  The impact of the expansion of a high-frequency radar (HFR) system in a
dynamic coastal area (the Ibiza Channel in the Western Mediterranean Sea) is
evaluated through an Observing System Simulation Experiment (OSSE). The
installation of two new antennas in the Iberian Peninsula would complement the
existing ones in the islands of Ibiza and Formentera, providing surface
currents observations of the full channel. Two different configurations of the
same model, validated to give realistic simulations, are used: i) a Nature Run
(NR) which is considered as the real ocean state and that is used to generate
pseudo-observations, and ii) a Control Run (CR) in which the
pseudo-observations are assimilated. The OSSE is first validated by comparison
against a previous Observing System Experiment (OSE). The impact of the new
antennas for forecasting surface currents is evaluated in two different periods
with different levels of agreement between NR and CR. The HFR expansion is
found to contribute to significantly correct the circulation patterns in the
Channel, leading to surface merdional velocity error reductions up to 19%. The
effects on the transport in the area are also analyzed from a Lagrangian
perspective, showing that DA can help to better represent the Lagrangian
Coherent Structures present in the NR and constrain the ocean dynamics.
