Assimilation of acoustic thermometry data in Fram Strait Free

The 2010i–2012 ACOBAR acoustic thermometry experiment in Fram Strait resulted in 2-year-long timeseries of sound speed and temperature. These timeseries are section- and depth-average along 4 sections covering the deep part of Fram Strait (77.9°–78.9°N, 4.2°W–8.7°E). They were obtained by inversion from the acoustic travel times measured in the thermometry experiment. Such large-scale average measurements in a highly turbulent environment at one of the important sites for ocean circulation in the Arctic are unique and will hopefully increase our understanding of volume and heat exchanges between the Arctic and Atlantic. A regional ocean model of Fram Strait was set up for the assimilation of the ACOBAR measurements using the 4-DVAR data assimilation technique. The forward model part was evaluated using both oceanographic and acoustic measurements (Geyer et al., 2019). First assimilation results show a cost reduction of 85% after 10 iterations. The resulting ocean state estimate will be evaluated using the long-term oceanographic mooring section in Fram Strait at 78°50’N. The aim of this study is to use the ocean state estimate to facilitate the interpretation of the acoustic thermometry results and to investigate how the circulation dynamics in Fram Strait are influencing the volume and heat exchanges through the strait.