Characteristics of sea ice

Sea ice forms from freezing of seawater. Dissolved salts— predominately NaCl, but many other ions as well—depress the freezing point of seawater by approximately 0.054°C ppt-1. For mean ocean water with a salinity of 34.5 ppt, sea ice forms at -1.86°C. Polar waters are often fresher than average seawater, due in part to limited evaporation. For salinities of 25 ppt and 30 ppt, water freezes at -1.35°c and -1.62°c, respectively.

Figure 5.1. A variety of Arctic sea-ice types and seasons. (a) Ridged landfast ice. (Photograph courtesy of Andy Mahoney, National Snow and Ice Data Center, University of Colorado.) (b) One-week-old ice—nilas—forming in Reykjavik Harbor, December 2000. (Photograph by author.) (c) Late-summer ice floes, Tanquary Fjord, northwestern Ellesmere Island, Canada. (Photograph courtesy of J. Dumas.) (d) Pancake ice in the Bering Sea. (Photograph by R. Behn, NOAA Corps.)

Figure 5.1. A variety of Arctic sea-ice types and seasons. (a) Ridged landfast ice. (Photograph courtesy of Andy Mahoney, National Snow and Ice Data Center, University of Colorado.) (b) One-week-old ice—nilas—forming in Reykjavik Harbor, December 2000. (Photograph by author.) (c) Late-summer ice floes, Tanquary Fjord, northwestern Ellesmere Island, Canada. (Photograph courtesy of J. Dumas.) (d) Pancake ice in the Bering Sea. (Photograph by R. Behn, NOAA Corps.)

First-year sea ice forms in the autumn in the polar regions. New ice has many colorful terms to distinguish it, including nilas, grease, slush, and pancake ice. Figure 5.1 illustrates a spectrum of sea-ice types in the Arctic, including fresh pancake ice in the Bering Sea, late-summer ice floes in the Canadian high Arctic, and a fully developed winter ice cover in the Beaufort Sea (landfast ice on Alaska's North Slope).

Sea ice is made up of a mixture of brine, ice crystals, air, and solid salts. Once nucleated, sea ice provides a platform for the deposition and accumulation of meteoric snow. Brine pockets trapped in first-year sea ice give it salinity values that are commonly in the range 5-15 ppt. During the summer melt season, sea ice reaches the melting point (0°C) and becomes permeable, with water and brine transport along intergranular veins. Brine rejection through this process freshens sea ice. For this reason, multiyear ice, which has survived the summer melt season, has only traces of salinity.

Sea ice thickens through the fall and winter, growing from below through basal accretion, or aggradation. Thermodynamic growth of first-year ice is self-limiting to a thickness of about 2 m, or less than this when sea ice is mantled in a thick snow cover. At thicknesses beyond this, the ocean is effectively insulated from cold atmospheric temperatures, and ocean heat flux into the base of the sea ice is balanced by upward heat conduction through the ice and snow. Thicker ice develops through mechanical ridging under convergence, compression, and overriding (rafting) of ice floes. Pressure ridges can reach thicknesses of 10-20 m.

Through the spring and summer in both the Arctic and Antarctic, a large fraction of first-year ice melts away or is advected to lower latitudes. That which survives the summer becomes multiyear ice, which goes through further growth stages (thermodynamically and through ridging), commonly reaching ice thicknesses of a few meters.

Figure 5.2 plots measured and modeled estimates of the winter thickness distribution of Arctic and Antarctic sea ice. The thick multiyear sea ice evident in figure 5.2a is a result of ice convergence against the Canadian Arctic Archipelago and northern Greenland, which gives thicknesses of about 5 m. These data represent a snapshot from February to March 2005 and are derived from satellite altimetry in the Arctic basin. Ice thicknesses vary from year to year, but this general geographic pattern is persistent. In Antarctica, a more radially symmetric ice thickness pattern is evident, associated with ice divergence away from the coast. Thicker ice along the coast indicates multiyear ice in regions of ice convergence, particularly evident in the western Weddell Sea where ice piles against the Antarctic Peninsula.

During the summer melt season, atmospheric heat flux is the primary driver of sea-ice melt, although there is also basal melting from ocean heat fluxes. Melt begins gradually due to the high albedo of the seasonal snow cover. once the snow ablates, melt ponds form on the ice surface (e.g., figure 1.2), the albedo is dramatically reduced, and melt proceeds swiftly. In the Arctic, more

Figure 5.2. Thickness distribution of winter sea ice in (a) the Arctic basin and (b) the Southern Ocean. (a) Arctic ice thickness from March 2006, derived from ICESat altimetry (Kwok et al., 2009). (Image courtesy of NASA's Goddard Scientific Visualization Studio.) (b) Mean September sea-ice thickness from a regional (circumpolar) configuration of a finite element sea-ice-ocean model. (Image courtesy of Ralph Timmermann, Alfred Wegener Institute.)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 l.S 2.0 3.0 4.0 S.0 Ice thickness (m)

than half of the ice pack typically melts away in the summer, and more than 85% of the Southern Hemisphere sea ice melts each summer (see table 1.1).

Although climate warming is being felt in the Arctic (chapter 9), this seasonal cycle is remarkably consistent from year to year. Figure 5.3 plots Arctic and Antarctic sea-ice extent for the period 1990-2000, based on microwave remote-sensing measurements of monthly mean ice extent. Sea-ice extent refers to the total area with at least 15% sea-ice cover; ice extent is therefore greater than the ice area. This figure testifies to the overall dominance of seasonal insolation cycles in governing sea-ice cover; other aspects of the climate system, such as variability in wind, pressure, and ocean conditions, influence year-to-year ice anomalies but these are difficult to discern in figure 5.3.

The geometry of the Arctic basin also plays a hand in the consistent maximum winter ice extent in the north; winters are cold enough that most of the Arctic basin freezes over each year, with ice extent limited and defined by the continents. Antarctic sea ice is not continen-tally constrained, but its annual maximum is also very consistent, governed by the "ablation wall" imposed by the relatively warm waters of the Antarctic circumpolar current. These plots indicate total ice cover in each hemisphere; regional ice cover is much more variable.

Figure 5.4 plots the geographic extent of sea ice for the times of minimum and maximum ice cover in each hemisphere. The white areas indicate the minimum and maximum ice extents in 2009, and the black lines a. Northern Hemisphere b. Southern Hemisphere a. Northern Hemisphere

Months (1990-2000)

Figure 5.3. Monthly sea-ice extent in the (a) Northern Hemisphere and (b) Southern Hemisphere, January 1990 to December 2000. (Data from the U.S. National Snow and Ice Data Center.)

Months (1990-2000)

Months (1990-2000)

Figure 5.3. Monthly sea-ice extent in the (a) Northern Hemisphere and (b) Southern Hemisphere, January 1990 to December 2000. (Data from the U.S. National Snow and Ice Data Center.)

indicate the median monthly ice extent for the period 1979-2000.

The polar oceans are not covered by a continuous, uniform sheet of ice. Rather, sea ice is made up of a mixture of open water, pack ice, and landfast ice, along with the occasional iceberg that has come off of a terrestrial ice mass and been entrained in the sea ice. Sea ice concentration refers to the areal fraction of ice cover in a region. Landfast ice, often called fast ice, is frozen to the shore, or it can be anchored to the seafloor in shallow, near-shore environments. It swells with the tides but is otherwise immobile. Ice cover in landfast ice is generally continuous. Open-water ice floes, in contrast, are discontinuous and highly mobile. Pack ice drifts at speeds of order 0.1 m s-1, driven by ocean currents and wind stress

Ic» extent: 29 x 106 km2 icetaxtemtn1:9ii x 10f{kmin2

Figure 5.4. Maximum and minimum sea-ice extent in the (a, b) Northern Hemisphere and (c, d) Southern Hemisphere. All plots show the monthly minimum and maximum ice extents in 2009, along with the median value for the period 1979-2000 (black lines). (Data from the U.S. National Snow and Ice Data Center.)

Ic» extent: 29 x 106 km2 icetaxtemtn1:9ii x 10f{kmin2

Figure 5.4. Maximum and minimum sea-ice extent in the (a, b) Northern Hemisphere and (c, d) Southern Hemisphere. All plots show the monthly minimum and maximum ice extents in 2009, along with the median value for the period 1979-2000 (black lines). (Data from the U.S. National Snow and Ice Data Center.)

and subject to Coriolis deflection. Pack ice, also called drift ice, consists of a mixture of ice floes and open water, with the latter taking the form of polynyas (open areas) or leads (cracks between ice floes). Open-water areas are sources of heat and moisture flux to the atmosphere. Many polynyas are persistent features from year to year, as open water is maintained by sea-ice divergence (due to prevailing winds or currents) or upwelling of warm ocean waters.

In the Arctic, pack ice that is entrained in the Beaufort gyre can circulate through the Arctic basin for several years before being exported to the North Atlantic through Fram Strait and the channels of the Canadian Arctic Archipelago. The East Greenland Current is a kind of "sea-fce alley" where huge volumes of sea ice are advected southward each year, including thick, mul-tiyear ice. The redoubtable Fridtjof Nansen recognized this and many other nuances of sea ice during his voyages in the Arctic in the late 1800s. Nansen described this ice upon approach to southeast Greenland in summer 1888, shortly before disembarking to complete the first crossing of the Greenland ice sheet:

It must not be supposed that this drifting ice of the Arctic seas forms a single continuous field. It consists of aggregations of larger and smaller floes, which may reach thicknesses of thirty or forty feet or even more. How these floes are formed and where they come from is not yet known with certainty, but it must be somewhere in the open sea far away in the north, or over against the Siberian coast, where no one has hitherto forced his way. Borne on the polar current, the ice is carried southwards along the east coast of Greenland.

Thick, multiyear ice is prevalent where winds pile up ice in parts of the basin, such as the northern coasts of the Canadian Arctic Archipelago (figure 5.2a). The thick ice floes that Nansen observed drifting southward would have been multiyear ice from the archipelago or the Beaufort Sea gyre that was caught up in the transpolar drift and exported through Fram Strait. The geography is much simpler in Antarctica. Offshore winds from the continent push ice northward, giving a divergent ice pack with little multiyear ice. This offshore export is very effective in melting first-year ice, giving the strong summer minimum in the Southern Ocean that is seen in figure 5.3b.

These processes also give systematic differences in ice concentration in each polar region; the average annual ice concentration in the Northern Hemisphere is 83%, versus 72% in the Southern Hemisphere. In the Northern Hemisphere, the average (1979-2010) ice area varied from 4.8 X 106 km2 in September to 13.6 X 106 km2 in March. Ice extents for the same period ranged from 6.6 X 106 to 15.5 X 106 km2. In the Southern Hemisphere, ice area and extent for the period 1979-2010 ranged from 1.9 X 106 to 14.5 X 106 km2 and 3.0 X 106 to 18.8 X 106 km2, respectively, with a minimum in February and maximum in September. It is difficult to estimate hemispheric-scale ice area and extent prior to satellite observations.

Whereas seasonal insolation cycles and land-sea geography shape the sea-ice extent in each hemisphere, in-terannual and decadal variability in sea-ice thickness and concentration are influenced by air temperature, ocean heat fluxes, ocean circulation, and atmospheric pressure patterns, which drive surface winds. As discussed earlier, prevailing winds can concentrate ice along coastlines, supporting thick, multiyear ice, or they can drive ice divergence and export. Because these meteorological controls are immediate, sea ice adjusts rapidly to climate variability and change. An anomalously warm summer can lead to loss of sea ice, with numerous positive feedbacks that include decreased local and regional-s cale albedo, solar radiative heating of open water, increased sensible and longwave heat transfer from open water to the atmospheric boundary layer, and effects of open water on cloud cover. In the Arctic, these positive feedbacks contribute to multiyear "memory" and decadal-scale variability in ice volume. There is less multiyear ice in the Southern Hemisphere, so sea-i ce volume in the Southern Ocean has less memory; thermodynamic processes here produce interannual variability but limited decadal variability.

Exceptionally detailed views of ice area, extent, and motion are available for the modern, satellite era (e.g., figures 5.2-5.4), whereas other aspects of sea ice are difficult to quantify. In particular, basin-scale measurements of ice thickness are elusive. Many local observations are available, and upward-directed sonar from submarine surveys provides good transect data, but sea ice is constantly shifting. Repeat sonar surveys along a particular transect provide information about ice-thickness changes over time, but they do not tell the complete story. Interannual variability in drift or convergence of the pack ice can give large differences in thickness in a region even though there may be little or no change in total basin ice volume.

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