GLOBAL LOW HARMONIC DEGREE MODELS OF THE SEASONAL VARIABILITY AND RESIDUAL OCEAN TIDES FROM TOPEX/POSEIDON ALTIMETER DATA

The large-scale seasonal variability is estimated jointly with the mean sea surface and the ocean tide signals not recovered by the Cartwright and Ray ocean tide model. This is done in a global analysis where spherical harmonic expansions to degree 18 of the components are estimated simultaneously using 34 cycles of TOPEX/POSEIDON altimeter data. The results show that the amplitude of the annual signal at these wavelengths has a magnitude of 5.1 cm. On the northern hemisphere the annual variability is found to reach its maximum values (10-15 cm) in October. On the southern hemisphere the phases are shifted 6 months. semiannual variability of 5 cm is found in the western part of the equatorial Pacific and has its maximum values in April and in October. The ocean tide residuals corrections are small: 3.0 cm (M(2)), 1.7 cm (S-2), 1.8 cm (K-1), and 1.0 cm (O-1), all root-mean-square, respectively. However, M(2) residuals of 6-8 cm are found. A computation of formal errors shows that the accuracies of the estimated quantities are about 1 cm. Comparisons with in situ data from open ocean tide gauge stations show that the annual cycle and the M(2) ocean tide fit within 3 cm and that the other quantities fit the in situ data within 2 cm.