Sea ice is frozen ocean water that forms, expands, and melts in the ocean. Unlike icebergs, which are made of freshwater from glaciers or ice sheets that have broken off into the ocean, sea ice is made of saltwater that has frozen.
How sea ice forms
Sea ice forms when the surface of the ocean cools to its freezing point, typically around -1.8°C (28.8°F) for seawater. This cooling process causes small ice crystals, known as frazil ice, to develop in the upper layer of the ocean. As these crystals accumulate, they form a thin, continuous sheet of ice on the ocean’s surface. Depending on conditions, this initial ice layer can thicken over time, eventually forming what is known as first-year sea ice or, if it survives through the summer, multi-year sea ice.
Sea ice differs from freshwater ice in its composition and structure. When seawater freezes, most of the salt is expelled, but some salt remains trapped in pockets within the ice, leading to a structure that is less dense than freshwater ice and often includes brine channels.
Types of sea ice
Sea ice can be classified into several types based on its age, thickness, and formation process:
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- New ice: This is the initial stage of sea ice formation, consisting of frazil ice or thin sheets like nilas.
- First-year ice: This ice forms during a single winter and usually reaches a thickness of 1 to 2 meters (3 to 6.5 feet). It is relatively smooth and is the most common type of sea ice in the polar regions.
- Multi-year ice: This type of ice has survived at least one summer melt season, making it thicker and more compact than first-year ice. It can reach a thickness of up to 4 meters (13 feet) or more and is more resistant to melting.
- Pack ice: This refers to sea ice that has been pushed together by winds and currents, creating a more consolidated and rugged surface. It can include both first-year and multi-year ice.
Types of sea ice visible in a satellite image
An image from November 20, 2021, captured by the Landsat 8 satellite, reveals a variety of ice types in the Weddell Sea, near the Ronne Ice Shelf. The thickest ice belongs to the Ronne Ice Shelf itself, a floating extension of Antarctica’s land-based ice sheet.
Winds pushing away the sea ice create an opening in the open ocean for new sea ice to form. Frazil’s accumulate into clusters of nilas. The pattern of the winds elongate nilas into the thin streaks stretching out from the sea ice layer towards the Ronne Ice Shelf.
What is a nilas?
Nilas is a thin, elastic layer of newly formed sea ice that usually ranges in thickness from 5 to 10 centimeters (2 to 4 inches). It is the first stage in the development of a more solid ice cover and forms in calm, cold conditions. Nilas has a characteristic dark, almost translucent appearance, as it contains a high percentage of unfrozen water between the ice crystals.
Dark nilas and light nilas
There are two types of nilas: dark nilas and light nilas. Dark nilas is thinner, typically less than 5 centimeters thick, and has a darker appearance due to the close proximity of the underlying ocean water. Light nilas, on the other hand, is slightly thicker and appears lighter in color as it starts to reflect more light.
The role of nilas in the sea ice cycle
Nilas is an important precursor to thicker, more stable sea ice. As nilas continues to cool, it gradually thickens and becomes more solid. When nilas is subjected to wind or ocean currents, it can raft, a process where thin layers of nilas slide over one another, increasing the overall thickness of the ice. This rafting is an essential step in the transition from nilas to young ice, which eventually leads to the formation of first-year sea ice.
As the ice thickens, it becomes less sensitive to changes in temperature and begins to accumulate snow on its surface. This snow cover further insulates the ice, slowing the rate of heat exchange between the ocean and the atmosphere. This insulation is critical in maintaining the integrity of the sea ice during the polar winter.
