COMBINING ALTIMETRY AND HYDROGRAPHY FOR GEODESY

It is well known that lack of knowledge of the geoid has been a limitation in the use of satellite altimetry for studies of mesoscale ocean currents. The ocean geoid is at least an order of magnitude larger than the signal from mesoscale ocean currents. This had lead to oceanographers concentrating on the variability of ocean signals from altimetry. Recent improvements in geoid determinations have allowed the large (gyre) scale portion of the mean ocean field to be determined to a fair degree of accuracy. However, at smaller spatial scales, the geoid is still poorly determined. A method used for some years at Southampton involves determining the surface geostrophic current along a satellite track using a combination of hydrography and ADCP. If the in situ data are collected during a satellite overpass, this current is the current measured by the altimeter. Hence, the total surface current at any time along this track can be determined by taking all other altimetry relative to this overpass. As a by-product of this process, we are able to determine the shape of the ocean geoid along the altimeter track at the time of the overpass. By removing the contribution of the surface ocean currents from the altimeter derived surface slope, we are left with the slope of the geoid. Integration of this gives the geoid, with an unknown constant offset. By analysing the geoid profiles determined from several such transects, taken at a variety of locations around the world'Ês oceans, we can predict the possible contribution that GOCE may make to determination of absolute surface currents at scales of 10s to 100s of km