GLOBAL OCEAN TIDES FROM ERS 1 AND TOPEX POSEIDON ALTIMETRY/

Ocean tide models representing all major diumal and semidiurnal tidal constituents with a spatial resolution of 0.75 degrees x 0.75 degrees have been estimated using the first 1.5 years of ERS 1 and TOPEX/POSEI DON altimetry. The ocean tide model was derived from the combined use of ERS 1 and TOPEX/POSEIDON data by using a modified orthotide formula tion that simultaneously solves for ail diumal and semidiurnal constit uents as well as the annual signal. An additional adjustment of the so lar semidiurnal harmonic of the gravitational potential was applied in order to account for radiational forcing, particularly in the S-2 con stituent. TOPEX/POSEIDON provides excellent ocean tide estimates in th e open ocean. However, especially in coastal regions, the track spacin g of TOPEX/POSEIDON (315 km at the equator) is too coarse to determine large parts of the ocean tide signal. For these regions the inclusion of data from the ERS 1 35-day repeat mission provides a valuable supp lement, as the ERS 1 satellite has a track spacing which is around 3.6 times better than that of the TOPEX/POSEIDON satellite. The combined ERS 1 and TOPEX/POSEIDON ocean tide solution exhibits distinct sectora l geographical pattern of highs and lows when compared with the Cartwr ight and Ray (1990, 1991) ocean tide model. This indicates the presenc e of small but fundamental orbit errors present in the Cartwright and Ray ocean tide solution. Compared with a new set of 104 tide gauge rea dings compiled by Le Provost, the RMS differences of the combined ERS 1 and TOPEX solution are 2.51, 1.67, 1.58, and 1.13 cm for the M(2), S -2, K-1, and O-1 constituents, respectively. The increased spatial res olution of the combined ERS 1 and TOPEX model as compared to a TOPEX-a lone model is seen to reduce RMS differences from 37 to 22 cm for the M2 constituent, when compared to a selection of 90 pelagic and c(o)ast al tide gauges in the northwest European shelf region.