This project is to analyze a unique space-time data set of continental shelf water properties obtained using autonomous underwater vehicle gliders. Since spring 2006, gliders have been at sea for over 1000 days, sampled over 400 cross-shelf sections, and covered 26,000 km off central Oregon, during a wide range of wind and buoyancy forcing. Glider data are primarily from along the Newport Hydrographic (NH) line, a location representative of regional upwelling processes and for which there is a 50-year data base. Since the gliders sample the upwelling front and jet region in just days, but do so nearly continuously and for multiple years, this analysis seeks to go beyond 2-3 week, ship-based snapshots of coastal upwelling processes and spatially limited moored time series. The PIs will investigate the observed complex water-column structure, in particular subsurface temperature and chlorophyll inversions associated with the coastal upwelling front, and a low-oxygen, near-bottom layer over the mid to inner shelf. Analyses will emphasize the interaction of wind-driven flow and the substantial relative vorticity near the coastal upwelling jet/front in determining the injection of surface water and the material it contains down into the water column. The project will also add Acoustic Doppler Current Profilers to the existing gliders in order to estimate the important ageostrophic secondary circulations. The team will quantify the relative contribution of physical processes (upwelling, horizontal advection, retention, source-water variability) to the structure and longevity of the near-bottom, low-oxygen layer. During 2008, a second E-W glider line south of the NH line and a N-S glider line connecting the two E-W lines were occupied. These concurrent lines will be used to estimate the cross-shelf flux of water and the material it contains as the coastal upwelling jet interacts with a submarine bank. All these analyses will combine glider data with wind measurements and remotely sensed surface data to extend our dynamical understanding of the above shelf processes.
The regular glider sampling on the NH line will be continued for two more years (2010-2011) to further understand the substantial interannual variability (El Niño, Pacific Decadal Oscillation) in the northern California Current. Glider data, will be used together with meteorological and satellite data, and numerical models of wind-driven shelf flows, to understand the dynamics and seasonal variability of upwelling as modulated by interannual variability. Intellectual Merit: The long, uninterrupted, high-resolution time series of four years plus two additional years will offer an unprecedented ability to finely characterize and analyze the dynamics of coastal processes, ranging from multi-day wind-driven to interannual time scales. Understanding flow topography interactions and the ageostrophic cross- frontal circulation associated with the upwelling jet are important, because they represent the mechanisms for transporting material across the shelf and from the surface layer to the interior of the ocean. Furthermore, the complex frontal circulation offers a potentially new formation mechanism for the formation of biologically important "thin-layers." Lastly, the multi-year, shelf-to-slope glider data will pinpoint the importance of interannual variations in upwelling source-water variability to local shelf ecosystem dynamics.
Broader Impact: Real-time glider observations provide essential in situ data over the shelf and slope to complement remotely sensed surface observations. Application of new glider technology to the study of the coastal ocean has captured the public's imagination and the education and outreach efforts of this team will continue to engage a range of ocean users. The subsurface glider observations are vital for data-assimilative modeling efforts. The subsurface frontal structure, chlorophyll and dissolved oxygen concentrations are of particular interest to other scientists and local fishermen, who are reached via the Oregon Sea Grant program. Finally, this project supports graduate research as well as summer undergraduate internships.
During this grant, we continued regular, year-round underwater glider sampling of the coastal ocean along the Newport Hydrographic line off central Oregon (44-deg 39.1’ N). Glider sampling from August 2010 through July 2013 included 76 deployments, 1572 days at sea, 301 cross-margin sections, 91,185 vertical profiles, and 31,841 km of horizontal distance traveled, equivalent to about three quarters around the Earth. This sampling roughly doubled the amount of glider data we have collected since spring 2006. To put the amount of new data in perspective, there are roughly 4000 vertical profiles collected in this region by ships from 1950 to the present. We analyzed the glider data to go beyond 2-3 week, ship-based snapshots of coastal upwelling processes and spatially limited moored time series. Using glider data obtained during the fall, winter and early spring, a period with historically low ship-based sampling due to bad weather, we obtained a detailed description of a freshwater-driven, northward flow hugging the Oregon coast. The gliders sampled through all sea states, including during 10-m waves, and 67 cross-shelf sections were analyzed to obtain, for the first time, the average characteristics of the Oregon Coastal Current (OCC): speeds of 0.1-0.5 m/s, widths of 10-30 km and northward transports of up to 500,000 meters cubed per second. The glider data were used together with wind and river input data to determine that the OCC is driven about 61% by freshwater input, 26% by the wind and 13% by a large-scale, broad, northward current identified as the Davidson Current. The OCC leaks freshwater offshore to freshen the continental shelf via cross-shelf, wind-driven transport and mixing, instabilities and eddies, and by flow-topography interaction. We used glider data to investigate the complex water-column structure associated with a low-oxygen, near-bottom layer over the mid to inner shelf. The gliders approached close to the bottom, often within 3 m, and a hypoxic – oxygen low enough to adversely impact marine organisms – layer extended up from to bottom to sometimes as much as half or two thirds of the water column at mid-shelf depths ~50-80 m. The near-bottom, hypoxic zone moved with seasonal and weather-band (3-5 days) changes in the alongshore winds. Lowest oxygen levels are found in the late summer and physical processes (advection, mixing) act to counteract the consumption of oxygen by microbial respiration to keep the observed oxygen levels above zero (anoxia) except in rare circumstances. An anticipated publication will show how inner-shelf (50 m), near-bottom oxygen levels can be predicted from knowing the upwelling source water oxygen levels and a measure of the local upwelling-favorable wind stress. Application of new glider technology to the study of the coastal ocean has captured the public’s imagination and we continued our education and outreach efforts during this grant. The glider-measured dissolved oxygen concentrations are of particular interest to other scientists and local fishermen. Our outreach included glider demonstrations and talks at middle schools, marine science centers, universities and publication in fields well outside of oceanography, for example, an article entitled "Robots Plumb the Depths" published in July 2011 by the American Society of Mechanical Engineers. PI Jack Barth organized and ran a panel entitled "The Dawn of the Robotic Exploration of our Planet Ocean" at the 2nd International Ocean Research Conference held during 2014 in Barcelona, Spain. Our glider work supported a range of educational activities including two Ph.D. graduates, two Ph.D. students presently enrolled at OSU, two M.S. graduates, six NSF Research Experience for Undergraduate participants, a Visiting Research Technician from Chile, and a Visiting Postdoctoral Scholar from Scotland. The glider and the data we collected are featured prominently in the OSU courses we teach ranging from a 300-student introduction to oceanography undergraduate class to our advanced coastal physical oceanography classes. We also taught 33 international early-career scientists about gliders during a 2013 Summer School on "Ocean Observing Systems and Ecosystem Monitoring." From our nearly continuous, around-the-clock glider sampling in shallow, coastal waters, we have learned these lessons: it takes 2 (or even 3) gliders to run a continuous glider line in order to "hot swap" gliders in the field and to deal with unanticipated instrument failures; work closely with the glider manufacturer to ensure success; avoid burnout of glider team members by scheduling piloting for one intensive week separated by a few weeks of less intensive work; reach out to coastal ocean users so that they know what the gliders look like and what they do, how the glider data might benefit them, and what they should do if they inadvertently contact the glider at sea; never give up on a "lost" glider because the fail safes built into the glider software and hardware are very effective; and, lastly, glider flight instruments and glider-borne sensors need careful attention through pre- and post-deployment testing and calibration.
