This study analyses the vertical structure and spatial associations of extreme events in ocean currents, for three sections in the vicinity of the Iceland-Scotland Ridge, to better understand the composition of extreme currents in terms of three components identified as: the low frequency component, tidal current and non-tidal surge residual.

The study is based on Acoustic Doppler Current Profiler records from the Variability of Exchanges In the Northern Seas project. Considerable variation in the relative magnitude of the different components as functions of depth and location is observed. It is found that the amplitude and direction of extreme currents are dominated by the low frequency component in deep water, whereas tidal currents and surges dominate near the surface and in shallow water regions.

In the section north of the Faroes the surge is smaller than the low frequency component in near-surface measurements, where the direction of extremes is generally eastward. Away from the sea-surface extreme currents here have a southward-preferred direction in the northernmost deployment, probably due to the effect of the Iceland-Faroe Front. In the Faroe-Bank Channel current speeds show largest values in bottom measurements where the low frequency component is dominant. These deep-water extremes are strongly directional, driven by topographic steering. Extremes are found at bearings near 315 degrees particularly below 500 m. In the Faroe-Shetland Channel the tide is generally larger than the surge and low frequency component, particularly in the deployments on the Faroe side of the channel. Here the preferred direction is southward.

It is also found that the Meteorological Office s FOAM model is unable to reproduce either the magnitude or the above form for the extremes, perhaps due to its coarse vertical and horizontal resolution, and is thus not suitable to model extremes on a regional scale.

More information is available at http://geosci.uchicago.edu/~ccarollo/index.html