| ||
 |
The West and East Spitsbergen Currents as represented by the Mariano Global Surface Velocity Analysis (MGSVA). This plot shows that these currents are fed by the East Iceland Current and the Norwegian Current. These currents are the polar limb of the North Atlantic circulation and contain source waters from the North Atlantic Current and North Atlantic Drift. Click here for example plots of seasonal averages. | |
The WSC is the northernmost extension of the Norwegian Atlantic Current. It flows poleward through eastern Fram Strait along the western coast of Spitsbergen. A mainly barotropic current, the WSC appears to be predominantly steered by the bathymetry (Bourke et al. 1988). It is about 100 km wide and is confined over the continental slope, where it reaches its maximum current speed of 24 to 35 cm s-1 at the surface (Boyd and D'Asaro 1994; Fahrbach et al. 2001; Saloranta and Svendsen 2001). Because it transports relatively warm (6 to 8°C) and salty (35.1 to 35.3) Atlantic Water, the WSC keeps this area free of ice (Aagaard et al. 1987; Maslowski 1994; Piechura et al. 2001).
At around 79°N the WSC splits in two. The Svalbard branch stays close to the continental shelf of Spitsbergen, flowing north and east and eventually sinking and spreading at intermediate depths (Perkin and Lewis 1984; Aagaard et al. 1987; Bourke et al. 1988). It travels around the polar basin and returns, after much modification, to the Atlantic with the EGC (Perkin and Lewis 1984; Aagaard et al. 1987). The Yermak branch first follows the western side of the Yermak Plateau. Then, north of 80°N, it detaches from the plateau and flows more directly westward (Aagaard 1987; Bourke et al. 1988). It usually loses its Atlantic Water signal quickly (Manley 1995) due to enhanced tidal mixing in the area (Padman 1995; Gascard et al. 1995; Saloranta and Haugan 2001). The Yermak Branch eventually recirculates southward as the Return Atlantic Current (Gladfelter 1964; Paquette et al. 1985; Aagaard and Coachman 1968; Bourke et al. 1988). The warm, saline Return Atlantic Current flows southward along the eastern edge of the EGC to a depth of about 300 m (Bourke et al. 1987; Quadfasel et al. 1987; Bourke et al. 1988). The sharp temperature and salinity gradients between the two form the East Greenland Polar Front (Bourke et al. 1988). In addition to the two major branches of the WSC, at least three others have been identified in the literature (Richez 1998; Schlichtholz and Houssais 1999), but they have not been found consistently in every study.
As Hopkins (1991) indicates, early transport estimates for the WSC included 2.5 Sv (Lemnov 1947), 1.1 Sv (Hill and Lee 1957), 3.2 ±1.5 Sv (Kislyakov 1962), and 3.1 ±0.6 (Timofeyev 1962), with minimum flow as low as 0.5 Sv in the summer and maximum flow as high as 5.5 in the winter. Later transport estimates showed 8.0 Sv (Aagaard et al. 1973), 7.0 Sv (Greisman 1976), and 5.6 Sv (Hanzlick 1983), with minimum flows of 1.4 Sv in March and maximum flows of 11.9 Sv in December. The later studies report higher transports because they used direct current observations, unlike earlier studies that assumed zero flow at some reference level. In addition, various attempts to measure the transport of the WSC have not produced consistent estimates because the flow is complex and has counter-currents and large variabilities (Hopkins 1991). The most recent transport measurement shows even higher values. Fahrbach et al. (2001) calculated monthly mean transports by integrating the interpolated monthly mean velocities fields. They found a strong annual cycle with a maximum in winter (February) of 20 Sv and a minimum in summer (August) of 5 Sv. The average yearly transport was 9.5 ±1.4 Sv to the north at 78°50'N. Monthly fluctuations were on the order of 2 Sv.
At the southernmost part of the current, the warm core reaches the surface and is thus cooled directly by the atmosphere. Some of the Atlantic Water in the current cools at a constant salinity and transforms into Lower Arctic Intermediate Water (Boyd and D'Asaro 1994). As the current flows north, ice from the Barents Sea is advected over the warm core, which then cools and freshens rapidly. Mixing at the surface occurs through wind, waves, and ice motion. This leads to the conversion of Atlantic water to Arctic Surface Water, whose temperature and salinity characteristics depend on the amount of ice melting and atmospheric cooling. Through this process, the warm core becomes separated from the surface by a layer of stratification, so it is then insulated from further surface mixing (Boyd and D'Asaro 1994).
According to Haugan (1999), heavy ice conditions west of Spitsbergen during spring could prevent the current from losing heat to the atmosphere by generating a surface layer of meltwater. This, in turn, could allow the current to transport more heat to the Arctic Ocean, which would result in warm summers north of Svalbard. Although the warm core becomes isolated from the surface by Arctic Surface Water, its temperature continues to cool downstream at a rate of about 200 W m-2, or about 0.5°C per 100 km from 74.5°N to 81°N (Boyd and D'Asaro 1994). The estimates of Boyd and D'Asaro showed that the WSC loses more heat than is gained by local air and ice. Therefore, they concluded that the heat spreads horizontally before it is lost to the atmosphere and ice.
The WSC can exhibit considerable variation in temperature. For example, Haugan (1999) found that the temperature in the upper 50 m was almost 2°C warmer in September 1998 than in September 1997. Although the survey in September was 16 days later in the year than in 1998, Haugan rejected this as an explanation for the difference because the cooling rate is only 0.5°C per month. Instead, he proposed that the difference was due to interannual variability.
The current also has areas of large mesoscale variability. The inshore side of the WSC appears to be confined to the continental slope due to its potential vorticity (Hanzlick 1983; Jonsson et al. 1992; Woodgate et al. 1998; Saloranta and Haugan 2001); however, the Lagrangian float trajectories of Gascard et al. (1995) and Richez (1998) indicate that the offshore side of the current seems to exhibit much more variability and eddying (Saloranta and Haugan 2001). The waters on this side apparently recirculate in or near Fram Strait and join the EGC (Bourke et al. 1988; Gascard et al. 1995; Saloranta and Haugan 2001). Piechura et al. (2001) also observed many mesoscale cyclonic and anticyclonic eddies in the main stream of the WSC. Boyd and D'Asaro (1994) investigated the WSC based on data from a 3-week cruise in late January and early February of 1989. A complex structure with many eddies was evident on horizontal scales smaller than 10-20 km. By averaging, they found a simpler structure: a core of warm water bounded on the offshore side by a density front that separates it from the cold Greenland Sea and on the onshore side by Spitsbergen shelf waters that are colder and fresher. The boundary between the onshore shelf waters and the WSC is the Arctic Front.
At the surface, the Arctic Front is a density front associated with fresh surface water, and it often resembles a wedge that thickens toward the shore. The bulk of this fresh water comes from glaciers and rivers, especially through the major fjords of Spitsbergen (Saloranta and Svendsen 2001). The subsurface portion of the front (below 50 m depth) is a temperature-salinity front between the core of the WSC and the Spitsbergen shelf waters. There is no density front at this depth because the changes in temperature and salinity are balanced. The heat exchange across this front can cause a heat loss in the WSC of the same magnitude as the current's heat loss to the atmosphere (Saloranta and Svendsen 2001).
There is also an East Spitsbergen Current, but very little is known about it. Apparently it carries Arctic Water between Spitsbergen and Frans Josef Land (an archipelago northwest of Spitsbergen) and then southward along the eastern coast of Spitsbergen until it reaches the Barents Sea (Loeng 1991; Pfirman et al. 1994).