A new offshore transport mechanism for shoreline‐released tracer induced by transient rip currents and stratification

Kumar, Nirnimesh; Feddersen, Falk

doi:10.1002/2017gl072611

Cited by 24 publications

(33 citation statements)

References 43 publications

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“…Surface gravity wave effects can be categorized into shelf and surf‐zone effects. In the surf zone (waters extending from the shoreline to ≈200–300 m across‐shore), cross‐shore transport can be induced by 3‐D transient rip currents (Kumar & Feddersen, ), which is not represented in our simulations. Such eddying circulations will be responsible for a large fraction of horizontal diffusivity in the surf zone and ultimately control exchanges with inner‐shelf waters.…”

Section: Discussionmentioning

confidence: 99%

“…To be more thorough, physical variability in coastal waters comprises tides (Ganju et al, ; Suanda et al, ), wind‐driven flows (Chapman, ; Feddersen et al, ; Lentz & Fewings, ), internal waves (Buijsman et al, ; Lerczak et al, ; Noble et al, ), surf‐zone eddies and rip currents (Feddersen, ; Hally‐Rosendahl & Feddersen, ; Kumar & Feddersen, ; Spydell, ), shelf‐break fronts (Chapman & Lentz, ; Zhang & Gawarkiewicz, ), buoyant river plumes (Horner‐Devine et al, ), mesoscale eddies, and submesoscale fronts and filaments (Dauhajre et al, ; Romero et al, ; Uchiyama et al, ). Together, these dynamical regimes encompass a range of space (

scriptO

(0.01–10 km)) and time scales (

scriptO

(hours‐months)) that makes comprehensive sampling in the real‐ocean extremely difficult.…”

Section: Introductionmentioning

confidence: 99%

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Nearshore Lagrangian Connectivity: Submesoscale Influence and Resolution Sensitivity

2019

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Realistic simulation of nearshore (from the shoreline to approximately 10‐km offshore) Lagrangian material transport is required for physical, biological, and ecological investigations of the coastal ocean. Recently, high‐resolution simulations of the coastal ocean have revealed a shelf populated with small‐scale, rapidly evolving currents that arise at resolutions ⪅100 m. However, many historical and recent investigations of coastal connectivity utilize circulation models with ≈1‐km resolution. Here we show a resolution sensitivity to simulated Lagrangian transport and coastal connectivity with a hierarchy of Regional Oceanic Modeling System simulations of the Santa Barbara Channel at Δx= 1, 0.3, 0.1, and 0.036 km. At higher resolution ( normalΔx⪅ 100 m), rapid alongshore and vertical transport occurs in regions less than 1 km from the shoreline due to submesoscale shelf currents that open up new transport pathways on the shelf: submesoscale fronts and filaments, topographic wakes, and narrow alongshore jets. Shallow‐water fronts and filaments induce early time downwelling and subsequent dispersal at depth of surface material; this is not captured at coarser resolution (Δx= 1 km). Differences in three‐dimensional and two‐dimensional transport are explored in a higher‐resolution simulation: In general, three‐dimensional trajectories are more dispersive than two‐dimensional, due to a separation in their respective trajectories.

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Section: Discussionmentioning

confidence: 99%

scriptO

(0.01–10 km)) and time scales (

scriptO

(hours‐months)) that makes comprehensive sampling in the real‐ocean extremely difficult.…”

Section: Introductionmentioning

confidence: 99%

Nearshore Lagrangian Connectivity: Submesoscale Influence and Resolution Sensitivity

2019

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“…Transient rip‐currents (TRCs) are strong, episodic offshore directed flows commonly generated within natural surf‐zones (e.g., Johnson & Pattiaratchi, ) that dominate tracer exchange between the surf‐zone and inner‐shelf (e.g., Clark et al, ; Hally‐Rosendahl et al, , , ; Reniers et al, ). Recently, based on idealized simulations, TRC‐induced mixing on the stratified inner‐shelf generated a subsurface baroclinic exchange pathway that transported surf‐zone released tracer across the inner‐shelf at ≈1.5 cm s −1 (Kumar & Feddersen, , ). This exchange mechanism depends critically on the inner‐shelf stratification.…”

Section: Introductionmentioning

confidence: 99%

Tracer Exchange Across the Stratified Inner‐Shelf Driven by Transient Rip‐Currents and Diurnal Surface Heat Fluxes

Grimes

Feddersen

Kumar

2020

Geophysical Research Letters

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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 12L SZ offshore (L SZ = 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.Plain Language Summary Cross-shore transport (also called exchange) of material, for example, pollutants, larvae, nutrients, and plankton, is important in coastal oceanography. Natural surf-zone wave breaking leads to transient rip-currents (TRCs), episodic, offshore flows onto the inner-shelf, which vertically mix stratified waters creating a cross-shore exchange pathway. In many regions, such as Southern California, daily surface heating/cooling, or diurnal surface heat-fluxes (SHF), also drive cross-shore exchange, because thermal response varies with water depth. However, the dominant exchange mechanism is not known. Impacts of combined TRC and SHF forcing on exchange and their relative strength are analyzed using idealized numerical model simulations. Cross-shore transport is quantified using a tracer released within the surf-zone. Tracer transport is strongest for simulations including TRCs, relative to SHF forcing alone, and transport induced by TRCs extends well offshore of the surf-zone. Analyses indicate that enhanced TRC-driven inner-shelf exchange is associated with the vertical mixing mechanism.

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“…Once the surfzone was saturated (at a distance 100 m to over 1 km alongshore from the dye release location), the dye was transported by advection in the alongshore direction (Harris et al, 1963;Inman et al, 1971;Grant et al, 2005;Clark et al, 2010). In some cases, dye was visually observed (Grant et al, 2005) and measured in-situ and remotely-sensed (Hally-Rosendahl et al, 2014 to be transported to the inner shelf by transient rip currents, which has also been numerically modeled (Suanda and Feddersen, 2015;Hally-Rosendahl and Feddersen, 2016;Kumar and Feddersen, 2017a, 2017b, 2017c.…”

Section: Introductionmentioning

confidence: 99%

“…There have been few field studies on mass transport and cross-shore exchange across the surfzone boundary, and most have occurred on wide, dissipative beaches in southern California (Inman et al, 1971;Boehm, 2003;Grant et al, 2005;Clark et al, 2010;Hally-Rosendahl et al, 2014 and on a rip-channeled beach in Monterey, California (MacMahan et al, 2010;Brown et al, 2015). A number of numerical experiments have evaluated material exchange and the offshore extent on dissipative beaches associated with stationary rip currents Reniers et al, 2010;Castelle and Coco, 2013;Fujimura et al, 2013;Castelle et al, 2014;Fujimura et al, 2014) and transient rip currents (Suanda and Feddersen, 2015;Hally-Rosendahl and Feddersen, 2016;Kumar and Feddersen, 2017a, 2017b, 2017c. Dissipative beaches support communities with large human populations, where an understanding of the mixing and transport of pollutants has both health and economic relevance (Grant et al, 2005;Given et al, 2006;amongst others).…”

Section: Introductionmentioning

confidence: 99%

Observations of mixing and transport on a steep beach

Brown

MacMahan

Reniers

et al. 2019

Continental Shelf Research

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A B S T R A C TSurfzone mixing and transport on a sandy, steep (∼1/8 slope), reflective beach at Carmel River State Beach, California, are described for a range of wave and alongshore flow conditions. Depth-limited wave breaking occurred close to the shore due to the steepness of the beach, creating a narrow surf/swash zone (∼10 m wide). Fluorescent Rhodamine dye was released as a slug in the surfzone, and the temporal and spatial evolution was measured using in-situ dye sensors. Dye concentration measured as a function of time reveals sharp fronts that quickly decay resulting in narrow peaks near the dye release, which subsequently broaden and decrease in peak concentration with alongshore distance. The measurements indicate two stages of mixing and transport occur inside the surfzone on the steep beach. 1) In the near-field (< 50 m downstream of the dye release location), the dye fully mixed throughout the water column after a few incident waves then continued to disperse in two dimensions, with both advection and diffusion processes being important. 2) In the far-field (> 50 m downstream from the dye release location), the mass transport was dominated by advection. The distance to the farfield is much shorter in the alongshore on a steep beach compared with a dissipative beach. Estimates of crossshore and alongshore diffusion coefficients (κ x , κ y ) were found to be similar in magnitude within the surfzone. Outside the surfzone in the far-field, the results suggest that the mixing processes are independent of those inside the surfzone. The mixing and transport of material observed on this steep beach are found to be analogous to that previously measured on dissipative beaches, however the diffusion coefficients within and outside the surfzone were found to be smaller on this steep beach.

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A new offshore transport mechanism for shoreline‐released tracer induced by transient rip currents and stratification

Cited by 24 publications

References 43 publications

Nearshore Lagrangian Connectivity: Submesoscale Influence and Resolution Sensitivity

Nearshore Lagrangian Connectivity: Submesoscale Influence and Resolution Sensitivity

Tracer Exchange Across the Stratified Inner‐Shelf Driven by Transient Rip‐Currents and Diurnal Surface Heat Fluxes

Observations of mixing and transport on a steep beach

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