Abstract In upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer and inner‐shelf mooring locations throughout the study. A poleward‐propagating plume of Southern California Bight water that increases shelf water temperatures by 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface‐trapped and slope‐controlled dynamics. Plume momentum balances are distinct between the offshore (>30 m depth) region where the plume is surface‐trapped, and onshore of the 30 m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence.
Abstract Exchange across the surf‐zone and inner‐shelf affects coastal water quality and larval recruitment. Surf‐zone generated transient rip‐currents (TRC) exchange shoreline released tracers onto and across a stratified inner‐shelf. Surface heat fluxes (SHF) modify inner‐shelf stratification and surf‐zone temperature, relative to the inner‐shelf, inducing nearshore thermally driven exchange. The coupled effect of TRC and diurnal SHF forcing on cross‐shore exchange is evaluated using idealized model surf‐zone tracer releases with TRC‐only, SHF‐only, and combined SHF+TRC forcing. For conditions representing Fall in Southern California, the TRC mechanism dominates cross‐shore exchange, relative to SHF, to 12 L S Z offshore ( L S Z = 100 m is the surf‐zone width). Tracer and velocity derived estimates of exchange velocity indicate that the TRC cross–inner‐shelf exchange mechanism is due to an alongshore mean baroclinic flow setup by TRC vertical mixing of inner‐shelf stratification.
Abstract This is Part II of a two-part study focused on Stokes drift and transient rip current (TRC) effects on the unstratified (Part I) and stratified (this paper) inner shelf. Part I focuses on funwaveC–Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) coupling and TRC effects on mixing and exchange on an unstratified inner shelf. Here, two simulations (R3 and R4) are performed on a stratified inner shelf and surfzone with typical bathymetry, stratification, and wave conditions. R3 is a COAWST-only simulation (no TRCs), while R4 has funwaveC–COAWST coupling (with TRCs). In R4, TRCs lead to patchy, near-surface cooling, vertical isotherm displacement, and increased water column mixing. For both R3 and R4, the mean Lagrangian circulation has two nearly isolated surfzones and inner-shelf overturning circulation cells, with a stronger, R4, inner-shelf circulation cell. The R4, inner-shelf, vertical velocity variability is 2–3 times stronger than a simulation with TRCs and no stratification. Relative to R3, R4 eddy diffusivity is strongly elevated out to three surfzone widths offshore due to TRCs and TRC-induced density overturns. The R4 inner-shelf stratification is reduced nearshore, and mean isotherms slope more strongly than R3 because of the TRC-enhanced irreversible mixing. At six surfzone widths offshore, both R3 and R4 are in geostrophic balance, explaining the stratified (summertime) observed deviation from Stokes–Coriolis balance. In this region, baroclinic pressure gradients induced by sloping isotherms induce an alongshore geostrophic jet offshore, strongest in R4. In R4, TRCs result in an enhanced (2–10 times) cross-shore exchange velocity across the entire inner shelf, relative to R3. Accurate, stratified, inner-shelf simulations of pollution, larval, or sediment transport must include transient rip currents.
As abyssal ocean properties are altered by climate change, density stratification may be expected to change in response. This shift can affect the buoyancy flux, internal wave generation, and turbulent dissipation, which may impact mixing and vertical transport. In this study, repeated surveys of three hydrographic sections in the Southwest Pacific Basin between the 1990s-2010s are used to estimate the change in buoyancy frequency N. We find that below Θ = 0.8◦C, N is reduced by a mean scaling factor of 0.88{plus minus}0.06 per decade. This reduction is intensified at depth, with the biggest change observed at Θ=0.63◦C by a scaling factor of 0.71{plus minus}0.07. Within the same time period, the magnitude of per unit area vertical diffusive heat flux is reduced by about 0.01 Wm, although this estimate is sensitive to the choice of estimated diffusivity. Finally, implications on heat budget and global ocean circulation are qualitatively discussed.
This archive contains SWASH model input and post-processing scripts presented in “Modeled Three-Dimensional Currents and Eddies on an A longshore-Variable Barred Beach." Support was provided by the Washington Royalty Research Fund, the National Science Foundation, the Office of Naval Research, a National Defense Science and Engineering Graduate Fellowship, a Vannevar Bush Faculty Fellowship, the United States Army Corps of Engineers, the United States Coastal Research Program, Sea Grant, and the WHOI Investment in Science Fund. The files are contained in two zip files: model_input.zip contains input files for all simulations presented in this paper. model_output_processing.zip contains the model output post-processing scripts SWASH is an open source code and can be download at http://swash.sourceforge.net/home.htm. Please contact C.M. Baker at cmbaker9@uw.edu with questions.
Abstract X‐band radar observations from the 2017 Inner Shelf Dynamics Experiment (ISDE) in central California show multiple persistent and pulsatory rip currents on a relatively straight coastline with alongshore‐varying bathymetry. Although past studies have assessed the characteristics of transient rip currents on alongshore uniform beaches, the relative balance of transient versus steady rip current behavior on nonuniform beaches in realistic wave conditions remains poorly understood. Here, a phase‐resolving Boussinesq‐type wave model ( funwaveC ) is used to assess the role of alongshore‐varying bathymetry and incident conditions in controlling mean and transient surf zone vorticity and velocity fields and their effect on surf zone exchange. The model simulates wave conditions chosen from the ISDE observations and utilizes both an alongshore‐varying bathymetry estimated from the ISDE radar observations and a uniform bathymetry. Results show that the variable bathymetry significantly increases the alongshore‐ and time‐averaged kinetic energy but that this increase is primarily due to the increase in the standing component resulting from mean circulation patterns, with only small changes in the transient component. A variable bathymetry also increases the spectral energy of surf zone vorticity and time‐averaged vorticity forcing at large spatial scales (>100 m). Wave directional spreading has a large impact on the alongshore‐ and time‐averaged enstrophy and on the spectral energy of surf zone vorticity and vorticity forcing at smaller spatial scales (<100 m). In the presence of a directionally spread wavefield, an alongshore‐varying bathymetry slightly increases the total exchange velocity but has little effect on its transient component.
Abstract Transport of shoreline-released tracer from the surfzone across the shelf can be affected by a variety of physical processes from wind-driven to submesoscale, with implications for shoreline contaminant dilution and larval dispersion. Here, a high-resolution wave–current coupled model that resolves the surfzone and receives realistic oceanic and atmospheric forcing is used to simulate dye representing shoreline-released untreated wastewater in the San Diego–Tijuana region. Surfzone and shelf alongshore dye transports are primarily driven by obliquely incident wave breaking and alongshore pressure gradients, respectively. At the midshelf to outer-shelf (MS–OS) boundary (25-m depth), defined as a mean streamline, along-boundary density gradients are persistent, dye is surface enhanced and time and alongshelf patchy. Using baroclinic and along-boundary perturbation dye transports, two cross-shore dye exchange velocities are estimated and related to physical processes. Barotropic and baroclinic tides cannot explain the modeled cross-shore transport. The baroclinic exchange velocity is consistent with the wind-driven Ekman transport. The perturbation exchange velocity is elevated for alongshore dye and cross-shore velocity length scales < 1 km (within the submesoscale) and stronger alongshore density gradient ∂ ρ /∂ y variability, indicating that alongfront geostrophic flows induce offshore transport. This elevated ∂ ρ /∂ y is linked to convergent northward surface along-shelf currents (likely due to regional bathymetry), suggesting deformation frontogenesis. Both surfzone and shelf processes influence offshore transport of shoreline-released tracers with key parameters of surfzone and shelf alongcoast currents and alongshelf winds.
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.
