Shelf-edge exchange in a numerical model of the Shetland shelf

Abstract

The fast Shetland slope current flows on one side of the Faroe-Shetland Channel (FSC). Mesoscale activity and bottom friction are mechanisms that affect the slope current, helping to mix water and tracers across the shelf-edge. We simulate the FSC dynamics using a high resolution (~ 2km) hydrostatic configuration of the Massachusetts Institute of Technology General Circulation Model (MITgcm). The model was validated against a variety of observations: surface currents; Sea Surface Temperatures (SSTs); Acoustic Doppler Current Profilers (ADCPs); hydrographic observations; and shelf-edge exchange estimates. It is considered suitable for studying shelf-edge exchange here. We investigated bottom boundary layer (BBL) fluxes on the Shetland shelf. Fluxes near the shelf-edge are found to have a seasonal variability with high/low volume transport in winter/summer respectively. Using a multivariate regression approach, we find the locally wind-driven Ekman transport plays no explicit role in explaining daily bottom fluxes. We can better explain the variability of the across-slope BBL flux as a linear function of the speed and across-slope component of the interior flow, corresponding to an Ekman plus vertical-mean-flow flux. The spatial heterogeneity of the BBL fluxes is attributed to the vertical-mean-flow. We investigated meanders of the Shetland slope current. We develop a metric that characterises when the slope current meanders and use it to evaluate maps of the Potential Vorticity Anomalies (PVA). When the slope current meanders, there is a cyclone–anticyclone dipole and the PVA isolines bend. Inversely, when there is no meander, the PVA isolines are better aligned with the shelf. We additionally released tracers. When a meander exists, tracers released near the shelf spread extensively off it into the open ocean. When there is no meander, the tracers released on-/off-shelf remain on-/off-shelf. Daily correlations between our metric and the wind are poor, suggesting the genesis for meanders is not wind-driven.