Residual transport within the Fair Isle Current observed during the Autumn Circulation Experiment (ACE)

Abstract

The Fair Isle Current was identified during the Autumn Circulation Experiment (ACE) as a vertically homogeneous current of salinity 35.05–35.15 lying close to the 100 m isobath both west of Orkney and within the northern North Sea. The mean residual current was stable (>80%) with magnitudes of the order of 7 cm s −1 west of Orkney and 10 cm s −1 within the North Sea. The mean volume transport within the current was found to be ∼1.3 × 10 5m 3 s −1 west of Orkney throughout the observation period (September–December 1987), while in the northern North Sea the mean transport exhibited a seasonal change from ∼4 × 10 5m 3 s −1 during the late summer months to ∼1.5 × 10 5m 3 s −1 during the winter months. A small additional transport towards the North Sea occurred to the west of Sherland and was approximately 25% of that within the Fair Isle Current itself (i.e. ∼0.3 × 10 5m 3 s −1). Transport from the area west of Orkney towards the North Sea occurred when wind stress was directed from the southwest quadrant, with the maximum transport occurring for winds from between 190° and 225°. A linear response to wind stress was found for winds from this quadrant and the coefficients of the regression agreed well with those obtained from a two-dimensional numerical model ( Pingree and Griffiths, 1980 , Oceanologica Acta, 3, 227–236). The observations did not confirm the results of the model when outflow from the North Sea occurred. Within the northern North Sea the dynamics of the Fair Isle Current change as stratification is eroded during the autumn. From September to November 1987 the transport was predominantly non-wind-driven. After the complete removal of residual stratification in December a linear response to wind stress was observed for winds from the southwest quadrant. A three-dimensional numerical model including baraclinic forcing is required if this response is to be correctly included in water quality modelling.