An innovative field study examining the dynamics of boundary-interior exchange events on the Monterey Bay shelf will be conducted in Monterey Bay. The main focus is on boundary layer intrusions, often seen in the form of intermediate nepheloid layers (INLs) detaching from the continental margin. The objectives of the project are to (1) identify processes responsible for resuspension of benthic material on the mid- to outer-shelf, (2) improve our understanding of intrusion formation mechanisms, (3) investigate the fate of material transported in intrusions, and (4) investigate event-scale and seasonal variability in intrusion dynamics. The driving hypothesis is that gravitational collapse after internal-wave-forced mixing drives intrusions; however, the experiment is designed to resolve benthic exchange generally, including advective mechanisms such as eddies, shelf currents, or convergent bottom Ekman fluxes.
Measurements will include a central benthic station, an offshore moored profiler station and cross-shelf autonomous underwater vehicle (AUV) surveys spanning the fixed stations. The observational strategy is to directly observe benthic exchange: from the seafloor interface through the stratified outer boundary layer, across the entire water column, and then into the cross-shelf dimension. New "hybrid" turbulence packages designed to measure turbulence via both scalar dissipation and eddy correlation techniques will be deployed on each of the measurement platforms. High-resolution boundary layer measurements, including bottom shear stress, turbulent dissipation rate, suspended particulates, and erosion events, will be made from a benthic tripod and mooring. Offshore of this platform, a moored profiler will continuously measure, over the entire water column, CTD, optics, and turbulence. Both fixed stations will include full-water-column velocity profiles, from which we will quantify internal wave energy density flux and estimate lateral transport of intrusions/INLs. A long-ranging AUV, newly developed by PI Bellingham, will map both turbulence and intrusions in cross-shelf sections, resolving diapycnal and isopycnal transport processes. Two deployments, one in the late spring upwelling season and the other during stratified fall conditions, will provide unprecedented temporal coverage of full-water-column mixing, internal wave energy, benthic response, and INLs at both event and seasonal scales. The fixed instrumentation will be deployed for one-month-long periods during which the AUV will operate continuously for two weeks.
Intellectual Merit. Fundamental questions about the dynamics of boundary-interior exchange motivate this project. On deeper continental slopes this process has important consequences for global abyssal mixing. On continental shelves, particularly those of eastern boundary upwelling coasts, benthic exchange plays a crucial role in ecosystem functioning by providing benthic iron (a limiting micronutrient) and, potentially, benthic resting cysts to the interior and upper water column. Benthic exchange events may explain some of the paradoxical decoupling between upwelling and coastal blooms at the event level. Intrusions/INLs are commonly observed, yet very little is known about their temporal variability, dispersal rates and duration, or forcing. There is ample evidence for an association between INLs and internal wave interaction with topography. Yet, to date there has been no study directly testing the relationship between mixing and boundary intrusion dispersal on a continental margin. This study is focused on the shelf but the findings will be transferrable to deeper continental margins. Better understanding of these dynamics may improve parameterization of boundary mixing in global circulation models.
Broader Impacts. A valuable, innovative technique that will be developed as part of this collaborative work is the incorporation of turbulence instrumentation into the payload of the Monterey Bay Research Institute (MBARI) long-range Autonomous Underwater Vehicle (AUV). The transfer of this technology to a cutting-edge, and routinely operated, AUV system means it will be used in years to come in investigations of fine-resolution fluxes and transports of coastal biological systems. Un-funded collaborator John Ryan will investigate potential links to surface blooms. The project will include training and mentoring of a postdoctoral researcher and two graduate students, and the research findings will be incorporated into the graduate curricula at Moss Landing Marine Laboratory (MLML) and the Naval Postgraduate School. Public outreach and education will benefit from the established MLML Teacher Enhancement Program and MBARI's summer internship program.
Our project used a new class of robot, a long-range Autonomous Underwater Vehicle (LRAUV), to understand how the ocean interacts with the coastal environment (see figure 1). The coastal oceans are dynamic regions, where wind, currents, waves, and the shallow seafloor interact in poorly understood ways. In some parts of the world, such as off of the West Coast of the United States, a combination of mixing and upwelling bring nutrients to the surface, creating conditions for phytoplankton blooms that feed large fisheries. While the surface waters can be observed from satellite and ships, the processes near the seafloor are effectively hidden. We know that the underwater equivalent of sand-storms carry sediments into the water column, but what causes these events, and where does the material which is lifted from the seafloor into the water column go? Our Tethys-class LRAUVs differs from earlier propeller-driven robots in that it can operate for much longer periods, extending deployments from roughly a day to a week or even a month in duration. This is important for two reasons. First, because these mixing events appear to be episodic and unpredictable, we would like our survey system to be able to stay in place for a long time, to increase our chances of detecting an event. Second, when an event occurs, we would like to observe that event as it evolves. Although there are systems with long endurances, namely buoyancy-driven gliders, the much higher speed of the LRAUVs (1 m/s versus 0.3 m/s) enables better surveys of fast-changing processes. The higher speed also allows the AUV to operate more effectively in the comparatively high currents near the shore, which can be faster than the speed of a glider. Finally, the LRAUV can carry high power payloads and can fly paths that hug the seafloor. We conducted three field programs in Monterey Bay to observe sediment suspension events (see figure 2 and 3). The Monterey Bay experiments deployed the LRAUV in tandem with bottom and mooring instrumentation by our collaborators at Moss Landing Marine Lab (Erika McPhee-Shaw) and Naval Postgraduate School (Tim Stanton and William Shaw). Flights of the vehicle near the seafloor clearly observe suspended sediments, and filaments of suspended material extending off the shelf. While mixing processes in the water column create features which are kilometers across and persist for a day or more, close to the seafloor events appear to be very small scale, probably less than a kilometer in extent. Further, the processes evolve very rapidly, coming into existence and then dissipating in a fraction of a day. To better observe these rapidly evolving, small scale processes near the seafloor, in the final phases of this experiment we deployed two LRAUVs. Our initial results were promising, however underwater navigation remains a source of error. Consequently, under separate funding we have begun experimenting with acoustic navigation methods that allow the vehicles to directly measure each other’s location. We believe that this capability will enable vehicles to cooperate with each other to make high resolution maps of dynamic processes. This will in turn have application to a host interesting ocean problems, including observing the evolution of plankton blooms, coastal fronts, and other fast but important ocean processes.
