Data assimilation of ocean sound speed in Fram Strait

The ACOBAR acoustic tomography experiment in Fram Strait measured acoustic travel times along three acoustic paths with high time resolution from September 2010 to July 2012. The path ranges of the three sections are 167 km, 181 km and 301 km, the longest path gave reciprocal measurements. Inversion of the observed travel times yielded range-and-depth-averaged time series of ocean sound speed and temperature for the ocean layer spanning 0-1000 m depth. A regional ocean model for Fram Strait with 52 vertical levels and 4.5 km horizontal resolution was set up interpret the measured acoustic arrival structure and to test some assumptions of the inverse method. Now daily range-and-depth-averaged sound speed from the ACOBAR acoustic experiment is assimilated in this regional ocean model using the 4D-Var (adjoint) method. First evaluation results show bias reductions at 750 m depth in comparison to a long-term oceanographic mooring section in Fram Strait.