Numerical studies of the current response at Ormen Lange to a travelling storm

Abstract

Mean temporal circulation at Ormen Lange (OL) off mid-Norway is strongly dominated by the Norwegian Atlantic Current. Tidal effects are weak. Observed events of short peak value in current speed along with peak in temperature close to the seabed at the shelf slope may be driven by strong pressure gradients. That is, strong atmospheric low pressures and/or internal pressure fronts between warmer Atlantic Water (AW) and colder Norwegian Sea Water (NSW). Along the shelf slope at OL there may be steepening of the isopycnals, separating AW and NSW, due to wind generated downwelling and strong Ekman veering during storms or approaching internal density fronts. During such events the density surfaces tend to undershoot their equilibrium level, and as the forcing weakens, the suppressed water may run up along the shelf slope. In this run up phase, peak values in the velocities are often found.Since the atmospheric forcing may be a major forcing mechanism behind the observed events, we have performed a numerical study of the sensitivity of near bottom velocities at OL to the pathway of low pressure disturbances and the strength and the radii of them.The results indicate how depth dependent currents at different locations respond to varying atmospheric forcing, with highest currents close to the surface, time delay of response at greater depths and highest impact of storms passing further out in the Nordic Seas than immediately over OL.