Large Events in Ocean Currents

The variability of current speed is studies with the aim of predicting the return period and amplitude of large magnitude events in a given location. Time series of current profiles above three passages of the Iceland-Scotland Ridge are investigated. Their long-term variability has been the subject of numerous studies, but no previous work has concentrated on instantaneous values, i.e. investigating large surge events, with the aim of understanding the vertical structures of extreme ocean currents. Spatial and temporal associations of large current events are also investigated. To understand the main features of large current events tidal and non-tidal components are separated using a harmonic analysis approach and a filter is applied to remove the long-term trend. This allows the directionality extremes to be studied. It is found that in these offshore locations the amplitude and directionality of extreme currents are mainly due to the non-tidal component. It is also observed, as to the vertical structure, that the amplitude of large currents decreases from the surface down to the sea floor.