Hydrographic Variability and Sound‐Channel Dynamics in the Nordic Seas: Implications for Predicting Acoustic Arrival Structure

Abstract Warm, saline Atlantic waters and fresher Arctic‐origin waters converge in the central Nordic Seas, creating strong mesoscale and submesoscale variability that influences both hydrography and sound propagation. During the Northern Ocean Rapid Surface Evolution 2022 experiment, high‐resolution temperature and salinity measurements were collected using shipboard profiling and drifting Wirewalker systems, alongside acoustic transmissions in the 500–1,500 Hz band. Water‐mass structure was characterized using two‐dimensional temperature–salinity histograms, which revealed distinct surface and intermediate water types and their spatial and temporal evolution. Two processes dominated variability: topographically trapped internal tides over the East Jan Mayen Ridge and a propagating anticyclonic eddy composed of modified Atlantic water. Both features produced measurable shifts in the depth and thickness of the intermediate Atlantic water layer, which forms a regional sound channel. These hydrographic changes led to predictable modulation of waterborne acoustic arrival patterns and, in the case of the eddy, downward refraction strong enough to eliminate the waterborne path through bathymetric blocking. The results demonstrate how evolving water‐mass structure in high‐latitude frontal systems directly governs mid‐frequency acoustic propagation, with implications for acoustic observing, environmental prediction, and interpretation of variability in Arctic‐influenced basins.