Water mass, front and meanders of the Brazil Current seen through acoustics: A preliminary study

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The Brazil Current (BC) is perhaps the least studied subtropical boundary current of the world's oceans. Within this region, the BC develops vigorous meanders and rings. A combination of numerical simulations and observational studies are important tools for unravelling these phenomena. Direct current measurements are rare and usually too short to depict the mean, long term circulation patterns. Similarly, quasi-synoptic hydrographic data in the region is sparse. Acoustic waves are an efficient tool for covering large regions of the water column in a synoptic way. Acoustic tomography can, therefore, be useful to better predict, through inversion for the effective sound speed field and its assimilation to a circulation model, the oceanographic fields of interest (temperature, salinity, density). Such information is particularly important for initialization and data assimilation to regional models for which small and meso-scale processes are of fundamental interest. In this paper, a preliminary study of acoustic propagation modelling through one vertical section off the Brazilian southeastern coast is presented. The acoustic rays are trapped in a minimum sound speed channel bounded by Antarctic Intermediate Water and Upper Circumpolar Deep Water. Between this so-created deep channel and the shel break, one interesting region from the acoustic viewpoint is identified. Notable variations in the transmission loss field are found in this region when the Brazil Current front is moving. In addition, the results show the baroclinic currents more sensitive to salinity variations than sound speed structure, as well as acoustic propagation.