The geoid EDIN2000 and mean sea surface topography around the British Isles

Astronomy and Astrophysics – Astronomy

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Geoid, Levelling, Ocean Currents, Sea Level, Sea Surface Topography

Scientific paper

This paper describes a new rationale for computing a gravimetric geoid and reports that the resulting geoid model, EDIN2000, is now good enough to resolve previously contradictory estimates for mean sea surface topography (MSST) around Great Britain. For at least 30 yr, it has been known that MSST derived from tide gauges, levelling and oceanography were mutually inconsistent and differed by an order of magnitude. When combined with an altimetric mean sea surface, EDIN2000 is also able to map realistic MSST over the northwest European shelf, and identify and quantify mean currents flowing in the deep ocean parallel to the shelf edge.
When compared with 11 GPS referenced tide gauges on the British mainland our estimate of MSST has a standard deviation of only 0.03 m and its variation with latitude is not significantly different from zero. For these sites, long distance levelling errors are larger than geoid errors. We identify a systematic jump (by 0.24 +/- 0.05 m) in the levelled transfer of Ordnance Datum Newlyn, affecting all sites in Great Britain to the north of latitude 53°N. Even after this correction, the variability of levelling remains larger than that of the geoid and GPS, implying a more complex structure to the levelling error than a simple step.
Away from coasts we find that the best way to validate marine gravity data is their ability to predict MSST with low local variability when combined with altimetric mean sea level. There is a large reduction in MSST variability when marine gravity data are subjected to an anti-aliased adjustment of the mismatch where ship tracks cross. We also report two MSST model comparisons from a shelf seas and a deep ocean model, both of high resolution. There is quantitative and qualitative agreement between our geoid-derived MSST and the model predictions, indicating significant improvements compared with earlier geoid models. We also show that with the improved accuracy of geoid-based MSST, it will become necessary to validate models and observations at matching epochs.

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