Near-Boundary Turbulent Dissipation, Suspended Particle Dispersal, and Stratified Turbulent Layers: a Comparison of Two Eastern Pacific Submarine Canyons

Erika E McPhee-Shaw, Western Washington University, Department of Environmental Sciences, Bellingham, WA, United States, Amy Frances Waterhouse, Scripps Institution of Oceanography, La Jolla, CA, United States, Eric L Kunze, U of Washington, Seattle, WA, United States, James B Girton, Applied Physics Laboratory, University of Washington, Seattle, WA, United States and Jingping Xu, Southern University of Science and Technology (SUSTech), Department of Ocean Science & Engineering, Guangdong, China
Submarine canyons may contribute a significant percentage of regionally averaged turbulent mixing at continental slope depths, and are important sinks for internal tide energy. Here we compare two canyons, Monterey Canyon and Eel Canyon, both on the eastern margin of the Pacific, with the goal of examining the relationship between elevated stratified turbulent layers extending several hundred meters above the seafloor, and spatial patterns of suspended particulates observed from transmissometer profiles co-sampled with turbulence and internal wave energy flux measurements. Fine suspended particulate (SP) settling velocities estimated via vertical advective-diffusive balance are 2 to 3 m day-1 (consistent with a Stokes estimate for collected particles of φ=8), meaning particles can be considered passive tracers for the time scale of the experiments. Lateral and vertical distribution of SP in these canyons is likely the result of isopycnal outcropping at the seafloor with subsequent dispersal, or advection into the canyon of external water masses. Observations within the canyons capture both diurnal and semidiurnal tidal cycles, allowing unique insight into the response and behavior of SP along canyon thalwegs. In Monterey Canyon, a persistent intrusion/intermediate nepheloid layer (INL) emanates from a region of internal wave energy flux convergence and inferred upslope advection convergence driven by stratified mixing. Eel Canyon does not show such a clear intrusion feature associated with internal wave flux divergence, however there are contiguous INLs at many Eel Canyon sites, and both canyons are characterized by strong SP signals throughout the stratified turbulent layers. We speculate on the significance of boundary-interior exchange possibly signified by the persistence of these features in regions of high dissipation yet maintained stratification