M2 model of the global ocean tide derived from SEASAT altimetry

Abstract

The recovery of ocean tides from satellite altimetry, an attractive alternative to the hydrodynamical‐numerical approach, is investigated to create a global model of the M2 tide. From the outline of the difficulties faced in altimetry interpretation, we bring out general guidelines to extract the tidal information from a short span of measurements. In particular, we discuss the choice of a reference surface and the effect of the orbit error and tidal aliasing on the recovery. From space‐time harmonic analyses of twenty‐four days of SEASAT altimetry, we derive M2 solutions expanded into series of surface spherical harmonics for the Indian, Pacific, and Atlantic Oceans separately and for the world ocean. The M2 cotidal maps we obtain feature qualitatively realistic tidal patterns and are consistent with the deep sea gages data. We then cast the bases to estimate the error budget of the altimeter tide solutions. The M2 fundamental harmonics involved in tidal energetics are evaluated from a spectral convolution of the global solutions with the ocean function and are used to test and discuss our results. The present tidal recoveries must still be considered as preliminary trials because they are strongly dependent on the limits of the SEASAT mission and subject to improvements via an updating of our analysis procedure. But the altimeter approach of the open ocean tide modelling proves to be efficient, and the objective—to produce highly reliable models with the support of the next generation of satellite altimeters—is reasonably optimistic.