Long-term variability of sound speed conditions in Hornsund fjord, Svalbard, between 2001 and 2019

The glacierized Arctic fjords are particularly sensitive to oceanic and atmospheric warming caused by climate shifts; the melting of glaciers and icebergs is one of the major indicators of this sensitivity. The meltwater delivery to the ocean changes the thermohaline structure of the water column, which not only affects water mixing but also controls the sound speed conditions; the latter is crucial for the variability of underwater sound propagation. Finally, changes in sound propagation conditions affect marine animals that rely on sound for their key biological functions, such as communication, navigation, and mating. Here, we investigate the long-term variability of sound speed conditions in the Hornsund fjord, Svalbard, together with its governing factors. We calculated the vertical sound speed profiles using temperature and salinity data collected along the fjord centreline from 2001 to 2019. Spatial and temporal variability in sound speed conditions are observed. We identify two major types of sound channels: (i) near-surface sound channel and (ii) deep sound channel. Potential physical mechanisms that govern the presence and position of these sound channels are discussed, including glacier melting, shelf-fjord water exchange, and atmospheric heat flux. In recent years, the climate-driven transformation of Hornsund has led to the disappearance of deep sound channels due to the intensified inflow of Atlantic Water, and an increased presence and extent of near-surface sound channels resulting from elevated freshwater input from melting glaciers. We suggest that climate warming-induced changes in sound speed conditions in Hornsund are likely to have far-reaching consequences for underwater sound pollution, potentially impacting the well-being of marine animals.