4.1 Ocean Circulation
The surface and subsurface movement (circulation) of waters has a strong influence on the formation and distribution of sea ice in the Arctic. Movement of warmer waters will inhibit ice formation. Likewise, the movement of colder, fresher waters into a region will encourage ice formation. Once sea ice has been formed, it will be moved (advected) by the combined influence of the winds and ocean currents.
In the Atlantic Ocean, warm, salty, tropical waters begin their journey toward the Arctic as the Gulf Stream, a narrow, rapid current flowing poleward from Florida to North Carolina where it moves offshore and flows toward England as the North Atlantic Current. As the North Atlantic Current approaches England, it flows increasingly poleward and enters the Greenland and Norwegian Seas, forming the Norwegian Current. These Atlantic waters flow into the Arctic region below the less dense surface waters flowing out of the Arctic. These warm waters have a tremendous impact on the ice conditions of the Greenland and Norwegian Seas. Nearly the entire Norwegian Sea and the eastern portion of the Greenland Sea is typically ice free year round because of the influx of heat carried into the region by these warm subsurface waters.
The flow into the Arctic from the Pacific is considerably different from that of the Atlantic. The waters flowing into the Arctic from the Pacific are fresher and consequently less dense, therefore they flow into the Arctic through the Bering Strait as surface waters.
The final source of waters flowing into the Arctic is the supply of fresh water from four major Canadian and Siberian rivers. The supply of fresh water from these rivers and the fresher surface waters flowing through the Bering Straits reduces the surface salinities and thus encourages sea ice formation in these regions.
Nearly all of the waters leave the Arctic between Greenland and Spitzbergen (or Svalbard) as a cold, fresh, surface flow, forming the East Greenland Current.
As we look at the ocean circulation in Baffin Bay, the Labrador Sea, the Bering Sea, and the Sea of Okhotsk, we see the same general pattern in these semienclosed basins. Warmer waters enter the basin along the eastern boundary and flow poleward, while cooler waters flow equatorward along the western boundary. Again, this circulation pattern has a major influence on the distribution of sea ice in each of these basins, with sea ice concentrations along the western portion of each basin heavier than those along the eastern portion in a typical year.
4.2 Seasonal Sea Ice Cycle
The structure and dynamics of the sea ice cover in the Northern Hemisphere are complex and variable. The Arctic Ocean is in fact a mediterranean sea bounded by land. In contrast, in the Southern Hemisphere most of the ice is bounded by land to the north and the ocean to the south. Many of the Northern Hemisphere peripheral seas (Bering, Greenland, and Barents), however, are also open to the oceans. Confinement of the Arctic Ocean ice pack by land limits its drift and divergence and strongly influences the distribution of ice types, growth and decay rates, and thicknesses. The Arctic Ocean also tends to have lower surface salinities because of the fresh water input from major rivers and confinement of fresh water from melting sea ice in spring and summer, as opposed to the Southern Hemisphere, which has no major river input and is open to the ocean.
The seasonal sea ice cycle is driven primarily by the annual cycle of solar energy received at Earth's surface and atmospheric conditions. The sea ice cycle lags the solar cycle by about 3 months; for example, in the Northern Hemisphere the maximum solar input occurs in late June while the minimum sea ice extent occurs in September. The distribution of sea ice is also heavily influenced by the ocean and atmospheric circulation (Gloersen et al., 1992).
On "average," the ice cover in the Northern Hemisphere varies from a maximum sea ice extent of 15.7 x 106 km2 in late March to a minimum sea ice extent of 9.3 x 106 km2 in early September (Figure 1.02). At the end of the melt season, the permanent ice pack retains about 60% of the ice covered area occurring at the winter maximum (Gloersen et al., 1992). An interesting characteristic of the overall growth and decay cycle is its nearly symmetrical pattern as seen in Figure 1.02; i.e., the fall-winter growth and spring-summer decay periods are about the same length. This contrasts with the marked asymmetry in the southern ocean, where the spring-summer melt is significantly faster than the fall-winter growth (Parkinson et al., 1987).
In September, ice covers much of the central Arctic Ocean and portions of the Canadian Archipelago and the Greenland Sea (Figure 1.03). By the end of October ice covers almost the entire Arctic Ocean and Canadian Archipelago and extends well south along the east coast of Greenland. The ice cover expands rapidly during the next 2 months (November and December) with most of Baffin Bay and the Kara Sea ice covered by the end of November, and Hudson Bay and the northern Bering Sea ice covered by the end of December. Expansion continues at a slower rate in January and February, with the ice by the March maximum (Figure 1.04) generally covering almost the entire Arctic Ocean, Canadian Archipelago, Kara Sea, Baffin Bay, and Hudson Bay plus large portions of the Sea of Okhotsk, Bering Sea, Greenland Sea, Davis Strait, and Barents Sea. Notice how asymmetrical the Northern Hemisphere sea ice distributions are in winter.
The warm Gulf Stream waters moving northeast across the North Atlantic help keep much of the North Atlantic free of ice year round, even at latitudes north of 65°N. In contrast, Hudson Bay is at much lower latitudes but is almost fully ice covered. The reason lies not only in the Bay's lack of access to warm waters from the south but also in its geographic placement in the midst of a continent. Because land surfaces heat and cool much more rapidly than do ocean surfaces subjected to the same intensity of solar radiation, continental areas tend to be warmer in summer and cooler in winter than midocean areas at the same latitude. This phenomenon is sometimes called the continentality effect. Hudson Bay, being landlocked, is affected by the land around it and thus tends to be colder in winter and more likely to have ice than its midocean, same latitudes counterparts.
Retreat generally proceeds slowly in March and April, when it tends to be most pronounced in the Sea of Okhotsk. By mid-May, noticeable retreat has also occurred in the Bering Sea and at the open-water ice edge in the North Atlantic vicinity. By mid-June major openings appear in the ice cover of the remaining regions, and by mid-July most of the ice of Hudson Bay, Davis Strait, and Baffin Bay has disappeared. By August, the ice cover is close to its September minimum, with significant coastal openings around much of the Arctic Ocean and little or no ice remaining in most of the peripheral seas and bays.
Another interesting contrast between the Northern and Southern Hemisphere sea ice distribution lies in the amount of open water area within the sea ice pack. Figures 1.05 and 1.06 show the areal sea ice extent and the amount of open water area within this ice covered area for the Northern Hemisphere from 1979 through 1987. The sea ice extent is defined as the area covered with at least 15% sea ice (or no more than 85% open water). Comparing these two figures, we see that the periods of maximum sea ice extent in March and minimum extent in September also have the least amount of open water area within the sea ice pack. In other words, the Northern Hemisphere sea ice cover is the most consolidated during these periods, composed of much sea ice with very little open water area.
The period of highest open water area occurs each year during the spring-summer sea ice retreat. This pattern in the Northern Hemisphere is a marked contrast to the Southern Hemisphere, where the maximum open water area coincides with the maximum sea ice extent. In other words, the Southern Hemisphere sea ice cover is less consolidated and has more open water area within the sea ice pack than that of the Northern Hemisphere. Again, this difference is due to the fact that the Northern Hemisphere polar region is bounded by land and has a lower surface salinity, while the Southern Hemisphere polar region is bounded by ocean. The Southern Hemisphere sea ice cover experiences more divergent forces from winds and currents, which results in the spreading out of the sea ice cover and the creation of areas of open water within the sea ice cover. Also, the lower surface salinities in the Arctic Ocean are more conducive to forming a consolidated sea ice cover.
These results were determined from the Scanning Multichannel Microwave Radiometer (SMMR) that operated onboard the Nimbus-7 satellite from 1978 to 1987 and was the predecessor of the Special Sensor Microwave Imager (SMM/I) currently operating onboard the DMSP polar orbiting satellites.
4.3 Location of Polynyas
See Figure 1.07.
Latent heat polynyas occur
Sensible heat polynyas occur