While freshwater freezes at 0°C, salt water (much like your salted sidewalk in winter) freezes at slightly lower temperatures. As a result, seawater doesn’t typically freeze before reaching -1.9°C. Once it begins to freeze, salt is excluded from the ice crystal in a process known as “brine rejection”, and the surrounding seawater becomes saltier and denser (Ruddimann 2001). The fresh ice floats on the dense water, creating a barrier between the ocean and the atmosphere. This prevents the transfer of heat and gasses across the interface, which is particularly important in the polar oceans. Water has a high heat capacity, and without a barrier, transfers a large amount of heat to the polar atmosphere, keeping the temperature of the lower atmosphere near that of the surface ocean (about 0°C). When sea-ice is present, the atmosphere can cool by up to 30°C (Ruddimann 2001) due to the lack of heat exchange and the increased albedo (reflectivity), discussed in the section on the radiation budget. Such a dramatic difference in air temperature is just one factor making sea-ice an important control on global climate.
PRESENT SEA-ICE EXTENT
Sea-Ice exists today in the polar oceans, in both the northern and southern hemispheres. The amount of sea-ice varies seasonally due to changes in the amount of incoming radiation, but this seasonality is expressed very differently in the northern and southern polar oceans. The Arctic Ocean stays fairly ice-covered year round. Only 10% of ice leaves each year, primarily due to geographical constraints imposed by the surrounding landmasses. As a result, much of the pack ice in the center of the ocean can persist for 4-5 years, and reach up to 4 meters in thickness- ice near the edge is generally closer to 1 meter in thickness. In contrast, the Southern Ocean’s sea ice is much more seasonal. With no landmasses to prevent flow to the north, nearly all of Southern Ocean sea-ice flows north, melts and forms again each year. Consequently, the latitude of northernmost sea-ice extent there varies by up to 20° over the course of a year.
Much of what we know about present sea-ice extent comes from satellite data. There are a number of programs run through the National Oceanic and Atmospheric Administration (NOAA), the National Aeronautics and Space Administration (NASA) and the National Snow and Ice Data Center (NSIDC). Each of these programs has documented a decrease in the amount of sea-ice cover in both polar oceans during the last decade. While the exact cause has yet to be identified, the importance of sea-ice in climate regulation means that this will certainly have implications for future climate.
Arctic and Antarctic sea ice concentration climatology from 1978-2002, at the approximate seasonal maximum and minimum levels. Image provided by National Snow and Ice Data Center, University of Colorado, Boulder.
SEA-ICE AND OCEAN CIRCULATION
Sea-ice affects ocean circulation in two main ways. The first involves the salt/freshwater budget of the ocean. Sea ice holds a large amount of freshwater, forcing salt into the surface layer of the ocean. This increases the density of the water (along with heat loss), and in the North Atlantic Ocean causes it to sink, forming North Atlantic Deep Water (NADW). If this freshwater were to be added to the ocean through melting sea-ice, many people believe that the strong density stratification would be enough to prevent NADW from forming. As NADW is a major part of the ocean conveyor circulation, this could greatly impact the ability of the ocean to take up CO2 from the atmosphere, transport nutrients, and transport heat to high latitudes.
A gap in sea-ice cover, called a polynya, can also influence ocean circulation. There are two varieties of these holes- coastal and open-ocean polynyas. Coastal polynyas generally form around the Antarctic continent when strong winds blow ice offshore for 50-100 km (University 2001). Open-ocean polynyas occur in pack ice, and can range from a few kilometers across to 1000 x 350 km, the size of the Weddell Polynya in the Weddell Sea near Antarctica (University 2001). Open-ocean polynyas occur in the Arctic Ocean as well, but are not linked to any water mass formation there. In the Southern Ocean, however, open-ocean polynyas potentially allow enough cooling of surface water for deep convection to occur. Coastal polynyas are also linked to circulation, since they are associated with sea-ice formation near the coast. As ice is blown offshore, cold air forms new sea-ice near shore, which is also blown away. This leads to an area of high salinity water due to brine rejection, which sinks once the density is higher than the surrounding water.
Schematic diagram of polynyas, from Open University.
SEA-ICE AND THE RADIATION BUDGET
Perhaps the most important effect of sea-ice it its impact on the global (and particularly polar) radiation budget. The simplified radiation budget is balanced by a few main factors- the incoming solar radiation, the amount of that radiation which is absorbed by some part of the Earth, and the portion reflected back out to space. The measure of a surface’s reflectivity is known as its albedo. Darker surfaces tend to have lower albedos since they absorb many wavelengths of light, while lighter surfaces reflect most light. As a result, snow and ice have very high albedos, meaning that snow and ice covered areas absorb far less sunlight than their uncovered counterparts. High latitude ocean albedo is typically near 10%, while snow or ice covered albedo at the same latitude can reach as high as 60% (Hartmann 1994).
The result of the albedo effect is a positive feedback with sea-ice cover. As sea-ice grows, albedo increases, decreasing absorbed radiation and cooling temperatures, allowing more sea-ice to form (Hartmann 1994). When sea-ice melts, the opposite is true. This means that while sea-ice is an important factor in radiation budgets at any time, its self-perpetuating nature is also important on longer time scales. This has possible implications for the response of sea-ice to changes in the amount of total incoming solar radiation, which is known to have changed with time through the Milankovitch orbital cycles. It also implicates sea-ice as a possible factor in perpetuating any changes triggered during transitional periods surrounding glaciations such as the Last Glacial Maximum (LGM), the focus of this project.