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Internal waves and tidal energy dissipation observed by satellite altimetry

Internal waves and tidal energy dissipation observed by satellite altimetry. E. Schrama, TU Delft / Geodesy The Netherlands schrama@geo.tudelft.nl. This talk. Altimetry to observe ocean tides Global energy dissipation Local energy dissipation Extraction of internal tide signals

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Internal waves and tidal energy dissipation observed by satellite altimetry

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  1. Internal waves and tidal energy dissipation observed by satellite altimetry E. Schrama, TU Delft / Geodesy The Netherlands schrama@geo.tudelft.nl

  2. This talk • Altimetry to observe ocean tides • Global energy dissipation • Local energy dissipation • Extraction of internal tide signals • Comparison to dissipation

  3. Satellite altimetry and tides • Altimetry: • Topex/Poseidon (and Jason), provide estimates of ocean tides at one second intervals in the satellite flight (along track) direction. • Quality Models: • The quality of these models can be verified by means of an independent comparison to in-situ tide gauge data, • RMS difference for M2: 1.5 cm, S2: 0.94, O1: 0.99, K1: 1.02, • Other consituents are well under the 0.65 cm level, • Assimilation: • There are various schemes that assimilate altimeter information in barotropic ocean tide models. (empirical, representer method, nudging)

  4. Satellite altimetry Source: JPL

  5. Global tidal energy dissipation • Integral values over the oceanic domain • Integral values over tidal cycles • Weak quality estimator for global ocean tides. • Independent astronomic and geodetic estimates. • Secular trend in Earth Moon distance • Earth rotation slow down • Here • Phase lags ocean, body or atmospheric tides

  6. 3.82 cm/yr M2 : 2.50 +/- 0.05 TW Tidal energy dissipation (Munk,1997)

  7. Global energy dissipation

  8. Recent Global Dissipations Estimates Units: TW

  9. Results Global Dissipation • High coherence between models, SW80 is an exception because it is pre-Topex/Poseidon. • M2: oceanic 2.42, astronomic 2.51 TW, the difference is dissipated in the solid Earth tide (Ray, Eanes and Chao, 1996) • S2: oceanic 0.40, geodetic 0.20 TW, the difference is mostly dissipated in the atmosphere (Platzman,1984)

  10. Local Dissipation (1) W: Work P: Divergence Energy Flux D: Dissipation

  11. Local dissipation (2) Notice: 1) Forcing terms are related to tide generating potential, self-attraction and loading, 2) the equations assume volume transport rather then velocity

  12. Local dissipation (3) • In order to compute local dissipations you must specify the forcing terms and the velocities • Altimetry only observes tidal elevations, it does not yield velocity estimates • The computation of barotropic velocities requires a numerical inversion scheme. • The forcing terms involve self-attraction and tidal loading.

  13. Internal tides (1) • High frequency oscillation is imposed on the along track tide signal, wavelength typically 160 km for M2, (Mitchum and Ray, 1997). • The feature stands above the background noise level. • The phenomenon is visible for M2 and S2 (hardly for K1). • There is some contamination in the T/P along track tides in regions with increased mesoscale variability. • “Clean” Along track tide features are visible around Hawaii, French Polynesia and East of Mozambique. • AT tides seem to appear near sub-surface ocean ridge systems.

  14. Mesoscale variability

  15. M2 ocean tide

  16. Track 223 Hawaii H dG D

  17. Internal tides (2) 160 km 5 cm 1 h1 20 m 2 h2

  18. Internal tides (3) (Apel, 1987)

  19. Area’s of interest

  20. Conclusions • Global dissipation: • there are consistent values for most models, • comparison to astronomic/geodetic values: • 0.2 TW at S2 for dissipation in the atmosphere • 0.1 TW at M2 for dissipation in the solid earth • Local dissipation: • values are more difficult to obtain and require an inversion of tidal elevations into currents, • AT tides: • appear as high frequency tidal variations in along track altimetry, • appear to be related to internal wave features, • coherence to local dissipations, • visibility: Hawaii, Polynesia, Mozambique, Sulu Celebes region.

  21. Discussion • Why relate internal tides to dissipation? • Mixing in the deep ocean is according to (Egbert and Ray, 2001) caused by internal tides. • Their main conclusion is that the deep oceanic estimate for M2 is about 0.7 TW. • According to Munk 2 TW is required for maintaining the deep oceanic stratification. • 1 TW could come from wind • The remainder could be caused by internal tides.

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