INTERCOMPARISON OF RECENT OCEAN TIDE MODELS

One of the tremendous achievements of the TOPEX/POSEIDON (T/P) satelli te mission is the release of 12 new global ocean tide models during 19 94. Virtually all of these models are based on the high-precision alti metry obtainable from the T/P satellite, and all of the models are sup erior to both the Cartwright and Ray (1990, 1991) model as well as to the Schwiderski (1980a, b) model when compared to tide gauges. In the present intercomparison, only global ocean tide models released during 1994 have been tested. As T/P continues to operate, new ocean tide mo dels will continue to be released and existing ocean models will be up dated. Hence this intercomparison can only present a snapshot of the ' 'state of the art'' within ocean tide modeling. However, the intercomp arison demonstrates the high accuracy and consistency of all these rec ent ocean tide models. From a comparison with a common 104 tide gauge data set compiled by Le Provost and others (unpublished data), the gen eral result is that, six of the new ocean tide models have RMS agreeme nt better than 3 cm. The obtainable accuracy with different tide gauge types raises the question about the accuracy of ''ground truth.'' As ocean tide models become increasingly more accurate, the accuracy of t he harmonic constants derived from the tide gauge recordings becomes s ignificant. Generally, the RMS agreement between models based on 2 yea rs of T/P altimetry is significantly better than the agreement between models based on 1 year of T/P altimetry. For models based on two year s of T/P altimetry the RMS agreement has values around 2.0, 1.4, 1.0, and 0.7 cm for the M(2), S-2, K-1 and O-1 constituents, respectively. As a preliminary evaluation, these values might be taken as a measure of the consistency of ocean tides estimated from the first part of the T/P mission, which is extremely promising for future ocean tide model s derived from T/P altimetry using 3 or more years of data. This also suggests that further improvement using 3 or more years of data will o nly be marginal. The differences arise when studying the ocean tides o n continental shelves surrounding the ocean. Major differences are see n, especially on the Patagonian shelf. As the continental shelves are critical for tidal dissipation, it will be very important to improve t ide models on the shelves in future global ocean tide models.