Observation-based evaluation of surface wave effects on currents and trajectory forecasts
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Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.
KeywordsWave–current interactions Trajectory forecasts Surface drifters iSphere Wave effects
We wish to thank Ilker Fer and Øyvind Sætra for helpful discussions. We also gratefully acknowledge financial support from the Research Council of Norway through the grants 196438 (BioWave) and 207541 (OilWave), as well as the Gulf of Mexico Research Initiative Deep-C Consortium. We also thank the Royal Norwegian Navy for supplying us with the ADCP instrument, as well as Steinar Hansen from the Norwegian Coastal Administration for providing necessary logistics. We are grateful for the constructive comments of the anonymous reviewers, who helped to increase the quality of this paper significantly.
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