Dynamic Topography Estimates UsingGeosatData and A Gravimetric Geoid In the Gulf Stream Region

Abstract

SUMMARY A gravimetric geoid undulation, on a 3' x 3' grid has been calculated in the Gulf Stream region: 5 @ I 45, 278 5 ,I 5 318. These undulations were calculated using two 360 potential coefficient models, land, ship and altimeter-derived gravity anomalies, and bathymetric data. Least-squares collocation and fast Fourier transform procedures were used with various data selection and gridding procedures. Results from five different undulation computations are described with comparisons made with synthetic geoid undulations along Geosut tracks in the region. The standard deviation of the undulation differences was f14cm when a cubic polynomial was used to model long-wavelength errors. A point verification of the geoid undulation at the laser tracking station on Bermuda was also made with a discrepancy ('ground truth' minus model undulation) of 40 cm, within the predicted standard deviation. The gravimetric geoid undulation was used to compute dynamic topography along Geosat tracks for comparison with existing estimates based on hydrographic data in the Gulf Stream region. The agreement between these two estimates is on the order of f15 cm although the discrepancies can reach 60 cm. The larger differences are usually associated with a location on a track that passes near a seamount where the gravity data may be inadequate to represent the high-frequency variations in the geoid undulations. This effect will be represented in the undulation standard deviations that have been calculated using the least-squares collocation procedure. The average accuracy is fl6cm with the range from f14 to f48cm. The dynamic height derived from the gravimetric undulations and altimeter data has been used to calculate characteristics of the Gulf Stream (width, velocity, centre location, height profile) using an implied velocity model for the set. The results are compared with previous estimates with generally good agreement. However, the maximum velocities and the jump function are approximately 30 per cent larger than previous studies that used an average of altimeter tracks to define the geoid undulation. The dynamic topography was calculated for the entire region using a mean sea surface based on Geos-3, Seasat and Geosat data. The results are compared with two hydrographic estimates due to Martel/Wunsch and LeTraon/Mercier. The agreement is at the f25cm level with the best correlation coefficient reaching 0.72 with the Martel/Wunsch model.