Internal waves and tidal energy dissipation observed by
Internal waves and tidal energy dissipation observed by satellite altimetry E. Schrama, TU Delft / Geodesy The Netherlands schrama@geo. tudelft. nl 1
This talk • • • Altimetry to observe ocean tides Global energy dissipation Local energy dissipation Extraction of internal tide signals Comparison to dissipation 2
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 M 2: 1. 5 cm, S 2: 0. 94, O 1: 0. 99, K 1: 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) 3
Satellite altimetry Source: JPL 4
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 5
Tidal energy dissipation 3. 82 cm/yr M 2 : 2. 50 +/- 0. 05 TW (Munk, 1997) 6
Global energy dissipation 7
Recent Global Dissipations Estimates Units: TW 8
Results Global Dissipation • High coherence between models, SW 80 is an exception because it is pre-Topex/Poseidon. • M 2: oceanic 2. 42, astronomic 2. 51 TW, the difference is dissipated in the solid Earth tide (Ray, Eanes and Chao, 1996) • S 2: oceanic 0. 40, geodetic 0. 20 TW, the difference is mostly dissipated in the atmosphere (Platzman, 1984) 9
Local Dissipation (1) W: Work P: Divergence Energy Flux D: Dissipation 10
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 11
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. 12
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Internal tides (1) • High frequency oscillation is imposed on the along track tide signal, wavelength typically 160 km for M 2, (Mitchum and Ray, 1997). • The feature stands above the background noise level. • The phenomenon is visible for M 2 and S 2 (hardly for K 1). • 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
M 2 ocean tide 16
Track 223 Hawaii H d. G D 17
Internal tides (2) 160 km 5 cm 1 2 20 m h 1 h 2 18
Internal tides (3) (Apel, 1987) 19
Area’s of interest 20
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Conclusions • Global dissipation: – there are consistent values for most models, – comparison to astronomic/geodetic values: • 0. 2 TW at S 2 for dissipation in the atmosphere • 0. 1 TW at M 2 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. 28
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 M 2 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. 29
- Slides: 29