Distribution and Formation of Secondary Subsurface Ducts in Global Ocean Model

In this study, we investigate quantitative characteristics and the statistics of secondary subsurface ducts on a global scale using sound speed
$c$
determined from operational global ocean model reanalysis products from the Hybrid Coordinate Ocean Model HYCOM for the purpose of underwater sound applications. Systematic diagnosis of secondary subsurface ducts from monthly averaged model hydrography is used to characterize their distribution and properties, including the depth of their central axis and duct strength. The results show that the secondary subsurface duct, with a typical central axis depth range of 10–300 meters, is prevalent in numerous regions of the worldwide ocean, particularly at mid-latitudes within the thermally stratified permanent pycnocline. The thickness of the duct exhibits significant variation across geographically separated regions of prevalence and undergoes seasonal fluctuations due to atmospheric forcing. Some areas may feature relatively thick O(100 m) ducts, while others may have thinner ones, measuring less than 50 m in thickness, potentially constrained by local model resolution. The acoustic cutoff frequency of the duct, which is a function of duct prominence and thickness, also exhibits regional variability, with higher values in the low-latitude regions and lower values in the high-latitude regions. The climatology of annual variations in monthly duct prevalence and properties presents a distinct seasonal cycle in the duct thickness, with thicker ducts in the winter and thinner ducts in the summer, corresponding to the impact of surface heat fluxes. Significant regional variations in duct thickness and depth diagnosed from HYCOM may therefore arise directly from variations in surface heat and freshwater fluxes. These variations may be expected to differ between models with different surface forcing, vertical resolution or model physics. In addition to responding to ocean surface fluxes, geographic and seasonal variability of duct prevalence and properties that are due to variations in upper ocean thermohaline stratification also reflect advection and modification by ocean currents, circulation, and mesoscale eddies, as well as mixing by smaller submesoscale and turbulence processes parameterized in ocean models. Both seasonal and interannual variabilities of duct properties correspond strongly to variations in surface fluxes of heat and salinity, driving the evolution of ocean surface mixed layer density and sound speed, combined with advection by ocean currents. We find weak correlations between duct properties and ENSO, an index of climate variability, with marginal impacts of note in a few regions.