A field program is proposed to examine exchange of mixed boundary fluid with the stratified interior by measuring turbulence, internal waves and nepheloid layers in Monterey Submarine Canyon and in several other canyons. Several questions will be addressed: 1. What are the net effects of continental-slope internal-wave-driven boundary mixing on the interior stratification of the ocean? 2. Do finescale parameterizations for turbulent mixing fall short by a factor of 5 6 in canyons because of (i) a more coherent internal wave field than found in the open ocean or (ii) interactions with topography? Measurements will include full-depth velocity-temperature profile surveys with XCPs, time series of currents and water properties with transmissometer-equipped LADCP/CTD, time-series of turbulent mixing with a microstructure profiler, and a velocity and temperature profile time series of the bottom 120 m with a mooring equipped with ADCP and thermistor chain. These data will be used (i) to quantify the BBL energy budget by determining internal wave energy fluxes into and out of the boundary along with boundary-layer turbulent dissipation rates, and (ii) to characterize the distribution of nepheloid layers. Energy-flux divergence will be used to identify internal wave sources and sinks within the canyon while potential external sources will be identified from deep-water fluxes. In addition, new analysis techniques will be explored, including (i) the use of energy-flux co-spectra to diagnose internal wave reflection from the near-critical bottom slope, (ii) bi-coherence to evaluate relationships between high and low vertical wavenumbers, (iii) momentum-fluxes to evaluate bottom form drag, and (iv) stratification flux and sediment flux to determine the exchange of mixed boundary water with the stratified interior. This work is in collaboration with Eric Kunze at the University of Victoria, who will make the microstructure measurements and participate in analysis and interpretation of the whole data set. It will benefit from recent modeling efforts and measurements on the slope south of Monterey Bay during ONR's 2006 AESOP program. Intellectual Merit: We are motivated by fundamental questions concerning (i) the exchange of fluid mixed on the boundary with that in the stratified interior and its role in global mixing and the thermohaline circulation, and (ii) how to better parameterize internal-wave-driven turbulent mixing near topography. The survey plan combines spatial coverage and high-frequency timeseries to allow rigorous testing of significance. The core XCP pattern spanning a full cycle of the semidiurnal tide will provide better resolution of a coherent internal wavefield in space and time than hitherto achieved outside the laboratory in order to better understand the cascade of energy through the internal wave field near sloping topography. In addition, exploratory measurements in other continental slope canyons will aid in expanding applicability to a global scope. Broader Impacts: The multi-institutional collaboration will advance interdisciplinary understanding of mixing processes in Monterey Canyon and other continental slope canyons, an important facet for local budgets of nutrients, sediment, and chemical tracers. Through the refinement of finescale mixing parameterizations, it will also aid large-scale mixing budgets and, potentially, climate model predictions. Education will be advanced through the support of graduate students and participation in outreach activities.
An important finding was the discovery of a region near 1000-m depth where fluid from the seafloor was persistently expelled towards the deep sea. This has important implications for mid-depth ocean mixing, which is an important driver of the basin-scale deep circulation that controls the rate of feedback between the ocean and the atmosphere. Better understanding the physics of this mixing is thus an extremely important part of improving global models of the ocean and climate system and predicting future climate scenarios. The biochemical implications may also be quite profound: we are suggesting that the physics of tidal energy entering submarine canyons determines the location of intense exchange between continental margins and the open ocean. This occurs near the depth of the oxygen minimum and other significant biochemical gradients and, importantly, such exchange "hot spots" are likely to be persistent over geological time. The project supported the training of several professionals entering the workforce in oceanographic and technological fields. A masters of science degree was completed at San Jose State University, Moss Landing Marine Laboratories, and a Ph.D. was completed at the University of Washington.
