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.
What are the Key Sea Ice Observations from Satellites?
The darkness during polar winter and the difficulty of operating within remote ice-covered regions have significantly limited the ability to collect in situ data in the polar regions. This has led to the extensive use of satellite remote sensing for studying sea ice in both poles. Microwave and active sensors have proven to be of great benefit, as they can operate all-year round, while visible and near-infrared sensors are limited by cloud cover and available sunlight. The longest sea ice time series has been obtained by passive microwave sensors, which have measured the sea ice extent and ice concentration (percentage of ice and open water) since 1979. Other key sea ice observables and sensors include the following: large-scale ice motion (primarily passive microwave); ice deformation (SAR); ice type (active and passive microwave); ice surface temperature and surface albedo (visible-near infrared, passive microwave); and derived thickness based on freeboard observations (radar altimeters and lidar). Freeboard is the portion of sea ice and overlying snow cover that extends above sea level. From freeboard, sea ice thickness is derived based on estimates of ice and snow density and snow depth, along with Archimedes principal of isostasy. Both ice motion and deformation observations require fine spatial resolution and dense temporal sampling. The identification of ice type (primarily seasonal and perennial ice) makes use of the varying scattering or emissivity resulting from salinity differences and internal properties of the two major types of ice. Snow cover and extent of melt water on the ice surface impact both the ice surface temperature and albedo measurements.
What is the Role of Sea Ice in Ocean Circulation and Climate?
The sea ice cover in the polar oceans play a fundamental role in the global climate and oceanographic system for several reasons. The comparatively thin sea ice cover, generally less than 5 m thick, provides a high albedo cover over the lower albedo ocean; alters the exchange of heat, gases, and momentum between the atmosphere and ocean; and impacts ocean circulation through the redistribution of both salt (during ice growth) and freshwater (as the relatively fresh sea ice melts). Recent observations indicate significant climate change is occurring in the Arctic Ocean due to global warming, as the sea ice has thinned and reduced in extent particularly at the end of summer.