Complementarity of ERS 1 and TOPEX/POSEIDON altimeter data in estimating the ocean circulation: Assimilation into a model of the North Atlantic

This study examines the possibility of improving ocean circulation estimates by using TOPEX/POSEIDON (T/P) altimetric measurements in conjunction with those obtained from ERS 1. An evaluation is carried out by assimilating these data sets into a circulation model of the North Atlantic. Experiments with simulated data show that the simultaneous use of both data sets results in clearly enhanced results, even for the estimate of the deep circulation, which is a key issue in ocean circulation modelling. However, the benefit is less clear when using real altimeter data, which could result from the fact that the spectral structure of the orbit error is not taken into account in the assimilation procedure. Comparisons of the model results with the altimeter measurements and with numerous independent in situ velocity measurements demonstrate that the estimates of the surface and subsurface circulations are still improved with a dual assimilation. However, this gain seems weaker than could be expected from experiments with simulated data. Moreover, comparisons with deep velocity measurements show no improvement at all, which probably indicates that measurement errors and nonmodelled physics are not well taken into account in the assimilation procedure. It is also observed that the combined use of the two data sets may help in limiting some systematic shortcomings of the numerical tools in exploiting the data. Finally, it is worth noting that the ERS 1 orbit characteristics almost systematically lead to better results than T/P ones for the range of temporal and spatial scales addressed in these experiments.