What is Sea Ice?
Sea ice forms within the polar oceans when the seawater temperatures reach the local freezing point. Salt is retained as concentrated brine between the pure ice crystals within the rapidly formed ice volume and the brine gradually makes its way through the ice by gravity drainage back into the ocean which locally increases the ocean salinity. This higher salinity water will sink down into the water column until it reaches an equilibrium state. Areas of rapid and sustained growth, particularly in the Southern Ocean, have been identified as contributing to deepwater formation of the global oceans.
During winter, sea ice will grow thermodynamically to a thickness of about 1-2 m, overlain with a comparatively thin snow cover. Sea ice moves by winds and currents and is subject to large-scale motion as well as local-scale deformation through collisions with adjacent and often thicker floes. This dynamic process of deformation results in the thinner ice being crushed into small pieces or blocks that pile up on top of adjacent ice floes to form ridges that may extend several meters both below and above the ice layer. When ice floes move away from each other, open water is exposed, where rapid ice formation may occur due to the increased heat flux between the comparatively warmer ocean and cooler atmosphere. During the summer, the snow and ice cover undergo melt. If the ice grown in the initial winter, called first-year or seasonal ice, makes it through the melt season to the next winter, it becomes multi-year or perennial ice. This older ice retains even lower amounts of salt compared to seasonal ice, as most of the remaining salt is flushed through the ice by draining summer melt water. The resulting ice cover is thus composed of both smooth and rough components with varying thickness and age.