Modelling the upper ocean dynamics of the north-west European shelf during storm events with the UK Met Office ocean-wave prediction system

Accurate modelling of the surface ocean dynamics is of paramount importance for many human activities such as search-and-rescue operations and offshore oil and wind power industry. During sea storm events, large waves can have a strong control on the surface ocean currents, making wave-current interaction a leading order process in the uppermost part of the ocean. North-west (NW) European shelf seas can be affected by extremely severe storms, increasing the need for precise predictions of the surface ocean dynamics.   

In this study we assess the impact of using a coupled ocean-wave modelling system to simulate the upper ocean dynamics of the NW European shelf during five storm events occurred in Winter 2016. Two versions of the eddy-resolving (1.5 km resolution) UK Met Office ocean-wave operational prediction system are compared: the first one uses the ocean and wave models in uncoupled mode; the second one is a coupled system including three ocean-wave interactions, namely the Stokes-Coriolis force, the modification of the surface stress by wave growth and dissipation and a wave height dependent ocean surface roughness. The assessment is carried out using the ocean currents and the Stokes’ drift reproduced by the two modelling systems to simulate the lagrangian trajectories of a number of iSphere (surface) and SVP (centered at 15m) drifters affected by the storms. The simulated trajectories are then compared with the observed drifters’ tracks. Some drifter trajectories representative of offshore, near the shelf-break and near the coast regimes have also been simulated switching on only one ocean-wave interaction per time, to better understand the relative impact of the three components we considered in the ocean-wave coupling.

Numerical results show that in the case of iSphere drifters, the trajectories simulated using ocean and wave-induced currents from the coupled system are much more accurate than the one obtained with the uncoupled system, especially near the shelf and the coasts, highlighting the importance of including wave feedback in the momentum equations of the ocean model.  For SVP drifters the effect of the ocean-wave coupling is less evident. This is probably due to the fact that the wave-current interactions considered in the current implementation of the coupled system mainly act in the proximity of the ocean surface, pointing out the need of including wave-induced effects able to influence also the sub-surface dynamics of the water column. However, results also seem to indicate that the reduced impact of the coupling might be related to some difficulties experienced by the ocean and wave models in properly representing some of the physical processes characterizing extreme storm events.

In conclusion, this study proves the importance of using a coupled ocean-wave system when simulating the ocean dynamics during storm events but also indicates where research efforts must be spent for improving the skills of the UK Met Office forecasting system.