Interannual changes in sound propagation across the Gulf Streama)

The effect of a changing ocean environment on sound propagation across the Gulf Stream near the New England Seamount chain is investigated using satellite altimetry and hydrographic profiles from a large set of Argo floats. Reconstruction of the Gulf Stream vertical structure is accomplished by use of an altimetry-informed gravest empirical mode (AGEM) method. The AGEMs act as transfer functions allowing for the estimation of 3D, interior sound speed fields from 2D, surface altimetry measurements. AGEMs are constructed using empirical relationships between the steric height and the temperature and salinity profiles in the region. Ray tracing and parabolic equation sound propagation models are employed to identify changes in propagation patterns over the 14-year interval between 2009 and 2023. It is found that there has been a reduction in the travel time to a range of 120 km caused by increased temperatures throughout the water column associated with a warming Gulf Stream. It is also observed that the structure of the sound speed gradient field has changed over the decadal window, causing differences in sound propagation interference patterns on the far side of the Gulf Stream.