The Adriatic Sea M2 and K1 tides by 3D model and data assimilation

In the present work we explore the impact of assimilating local tide-gauge and altimetric data on the quality of predicting the major Adriatic tides (M2 and K1). To that end we compute optimal tidal open boundary conditions for a 3D high-resolution finiteelement model by using an incremental assimilation formalism. The essence of the method is the use of two dynamical models where the solution in the complex 3D high-resolution model is sought via assimilation of prediction errors into the simpler 2D model with explicit inverse. In the central numerical experiment, harmonic constants from 12 tide gauges are assimilated and the results are analysed at 31 locations, hence 19 independent ones. The data assimilation contributes to the reduction of maximum amplitude error from 5.6 to 0.5 cm for M2 and from 3.9 to 0.1 cm for K1. The assimilation procedure is repeated by assimilating suitably processed Topex/Poseidon altimeter data, again validating the outcome at 31 tide gauge locations. The result was very similar to the gauge-data assimilation outcome. The model output is also validated with the current data, not used in the assimilation. At two locations and at three depths the model was able to reproduce the major and the minor semi-axes of tidal ellipses, as well as their orientations very well.