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A Global Mean Dynamic Ocean Topography
  • Frank Siegismund
Frank Siegismund
Technical University Munich

Corresponding Author:frank.siegismund@tum.de

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The space-born geodetic temporal Mean Dynamic Topography (MDT) is obtained from the difference of altimetric Mean Sea Surface (MSS) $h$ and the geoid height $N$. With the geostrophic surface currents obtained from its gradient the MDT is an essential parameter when discribing the ocean dynamics. Spectral consistency of $h$ and $N$ is crucial to minimize MDT errors. Usually, $h$ is globalized to allows for a Spherical Harmonic (SH) analysis and small scales beyond maximum degree and order (d/o) resolved in the geoid are cut-off. However, the usual globalization causes ocean-land steps in $h-N$ and spectral inconsistencies of $N$ and $h$ over land. To overcome both issues a new methodology is proposed based on globalization of the MDT. A Laplacian smoother with the coastal MDT values as boundary condition is applied resulting in a flat surface over land and a continuous ocean-land transition. The new methodology strongly reduces Gibbs effects and the need to work with high resolution MDTs to minimize them. Reduction of resolution is tested to reduce MDT uncertainties caused by the commission error expected to increase whith decreasing scale. Applying drifter data and a high resolution hydrodynamic ocean model it is shown, that for the Gulf Stream and the Kuroshio geodetic MDTs applying recent combined geoid models contain physical information up to at least d/o 420 (48km spatial scale). Since for oceanic regions with strong gradients in $N$ still inconsistencies between the geoid and the MSS exist, it depends on application/region if a higher resolution MDT is needed.