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Impacts of surface gravity waves on a tidal front: a coupled model perspective

Journées Scientifiques LEFE/GMMC 2019

Sophia E. BrumerV. Garnier, J.-L. Redelsperger, M.-N. Bouin, F. Ardhuin, and M. Accensi

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The AMICO Model Framework

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Model Framework

MesoNH

WaveWatchIII

MARS3D

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Model Framework

MesoNH

WaveWatchIII

MARS3D TOY

Sea Surface TemperatureSea Surface Currents

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Model Framework

MesoNH TOY

WaveWatchIII

MARS3D

Mean PeriodSignificant Wave Height

Surface Stokes DriftsStokes TransportsSurface Stresses

Wave-Ocean Momentum FluxesWave breaking Dissipation

Wave induced PressureNear-bottom rms Wave VelocitiesBottom Wave Diss. Energy Flux

Bottom Wave Diss. Stresses

Wind StressesHeat Fluxes

Pressure

10-m Wind

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2

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3

Sea Surface HeightSea Surface Currents

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Model Framework

MesoNH

WaveWatchIII

MARS3D

Mean PeriodSignificant Wave Height

Surface Stokes DriftsStokes TransportsSurface Stresses

Wave-Ocean Momentum FluxesWave breaking Dissipation

Wave induced PressureNear-bottom rms Wave VelocitiesBottom Wave Diss. Energy Flux

Bottom Wave Diss. Stresses

Wind StressesHeat Fluxes

Pressure

Sea Surface TemperatureSea Surface Currents

10-m Wind Charnock Coefficient

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2

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3

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Sea Surface HeightSea Surface Currents

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Iroise Sea Application

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The Iroise Sea

Physical Features:▪ SST Front

▪ Strong tidal currents

▪ Swells

FROMVAR:▪ 2-4 September 2011

▪ Low Winds (2 to 14 m s-1)

▪ Swell from the west-southwest

▪ Atmospheric front moving in from the west on Sept 2nd around 1800

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Model Setup

Models:▪ Ocean → Mars3D (500 m)

▪ Waves → WavewatchIII (1.5 km)

▪ Atmos. → MésoNH (1.25 km)

▪ Coupler → OASIS3MCT

Data:▪ FROMVAR field campaign

(wave buoys, scan fish, ...)

▪ Wave buoy @ Les Pierres Noires

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Model Setup

Models:▪ Ocean → Mars3D (500 m)

▪ Waves → WavewatchIII (1.5 km)

▪ Atmos. → MésoNH (1.25 km)

▪ Coupler → OASIS3MCT

Data:▪ FROMVAR field campaign

(wave buoys, scan fish, ...)

▪ Wave buoy @ Les Pierres Noires

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Experimental Design

02/09/2011 03/09/2011

00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21

WW3

MésoNH - D1

MARS3D

MésoNH - D2

Coupled Runs& TOY forced

▪ >15 RUNS:

▪ 4 stand-alone runs

▪ 7 forced with TOY

▪ 3 coupled (2 models)

▪ 1 coupled (3 models)

▪ Multiple partially coupled runs (2 models)

▪ Hourly forcing

▪ 10 min coupling

▪ 10 min & Hourly output

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Model Validation

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Excellent agreement with FROMVAR dataat surface and throughout water column

Ocean Model Validation

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Pierres Noires Treffle

Wave Model Validation

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Impacts of Waves on a Tidal Front

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Wave-Ocean Coupling

[1] Ardhuin et al. (2008), [2] Bennis et al. (2011), [3] Walstra et al. (2000)

1, 2

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Wave-Ocean Coupling

1, 2

[1] Ardhuin et al. (2008), [2] Bennis et al. (2011)

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● Semi-diurnal tide

○ Flood → eastward

○ Ebb → westward

● Waves grow gradually,

propagating northeast and

east in response to wind

● wave following (opposing)

current → blue (coral)

shading

Environmental Conditions

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Contrasting MARS3D runs @ 48.8 °N

● Cooling of 1°C on stratified side of front (c).

● Westward shift of the temperature front, opposite to Stokes drift advection (c).

● Reduction (increase) of the westward (eastward) tracer advecting velocity (uL ) around front (e)

Results - Surface Temperature

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Contrasting MARS3D runs @ 48.8 °N

Diagnosis of temperature equation:

● Difference in trend is reflected by difference in advection terms (c, d).

● Difference in diffusion smaller than in advection (e)

Results - Surface Temperature

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Results - Currents

Average profiles in Z1 and Z2:

● quasi-eulerian horizontal current components in coupled run are reduced (increased) in wave following (opposing) situations compared to eulerian currents in stand-alone run

● vertical momentum eddy diffusion (Ku ) increased within surface layer due to waves

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Results - Currents

• Quasi-eulerian horizontal current components in coupled run are reduced (increased) in wave following (opposing) situations compared to eulerian currents in stand-alone run

Terms of current equations for module of (quasi-) eulerian velocity at surface show:

• As waves grow, balance betweenStokes-Coriolis and vertical diff. Terms (+ apparent P grad terms) drives changes in currents.

NB: pressure gradient remains unchanged; apparent influence in (c)solely reflect differences in velocity modules

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1. Waves propagating towards the shore lead to offshore displacement of the SST front. Due to:a. reduced (enhanced) eastward

(westward) advection.b. reduction (enhancement) of

currents when waves follow (oppose) tides.

c. wave induced mixing (vertical diffusion) that works to reduce vertical gradients in current profiles.

2. Increased vertical diffusion partially balanced by Stokes-Coriolis effect.

3. Wave induced mixing has secondary effect on tracers compared to advection.

Conclusions

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Results - Currents

• Quasi-eulerian horizontal current components in coupled run are reduced (increased) in wave following (opposing) situations compared to eulerian currents in stand-alone run

Terms of current equations for module of (quasi-) eulerian velocity at surface show:

• As waves grow, balance betweenStokes-Coriolis and vertical diff. Terms (+ apparent P grad terms) drives changes in currents.

NB: pressure gradient remains unchanged; apparent influence in (c)solely reflect differences in velocity modules

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Results - Currents

• Quasi-eulerian horizontal current components in coupled run are reduced (increased) in wave following (opposing) situations compared to eulerian currents in stand-alone run

Terms of current equations for module of (quasi-) eulerian velocity at surface show:

• As waves grow, balance betweenStokes-Coriolis and vertical diff. Terms (+ apparent P grad terms) drives changes in currents.

NB: pressure gradient remains unchanged; apparent influence in (c)solely reflect differences in velocity modules

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