INTERNAL WAVE GENERATION , BREAKING, MIXING AND MODEL VALIDATION

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INTERNAL WAVE GENERATION , BREAKING, MIXING AND
MODEL VALIDATION

• ALAN DAVIES (POL)
• JIUXING XING (POL)
• JARLE BERNTSEN (BERGEN)

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EXTERNAL FORCING

• Tides (Moon , Sun ) , with stratification (T or
  S origin) topography gives internal waves.
• Meterological , solar heating gives
  stratification , with wind forcing
  stratification internal waves

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LOCAL MIXING INFLUENCE LARGE SCALE CIRCULATION

• Significant Ocean circulation in lateral boundary
  layers
• Topographic gradients Density gradients in
  these regions , source of internal wave
  generation , mixing which influences their
  lateral extent , Hence boundary layer flow.

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MIXING SOURCES

• Energy cascade through breaking internal waves
• Internal waves generated in one region propagate
  to another
• Energy loss to mixing during propagation
• Energy loss to mixing , due to non-linear
  processes giving rise to wave breaking

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HOW DO WE VALIDATE THAT WE HAVE CORRECT INTERNAL
WAVE MIXING

• INTERNAL WAVE SPECTRA AT KEY LOCATIONS
• DETAILED COMPREHENSIVE TURBULENCE MEASUREMENTS

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MODEL NEEDS

• DETAILED SMALL SCALE TOPOG.
• PRECISE SPECTRA OF FORCING AND ITS AMPLITUDE
• ACCURATE INITIAL STRATIFICATION AND DETAILS OF
  ITS EVOLUTION FOR VALIDATION

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HOW TO PARAMETERIZE AND UPSCALE TO LARGE AREA
MODELS

• Topographic gradients dh/dx
• Details of stratification
• Details of small scale wind forcing

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TWO EXAMPLES INTERNAL WAVE MIXING

• Wind forced internal waves trapped in cold water
  dome
• Tidally forced internal waves over a sill.

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Format

• (A) Internal Wave trapping in Domes
• (B) Mixing over abrupt topog.
• Conclusions and future Developements

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BAROCLINIC IRISH SEA MODEL

• Simulation 3D baroclinic model
• Dome formation and breakdown
• Dome circulation published JPO

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Non-Linear effects on Inertial Oscillations

• Unbounded Ocean Eqts
• Effect of external shear is to change Amp.
  Freq. of I.O.
• Frontal Shear Changes I.O. amp./Freq at depth so
  conv/divg. Gives internal wave at level of
  thermocline.
• Freq. int. wave above inertial propogates away ,
  if below trapped

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Super-inertial wind forcing
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• Wavelength ?f from Dispersion Relation
• ?f forcing frequency
• So ?f/Leff gives nodal structure where Leff is
  effective length of dome

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Sub-inertial wind forcing
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CONCLUSIONS

• 1. Non-linear effects associated with along
  frontal flows produce near-inertial internal
  waves in presence of wind forcing
• 2. Super-inertial internal waves propagate away
  from generation region (front)
• 3. Sub-inertial are trapped and enhance mixing in
  frontal region
• 4. In a cold water bottom dome, super-inertial
  internal waves are trapped as standing waves, can
  modify GM spectrum
• 5. Response in centre of dome different from 1D
  model, must account for internal wave
• 6. Sub-inertial wave confined to front, and
  response in centre of dome as in 1D model

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TIDAL MIXING AT SILLS

• Idealized Loch Etive
• Recent measurements Inall et al
• Non-hydrostatic model
• High resolution
• Idealized M2 forcing idealized T profile
• Example of internal tidal mixing

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Initial Conditions
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Influence of small scale topog.

• Lee wave characteristics influenced by
• Buoyancy frequency
• Velocity over sill.. Froude Number
• Fourier transform of topog.
• So How small scale effect mixing ?????

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CONCLUSIONS.. Sill

• Internal tide little mixing
• Lee Wave not advected back over sill
• Lee Wave major source of mixing
• Lee wave distribution influenced by non-hydro.
  nature of model
• Lee wave spectrum/mixing influenced by small
  scale topog.
• Assumptions in b.b.l. also infulence lee wave
  hence mixing

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Future

• Role surface stratification / fresh water , wind
  mixing
• Detailed distribution of Topog.
• Sill b.b.l effects
• Lateral across sill form drag

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Model Skill Assessment

• Model Validation in highly variable undersampled
  domain.
• Spectral Decompostion.. Hans van Haren

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SPECTRA
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Conclusions

• Details of wind field frequency composition
• Precision in stratification
• Accurate tidal forcing
• Precise small scale topog. Variations.
• MAJOR PROBLEMS IN VALIDATION
• HOW TO UPSCALE WITHOUT LOOSING ACCURACY
  !!!!!!!!!