Abstract
AbstractAdvances in the understanding and modeling of surface currents have revealed the importance of internal waves, mesoscale and submesoscale features. Indeed, all these features should have a large influence on wind waves and in particular on wave heights. Still, the quantitative impact of currents on waves is not well known due to the complexity of the random wavefields and currents that are found in the ocean and the lack of observations of both currents and waves at scales shorter than 150 km. Here, we compare novel satellite altimetry data and state of the art phase‐averaged numerical wave models forced both by wind and currents. Currents used are taken from the oceanic model Coastal and Regional Ocean COmmunity, run at high resolution. The influence of current field resolution is investigated by applying Gaussian filters of different width to that same high‐resolution current field. We find that a numerical wave model that uses currents with resolutions of ∼30 km or less and a directional resolution of 7.5° can provide accurate representations of the significant wave height gradients found in the Agulhas current. Using smoother current fields such as derived from altimeters measurements alone, coarse directional spectral resolution or larger directional spread of the wave model generally underestimates gradients and extreme wave heights. Hence, satellite altimetry provides high‐resolution wave height with a gradient magnitude that is highly sensitive to underlying surface current gradients, at resolutions that may not be resolved by today's altimeters measurements. This is demonstrated here for relatively steady currents averaged over 3 years.
Highlights
Surface gravity waves generated by wind interact with surface currents at all scales due to a wide range of processes (Phillips, 1977)
We find that a numerical wave model that uses currents with resolutions of ∼30 km or less and a directional resolution of 7.5° can provide accurate representations of the significant wave height gradients found in the Agulhas current
Surface currents modify the wavefield in a complex way that is not just local (Ardhuin et al, 2017; Kudryavtsev et al, 2017; White & Fornberg, 1998), creating a spatial pattern of wave properties that can be important for applications and that may reveal properties of the ocean currents that are otherwise difficult to obtain
Summary
Surface gravity waves generated by wind (hereinafter waves) interact with surface currents at all scales due to a wide range of processes (Phillips, 1977). Wave simulations in the Gulf Stream and Drake Passage suggest that the patterns of Hs field induced by surface currents are dominated by the refraction (Ardhuin et al, 2017), with a significant impact of small-scale currents These modeling results could not be validated using standard satellite altimeter data that is dominated by noise for along-track wavelengths shorter than 100 km (Dibarboure et al, 2014). The wave spectrum, with stronger Hs gradients obtained for narrower incident wave spectra even when only large-scale currents, as derived from gridded altimetry data were used (Quilfen et al, 2018) These two previous studies by Ardhuin et al (2017) and Quilfen et al (2018) have suggested two possible reasons for sharp Hs gradient: namely the presence of sharp current gradients or the strong local focalization of waves on a smooth current field.
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