Upwelling Dynamics from Days to Years Using Underwater Gliders

Oregon State University, Corvallis OR

Investigators

Abstract

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.