Testing ocean tide models in the Nordic seas with tidal gravity observations

Tidal gravity observations at Ny-Alesund on Spitzbergen, made with a LaCoste & Romberg gravimeter with electrostatic feedback, have been used to assess the accuracy of ocean tide models in the Nordic seas. Eight ocean tide models have been used to compute the ocean tide loading at Ny-Alesund. The station is very close to the coast and therefore experiences a significant direct gravitational attraction of the nearby tidal water mass. For the bay adjacent to the station, the model values have been replaced by the tide gauge values and a special effort has been made to fit the models correctly to the coastline. The ocean tide models for M-2 and O-1 have also been compared with observations made over 30 years ago at Longyearbyen, Spitzbergen, with Askania gravimeters. The comparisons of the tidal gravity observations with computed ocean tide loading show that for the harmonic M-2 the phase lag of the ocean tide model of Schwiderski and the local tide model of Gjevik et al. are wrong by +15degrees and -5degrees in the Norwegian Basin. For the harmonic S-2, the models FES94.1, GOT00.2 and Schwiderski give a poor fit to the observed tidal loading at Ny-Alesund because of phase lags that are too large in the Norwegian Basin by similar to10degrees. FES94.1 and GOT00.2 also have S-2 amplitudes that are similar to5 cm too large in the Norwegian Basin. For N-2, FES94.1, FES95.2, GOT00.2 and Schwiderski have phase lags that are 10 too large in the Norwegian Basin and FES94.1 and 95.2 also have N-2 amplitudes that are 2-4 cm too large in that region. Overall, for these three harmonics the global model FES99 gives the best fit to the tidal gravity observations at Ny-Alesund and this agreement is even closer than that obtained using regional Arctic Ocean tide models.