Small‐scale open ocean currents have large effects on wind wave heights

Abstract

Tidal currents and large‐scale oceanic currents are known to modify ocean wave properties, causing extreme sea states that are a hazard to navigation. Recent advances in the understanding and modeling capability of open ocean currents have revealed the ubiquitous presence of eddies, fronts, and filaments at scales 10–100 km. Based on realistic numerical models, we show that these structures can be the main source of variability in significant wave heights at scales less than 200 km, including important variations down to 10 km. Model results are consistent with wave height variations along satellite altimeter tracks, resolved at scales larger than 50 km. The spectrum of significant wave heights is found to be of the order of 70〈Hs〉2/(g2〈Tm0,−1〉2) times the current spectrum, where true〈Hstrue〉 is the spatially averaged significant wave height, true〈Tm0,−1true〉 is the energy‐averaged period, and g is the gravity acceleration. This variability induced by currents has been largely overlooked in spite of its relevance for extreme wave heights and remote sensing.

Ocean Surface Wave‐Current Signatures From Satellite Altimeter Measurements

“…Indeed, currents impact the dispersion of surface waves, and surface current fluctuations can lead to the formation of anomalous large-amplitude waves. Joint analysis of numerical modeling outputs and satellite measurements have confirmed that spatial wave height variations result from these refraction effects (Ardhuin et al, 2017;Kudryavtsev et al, 2017;Quilfen et al, 2018). Ultimately, these impacts can lead to significant ray deflection (Dysthe, 2001;Gallet & Young, 2014;Kenyon, 1971;White & Fornberg, 1998).…”

Measuring currents, ice drift, and waves from space: the Sea Surface KInematics Multiscale monitoring (SKIM) concept

“…In practice we use the locations (x , y ) that have the right azmuth θ , and that are closest to (x, y), separated by a distance r(θ ). The variations of U S,1D (θ ) over the distance r(θ ), which is typically 25 less than 50 km, is mostly due to the effect of currents on waves (Ardhuin et al, 2017b).…”

“…These will be discussed in other 25 publications. We only point out here that the sea state variability at small scales is probably dominated by the effect of ocean currents (Ardhuin et al, 2017b). It is thus logical to measure waves and currents together, and possibly further use the measured variability of sea state parameters to further constrain the magnitude of current gradients.…”

“…The other important source of error, caused by inaccuracies in the wave bias correction is a function of the beam geometry but also of the strength of gradients of the Stokes drift, which are mostly caused by current gradients (Ardhuin et al, 2017b).…”

“…We have 10 estimated all errors in particular ocean conditions, using state of the art models for the ocean circulation (e.g. Rocha et al, 2016;Gula et al, 2015) and associated ocean waves (Ardhuin et al, 2017b). Model simulations were performed at resolutions on the order of 1.5 km for a set of regions for which we expect SKIM to have a strong contribution, resolving processes that are not accessible with today's observing systems.…”