Distribution, Formation, and Evolution of Subsurface Secondary Acoustic Ducts From Global Ocean Modeling and Observations

Abstract Monthly mean reanalysis from assimilating global ocean circulation models spanning 27 years is used to study subsurface secondary acoustic ducts, which provide waveguides for the transmission of mid‐frequency sound. A systematic diagnosis of secondary ducts from monthly mean temperature and salinity fields characterizes their distribution and properties in two global ocean models. Results from both models are compared against a monthly gridded product derived from Argo float observations to evaluate the climatology, distribution, and formation mechanisms of these ducts. Geographical and seasonal patterns reveal two distinct formation mechanisms for subsurface ducts. Regions dominated by subducted pycnostads, associated with mode waters, exhibit well‐mixed layers with weak stratification dominated by temperature. In contrast, ducts formed within the permanent pycnocline are characterized by stratification dominated by salinity, especially in subpolar regions. A constraint limiting bulk stratification of the upward‐refracting layer as a function of density ratio or of Turner angle across the layer is obtained from linearized equations of state for density and sound speed. Subsurface ducts diagnosed from nonlinear equations for density and sound speed conform to this approximated constraint, which accounts for the global decomposition of modeled ducts into two partially overlapping branches: one with the upward‐refracting layer stratified primarily by salinity and the other, more weakly stratified. The distribution of weakly stratified layers largely conforms to known mode waters. The formation of salinity‐dominated upward‐refracting layers in ducts is linked to stratification generated annually by one‐dimensional processes at the base of deep winter mixed layers, freshened by precipitation and runoff.