Tidal-Band and High-Frequency Internal Waves on the Inner Shelf

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This study will characterize and understand the variability of internal waves on the Oregon inner shelf through three analysis approaches using historical data, targeted high-frequency inner-shelf observations and realistic numerical model output. The historical data come from a 10+ year observational record of the central Oregon inner shelf collected by the Partnership for Interdisciplinary Studies of Coastal Oceans, and a mid-shelf mooring maintained by the Northwest Association of Networked Ocean Observing Systems since 2007. Data include moored water-column velocity and water properties (temperature, salinity). The inner-shelf data set also includes a combination of high-frequency sampling in-situ moorings and Argus video remote sensing to capture the high-frequency nonlinear internal wave field around Yaquina Head, OR. Data from experiments conducted in summer 2010 and 2011 will be analyzed to establish the relationship between high-frequency internal motions on the inner shelf and offshore continental shelf processes (e.g. internal tides, buoyant plumes, upwelling fronts). Since moored observational records are necessarily limited in space, output from a hydrostatic, 3-dimensional numerical model (ROMS) with realistic bathymetry and forcing conditions will be analyzed in conjunction with the observational data to study the propagation and arrival of internal tides to the Oregon inner shelf.The inner shelf is the last stretch of stratified coastal ocean and is potentially the termination point for internal waves. This study will investigate how various oceanic background conditions and external forcing induce temporal and spatial variability in internal wave evolution as they propagate across the shelf. This includes the effects of variable wind, stratification, subtidal currents, tidal forcing, and surface wave conditions. It will also provide estimates of how internal wave energy and potential mass transport to the inner shelf are modified by these background conditions. Because the internal tide is intermittent in time, complex demodulation or wavelet analysis will be used to isolate the internal tidal component from velocity and temperature data. Time series techniques (e.g., lagged correlations, empirical orthogonal functions) will be employed to explore a number of hypotheses of how inner-shelf internal tide energy relates to offshore processes, external forcing and background oceanic conditions.Intellectual Merit: Internal waves are an important link connecting nearshore environments to offshore shelf circulation. With the extensive historical data record, tidal-band internal wave variability will be described for a wide range of background oceanic and forcing conditions over regions of both simple and complex topography. This work will uncover interannual variability in these mechanisms and begin to address the question of how these processes might change over time. Understanding these processes will also provide insight into locating regions of energy dissipation and mixing on the continental shelf. Lastly, the use of Argus technology will address the utility of shore-based video remote sensing to monitor high-frequency internal waves across the inner shelf.Broader Impact: Understanding inner-shelf internal tides is of interest in marine ecology and biogeochemistry. For example, recruitment of intertidal invertebrates has been linked to internal tide events. Furthermore, the study region is near the entrance to the Yaquina river - an example of a small coastal estuary with active oyster farming and commercial fishing industries. Understanding the processes that determine the source water content to these estuaries is crucial for effective estuarine management especially as shelf waters have been found to be acidic and low in dissolved oxygen. Internal waves are often unresolved in coastal numerical models and collaborating with modelers will allow for a feedback between this research and the modeling community. Finally, this project primarily supports the dissertation research and scientific training of a graduate student.