What Is Sea Ice? Q&A
The simple answer is that sea ice forms in the sea.
That's when it starts to get interesting.
A synthetic aperture radar (SAR) image
Fast ice, sometimes called landfast ice, freezes to the shore, the ocean bottom, an ice wall, or a grounded iceberg. If it rises more than about two meters above sea level, it is called an ice shelf. In the animation below, radar captures fast ice losing its hold on the shoreline of Barrow, Alaska, the most northern point in the United States. Credit: Sea Ice Group, Geophysical Institute, University of Alaska Fairbanks.
First-year sea ice has lasted for no more than one winter, but it is thicker than the thin films of new ice called frazil, grease ice, and nilas. Although first-year ice ejects salt from its ice crystals in the freezing process, it still contains pockets of saltwater. If first-year ice survives to become multiyear ice, the saltwater eventually drains out, and the ice becomes denser. As Arctic sea ice has diminished in recent decades, first-year ice has become more prevalent than multiyear ice.
Frazil is the start of sea ice — a collection of ice crystals that form a film on the water. As the crystals form, they eject salt into the surrounding water. The saltier the water, the heavier it is, so it sinks. The ocean becomes layered with sea ice on top of cold water, on top of heavier, saltier water. Those layers move in currents around the world, playing a large role in global climate.
|© WHOI 2014.|
Ice shelves are landfast sea ice (also known as fast ice) that rise more than about two meters above sea level.
Lagrangian sea-ice data products contain monthly measurements of dynamic and kinematic parameters over the Arctic Ocean sea-ice cover. "Lagrangian" refers to a mathematical way to study ice dynamics by noting changes in position and velocity of points over time. The sea-ice analysis in the dataset available here is initialized by laying a 10km-by-10km grid over a set of RADARSAT images of the sea ice during an initial 3-day period of a season. The grid forms cells for which a number of properties can be derived that describe the sea-ice dynamics. The grid points of the sea ice and related cell properties are then tracked throughout the season.
Leads are long, linear areas of open water that develop as sea ice pulls apart, moved by winds or currents. A lead in sea ice can range from a few meters to over a kilometer in width and tens of kilometers long. Leads and polynyas (see below) are important for wildlife. The world’s population of spectacled eiders, for example, spends each winter in the northern Bering Sea, depending on the water in leads and polynyas when most other open water is frozen.
|Credit: NASA 2011|
|Pack ice off of Baffin Island, Canada. |
© 2013 Paul Gierszewski.
Polynyas, like leads (see above), are large open areas of water surrounded by sea ice -- or by sea ice on one side and land on the other. Polynyas, however, are wide, not long and narrow. Polynyas and leads are important for wildlife.
Sea-ice pressure ridges form when wind and currents press ice against sea ice, causing the sea ice to buckle and/or layer on top of itself.
Rafting takes place when winds or currents push pieces of sea ice on top of each other.
The RADARSAT-1 Geophysical Processor System (RGPS) was designed to analyze the large volumes of SAR imagery required to produce basin-wide fields of sea-ice motion and estimates of sea-ice age and thickness from sequential RADARSAT-1 maps of the Arctic Ocean. From the RGPS, researchers have been able to obtain routine estimates of the seasonal sea ice produced by deformation of the sea-ice cover.