Wave-circulation Model Research Articles

The role of wave setup on extreme water levels around Australia

Estimates of extreme sea levels have mainly been derived using atmospheric and tidal forcing only, but surface gravity waves are a key process that can cause a substantial elevation of mean sea level at the coast. In this paper a coupled wave-circulation model was used to simulate storm surge and wave setup across a broad range of coastal topographies and storm types around Australia. The main aim was a practical assessment of the benefits and limitations of using a coupled wave-surge model to determine wave effects for extreme sea level studies on the continental scale. The simulations included: tropical cyclones Yasi (2011) and George (2007) on the northeast and northwest coasts of Australia; Cyclone Alby (1978) that underwent extratropical transition in southwest Australia; and a large extratropical winter storm in the Southern Ocean (2016). Predicted wave setup was highly variable both spatially and temporally, with values up to 0.45 m, 35% of maximum storm surge height, and 9% of incident significant wave heights. In general, where storm surge heights were smaller, the relative importance of wave setup increased in the presence of large waves. The model best resolved wave setup in shallow gently sloping coastal areas, whilst unrealistically low values were predicted for steep coastlines. Predictions improved when wave setup was included for most sites and events, however, these wave effects were often secondary to other assumptions implicit in modeling extreme events. This was especially true for tropical cyclones where reliable wind forcing was not available. The results showed similar contributions from wave setup to other studies but highlighted the high temporal and spatial variability of wave setup across a wider variety of storms and coastal morphologies. The results also revealed that the maximum wave setup did not always coincide with the maximum storm surge. Overall, the coupled wave-surge model presented a useful tool to predict extreme water levels including wave setup when accurate depth and atmospheric data are available, however, significant challenges still exist to model this process at the continental scale.

Conservative Surface Wave Effects on a Wind-Driven Coastal Upwelling System

Abstract Upwelling brings deep, cold, and nutrient-rich water to the euphotic zone, enhancing biological primary productivity. Coastal upwelling is affected by various factors, such as winds, topography, and tides. However, it remains unclear how the upwelling is affected by surface waves, particularly the Stokes drift and its related forces, that is, conservative wave effects. Here using a coupled wave–circulation model, we examined how conservative wave effects impact the wind-driven coastal upwelling system over an idealized continental shelf. We showed that conservative wave effects reduce upwelling but enhance downwelling; consequently, the amount of deep cold water brought up to the surface by upwelling is reduced with waves, leading to a weaker upwelling front than that without waves. Conservative wave effects also change the potential vorticity (PV) fluxes across the sea surface/bottom and alter the thickness of surface/bottom negative-PV layers. In addition, conservative wave effects modify the turbulent thermal wind (TTW) associated with the upwelling front, forming a Stokes–TTW balance. Further, we studied sensitivities of the upwelling and downwelling magnitudes to four parameters: wave height, wind stress, shelf slope, and wave incident angle. We combined these parameters into a single nondimensional number that can indicate when conservative wave effects need to be included in the upwelling and downwelling. Significance Statement Upwelling is important to the marine ecosystem because it enhances biological primary productivity by bringing nutrient-rich water to the euphotic zone from depths. However, it remains unclear how the upwelling is affected by ubiquitous surface waves. Here using numerical simulations, we showed that Stokes drift and its related forces due to surface waves reduce upwelling but enhance downwelling. It implies that there could be a substantial bias in the estimation of upwelling and downwelling if surface waves are not considered. Further, we proposed a nondimensional number to indicate when surface waves need to be considered in the upwelling and downwelling.

Sediment and Radioactivity Transport in the Bohai, Yellow, and East China Seas: A Modeling Study

This paper is concerned with the development of a radionuclide dispersion model for the nuclear power plants in the Bohai, Yellow, and East China seas (BYECS) characterized by high turbidity and multi-scale circulations, focusing on the comparison of dispersion processes of 137Cs depending upon, in particular, the suspended sediment concentration and erosion/sedimentation processes. The simulations were carried out using a multi-fraction sediment transport model embedded in the semi-implicit Eulerian–Lagrangian finite-element coupled wave-circulation model linked with the model of radionuclide transport, which describes the key radionuclide transfer processes in the system of water–multi-fraction sediments. In contrast to the Eulerian models used for hydrodynamics and sediment transport processes, the Lagrangian technique was applied to simulate the transport of radionuclides. The simulation results for total suspended concentration agreed with in situ measurements and the Geostationary Ocean Color Imager data. The results of the simulation of hypothetical releases of 137Cs from four nuclear power plants (NPPs) placed in BYECS essentially differ from the real release of activity in the Pacific Ocean shelf due to the Fukushima Daiichi accident, which took place at the same time and released activity that was similar. The total amount of bottom contamination of 137Cs in releases from the Sanmen, Hanbit, and Hongyanhe NPPs was about 40% of dissolved component, and the total amount of suspended component was about 20% of dissolved component, in contrast with the Fukushima Daiichi accident, where the particulate component was only 2%. The results demonstrate the importance of erosion processes in the budget of 137Cs in shallow areas around the Sanmen and Hanbit NPPs, where strong wind and tidal currents took place.

Storm damage risk assessment for offshore cage culture

This study establishes a practical methodology for storm damage risk assessment and zoning for offshore cage culture by superimposing the vulnerability theory onto hydrodynamic modeling. The peak significant wave height is statistically correlated to the normalized fatigue load criteria at specified loading cycles to classify the destruction duration and damage state during storm events. The yield damage of the flotation pipe under cyclical wave oscillations was identified as the major failure mechanism of the sea cage. The structural failure risk would significantly decrease to 4.5-m waves, which is much lower than the manufacturing standard due to cumulative damage in a relatively short operation time. The wave-circulation model was applied to hindcast 112 high-impact typhoon scenarios in the study area for the return period analysis of the storm wave field. The damage risk distribution of the cage culture was mapped across the landscape to increase storm strength according to the vulnerability classification. The effectiveness of island shielding was explicitly revealed, and the leeward side was overall less risky as the return period increased, with the safe area shrinking inshore. For extreme conditions over a 100-year return period, the shield performance of leeward bays varied significantly due to the shoreline profile differences. Some existing cage deployment locations are at-risk relative to the structural resistance capability, whereas other unrecognized areas might be suitable for cage culture. These findings imply the necessity of storm risk assessment before proactive prevention measures and deployment zoning. The methodology may also be adapted to storm risk predictions for other facilities in the coastal ocean.

Effect of Depth-Induced Breaking on Wind Wave Simulations in Shallow Nearshore Waters off Northern Taiwan during the Passage of Two Super Typhoons

Super Typhoons Maria (2018) and Lekima (2019) were adopted for this case study, although they only passed the northern offshore waters of Taiwan without making landfall. A direct modification technique was employed to create the atmospheric conditions for a wave-circulation model to hindcast large typhoon-driven waves. The radius of the modified scale (Rtrs) for a hybrid typhoon wind plays an important role in the significant wave height (SWH) simulations during the passage of typhoons. The maximum increment in peak SWH reached 3.0 m and 5.0 m in the deep ocean for Super Typhoons Maria (2018) and Lekima (2019), respectively if the Rtrs was increased from 4 × Rmax (radius of the maximum wind) to 7 × Rmax. The SWHs induced by the typhoon winds in the surf zone were more sensitive to different wave-breaking formulations used in the wave-circulation model. The maximum difference in peak SWH reached 2.5 m and 1.2 m for Super Typhoons Maria (2018) and Lekima (2019), respectively, when the wave-breaking formulations of BJ78 (proposed by Battjes and Janssen in 1978) and CT93 (proposed by Church and Thornton in 1993) were introduced to the wave-circulation model. The SWH simulations in the surf zone were insensitive to the wave-breaking criterion (γ) during the passage of typhoons. In shallow nearshore waters, the utilization of a constant γ for the wave-circulation model always produces peak SWHs that are smaller than those using γ based on local steepness or peak steepness.

Effects of Wave-Induced Processes in a Coupled Wave–Ocean Model on Particle Transport Simulations

This study investigates the effects of wind–wave processes in a coupled wave–ocean circulation model on Lagrangian transport simulations. Drifters deployed in the southern North Sea from May to June 2015 are used. The Eulerian currents are obtained by simulation from the coupled circulation model (NEMO) and the wave model (WAM), as well as a stand-alone NEMO circulation model. The wave–current interaction processes are the momentum and energy sea state dependent fluxes, wave-induced mixing and Stokes–Coriolis forcing. The Lagrangian transport model sensitivity to these wave-induced processes in NEMO is quantified using a particle drift model. Wind waves act as a reservoir for energy and momentum. In the coupled wave–ocean circulation model, the momentum that is transferred into the ocean model is considered as a fraction of the total flux that goes directly to the currents plus the momentum lost from wave dissipation. Additional sensitivity studies are performed to assess the potential contribution of windage on the Lagrangian model performance. Wave-induced drift is found to significantly affect the particle transport in the upper ocean. The skill of particle transport simulations depends on wave–ocean circulation interaction processes. The model simulations were assessed using drifter and high-frequency (HF) radar observations. The analysis of the model reveals that Eulerian currents produced by introducing wave-induced parameterization into the ocean model are essential for improving particle transport simulations. The results show that coupled wave–circulation models may improve transport simulations of marine litter, oil spills, larval drift or transport of biological materials.

Assessment of Offshore Wave Energy Resources in Taiwan Using Long-Term Dynamically Downscaled Winds from a Third-Generation Reanalysis Product

In this study, long-term wind fields during 1991–2010 from the Climate Forecast System Reanalysis (CFSR) were dynamically downscaled over Taiwan and its offshore islands at a 5 km horizontal resolution using the Weather Research and Forecasting (WRF) model. Simulations of the 10 m (above sea level) dynamically downscaled winds served as the atmospheric forcing for driving a fully coupled wave-circulation model. The sea states of the waters surrounding Taiwan during 1991–2010 were hindcasted to evaluate the offshore wave energy resources and optimal wave energy hotspots. This study reveals that the southeastern offshore waters of Taiwan and the Central Taiwan Strait exhibited the highest mean wave power density (WPD), exceeding 20 kW/m. The annual mean WPD, incidence of the hourly WPD greater than or equal to 4 kW/m, monthly variability index and coefficient of variation of the WPD indicated that the sea areas located between Green Island and Orchid Island (OH_1), southeast of Orchid Island (OH_2), south of the Hengchun Peninsula (OH_3), and north of the Penghu Islands (OH_4) were the optimal hotspots for deploying wave energy converters. The most energetic months were October for OH_1 and OH_2 and November for OH_3 and OH_4, while the wave power was weak from March to June for OH_1, OH_2 and OH_3 and in May for OH_4. The wave direction is prevailingly east-northeast for OH_1, OH_2 and OH_3 and nearly northeast for OH_4. These phenomena reveal that wave power in the waters offshore Taiwan is induced primarily by the northeast (winter) monsoon. The exploitable annual WPD was estimated to be 158.06, 182.89, 196.39 and 101.33 MWh/m for OH_1, OH_2, OH_3 and OH_4, respectively.

BENEFITS AND LIMITATIONS OF A COUPLED WAVE-SURGE MODEL FOR AUSTRALIAN EXTREMES

Extreme sea levels result from a combination of a range of factors that include long term mean sea level variability, astronomical tides, storm surges due to atmospheric pressure and wind, wave breaking, and other regional dynamics. Numerical circulation/storm-surge models are frequently used to predict water levels over broad areas with the outputs used for planning or emergency management applications. Recently, coupled wave-circulation models have been shown to improve extreme sea level predictions through the inclusion of wave setup that results from the transfer of momentum of breaking waves into sea level at the shoreline. Other studies have shown that the representations of surface wind drag can be improved when the sea state is considered, and this can directly influence the amplitude of storm surges at the coast. However, most coupled wave-circulation model studies have been undertaken for relatively small computational domains and for a limited range of coastal morphologies and storm types. In this paper we assess the benefits and limitations of using a coupled wave-circulation model to predict extreme sea levels and determine wave effects for a broad range of coastal morphologies and extreme storm events all around Australia. Simulated events occurred in three oceans and considered tropical cyclones, a cyclone undergoing extratropical transition, and a large mid-latitude extratropical low-pressure system.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/UfyWHI4OHBA

Estimating the Annual Exceedance Probability of Water Levels and Wave Heights from High Resolution Coupled Wave-Circulation Models in Long Island Sound

Accurately estimating the probability of storm surge occurrences is necessary for flood risk assessments. This research models Long Island Sound using a coupled coastal circulation and wave model (FVCOM-SWAVE) to hindcast the 44 highest storms between 1950–2018 and fitted Poisson-GPD distributions to modelled water levels and wave heights. Floodwater elevations and significant wave heights for 10% (1/10), 3% (1/30), 2% (1/50), and 1% (1/100) annual exceedance probabilities are provided for all Connecticut coastal towns. The results show that both water levels and their corresponding return intervals are higher along the western coast of Connecticut than the eastern coast, whereas significant wave heights increase eastward. Comparing our model results with those from the North Atlantic Coast Comprehensive Study (NACCS) shows that the mean NACCS results are higher for water levels and lower for significant wave heights for longer return periods. Likewise, the Federal Emergency Management Agency (FEMA) results in large errors compared to our results in both eastern and western coastal Connecticut regions. In addition to evaluating historical risks, we also added a sea-level height offset of 0.5 m for 2050 estimates in order to examine the effect of rising sea-levels on the analysis. We find that sea-level rise reduces the return period of a 10-year storm to two years. We advise periodically updating this work as improved sea-level rise projections become available.

Numerical Simulation of Large Wave Heights from Super Typhoon Nepartak (2016) in the Eastern Waters of Taiwan

Super Typhoon Nepartak (2016) was used for this case study because it is the most intense typhoon that made landfall in Taiwan in the past decade. Winds extracted from the Climate Forecast System version 2 (CFSV2) and ERA5 datasets and merged with a parametric typhoon model using two hybrid techniques served as the meteorological conditions for driving a coupled wave-circulation model. The computed significant wave heights were compared with the observations recorded at three wave buoys in the eastern waters of Taiwan. Model performance in terms of significant wave height was also investigated by employing the CFSV2 winds under varying spatial and temporal resolutions. The results of the numerical experiments reveal that the simulated storm wave heights tended to decrease significantly due to the lower spatial resolution of the hourly winds from the CFSV2 dataset; however, the variations in the storm wave height simulations were less sensitive to the temporal resolution of the wind field. Introducing the combination of the CFSV2 and the parametric typhoon winds greatly improved the storm wave simulations, and similar phenomena can be found in the exploitation of the ERA5 dataset blended into the parametric wind field. The overall performance of the hybrid winds derived from ERA5 was better than that from the CFSV2, especially in the outer region of Super Typhoon Nepartak (2016).

A comparative study of storm surge and wave setup in the East China Sea between two severe weather events

The coastal regions of the East China Sea are frequently threatened by the increased sea level induced by intense tropical cyclones. In this study, a coupled wave-circulation model ADCIRC + SWAN is used to investigate the spatial and temporal characteristics of storm surges and wave setup heights in the southeastern coastal area of China during two severe weather events with different tracks: Typhoon Saomai, which made direct landfall, and the bypassing Typhoon Chan-hom. By definition, the storm surge is attributable to the wind forcing, while the wave setup results from the wave radiation stress. The simulated sea levels agree well with the observations, with improved results when the wave setup was considered. The simulation results showed clearly different spatial patterns depending on the track type. The maximum storm surge resulted from the cumulative effect of the local onshore wind forcing, occurring on the right side of the Typhoon Saomai track and the left side of the Typhoon Chan-hom track. Significant surge levels along the coast on the left side of the typhoon track were well observed in both cases, resulting from the coastal-trapped shelf waves. The Saomai track type is more likely to cause extremely high storm surges along the coast due to the stronger cumulative effect of the onshore wind forcing. The maximum wave setup was governed by the swell and slope of the sea floor. The locations of the maximum wave setup and surge level were spatially close during Typhoon Saomai, but they were separated during Typhoon Chan-hom.

Mechanistic analysis of the wave-current interaction in the plume region of a partially mixed tidal inlet

In this research, a three-dimensional coupled wave-circulation model, including meteorological forcing, freshwater inflow and time varying open boundary conditions, for New River Inlet is validated. A mechanistic approach is taken to investigate how various wave-current interaction mechanisms affect the nearshore circulation, plume expansion and surface wave field in the plume region of a relatively small partially mixed tidal estuarine system. More specifically, focus is comparing four different modeling cases including: (1) a three-dimensional ocean circulation model (no wave effects), (2) a coupled wave-circulation model, (3) a coupled wave and circulation model including vertical mixing enhancement due to wave breaking, and (4) a wave model without surface current effects. Findings reveal forces are applied by incoming waves due to various wave-current interaction mechanisms. Wave momentum released by incoming waves pushes the outgoing freshwater ebb plume back to the shoreline and prevents the plume from expanding freely towards the open ocean. Findings also reveals that releasing wave-dissipated energy in the expanding plume region enhances vertical mixing, mixes down freshwater, and therefore thickens the plume. These results are congruent with observations at the mouth of the Columbia River.

Tidal Modulation of Surface Gravity Waves in the Gulf of Maine

AbstractThis study examines the tidal modulation of surface gravity waves in the Gulf of Maine (GoM) by using in situ observations and numerical model results. Analysis of observational data demonstrates significant semidiurnal tidal modulations in the mean wave variables for swell-dominated waves in the region. The observed tidal modulation features significant spatial–temporal variabilities, with large magnitudes near the mouth of the GoM. Observations also demonstrate unusual timing of the maximum modulation of significant wave height Hs in the following tidal currents. The coupled wave–circulation model successfully reproduces the observed tidal modulation and the associated spatial–temporal variabilities. Model results demonstrate that the maximum Hs modulations are first generated during the maximum flood tide or ebb tide near the mouth of the GoM and then propagate onto the inner gulf. Around the mouth of the GoM, tidal currents have strong spatial gradients, resulting in great effects of current-induced convergence, refraction, and wavenumber shift. The tidal modulation in Hs generated by convergence (10%–14%) is less affected by the wave propagation direction than the modulation generated by the wavenumber shift (6%–10%) and refraction (4%–20%). The latter modulation varies significantly with changes in the wave propagation direction. In addition, current-enhanced dissipation becomes important during high winds, which reduces at least one-half of the Hs modulation during the study period. The observed unusual timing of the maximum Hs modulation in the following tidal currents can be mostly explained by the convergence and wavenumber shift associated with wave-energy convergence and energy transfer from currents to waves in spatially decelerating currents.

Numerical study of the secondary circulations in rip current systems

To investigate the mechanism of secondary circulations in rip current systems, and to explore the relationship between wave conditions and secondary circulation intensity, a series of numerical experiments is performed using coupled nearshore wave model and circulation model. In these experiments, the rip currents and secondary circulations generated above barred beaches with rip channels are simulated. A comparison experiment is conducted to investigate the formation and hydrodynamics of the secondary circulations. Model results indicate that the secondary circulations consist of alongshore flows driven by wave set-up near the shoreline, part of the feeder currents driven by the wave set-up over the bars, and onshore flows at the end of the rip channel driven by wave breaking and convection. The existence of the secondary circulation barely affects the rip current, but narrows and intensifies the feeder currents. Three groups of experiments of varying incident wave conditions are performed to investigate the relationship between wave conditions and secondary circulation intensity. The velocity of the alongshore flow of the secondary circulation is sensitive to the variation of the incident wave height and water depth. It is also found that the alongshore flow intensity is in direct proportion to the alongshore variation of the wave height gradient between the bars and the shoreline.

Pressure–Slope Momentum Transfer in Ocean Surface Boundary Layers Coupled with Gravity Waves

AbstractThe paper focuses on the consequences of including surface and subsurface, wind-forced pressure–slope momentum transfer into the oceanic water column, a transfer process that competes with now-conventional turbulence transfer based on mixing coefficients. Horizontal homogeneity is stipulated as is customary when introducing a new surface boundary layer model or significantly new vertical momentum transfer physics to an existing model. An introduction to pressure–slope momentum transfer is first provided by a phase-resolved, vertically dependent analytical model that excludes turbulence transfer. There follows a discussion of phase averaging; an appendix is an important adjunct to the discussion. Finally, a coupled wave–circulation model, which includes pressure–slope and turbulence momentum transfer, is presented and numerically executed. The calculated temperatures compare well with measurements from ocean weather station Papa.

Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin

Sclavo, M., Benetazzo, A., Carniel, S., Bergamasco, A., Falcieri, F.M., and Bonaldo, D., 2013. Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basinThe shallow northern Adriatic Sea (namely Gulf of Venice) serves as a model for exploring the interaction of surface gravity waves and oceanic currents and how they influence bottom sediment dispersal and bathymetry evolution. This wave-current interaction effect is investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the Regional Ocean Modeling System (ROMS), the Simulating WAves Nearshore (SWAN) model, and the CSTMS (Community Sediment Transport Modeling System) models and routines. The 2-way data transfer between wave and ocean circulation models is synchronous via MCT (Model Coupling Toolkit), with ROMS providing to SWAN the 2-D current field, free surface elevation, and bathymetry. COAWST modeling system is implemented on two computational grids at different horizontal resolution: a parent grid (with resolution of 2.0 km) covering the whole Adriatic Sea and a child grid resolving the Gulf of Venice at a resolution of 0.5 km. Simulated waves and currents are validated against in-situ observations at the CNR-ISMAR Acqua Alta oceanographic tower, located 15 km off the Venice lagoon. The analysis of wave-current interaction effect on sediment dispersal and sea bottom evolution are performed over the 2011 winter season (January-March) with particular focus on the waves generated by dominant and prevailing winds blowing on the Adriatic Sea: Bora and Sirocco. Results show that while the effects on bottom stress may vary depending on wave propagation and current direction, the effects on advective dynamics may become dominant particularly in presence of severe storms with parallel wave propagation and global circulation, which is the case of Bora storms in northern Adriatic Sea.

Hurricane Sea Surface Inflow Angle and an Observation-Based Parametric Model

Abstract This study presents an analysis of near-surface (10 m) inflow angles using wind vector data from over 1600 quality-controlled global positioning system dropwindsondes deployed by aircraft on 187 flights into 18 hurricanes. The mean inflow angle in hurricanes is found to be −22.6° ± 2.2° (95% confidence). Composite analysis results indicate little dependence of storm-relative axisymmetric inflow angle on local surface wind speed, and a weak but statistically significant dependence on the radial distance from the storm center. A small, but statistically significant dependence of the axisymmetric inflow angle on storm intensity is also found, especially well outside the eyewall. By compositing observations according to radial and azimuthal location relative to storm motion direction, significant inflow angle asymmetries are found to depend on storm motion speed, although a large amount of unexplained variability remains. Generally, the largest storm-relative inflow angles (<−50°) are found in the fastest-moving storms (>8 m s−1) at large radii (>8 times the radius of maximum wind) in the right-front storm quadrant, while the smallest inflow angles (>−10°) are found in the fastest-moving storms in the left-rear quadrant. Based on these observations, a parametric model of low-wavenumber inflow angle variability as a function of radius, azimuth, storm intensity, and motion speed is developed. This model can be applied for purposes of ocean surface remote sensing studies when wind direction is either unknown or ambiguous, for forcing storm surge, surface wave, and ocean circulation models that require a parametric surface wind vector field, and evaluating surface wind field structure in numerical models of tropical cyclones.

A numerical study of the dynamics of the wave-driven circulation within a fringing reef system

The circulation driven by wave breaking, tides and winds within a fringing coral reef system (Ningaloo Reef) in Western Australia was investigated using the ocean circulation model ROMS two-way coupled to the wave model SWAN. Currents within the system were dominantly forced by wave breaking, with flow driven over the shallow reefs and towards the lagoon, which returned to the ocean through channels in the reef. Hindcast model simulations were compared against an extensive field dataset, revealing that the coupled wave–circulation model could accurately predict the waves and currents throughout this morphologically complex reef–lagoon system. A detailed momentum budget analysis showed that, over the reef, a dominant cross-shore balance was established between radiation stress gradients and a pressure (mean water level) gradient (similar to a beach). Within the lagoon, alongshore currents were primarily balanced by alongshore gradients in wave setup, which drove flow towards (and ultimately out) the channels. The importance of these wave-driven currents to Ningaloo Reef was quantified over a full seasonal cycle, during periods when wave and wind conditions significantly differed. These results showed that wave breaking still overwhelmingly dominated the circulation and flushing of Ningaloo Reef throughout the year, with winds playing an insignificant role.

Pressure-gradient-driven nearshore circulation on a beach influenced by a large inlet-tidal shoal system

[1] The nearshore circulation induced by a focused pattern of surface gravity waves is studied at a beach adjacent to a major inlet with a large ebb tidal shoal. Using a coupled wave and wave-averaged nearshore circulation model, it is found that the nearshore circulation is significantly affected by the heterogeneous wave patterns caused by wave refraction over the ebb tidal shoal. The model is used to predict waves and currents during field experiments conducted near the mouth of San Francisco Bay and nearby Ocean Beach. The field measurements indicate strong spatial variations in current magnitude and direction and in wave height and direction along Ocean Beach and across the ebb tidal shoal. Numerical simulations suggest that wave refraction over the ebb tidal shoal causes wave focusing toward a narrow region at Ocean Beach. Due to the resulting spatial variation in nearshore wave height, wave-induced setup exhibits a strong alongshore nonuniformity, resulting in a dramatic change in the pressure field compared to a simulation with only tidal forcing. The analysis of momentum balances inside the surf zone shows that, under wave conditions with intensive wave focusing, the alongshore pressure gradient associated with alongshore nonuniform wave setup can be a dominant force driving circulation, inducing heterogeneous alongshore currents. Pressure-gradient-forced alongshore currents can exhibit flow reversals and flow convergence or divergence, in contrast to the uniform alongshore currents typically caused by tides or homogeneous waves.

On the coupling of wave and three-dimensional circulation models: Choice of theoretical framework, practical implementation and adiabatic tests

Many theoretical approaches and implementations have been proposed for the coupling of the three-dimensional ocean circulation with waves. The theoretical models are reviewed and it is shown that the formulation in terms of the quasi-Eulerian velocity circumvents the essential difficulty of alternative formulations for the Lagrangian mean velocity. Namely, models based on this Lagrangian velocity require an estimation of wave-induced motions to first order in the horizontal gradients of the wave field in order to estimate the vertical flux of wave pseudo-momentum. So far, only three-dimensional wave models have been able to provide these estimates, and all published theories based on the simpler Airy theory are not consistent at the leading order, because they ignore or incorrectly estimate the vertical momentum flux. With an adiabatic example on a sloping bottom it is shown that this inconsistency produces very large spurious velocities. These errors are independent of the slope for the inviscid case, and are still significant when a realistic vertical mixing is applied. A quick diagnostic of the potential accuracy of a theoretical model is the vertical profile of the wave-induced forcing terms: if it is not uniform over depth in adiabatic conditions then it will produce spurious artificial flow patterns in conditions with shoaling waves. Although conceptually more challenging, the quasi-Eulerian velocity theories only introduce minor modifications of the solution procedure for the standard primitive equations: a modification of the surface boundary condition for the mass conservation, the addition of the Stokes drift in the tracer advection equations, and sources of momentum and turbulent kinetic energy with associated surface and bottom fluxes. All the necessary modifications of primitive equation models are given in detail. This implementation is illustrated with the MARS3D model, which passes the test of the adiabatic shoaling waves.