Drifter observations of the Hebrides slope current and nearby circulation patterns

The mean flow at and around the Hebrides and Shetland Shelf slope is measur ed with ARGOS tracked drifters. Forty-two drifters drogued at 50 m were dep loyed in three circles over the Hebrides slope at 56.15 degrees N in two re leases, one on 5th December, 1995 and the second on 5-9th May, 1996. The ci rcles span a distance of some 20 km from water depths of 200 m to 1200 m. D rifters are initially advected poleward along-slope by the Hebrides slope c urrent at between 0.05 and 0.70 In s(-1) in a laterally constrained (25-50 km wide) jet-like flow. Drifters released in winter remained in the slope c urrent for over 2000 km whilst summer drifters were lost from the slope cur rent beyond the Wyville-Thomson Ridge, a major topographic feature at 60 de grees N. Dispersion from the slope region into deeper waters occurs at bath ymetric irregularities, particularly at the Anton Dohrn Seamount close to w hich the slope current is found to bifurcate, both in summer and winter, an d at the Wyville-Thomson Ridge where drifters move into the Faeroe Shetland Channel. Dispersion onto the continental shelf occurs sporadically along t he Hebrides slope. The initial dispersion around the Hebrides slope is rema rkably sensitive to initial position, most of the drifters released in shal lower water moving onto the shelf, whilst those in 1000 m or more are mostl y carried away from the slope into deeper water near the Anton Dohrn SI:amo unt. The dispersion coefficients estimated in directions parallel and norma l to the local direction of the 500 m contour, approximately the position o f the slope current core, are approximately 8.8 x 10(3) m(2) s(-1) and 0.36 x 10(3) m2 s(-1), respectively, during winter, and 11.4 x 10(3) m(2) s(-1) and 0.36 x 10(3) m(2) s(-1), respectively, during summer. At the slope the re is a minimum in across-slope mean velocity, Reynolds stress, and across- slope eddy correlations. The mean across-slope velocity associated with mas s flux is about 4x10(-3) ms shelfward across the shelf break during winter and 2 x 10(-3):m s(-1) during summer. The drifters also sam-pled local patt erns of circulation, and indicate that the source of water for the seasonal Fair Isle and East Shetland currents are the same, and drawn from Atlantic overflows at the Hebrides shelf.