The Norwegian Sea(; ; ) is a marginal sea, grouped with either the Atlantic Ocean or the Arctic Ocean, northwest of Norway between the North Sea and the Greenland Sea, adjoining the Barents Sea to the northeast. In the southwest, it is separated from the Atlantic Ocean by a submarine ridge running between Iceland and the Faroe Islands. To the north, the Jan Mayen Ridge separates it from the Greenland Sea.
Unlike many other seas, most of the bottom of the Norwegian Sea is not part of a continental shelf and therefore lies at a great depth of about on average. Rich deposits of oil and natural gas are found under the sea bottom and are being explored commercially, in the areas with sea depths of up to about . The coastal zones are rich in fish that visit the Norwegian Sea from the North Atlantic or Barents Sea (cod) for spawning. The warm North Atlantic Current ensures relatively stable and high water temperatures, so that unlike the Arctic seas, the Norwegian Sea is ice-free throughout the year. Recent research has concluded that the large volume of water in the Norwegian Sea with its large heat absorption capacity is more important as a source of Norway's mild winters than the Gulf Stream and its extensions.Westerly storms warm Norway . The Research Council of Norway. Forskningsradet.no (3 September 2012). Retrieved on 2013-03-21.
Extent The International Hydrographic Organization defines the limits of the Norwegian Sea as follows:
On the Northeast. A line joining the southernmost point of West Spitzbergen to North Cape of Bjørnøya (Bear Island), through this island to Cape Bull and thence on to North Cape, Norway (25°45'E).
On the Southeast. The West coast of Norway between North Cape and Cape Stadt ().
On the South. From a point on the West coast of Norway in Latitude 61°00' North along this parallel to Longitude 0°53' West thence a line to the NE extreme of Fuglö () and on to the East extreme of Gerpir () in Iceland.
On the West. The Southeastern limit of Greenland Sea [A line joining the southernmost point of West Spitzbergen to the Northern point of Jan Mayen Island, down the West coast of that island to its Southern extreme, thence a Line to the Eastern extreme of Gerpir () in Iceland].
Formation and geography
The Norwegian Sea was formed about 250 million years ago, when the Eurasian Plate of Norway and the North American Plate, including Greenland, started to move apart. The existing narrow shelf sea between Norway and Greenland began to widen and deepen.Terje Thornes & Oddvar Longva "The origin of the coastal zone" in: Sætre, 2007, pp. 35–43 The present continental slope in the Norwegian Sea marks the border between Norway and Greenland as it stood approximately 250 million years ago. In the north it extends east from Svalbard and on the southwest between Britain and the Faroes. This continental slope contains rich fishing grounds and numerous coral reefs. Settling of the shelf after the separation of the continents has resulted in landslides, such as the Storegga Slide about 8,000 years ago that induced a major tsunami.
The coasts of the Norwegian Sea were shaped during the last ice age. Large glaciers several kilometres high pushed into the land, forming fjords, removing the crust into the sea, and thereby extending the continental slopes. This is particularly clear off the Norwegian coast along Helgeland and north to the Lofoten Islands. The Norwegian continental shelf is between wide, and has a different shape from the shelves in the North Sea and Barents Sea. It contains numerous trenches and irregular peaks, which usually have an amplitude of less than , but can reach up to . They are covered with a mixture of gravel, sand, and mud, and the trenches are used by fish as spawning grounds. Deeper into the sea, there are two deep basins separated by a low ridge — its deepest point at — between the Vøring Plateau and Jan Mayen island. The southern basin is larger and deeper, with large areas between deep. The northern basin is shallower at , but contains many individual sites going down to . Submarine thresholds and continental slopes mark the borders of these basins with the adjacent seas. To the south lies the European continental shelf and the North Sea, to the east is the Eurasian continental shelf with the Barents Sea. To the west, the Scotland-Greenland Ridge separates the Norwegian Sea from the North Atlantic. This ridge is on average only deep, only in a few places reaching the depth of . To the north lie the Jan Mayen Ridge and Mohns Ridge, which lie at a depth of , with some trenches reaching depths of about .
Hydrology
Four major water masses originating in the Atlantic and Arctic oceans meet in the Norwegian Sea, and the associated currents are of fundamental importance for the global climate. The warm, salty North Atlantic Current flows in from the Atlantic Ocean, and the colder and less saline Norwegian Current originates in the North Sea. The so-called East Iceland Current transports cold water south from the Norwegian Sea toward Iceland and then east, along the Arctic Circle; this current occurs in the middle water layer. Deep water flows into the Norwegian Sea from the Greenland Sea. The tides in the sea are semi-diurnal; that is, they rise twice a day, to a height of about .
Surface currents The hydrology of the upper water layers is largely determined by the flow from the North Atlantic. It reaches a speed of 10 Sv (1 Sv = million m3/s) and its maximum depth is at the Lofoten Islands, but normally it is within . Part of it comes through the Faroe-Shetland Channel and has a comparatively high salinity of 35.3‰ (parts per thousand). This current originates in the North Atlantic Current and passes along the European continental slope; increased evaporation due to the warm European climate results in the elevated salinity. Another part passes through the Greenland-Scotland trench between the Faroe Islands and Iceland; this water has a mean salinity between 35 and 35.2‰.Aken, 2007, pp. 119–124 The flow shows strong seasonal variations and can be twice as high in winter as in summer. While at the Faroe-Shetland Channel it has a temperature of about , it cools to about at Svalbard and releases this energy (about 250 terawatts) to the environment.Roald Sætre Driving forces in: Sætre, 2007, pp. 44–58
The current flowing from the North Sea originates in the Baltic Sea and thus collects most of the drainage from northern Europe; this contribution is however relatively small.Sætre, 2007, pp. 44–58 The temperature and salinity of this current show strong seasonal and annual fluctuations. Long-term measurements within the top near the coast show a maximum temperature of at the 63° N parallel in September and a minimum of at the North Cape in March. The salinity varies between 34.3 and 34.6‰ and is lowest in spring owing to the inflow of melted snow from rivers. The largest rivers discharging into the sea are Namsen, Ranelva, and Vefsna. They are all relatively short, but have a high discharge rate owing to their steep, mountainous nature.
A portion of the warm surface water flows directly, within the West Spitsbergen Current, from the Atlantic Ocean, off the Greenland Sea, to the Arctic Ocean. This current has a speed of 3–5 Sv and has a large impact on the climate.Tyler, 2003, pp. 45–49 Other surface water (~1 Sv) flows along the Norwegian coast in the direction of the Barents Sea. This water may cool enough in the Norwegian Sea to submerge into the deeper layers; there it displaces water that flows back into the North Atlantic.Tyler, 2003, pp. 115–116
Arctic water from the East Iceland Current is mostly found in the southwestern part of the sea, near Greenland. Its properties also show significant annual fluctuations, with long-term average temperature being below and salinity between 34.7 and 34.9‰. The fraction of this water on the sea surface depends on the strength of the current, which in turn depends on the pressure difference between the Icelandic Low and Azores High: the larger the difference, the stronger the current.ICES, 2007, pp. 2–4
Deep-sea currents The Norwegian Sea is connected with the Greenland Sea and the Arctic Ocean by the deep Fram Strait.Tyler, 2003, pp. 240–260 The Norwegian Sea Deep Water (NSDW) occurs at depths exceeding ; this homogeneous layer with a salinity of 34.91‰ experiences little exchange with the adjacent seas. Its temperature is below and drops to at the ocean floor. Compared with the deep waters of the surrounding seas, NSDW has more nutrients but less oxygen and is relatively old.Aken, 2007, pp. 131–138
The weak deep-water exchange with the Atlantic Ocean is due to the small depth of the relatively flat Greenland-Scotland Ridge between Scotland and Greenland, an offshoot of the Mid-Atlantic Ridge. Only four areas of the Greenland-Scotland Ridge are deeper than : the Faroe-Bank Channel (about ), some parts of the Iceland-Faroe Ridge (about ), the Wyville-Thomson Ridge (), and areas between Greenland and the Denmark Strait () – this is much shallower than the Norwegian Sea. Cold deep water flows into the Atlantic through various channels: about 1.9 Sv through the Faroe Bank channel, 1.1 Sv through the Iceland-Faroe channel, and 0.1 Sv via the Wyville-Thomson Ridge.Skreslet & NATO, 2005, p. 93 The turbulence that occurs when the deep water falls behind the Greenland-Scotland Ridge into the deep Atlantic basin mixes the adjacent water layers and forms the North Atlantic Deep Water, one of two major deep-sea currents providing the deep ocean with oxygen.Ronald E. Hester, Roy M. Harrison Biodiversity Under Threat, Royal Society of Chemistry, 2007 , p. 96
Climate The thermohaline circulation affects the climate in the Norwegian Sea, and the regional climate can significantly deviate from average. There is also a difference of about between the sea and the coastline. Temperatures rose between 1920 and 1960,Gerold Wefer, Frank Lamy, Fauzi Mantoura Marine Science Frontiers for Europe, Springer, 2003 , pp. 32–35 and the frequency of storms decreased in this period. The storminess was relatively high between 1880 and 1910, decreased significantly between 1910 and 1960, and then recovered to the original level.Matti Seppälä The Physical Geography of Fennoscandia, Oxford University Press, 2005 , pp. 121–141
In contrast to the Greenland Sea and Arctic seas, the Norwegian Sea is ice-free year round, owing to its warm currents. The convection between the relatively warm water and cold air in the winter plays an important role in the Arctic climate.Schaefer, 2001, pp. 10–17 The July isotherm (air temperature line) runs through the northern boundary of the Norwegian Sea and is often taken as the southern boundary of the Arctic.Kieran Mulvaney At the Ends of the Earth: A History of the Polar Regions, Iceland Press, 2001 , p. 23 In winter, the Norwegian Sea generally has the lowest air pressure in the entire Arctic and where most Icelandic Low depressions form. The water temperature in most parts of the sea is in February and in August.
Flora and fauna The Norwegian Sea is a transition zone between boreal and Arctic conditions, and thus contains flora and fauna characteristic of both climatic regions.Blindheim, 1989, pp. 366–382 The southern limit of many Arctic species runs through the North Cape, Iceland, and the center of the Norwegian Sea, while the northern limit of boreal species lies near the borders of the Greenland Sea with the Norwegian Sea and Barents Sea; that is, these areas overlap. Some species like the scallop Chlamys islandica and capelin tend to occupy this area between the Atlantic and Arctic oceans.Skreslet & NATO, 2005, pp. 103–114
Plankton and sea bottom organisms