A PROUDMAN-FUNCTION EXPANSION OF THE M2 TIDE IN THE MEDITERRANEAN-SEA FROM SATELLITE ALTIMETRY AND COASTAL GAUGES

An empirical model has been derived for the semidiurnal principal lunar tide M2 in the Mediterranean Sea, excluding the Aegean Sea. The work is primarily an exercise in combining satellite altimeter data with tide-gauge data, through the use of specially constructed spatial basis functions. The tide-gauge data consist of M2 harmonic constants previously derived at 43 chosen coastal locations. The altimeter data consist of approximately two years of Geosat data in the form of collinear differences of sea-surface heights, corrected for orbit error by adjusting long sequences of global data. The tide is expressed as an expansion in Proudman functions - numerically constructed, orthogonal eigenfunctions of the velocity potential. The most important mode is found to be one of period 11.2 hours, presumably owing to its closeness to the tidal excitation frequency. The resulting M2 model is shown to be superior to a previous altimeter-based model that had been derived without benefit of spatial basis functions.