Sea ice is a constantly changing cover of frozen seawater that floats on the surface of the polar oceans, distinct from glacial ice or icebergs which originate from land-based freshwater snow and ice. This frozen layer is a major component of the global climate system because its bright white surface reflects solar energy back into space, helping to regulate the planet’s temperature. The annual formation and melt of sea ice also drives the circulation of the world’s deep ocean currents, and its presence provides a habitat for a diverse polar ecosystem. Understanding the complex process of sea ice formation is a prerequisite for studying its overall influence on the polar environment and global climate.
The Chemical Prerequisites for Freezing
The presence of dissolved salts makes freezing seawater a more complex process than freezing freshwater. Pure water freezes at 0°C, but the solutes in seawater lower its freezing point, a phenomenon known as freezing point depression. Typical ocean water with a salinity of about 35 parts per thousand (ppt) will not begin to freeze until the temperature drops to approximately -1.8°C.
The density behavior of saltwater also significantly impacts the start of freezing. Because the salt content means its temperature of maximum density is below its freezing point, cooling surface water becomes denser and sinks, replaced by warmer water from below in a continuous process of convection.
This convection process must cool the entire surface layer, often down to 100 to 150 meters, to the freezing point before ice can begin to form. This requirement demands sustained cold air temperatures, causing sea ice to form more slowly than freshwater ice.
Initial Stages of Ice Crystal Formation
The first physical indication of sea ice formation is frazil ice, tiny, needle-like ice crystals only 3 to 4 millimeters in diameter. These crystals form in the upper few centimeters of the water column once the supercooled surface layer reaches its freezing point. Since salt cannot be incorporated into the pure ice crystal structure, frazil crystals are essentially freshwater ice, expelling salt into the surrounding water.
The accumulation of frazil crystals on the surface leads to the next stage, which depends on ocean conditions. In very calm waters, the frazil crystals coagulate to form a soupy, opaque layer called grease ice. This layer gives the ocean surface a matte or oily appearance and behaves like a viscous fluid.
If the water remains calm, the grease ice matures into nilas, a thin, elastic sheet of new ice that can be up to 10 centimeters thick. Nilas is flexible and can easily bend on waves and swell without breaking, forming a continuous, smooth surface layer.
Growth, Consolidation, and Salt Segregation
When the ocean surface is agitated by winds and waves, the initial frazil and grease ice develop along a different pathway. The slushy mixture is broken up and consolidated by the turbulence, forming circular disks with raised edges known as pancake ice.
As the pieces of pancake ice jostle and collide, their raised rims allow them to freeze together and consolidate into larger, rough-bottomed sheets. Nilas sheets in calm water thicken by rafting, where thin layers slide over one another. These merging processes eventually form expansive, continuous sheets known as ice floes.
A fundamental process accompanying the freezing of seawater is brine rejection, which determines the ice’s salinity. Since the ice crystal lattice excludes salt, the freezing process forces dissolved ions into pockets of highly concentrated saltwater called brine inclusions. This brine remains liquid because its high salinity significantly lowers its freezing point.
Over time, this trapped brine begins to drain out of the ice sheet through microscopic vertical channels, driven by gravity and temperature changes. This downward migration purifies the sea ice, leaving air pockets and reducing the ice’s overall salinity. Brine rejection is a continuous process, making older, thicker ice significantly less salty than young ice.
Defining Mature Sea Ice Types
Once the initial stages of formation are complete, sea ice is classified based on its age and mobility. Ice that forms during one winter season but melts completely during the subsequent summer is classified as first-year ice. This ice typically reaches thicknesses between 0.3 and 2 meters.
Ice that has survived at least one summer melt season is defined as multi-year ice. Surviving the summer allows the ice to become much thicker, often ranging from 2 to 4 meters, and its continued brine drainage makes it nearly salt-free.
Ice is also categorized by its mobility. Fast ice is sea ice that remains anchored to the coast, an ice wall, or grounded icebergs. Conversely, ice that is free to move under the influence of currents and wind is called drift ice or pack ice.