Presentation on theme: "1 Internal waves and tidal energy dissipation observed by satellite altimetry E. Schrama, TU Delft / Geodesy The Netherlands"— Presentation transcript:
1 Internal waves and tidal energy dissipation observed by satellite altimetry E. Schrama, TU Delft / Geodesy The Netherlands
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
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.
14 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.
15 Mesoscale variability
16 M2 ocean tide
17 Track 223 Hawaii H dG D
18 Internal tides (2) 20 m 5 cm 160 km 11 22 h1h1 h2h2
19 Internal tides (3) (Apel, 1987)
20 Area’s of interest
28 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.
29 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.