Inversion of three-dimensional tidal currents in marginal seas by assimilating satellite altimetry

The four-dimensional variational data assimilation technology based on the theory of inverse problem is applied to simulate the three-dimensional tidal currents in the marginal seas by assimilating the satellite altimetry. The model is calibrated by the twin experiments where the prescribed open boundary conditions for a three-dimensional barotropic tidal model are successfully inverted. By assimilating the tidal harmonic constants derived from TOPEX/Poseidon altimeter data, the open boundary conditions are optimized and the M₂ tidal currents in the Bohai and Yellow Seas (BYS) are simulated in the practical experiment. During the assimilation, the cost function and the gradients of cost function with respect to the open boundary conditions have been decreased significantly. Although the current observations are not assimilated into the model, the cost function composed of the data misfit between model-produced and observed currents is still decreased from 1.00 to 0.09, which demonstrates the reasonability and feasibility of inverting tidal currents from satellite altimetry or other elevation measurements. The co-tidal charts and the near-surface M₂ tidal current ellipses obtained in the practical experiment are in good agreement with the observed tides and tidal currents in BYS.