the ocean floor, and contributes to the formation of deep ocean water that drives the planet-wide thermohaline circulation.

The thickness of sea ice growing solely through thermal processes—undisturbed by other factors—increases at a progressively slower rate as freezing degree-days accumulate throughout the winter; this is because the existing ice acts as an increasingly effective insulator, shielding the underlying liquid water from the cold at the surface. This self-limiting growth pattern applies only to stationary sea ice in the absence of an abnormally warm underlying ocean.

In addition to growing, sea ice drifts under the combined influence of wind and ocean currents; this drift does not follow the exact direction of the driving wind but instead deviates from it due to the Earth's rotation. When distinct blocks of sea ice converge during this drift, they mechanically deform against one another, forming ridges of compressed ice on the surface and keels of compressed ice underwater.

Sea ice also has a critical effect on the polar surface albedo: the sea ice surface reflects between fifty and ninety percent of the incoming solar radiation, whereas the open ocean reflects a mere six percent of that same radiation. When human-induced warming reduces the summer minimum of Arctic sea ice, a larger area of ​​open ocean is exposed; this area absorbs far more solar radiation than the lost sea ice did, causing the exposed ocean to warm. This additional warming melts even more sea ice, and this positive ice-albedo feedback is precisely what amplifies warming in polar regions compared to the rest of the planet.

Finally, sea ice acts as a barrier separating the ocean from the atmosphere: beneath a continuous cover of sea ice, the exchange of heat, carbon dioxide, and momentum between the ocean and the overlying atmosphere is drastically reduced. Thus, the very sea ice whose formation, growth, and drift were described earlier also ends up regulating the interaction between the ocean and the atmosphere in polar regions.

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