INTERNAL WAVE GENERATION, BREAKING, MIXING AND MODEL VALIDATION ALAN DAVIES (POL) JIUXING XING (POL) JARLE BERNTSEN (BERGEN)

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

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.

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

HOW DO WE VALIDATE THAT WE HAVE CORRECT INTERNAL WAVE + MIXING INTERNAL WAVE SPECTRA AT KEY LOCATIONS DETAILED + COMPREHENSIVE TURBULENCE MEASUREMENTS

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

HOW TO PARAMETERIZE AND UPSCALE TO LARGE AREA MODELS Topographic gradients dh/dx Details of stratification Details of small scale wind forcing

TWO EXAMPLES INTERNAL WAVE MIXING Wind forced internal waves trapped in cold water dome Tidally forced internal waves over a sill.

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

BAROCLINIC IRISH SEA MODEL Simulation 3D baroclinic model Dome formation and breakdown Dome circulation published JPO

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

Super-inertial wind forcing

Wavelength λf from Dispersion Relation ωf = forcing frequency So λf/Leff gives nodal structure where Leff is effective length of dome

Sub-inertial wind forcing

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

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

Initial Conditions

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 ?????

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

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

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

SPECTRA

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 !!!!!!!!!