Intercomparison of recent ocean tide models

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Abstract
One of the tremendous achievements of the TOPEX/POSEIDON (T/P) satellite mission is the release of 12 new global ocean tide models during 1994. Virtually all of these models are based on the high‐precision altimetry obtainable from the T/P satellite, and all of the models are superior 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 models will continue to be released and existing ocean models will be updated. Hence this intercomparison can only present a snapshot of the “state of the art” within ocean tide modeling. However, the intercomparison demonstrates the high accuracy and consistency of all these recent ocean tide models. From a comparison with a common 104 tide gauge data set compiled by Le Provost and others (unpublished data), the general result is that, six of the new ocean tide models have RMS agreement better than 3 cm.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 the harmonic constants derived from the tide gauge recordings becomes significant. Generally, the RMS agreement between models based on 2 years 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 years of T/P altimetry the RMS agreement has values around 2.0, 1.4, 1.0, and 0.7 cm for the M2, S2, K1and O1constituents, 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 models 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 only be marginal. The differences arise when studying the ocean tides on continental shelves surrounding the ocean. Major differences are seen, especially on the Patagonian shelf. As the continental shelves are critical for tidal dissipation, it will be very important to improve tide models on the shelves in future global ocean tide models.