Extreme events in ocean currents through depth in the vicinity of the Iceland-Scotland Ridge

This study analyses the vertical structure of extreme currents for three sections in the vicinity of the Iceland-Scotland Ridge. The variation of speed and direction with depth of observational currents from the Variability of Exchanges in the Northern Seas (VEINS) project is studied. Currents have been divided into (1) the mean residual flow, (2) tidal current and (3) nontidal surge residuals to assess their relative importance in contributing to extremes. The results show considerable variation in the relative magnitude of the different components as function of both depth and location, which raises a number of fundamental questions concerning the structure of extreme currents and dominance of component effects. It is found that in these locations the amplitude and direction of extreme currents are dominated by the nontidal component in deep water, whereas tidal currents and surges are dominant near the surface and in shallow-water regions. In the section north of the Faroes the mean residual flow is equal to or larger than surge effects in near-surface measurements, where the direction of extreme currents is eastward for all the deployments; away from the sea-surface extremes have a southward-preferred direction. Here, the Faroe Current (FC) and the Iceland-Faroe Front (IFF) probably influence extremes. In the Faroe-Bank Channel current speed shows larger values in bottom measurements where the mean residual flow is dominant; in near-surface measurements the surge is dominant; extremes are found at bearings near 315°, particularly below 500 m. In the Faroe-Shetland Channel tide is generally larger than the surge and mean residual flow, particularly in the deployments on the Faroe side of the channel; the preferred direction is southward.