The combined attraction of the Moon and the Sun is behind the tides and their variations. Ocean tides represent more than 80% of the variability of the surface in the open ocean. In most regions of the world oceans, the tides periods are semidiurnal or diurnal (periods near 0.5 or 1 day) so they are shorter than the repeat periods of an altimeter satellite orbit. Tidal corrections are very important for oceanographic studies because tidal signals contaminate the low-frequency part of raw altimetric signals.
|fig 1. Amplitude in metres of the two main tide components from FES2004 model, one semi-diurnal (top, M2 wave), the other diurnal (bottom, K1 wave). While M2 is clearly dominant on the French coasts (two tides per day), K1 is dominant in the China Sea where there is only one tide per day. (Credits CLS/Legos)|
The standard deviations of tidal variations in the open ocean are 10-60 cm with larger values near coastal regions (they can reach 10 metres in some ports) and in marginal seas. Tidal variations are thus larger in magnitude than the 5-30 cm standard deviations of the dynamic sea surface height. In this respect, ocean tides can be regarded as noise and must, therefore, be removed to estimate the dynamic sea surface height. It is imperative that the tide model used to correct the altimeter data be accurate. If errors in the tide model are large compared with oceanographic variability at the alias frequencies, then oceanographic applications will be compromised at these frequencies. Prior to the launch of Topex/Poseidon, knowledge of tides over the global ocean was based primarily on hydrodynamical models. The accuracies of these models were limited by some uncertainties and were constrained by empirically determined ocean tides from a worldwide network of coastal and inland tide gauges and bottom pressure gauges. Tides errors exceeded 10 cm in many areas of the world ocean.
The altimeter data at the aliased tidal periods have been analysed to extract estimates of tidal amplitudes and phases at the altimeter satellites locations. These empirical estimates have been assimilated into sophisticated hydrodynamical and statistical models to estimate the tides globally with high spatial resolution. One of the important accomplishments of the altimeter missions has been the global estimation of tides to an accuracy of 2-3 cm.
- Le Provost C., Ocean tides, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001
- Ray, R., A Global Ocean Tide Model From TOPEX/Poseidon Altimetry/ GOT99.2 – NASA/TM-1999-209478. Greenbelt, MD, Goddard Space Flight Center/NASA: 58, 1999.
- Lebedev, S., A. Sirota, D. Medvedev, et al., Exploiting satellite altimetry in coastal ocean through the ALTICORE project, Russ. J. Earth Sci., 10, ES1002, 2008 doi:10.2205/2007ES000262.