COTAGA (Combined Ocean Tide Analysis by GRACE and Altimetry Data) is a joint project between the German Geodetic Research Institute (DGFI) in Munich, the Institute of Theoretical and Satellite Geodesy (ITSG) of the Technical University Graz and GFZ within the DFG Special Priority Program 1257 "Mass Transport and Mass Distribution in the Earth System".
Precise models of ocean tides are crucial because (a) the effect of ocean tides has to be removed from CHAMP, GRACE and GOCE observations in order to obtain models describing exclusively the Earth gravity field and (b) altimetric sea surface heights should be "tide-free" in order to be comparable with each other and to be assimilated into numerical models. As deficiencies in the currently available ocean tide models, especially over continental shelves and in the polar oceans, are well known, the COTAGA project has the following strategies:
Empirical improvements to the widely used state-of-the-art ocean tide model FES2004 will be derived by (a) solving residuals for major tidal constituents with a spatial resolution as short as 500 km from GRACE data only (long wavelength) and (b) applying response and harmonic analysis to multi-mission altimeter data to estimate in particular for shallow water areas corrections for the dominant tidal constituents and non-linear effects like M4 (short wavelength). Both corrections will be validated and combined to an upgraded FES2004 model, which is called EOT??a (Empirical Ocean Tide model where ?? denotes the generation year e.g. 08,10 or 11) and which is subsequently used for a more precise de-aliasing of the gravity field models and for improved corrections of altimetry data. The optimal combined solution of altimetry information and GRACE information (using variance component estimation) will lead to an ocean tide model EOT12ag.
Several methods to validate the latter will be used.
The current project achievements are:
- A series of new global Empirical Ocean Tide models, EOT08a, EOT10a, EOT11a, (from the years 2008, 2010, 2011) has been estimated by empirical analysis of (harmonized, upgraded and cross-calibrated) multi-mission satellite altimeter data. Analysis yield shallow-water amplitude corrections up to 15 cm, identified large scale corrections for M2 and K2 with 1-2 cm amplitude with a clear de-correlation of critical constituents (mean & S2, K2 & Ssa). EOT11a will serve as the basis for a GRACE combined EOT12ag which will become available soon.
- Residuals of 4.5 years of K-band range-accelerations have been used to adjust amplitudes and phases of different partial tides. Using EOT08a most of the M2 amplitudes disappear, successfully validating the EOT08a model by the K-band range data (see Figure). The potential of GRACE to observe long-wavelength tides is also confirmed by a sensitivity study.
© GFZ Potsdam
- Harmonic analysis for M2 of residual GRACE K-band range accelerations for 2°x 2° global grid cells using 4.5yr of GRACE data between August 2002 and April 2007 [nm/s2] using the models FES2004 (top) and EOT08a (bottom). The reduction of spurious signals in different shelf areas can be clearly seen (courtesy ITSG).