Time variability of hydrodynamics and potential use of surface water and ocean topography mission in estuarine macrotidal system: example of Seine estuary

Abstract : This work was supported by the National Center for Space Studies in the framework of the future mission surface water and ocean topography (SWOT). The estuarine zones with their complex hydrodynamics represent a great challenge for the future SWOT mission. The Seine estuary was selected by the SWOT Science Definition Team for the calibration/validation of the mission. Thus the aim of this study is to investigate the temporal hydrodynamic variability in the Seine estuary and the SWOT ability to reproduce this variability and hydrodynamic phenomena. For this, we used several statistical techniques and frequency analyses applied on the tide-gauge measurements of the Seine estuary and on SWOT simulated data. SWOT samples (SWOT simulated data) have been extracted synthetically from observations using the number of overpasses per repeat cycle [21 days (d)] in order to study the ability of SWOT to reproduce the time variability of the hydrodynamics. Results have demonstrated similar frequencies of multiyear, annual, and intraannual along the estuary from the Seine discharge to the Cherbourg sea level. The 11- to 22-d frequency has been only observed for estuarine and coastal time series suggesting the tide effect in these contexts. The distribution of levels has shown a high influence of tides and discharges at the downstream and the upstream, respectively. Low frequencies of water level are explained by the climate patterns of North Atlantic Oscillation with a mean range of 65%, whereas the high frequencies are associated to the periods of flooding and stormy events. Finally, simulated SWOT samples of water level show a better restitution of observations for high overpasses number. The results have shown a better restitution of hydrodynamic variability by SWOT in the river and upstream of estuary than in the downstream of estuary and coastal zone: in the estuary downstream and the coast, the annual mode is underexpressed while the 3-month (m) mode is overexpressed, which means the annual mode is underestimated and the 3-m mode is amplified by SWOT.