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The Slope Current as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The Slope Current is just off-shore of the British Isles and transports Northeast Atlantic water into the Norwegian Sea. Click here for example plots of seasonal averages. | |
Above the Porcupine Bank, the slope current transports Subpolar Mode Water, which has a salinity greater than 35.5, northwards, with peak surface speeds of 20 to 35 cm s-1. Maximum surface speeds are found above the 2000 to 3000 m isobaths and the speed of the SC decreases over deeper ocean water. Below 1500 m depth, the speed decreases to below 10 cm s-1, and near the bottom in 2500 m depth, Dickson et al. (1985) found a mean northward flow of 3.4 cm s-1 at 52.5°N, 15.4°W (Bersch 1995). Huthnance (1986) measured an average speed of 9 cm s-1 near the 3500 m isobath. Above the 1000 m isobath at 57°N, long-term current meter measurements in the upper 500 m indicate average speeds of 9 to 16 cm s-1 (Booth and Ellett 1983).
At the entrance of the Rockall Trough the flow field meanders with speeds of 30 to 35 cm s-1. There is a mean northward flow at the western and eastern sides, while in the central region, the mean flow is southward. The northward flow at the western side was documented by the trajectories of four drouged buoys released in July 1990 (see Fig. 3 in Pingree 1993). The velocity of the current increases northward along the Rockall Channel, but it is not known whether the transport also increases (Huthnance 1986; Pingree and Le Cann 1989; Hackett and Roed 1998). Near the axis of the Rockall Trough at 17°W, Acoustic Doppler Current Profiler (ADCP) data also indicate a relatively strong southward flow. Ellett et al. (1986) present a circulation scheme for the Rockall Trough that consists of four anticyclonic gyres with the southernmost gyre situated just to the north of the ADCP section (Bersch 1995). South of 50°N there is a broad southward flow between 10 and 20°W that feeds the Portugal Current.
16 months of ADCP and current meter observations across the Hebridean shelf edge during the Shelf Edge Study (SES) programme found that the mean flow over the upper slope is approximately parallel to the topography with average speeds of about 20 cm s-1 (Souza et al. 2001). They found that the SC extends down to a depth of 500 m and is predominantly barotropic, especially in winter when the flow is practically uniform between 350 m and the surface. In summer, there is a significant baroclinic component with a pronounced maximum in current speeds at a depth of about 200 m, but there is more than 80% of the eddy kinetic energy is in the barotropic component. The flow is generally more energetic and variable in the winter.
Burrows (1999) tracked drifters for up to 240 days during the summer and winter seasons over the Hebrides slope and found SC speeds between 10-30 cm s-1, with lower speeds in the summer. In summer the drogues are in the seasonal thermocline which forms in early May, and are therefore removed from direct surface forcing to which they are exposed to in the surface mixed layer in winter. Gould et al. (1985) identified a strong seasonality in the slope current west of Shetland, while Poulain et al. (1996), from float measurements made throughout the Greenland, Iceland, and Norwegian Seas estimate that only 10-30% of the eddy kinetic energy could be explained by seasonal effects. This latter estimate is suspect because of data sparseness and eddy aliasing (Heathershaw et al. 1998)
The SC enters both the North Sea and the Norwegian Sea, where it feeds the Norwegian Coastal Current. However, Sherwin et al. (1999) found that in the summer of 1996, there was a significant flow, peak speed of 70 cm s-1, offshore into the Atlantic Ocean. The SC advects warm, saline North Atlantic Water (NAW, pot temp:>9.5°C, S:>35.3) from along the edge of the Scottish continental shelf edge into the Norwegian Sea (Hill and Mitchelson-Jacob 1993; Sherwin et al. 1999). In the region between the southern tip of the Faroe Shelf at 6°W and the slope on the Shetland side at 4°W, the NAW, which occupies the upper 200-300 m of the water column, is in a strong front associated with the SC, and is bounded by the colder Modified North Atlantic WAter (MNAW, potential temperature of 6.5-8.5°C and a salinity of 35.15-35.28) observed at about 4.5°W by Sherwin et al. (1999).
Both current meter and drifter observations from the slope west of Vestfjorden, Norway at approximately 68.5°N, obtained during the NEAT GIN 1989 experiment, indicate a mean flow to the northeast with average speeds between 20 to 40 cm s-1 and peak velocities of 50 to 70 cm s-1. The SC flows approximately parallel to the isobaths in this region (Booth and Meldrum 1987; Poulain et al. 1996; Heathershaw et al. 1998). The measured slope current appears to be consistent with other observations that are towards the northern end of the NW European continental margin and which are generally thought to be steady throughout the year from comparisons with drouged buoy measurements (e.g. Booth and Meldrum 1987), although the question of seasonality in slope currents does not yet appear to be fully resolved.
The ENAW (Eastern North Atlantic Water), more saline than the North Atlantic Central Water (NACW), forms in the Bay of Biscay (Pollard et al. 1996), and is advected northwards, around the Porcupine Bank into the southern Rockall Trough (Ellett and Martin 1973), by the poleward Shelf Edge Current (SEC) with average speeds of 15-30 cm s-1 (New and Smythe-Wright, 2001). The 50 km wide SEC is the shoreward most and strongest filament of the SC. The SEC is associated with a high-salinity core in the upper 400 m of the water column with maximum salinities near 35.4 that decrease to the north because of mixing (Ellett and Martin 1973; Hill and Mitchelson-Jacob 1993; White and Bowyer 1997). White and Bowyer (1997) found peak speeds of 50 cm s-1, average speeds of 10 and 21 cm s-1, and evidence of cross-stream meandering at a depth of 620 m above the 660 m isobath, northwest of Ireland. Transport estimates for the SEC range between 1.2-3.0 Sv (Huthnance 1986; Holliday et al. 2000; New and Smythe-Wright, 2001). In general, current speeds increase northward over the steeper bottom slopes. Pingree et al. (1999) deployed a drifting buoy at 54°N on the Irish shelf and it ended up on the West Shetland shelf at 61°N indicating a continuous SEC (New and Smythe-Wright, 2001).
The mesoscale dynamics of the Scottish side of the Faroe-Shetland Channel have been investigated using synoptic in-situ and remote sensing observations. A cold core cyclonic eddy, identified from an AVHRR image, had a diameter of about 50 km, surface current speeds of up to 50 cm s-1, and it propagated along the 800 m isobath at about 8 cm/s to the northeast (Sherwin et al. 1999). Eddy stirring is important for mixing water between the shelf and the deeper ocean. Northwest of the Shetland Islands, eddy-induced deflections from the mean path of the current are as large as 70 km (Sherwin et al., 1999).