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Arctic Ocean Circulation ARCTIC OCEAN CIRCULATION B. Rudels, Finnish Institute of Marine Research, The physical oceanography of the Arctic Ocean is Helsinki, Finland shaped by the severe high-latitude climate and the B & 2009 Elsevier Ltd. All rights reserved. large freshwater input from runoff, 0.1 Sv (1 Sv ¼ 1 Â 106m3 s À 1), and net precipitation, B0.07 Sv. The Arctic Ocean is a strongly salinity stratified ocean that allows the surface water to cool to freezing temperature and ice to form in winter and to remain throughout the year in the central, deep part of the Introduction ocean. The Arctic Ocean is the northernmost part of the The water masses in the Arctic Ocean can, because Arctic Mediterranean Sea, which also comprises the of the stratification, be identified by different vertical Greenland Sea, the Iceland Sea, and the Norwegian layers. Here five separate layers will be distinguished: Sea (the Nordic Seas) and is separated from the 1. The B50-m-thick upper, low-salinity polar mixed North Atlantic by the 500–850-m-deep Greenland– layer (PML) is homogenized in winter by freezing Scotland Ridge. The Arctic Ocean is a small, and haline convection, while in summer the upper 6 2 9.4 Â 10 km , enclosed ocean. Its boundaries to 10–20 m become diluted by sea ice meltwater. The the south are the Eurasian continent, Bering Strait, salinity, S, ranges from 30 to 32.5 in the Canadian North America, Greenland, Fram Strait, and Basin and between 32 and 34 in the Eurasian Svalbard. The shelf break from Svalbard southward Basin. to Norway closes the boundary. The Arctic Ocean 2. The 100–250-m-thick halocline, with salinity in- lies almost entirely north of and occupies most of the creasing with depth, while the temperature re- region north of the polar circle. More than half of mains close to freezing, 32.5oSo34.5 (Figure 2). its area, 53%, consists of large, shallow shelves, 3. The 400–700-m-thick Atlantic layer historically the broad Eurasian marginal seas: the Barents Sea defined as subsurface water with potential tem- (200–300 m), the Kara Sea (50–100 m), the Laptev perature, y, above 0 1C, 34.5oSo35. Sea (o50 m), the East Siberian Sea (o50 m) and the 4. The intermediate water below the Atlantic layer Chukchi Sea (50–100 m), and the narrower shelves that communicates freely across the Lomonosov north of North America and Greenland. The deep Ridge, À 0.5 1Coyo0 1C, 34.87oSo34.92. Arctic Ocean comprises two major basins, the 5. The deep and bottom waters in the different Eurasian Basin and the Canadian (also called basins, À 0.55 1Coyo À 0.5 1C, 34.92oSo34.96 Amerasian) Basin, separated by the approximately (Canadian Basin), À 0.97 1Coyo À 0.5 1C, 34.92o 1600-m-deep Lomonosov Ridge. The Eurasian Basin So34.945 (Eurasian Basin). is further divided into the Nansen and Amundsen basins by a mid-ocean ridge (the Gakkel Ridge), There are large lateral variations of the character- while the Canadian Basin is separated by the Alpha istics in these layers that depend upon the circulation Ridge and the Mendeleyev Ridge into the Makarov and upon the mixing processes in the Arctic Ocean and the Canada Basins. The Amundsen Basin is the (Figures 2 and 3). A more detailed classification, es- deepest (B4500 m), while the maximum depths of pecially for the deeper water masses, is given in the Makarov and the Nansen Basins are B4000 m. Table 1. It should be kept in mind that this classifi- The Canada Basin is slightly shallower (B3800 m) cation is not unique and that several others exist in but by far the largest (Figure 1). the literature. The Arctic Ocean water masses are primarily of Atlantic origin. Atlantic waters (AWs) enter the Circulation Arctic Ocean from the Nordic Seas through the 2600-m-deep Fram Strait and over the B200-m-deep The circulation of the uppermost layers of the sills in the Barents Sea. The Arctic Ocean also re- Arctic Ocean has mainly been inferred from the ice ceives low-salinity Pacific water through the shallow drift, determined from satellites and from drifting (45 m) and narrow (50 km) Bering Strait. The out- buoys, while at deeper levels primarily the distri- flows occur through Fram Strait and through the butions of temperature and salinity and more re- shallow (150–230 m) and narrow channels in the cently of other tracers have been used to deduce the Canadian Arctic Archipelago. movements of the different water masses, often with 211 212 ARCTIC OCEAN CIRCULATION 135° E90° E 180° 45° E 35° W 0° 90° W 45° W Figure 1 Map of the Arctic Mediterranean Sea showing geographical and bathymetric features. The bathymetry is from IBCAO (the International Bathymetric Chart of the Arctic Ocean; Jakobsson et al., 2000; 2008) and the projection is Lambert equal area. The 500 and 2000 m isobaths are shown. All maps used here are made by Martin Jakobsson (personal communication). BIT, Bear Island Trough; CB, Canadian Basin; EB, Eurasian Basin; GFZ, Greenland Fracture Zone; MJP, Morris Jessup Plateau; JMFZ, Jan Mayen Fracture Zone; SAT, St. Anna Trough; YM, Yermak Plateau; VC, Victoria Channel; VS, Vilkiltskij Strait; FJL, Franz Josef Land; BS, Barrow Strait; HG & CS, Hell Gate and Cardigan Sound. the assumption that the circulation is largely con- the Laptev Sea, carries ice across the Eurasian Basin. trolled by the bathymetry. Direct, moored current About 90% of the ice export (0.09 Sv) passes measurements have been scarce and mostly confined through Fram Strait (Figure 4). to the continental slope and to the Lomonosov The PML and the halocline are maintained by Ridge. These measurements have confirmed the im- river runoff, ice melt, and the inflow of low-salinity portance of the bathymetry for the circulation. In the water through Bering Strait. The lowest surface deep basins current measurements have been made salinities and the thickest halocline are therefore from drifting ice camps and more recently also from observed in the Canada Basin. The inflow through autonomous ice-tethered platforms, relaying the ob- Bering Strait, although affected by local winds, is in servations via satellite to shore. Subsurface drifters the last instance driven by a higher sea level in the are just beginning to be used and observational ef- North Pacific as compared to the Arctic Ocean. This forts during the International Polar Year (IPY) 2007– creates a pressure gradient that forces the Pacific 09 are likely to significantly increase the knowledge water northward into the Arctic Ocean. The Bering of the circulation in the Arctic Ocean. Strait inflow continues across the Chukchi Sea in The motions of the ice cover and the surface water four branches. One branch enters the East Siberian are predominantly forced by the wind, and the at- Sea, while one of the central inflow branches enters mospheric high-pressure cell over the Arctic creates the Arctic Ocean along the Herald Canyon west of the anticyclonic Beaufort gyre in the Canada Basin. the Chukchi Plateau, and the other passes via the Ice leaks from the offshore side of the gyre and joins Central Gap east of the Chukchi Plateau into the the Transpolar Drift (TPD) that brings ice from the Canada Basin. The easternmost branch reaches the Canada Basin toward Fram Strait. A second branch Canada Basin along the Barrow Canyon close to originating from the Siberian shelves, mainly from Alaska. River runoff, mainly from the Mackenzie ARCTIC OCEAN CIRCULATION 213 0 PML NB 200 AB Halocline MB CB 400 Pressure (dbar) 600 −2 −1 0 1 2 32.0 33.0 34.0 35.0 Potential temperature (°C) Salinity 3 C) ° 2 26 1 27 -27.5 0 26.5 −1 28 Potential temperature ( −2 32.0 32.5 33.0 33.5 34.0 34.5 35.0 Salinity Figure 2 Potential temperature and salinity profiles and yS curves from the upper layers of the Nansen Basin (NB, dark yellow), Amundsen Basin (AB, green), Makarov Basin (MB, magenta), and Canada Basin (CB, blue). The PML and the halocline are indicated in the salinity profiles. Above the PML, the low-salinity layer due to seasonal ice melt is seen. The temperature maximum in the Canada Basin is due to the presence of Bering Strait Summer Water (BSSW) and the temperature minimum below indicates the upper halocline with S B 33.1, deriving from the colder, more saline Bering Strait Winter Water (BSWW) and from brine release and haline convection in the Chukchi Sea. No halocline is present in the Nansen Basin, only a deep winter mixed layer between the thermocline and the seasonal ice melt layer with temperature close to freezing. The curved shape of the Nansen Basin thermocline as seen in the yS diagram suggests wintertime haline convection with dense, saline parcels penetrating into the thermocline. Adapted from Rudels B, Jones EP, Schauer U, and Eriksson P (2004) Atlantic sources of the Arctic Ocean surface and halocline waters. Polar Research 23: 181–208. River, adds freshwater to the Canada basin. The recirculates westward in the strait to join the south- runoff peaks in early summer (June). The river runoff ward-flowing East Greenland Current, and the rest as well as most of the Pacific water becomes trapped enters the Arctic Ocean in two streams. One stream in the anticyclonic Beaufort gyre, forming an oceanic flows over the Svalbard shelf and slope, the other high-pressure cell in the southern Canada Basin. The passes west and north around the Yermak Plateau Pacific water leaves the Beaufort gyre and the Arctic and then continues eastward, eventually joining Ocean mainly through the Canadian Arctic Archi- the inner stream at the continental slope east of pelago, but a smaller fraction also exits, at least Svalbard.
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