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Explore the spectral response and vertical mixing parameterizations in the Indonesian Throughflow Domain, evaluating different models and their impact on energy transfer between frequencies. Comparing observations to model results, the study assesses the diffusivity of momentum and vertical diffusivities while discussing the implications for tidal velocities and dissipation.
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Vertical Mixing from ROMS: Spectral Response of Velocities Dr. Robin RobertsonSchool of PEMSUNSW@ADFACanberra, Australia
Indonesian Throughflow Domain Transects
Elevation Fields Rms differences: M2 7.6 cm S2 5.8 cm K1 11.7cm O1 8.8 cm With site 8 excluded
Vertical Mixing • Internal Waves are a significant mixing mechanism • 3.2 TW (TW=1012 W) [Garrett, 2003] • Interest in estimates of vertical mixing • Measurements • Difficult and expensive • Small scale • Episodic • From Models • Depends on the parameterization used
Vertical Mixing ParameterizationsIn ROMS • Many available in Roms • Mellor-Yamada 2.5 level turbulence closure (MY2.5) • Kpp – Large-McWillamns-Doney (LMD) • Brunt-Väisälä Frequency Based (BVF) • Pacanowski-Philander (PP) • General Ocean Mixing (GOM) • Generic Length Scale (GLS) • - • - • -l • generic
Evaluated the Performance of theseMixing Parameterizations • Internal Tidal model for Fieberling Guyot • Accessible data set for both velocities and dissipation • Good agreement for major axes of semidiurnal tidal ellipses between model estimates and observations
Comparison to Observations: Observations Model Results rms: 3.7 cm s-1 rms: 6.5 cm s-1 rms: 7.1 cm s-1
Vertical Diffusivities Averaged over the Region • MY25* 1.1x10-4 m2 s-1 • GOM 7.0x10-5 m2 s-1 • BVF 3.4x10-3 m2 s-1 • LMD* 1.9x10-4 m2 s-1 • LMD-SCCW 1.7x10-4 m2 s-1 • PP 4.8x10-5 m2 s-1 • GLS- kkl 1.7x10-3 m2 s-1 • GLS- k 1.7x10-3 m2 s-1 • GLS- k* 9.9x10-5 m2 s-1 • GLS- gen* 8.9x10-5 m2 s-1
Is there really no difference in the velocities? • Maybe the tidal frequencies aren’t the place to look • Vertical mixing transfers energy from tides to high frequencies, especially harmonics
Spectral Response • Background Location • Follows Garrett-Munk spectra (N=5 cph)
Leads to 2 questions • Why do the spectra follow Garrett-Munk? • Which one is the best performer?
Garrett-Munk and Vertical Diffusion • Vertical Diffusion plays a big role in spectral shape • Is not responsible for the transfer of energy between frequencies or energy cascade • Does remove the higher frequency energy • Background location • Black – with vertical diffusion • Red – without vertical diffusion
What about Higher Frequencies? • Diurnal and Semidiurnal tidal peaks clearly visible • 0.04 hr-1 (24 hr) • 0.08 hr-1 (12 hr) • GOM has increased energy at highest frequency (midlevel)
Focusing on Higher Frequencies • Internal Wave Generation Site
Normalizing by MY25 • Internal Wave Generation Site
Focusing on Higher Frequencies • Background Site
Normalizing by MY25 • Background Site
Observations • Correlation between high vertical diffusivity and low spectral energy • GOM showed enhanced energy at high frequencies; exceeded the 95% confidence • PP shows very weak vertical diffusivity • BVF, GLS-kkl, and GLS- have high average KM • MY25 has bump in energy at high frequency end • Differences at tidal frequencies are small • Differences occur in high frequencies.
Summary • All 10 vertical mixing schemes generated spectra roughly following G-M • Believed to be due to non-linear interactions and the vertical mixing parameterization • No answer as to which is “best” • Best performers are: GLS-, GLS-gen then MY25, LMD • Based on matching • tidal velocities • Dissipation: both with depth and area average • Spectral response
