Abstract
The influence of ocean surface currents on the global wind-wave field is revisited. State-of-the-art numerical spectral wave model simulations with and without surface currents taken from an eddy resolving global ocean reanalysis were compared. As a global average, simulations forced with currents display significantly better agreement with altimeter derived wave heights. The bias and root mean square error in significant wave heights are mostly reduced when including current forcing, especially in the Southern Ocean. An overall improvement in wave periods and wave direction is also seen when comparing model outputs with the Australian and United States buoy network observations. Including surface ocean current forcing in wave simulations reduces the simulated wave heights in most areas of the world, due to a decreased relative wind given by co-flowing winds and currents. Current-induced refraction generates important changes in wave direction in western boundary current and tropical regions. Furthermore, large and broad changes in friction velocity, atmosphere-to-ocean energy flux, whitecap cover and Stokes drift velocities are observed in equatorial regions. Finally, the importance of the wave model resolution for representing wave–current interactions was tested by comparing results from eddy-permitting (lower resolution) and eddy-resolving (higher resolution) configurations. We conclude that the main patterns of current-induced refraction are well represented in both cases, albeit that the higher resolution simulation represents these in a more detailed manner. Finally, the implications that the observed wave–current interactions have on several ocean processes are discussed.
Highlights
Since the pioneering studies of Longuet-Higgins and Stewart (1960, 1961, 1964), it has been understood that there are complex non-linear interactions and energy transfers between ocean wind-waves and the mean flow
We have shown that a significant improvement in the performance of wave simulations can be achieved by including ocean surface current data from a global reanalysis as an additional forcing to the wave model
We found there is an important reduction of the mean bias and RMSE in most areas of the world, and in the Southern Ocean: here, global wave models tend to overestimate the observed wave heights, in part due to biases in the wind data used as input, drifting icebergs that block the wave energy but not accounted for in the model, and not considering surface current forcing
Summary
Since the pioneering studies of Longuet-Higgins and Stewart (1960, 1961, 1964), it has been understood that there are complex non-linear interactions and energy transfers between ocean wind-waves and the mean flow. Rapizo et al (2018) showed that a significant improvement in the performance of global wave simulations can be achieved by including surface current forcing from a global reanalysis They showed that the main effect of including currents is the reduction of wave heights due to a diminished relative wind in the presence of co-flowing current fields (this is especially evident in the Southern Ocean where the westerly winds blow in the same direction as the ACC). Quilfen et al (2018) found that their wave model simulations underestimated the magnitude of the wave height variability, which they attributed to their model resolution (0.25◦) being too coarse, as well as to errors in the ocean current data Overall, these recent studies suggest that increased wave model resolution is an important factor to consider in order to properly represent wave–current interactions.
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