Sea Ice

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 (Comiso et al., 2008).

 
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. 

 

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.
 

Historically, the Arctic Ocean is composed of a higher percentage of perennial ice than seasonal ice, while the sea ice in the Southern Ocean is largely composed of seasonal ice (http://nsidc.org/seaice/characteristics/difference.html).  For decades, it has been considered that the polar regions would be particularly sensitive to the potential of global warming and thus provide an early indicator of a changing climate. This is based on the fact that the circulation of the Earth’s atmosphere transports heat towards the polar regions. One strong climate interaction is the ice albedo-temperature feedback where higher temperatures lead to enhanced melt, which leads to reduced albedo and further warming. The ice albedo-temperature feedback preferentially impacts the summer melt processes (low albedo) and duration, compared to the high albedo winter surfaces.  The extensive cold, ice-covered continental land mass of the Antarctic makes the southern region less sensitive to warming than the comparatively warmer Arctic Ocean.

 
Over the past several decades, there has been a decline in Arctic sea ice extent, with the smallest extent at the end of summer in 2007 followed by 2011, in contrast to the Southern Ocean where little change in extent has occurred since 1979 (http://nsidc.org/arcticseaicenews/). In the Arctic, there has also been a significant reduction in the extent of perennial ice while the relative percentage of seasonal ice has increased (determined by scatterometer data), along with a measured thinning and loss of volume of the overall ice cover as measured by ICESat (Kwok et al., 2009).  The thinning found in the satellite record of ice thickness since 2002 continued the thinning found between the periods of 1958-1976 and the mid-1990s, when compared with submarine-derived thickness measurements using upward-looking sonar (Kwok and Rothrock, 2009).

Today, earth observing satellites are continuing to demonstrate their fundamental role in understanding the role of the sea ice cover in climate change and possibly the prediction of future impacts related to warming