Estimating the circulation from hydrography and satellite altimetry in the Southern Ocean: limitations imposed by the current geoid models

Sea-surface height data from satellite altimetry provide a very powerful means of determining the general ocean circulation. For oceanographic studies in which the absolute sea-surface height is required, one has to use the equipotential height of a geoid model as a reference surface. Earlier studies have shown that with recent geoid models, this reference surface is not known to an accuracy sufficient for ocean state estimation. It is demonstrated with an analysis of a hydrographic section between Australia and Antarctica that the combination of altimetry data and the geoid height of a state-of-the-art geoid model is not only inaccurate, but also inconsistent with a hydrographic estimate of the flow field. The conclusion is drawn that this is so because the formal errors of the geoid model underestimate its true errors.