How does sea ice form?

Formation of sea ice

Sea ice changes within large temporal and spatial scales and interlocks with many other spheres. The brief presentation of the processes and phenomena at this point can only provide a superficial impression and capture a few individual aspects, but cannot depict the topic in its complexity. For a detailed and more complete description, refer to the relevant specialist literature.

In order to be able to understand many connections around the sea ice, various basic information on the subject of water and ice is important. This concerns the properties of the element water in all its aggregate states, but especially the crystal structure of frozen water, the ice. The salinity of ocean water in particular influences its freezing point, which decreases with increasing salinity. The solubility of salts is in turn dependent on the temperature of the sea water and the respective ion concentration. If the temperature drops, part of the sea water freezes and the salt content in the still liquid water rises. This dependence of the sea ice composition on the temperature can be shown in a phase diagram.

Changes in the structure of the sea ice are mainly caused by the desalination of sea ice. This can take place in three different ways, heavy-duty drainage, brine discharge and leaching. The following applies to all three processes: the older the ice, the less salt it contains. Due to the fact that the salt is released into the ocean in the form of a highly concentrated brine towards the bottom of the ice, its salinity increases.

Sea ice microstructures are of great importance for the optical and thermodynamic properties and are a result and a documentation of the growth process. The texture of sea ice describes its size, shape and the spatial arrangement of the individual ice crystals. It depends on the atmospheric and oceanographic conditions during thermodynamic ice growth and therefore allows conclusions to be drawn about the history of the ice formation. The grain size is a function of the temperature gradient and the rate of growth. Smaller crystals indicate a high growth rate, larger ones a lower one. The shape and arrangement of the crystals in the ice reflect oceanographic conditions and are dependent on the turbulence within the water column. Deformation processes (dynamic processes) caused by wind and ocean currents play another important role.

Since sea ice structures have different phases (solid ice, gaseous air, liquid brine and a few solid inclusions) and these different phases have different thermal, optical and mechanical properties, the ice is anisotropic. This means that the properties of the ice are directional. The heterogeneous spatial arrangement of the phases (e.g. microstructure and texture), i.e. their uneven distribution in the sea ice, affects the growth rate of the ice.

Along with the complex formation processes, the appearance of sea ice can also vary greatly, so that different sea ice forms exist and this ice has different properties than mainland ice. This is how wind and ocean currents affect the growth process of sea ice. When the weather is calm, what is known as frazil-ice forms a continuous thin layer of ice at the interface between the ocean and the atmosphere, known as the Nilas. In contrast to this, when the weather is unsteady, non-contiguous pancake-shaped ice sheets are created, which are called pancake ice cream because of their appearance. Over time, the volume of the sea ice increases, the Nilas, which are initially so thin that they appear black because the dark sea shines through, become increasingly thicker and therefore also brighter. At the same time, the ice surface remains regular. Pancake ice, on the other hand, can collide and slide over one another, creating a thick, but irregular, surface of the ice.

Regardless of how the sea ice is formed, it is subject to great external influences and is constantly changing. A significant change is the melting process in spring and summer, when part of the annual sea ice is turned back into water. The ice that survives this period is called perennial sea ice. Since it exists for another winter, this ice is also much thicker than annual ice.