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We present a new modeling system for wave‐current interaction based on unstructured grids and thus suitable for very large‐scale high‐resolution multiscale studies. The coupling between the 3D current model (SELFE) and the 3rd generation spectral wave model (WWM‐II) is done at the source code level and the two models share same sub‐domains in the parallel MPI implementation in order to ensure parallel efficiency and avoid interpolation. We demonstrate the accuracy, efficiency, stability and robustness of the coupled SELFE‐WWM‐II model with a suite of progressively challenging benchmarks with analytical solution, laboratory data, and field data. The coupled model is shown to be able to capture important physics of the wave‐current interaction under very different scales and environmental conditions with excellent convergence properties even in complicated test cases. The challenges in simulating the 3D wave‐induced effects are highlighted as well, where more research is warranted. Key Points A fully parallel highly efficient numerical framework for 3d wave current model The 3D model is efficient, stable and convergent on many different scales Importance of wave‐enhanced bottom friction for field applications

In this work we present a unique set of coincident and collocated high‐resolution observations of surface currents and directional properties of surface waves collected from an airborne instrument, the Modular Aerial Sensing System, collected off the coast of Southern California. High‐resolution observations of near surface current profiles and shear are obtained using a new instrument, “DoppVis”, capable of capturing horizontal spatial current variability down to 128 m resolution. This data set provides a unique opportunity to examine how currents at scales ranging from 1 to 100 km modulate bulk (e.g., significant wave height), directional and spectral properties of surface gravity waves. Such observations are a step toward developing better understanding of the underlying physics of submesoscale processes (e.g., frontogenesis and frontal arrest) and the nature of transitions between mesoscale and submesoscale dynamics. Plain Language Summary In recent years, through improvement of computational resolution of global ocean models, scientists have begun to suspect that kilometer‐scale eddies, whirlpools and fronts, called “submesoscale” variability, make important contributions to horizontal and vertical exchange of climate and biological variables in the upper ocean. Such features are challenging to analyze, because of their size (and how quickly they evolve; within hours), they are too large to study from a research vessel but smaller than regions typically studied with satellite measurements. In this work, we use a research aircraft instrumented to characterize ocean currents, temperature, color (in turn chlorophyll concentration) and the properties of surface waves over an area large enough to capture submesoscale processes. This approach is a step forward in understanding and quantifying the underlying physics of submesoscale processes, and in turn develops parameterization that can help improve the fidelity of weather and climate models. Key Points Unique coincident and collocated airborne observations of Sea Surface Temperature, surface currents and properties of surface waves across submesoscales features A new airborne instrument enables observations of surface currents, vertical and horizontal shear to capture quickly evolving ocean features Such observations are crucial to develop better understanding of the physics of submesoscale processes and wave‐current interaction

This paper describes the importance of wave‐current interaction in an inlet‐estuary system. The three‐dimensional, fully coupled, Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) modeling system was applied in Willapa Bay (Washington State) from 22 to 29 October 1998 that included a large storm event. To represent the interaction between waves and currents, the vortex‐force method was used. Model results were compared with water elevations, currents, and wave measurements obtained by the U.S. Army Corp of Engineers. In general, a good agreement between field data and computed results was achieved, although some discrepancies were also observed in regard to wave peak directions in the most upstream station. Several numerical experiments that considered different forcing terms were run in order to identify the effects of each wind, tide, and wave‐current interaction process. Comparison of the horizontal momentum balances results identified that wave‐breaking‐induced acceleration is one of the leading terms in the inlet area. The enhancement of the apparent bed roughness caused by waves also affected the values and distribution of the bottom shear stress. The pressure gradient showed significant changes with respect to the pure tidal case. During storm conditions the momentum balance in the inlet shares the characteristics of tidal‐dominated and wave‐dominated surf zone environments. The changes in the momentum balance caused by waves were manifested both in water level and current variations. The most relevant effect on hydrodynamics was a wave‐induced setup in the inner part of the estuary. Key Points The three‐dimensional, wave‐current, COAWST modeling system was applied in Willapa Bay Interaction between waves and currents represented by the vortex‐force method The inlet shares the characteristics of tidal and surf zone environments

The Surface Water and Ocean Topography (SWOT) satellite mission enables, for the first time, two‐dimensional (2D) mapping of significant wave height Hs$\\left({H}_{s}\\right)$at kilometer‐scale resolution. Using data from SWOT's Ka‐band Radar Interferometer (KaRIn), this study investigates interactions between surface waves, winds, and currents across diverse dynamic regimes, including western boundary currents, mesoscale turbulence, tropical cyclones, and wave group modulation. SWOT reveals unprecedented 2D spatial gradients in Hs${H}_{s}$ , capturing fine‐scale variability previously identified only in numerical models. These observations highlight the critical role of currents in shaping the wave field and show strong agreement with theoretical predictions. SWOT's high‐resolution wave data represent a transformative advance in understanding Hs${H}_{s}$gradients, paving the way for refining operational models and addressing challenges in characterizing the influence of sea state gradients on coupled air‐sea processes.

Mesoscale eddy currents influence ocean surface waves, but their imprints on wave height remain poorly described by observations. Here, we examine significant wave height (SWH) variations associated with more than 42,000 mesoscale eddies in the Southern Ocean using along‐track Jason‐3 altimeter measurements. Altimeter composites reveal a pronounced meridional dipole, with reduced SWH where wave propagation aligns with eddy currents and enhanced SWH on the opposing side. Typical eddies (radius ∼45 km, geostrophic velocity anomalies ∼0.15 m s−1) are associated with SWH anomalies of ∼5 cm. Reanalysis data and idealized simulations further suggest that eddy‐related wind anomalies, the relative wind effect and direct current effects all contribute to the observed SWH patterns, with refraction playing a key role. These results highlight the role of mesoscale currents in redistributing wave energy and shaping the spatial distribution of wave height.

A monthlong time series of wave, current, salinity, and suspended‐sediment measurements was made at five sites on a transect across the Mouth of Columbia River (MCR). These data were used to calibrate and evaluate the performance of a coupled hydrodynamic and wave model for the MCR based on the Delft3D modeling system. The MCR is a dynamic estuary inlet in which tidal currents, river discharge, and wave‐driven currents are all important. Model tuning consisted primarily of spatial adjustments to bottom drag coefficients. In combination with (near‐) default parameter settings, the MCR model application is able to simulate the dominant features in the tidal flow, salinity and wavefields observed in field measurements. The wave‐orbital averaged method for representing the current velocity profile in the wave model is considered the most realistic for the MCR. The hydrodynamic model is particularly effective in reproducing the observed vertical residual and temporal variations in current structure. Density gradients introduce the observed and modeled reversal of the mean flow at the bed and augment mean and peak flow in the upper half of the water column. This implies that sediment transport during calmer summer conditions is controlled by density stratification and is likely net landward due to the reversal of flow near the bed. The correspondence between observed and modeled hydrodynamics makes this application a tool to investigate hydrodynamics and associated sediment transport. Key Points Investigate the hydrodynamic processes at the mouth of the Columbia River Present the Mega Transect data set Present a validation of the Delft3D flow‐wave model

This paper reviews some recent mathematical research activity in the field of nonlinear geophysical water waves. In particular, we survey a number of exact Gerstner-like solutions which have been derived to model various geophysical oceanic waves, and wave-current interactions, in the equatorial region. These solutions are nonlinear, three-dimensional and explicit in terms of Lagrangian variables. This article is part of the theme issue ‘Nonlinear water waves’.

Over recent decades, the exploitation of wave energy resources has sparked a wide range of technologies dedicated to capturing the available power with maximum efficiency, reduced costs, and minimum environmental impacts. These different objectives are fundamental to guarantee the development of the marine wave energy sector, but require also refined assessments of available resource and expected generated power to optimize devices designs and locations. We reviewed here the most recent resource characterizations starting from (i) investigations based on available observations (in situ and satellite) and hindcast databases to (ii) refined numerical simulations specifically dedicated to wave power assessments. After an overall description of formulations and energy metrics adopted in resource characterization, we exhibited the benefits, limitations and potential of the different methods discussing results obtained in the most energetic locations around the world. Particular attention was dedicated to uncertainties in the assessment of the available and expected powers associated with wave–climate temporal variability, physical processes (such as wave–current interactions), model implementation and energy extraction. This up-to-date review provided original methods complementing the standard technical specifications liable to feed advanced wave energy resource assessment.

This paper considers the pressure-streamfunction relationship for a train of regular water waves propagating on a steady current, which may possess an arbitrary distribution of vorticity, in two dimensions. The application of such work is to both near shore and offshore environments, and in particular, for linear waves we provide a description of the role which the pressure function on the seabed plays in determining the free-surface profile elevation. Our approach is shown to provide a good approximation for a range of current conditions. This article is part of the theme issue ‘Nonlinear water waves’.

To investigate changes in the instability of Stokes waves prior to wave breaking in shallow water, pressure data were recorded vertically over the entire water depth, except in the near-surface layer (from 0 cm to −3 cm), in a recirculating channel. In addition, we checked the pressure asymmetry under several conditions. The phase-averaged dynamic-pressure values for the wave-current motion appear to increase compared with those for the wave-alone motion; however, they scatter in the experimental range. The measured vertical distributions of the dynamic pressure were plotted over one wave cycle and compared to the corresponding predictions on the basis of third-order Stokes wave theory. The dynamic-pressure pattern was not the same during the acceleration and deceleration periods. Spatially, the dynamic pressure varies according to the faces of the wave, i.e. the pressure on the front face is lower than that on the rear face. The direction of wave propagation with respect to the current directly influences the essential features of the resulting dynamic pressure. The results demonstrate that interactions between travelling waves and a current lead more quickly to asymmetry. This article is part of the theme issue ‘Nonlinear water waves’.