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Xiaodong Wu, Douglas Cahl, and George Voulgaris

Abstract

The formation of coastal dense shelf water in winter provides the available potential energy (APE) to fuel baroclinic instability. The combined effects of baroclinic instability and wind forcing in driving cross-shelf exchange are investigated using idealized numerical simulations with varied bottom slope, wind stress, and heat loss rate. The results show that under upwelling-favorable winds, the intensity of the instability decreases as the wind stress increases. This is caused primarily by enhanced turbulence frictional dissipation. Under downwelling-favorable winds, an increase in wind stress and/or a decrease in heat loss rate tends to constrain the baroclinic instability, leading to a circulation resembling that driven purely by wind forcing. In the latter case, once a critical value of cross-shore density gradient is reached, isopycnal slumping is initiated, leading to increased vertical stratification and narrowing of the inner shelf. The change in depth of the inner-shelf outer boundary, defined as the location corresponding to the maximum cross-shore gradient of the surface Ekman transport, is proportional to an empirically derived multiparametric quantity , where a 2 is a dimensional constant, B 0 is a constant heat loss rate, γ = 0.43, f is the Coriolis parameter, α is the shelf slope, B is the heat loss rate, and τ is the wind stress. This relationship is found to hold for cases when instabilities are present.

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Xiaodong Wu, George Voulgaris, and Nirnimesh Kumar

Abstract

Numerical simulations are used to study the response of Long Bay, South Carolina, a typical coastal embayment with curved coastline located on the South Atlantic Bight, to realistic, climatologically defined, synoptic storm forcing. Synoptic storms, consisting of cold and warm fronts as well as tropical storms, are used as forcing under both mixed and stratified initial conditions. The analysis focuses on the development of cross-shore shelf circulation and the relative contributions of regionally defined cross-shore winds and alongshore bathymetric variation. The simulation results show that, under stratified conditions, the regionally defined offshore-directed wind component promotes upwelling during the developing stage of the cold front and enhances mixing during the decaying stage. No significant effect is found for warm front and tropical storm forcing conditions. Net cross-shore transports are induced at the southern and northern sides of the embayment that have opposing signs. Besides the surface and bottom Ekman transports, geostrophic transport due to alongshore shelf bed slope and horizontal advection are found to be important contributors to cross-shore flow development. Sea level variability along the curved coastline is driven by the regional alongshore wind, but a spatial variability is identified from the locally defined components of along- and cross-shore winds controlled by coastline orientation.

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Xiaodong Wu, Falk Feddersen, and Sarah N. Giddings

Abstract

Here, we explore the kinematics and dynamics of coastal density fronts (within 10 km from shore and <30-m depth), identified using an edge detection algorithm, in a realistic high-resolution model of the San Diego Bight with relatively weak winds and small freshwater input. The density fronts have lengths spanning 4–10 km and surface density gradients spanning 2–20 × 10−4 kg m−4. Cross-shore-oriented fronts are more likely with northward subtidal flow and are 1/3 as numerous as alongshore-oriented fronts, which are more likely with onshore surface baroclinic diurnal flow. Using a subset of the cross-shore fronts, decomposed into cross-front mean and perturbation components, an ensemble front is created. The ensemble cross-front mean flow is largely geostrophic in the cross- and alongfront directions. The ensemble cross-shore front extends several kilometers from shore, with a distinct linear front axis and downwelling (upwelling) on the dense (light) side of the front, convergent perturbation cross-front flow within the upper 5 m, strengthening the ensemble front. Vertical mixing of momentum is weak, counter to the turbulent thermal wind mechanism. The ensemble cross-shore front resembles a gravity current and is generated by a convergent strain field acting on the large-scale density field. The ensemble front is bounded by the shoreline and is alongfront geostrophic and cross-front ageostrophic. This contrasts with the cross-front geostrophic and alongfront ageostrophic balances of classic deformation frontogenesis, but is consistent with semigeostrophic coastal circulation.

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Xiaodong Wu, Falk Feddersen, Sarah N. Giddings, Nirnimesh Kumar, and Ganesh Gopalakrishnan

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.

Open access