Event-Scale to Interannual Variations in Shelf Water Properties and Dynamics

Project Summary

This research addresses a fundamental question in coastal oceanography: What are the relative contributions of the interaction between wind-driven flow and bottom topography, and offshore source water variations, to event-scale, seasonal and interannual variations in water properties and flow dynamics over the continental shelf and slope? While much has been learned about flow-topography interaction during process-oriented cruises of up to one-month duration, these provide only a few realizations of the weather-band (2-10 days) variability and lack seasonal coverage. Moored observations, while more continuous and sometimes year-round, are rarely of sufficient horizontal resolution to describe the interaction of coastal fronts and jets with topographic features. We are using year-round Autonomous Underwater Vehicle Glider (AUVG)-based observations, together with remotely sensed data, to understand the annual cycle of flow-topography interaction between shelf flows and a submarine bank. A northern E-W glider section, north of Heceta Bank, Oregon, is along the Newport Hydrographic (NH) Line (44 39.1'N), for which there is a substantial historical data base. A second E-W glider line, south of Heceta Bank off the Umpqua River (43 45'N), sampled in combination with the NH line will allow us to quantify the year-round, time-dependent cross-shelf flux of water and the material it contains. We started pilot glider operations on the NH line in summer 2005, while routine, nearly continuous sampling began in April 2006. We expanded AUVG sampling in summer 2008 to include concurrent sampling of both E-W sections using two gliders. We are using the glider data, together with meteorological and satellite data and numerical models of wind-driven shelf flows, to understand the dynamics and seasonal variability of flow-topography interaction. By sampling over multiple years in a region with significant interannual variability (El Niño, Pacific Decadal Oscillation), we are exploring the modulation of flow-topography interaction byinterannual variability.

Intellectual Merit: Understanding the processes that generate complex three-dimensional structure in shelf circulation and properties (e.g., flow-topography interaction and interannual changes in offshore source water properties) is critical for extending our knowledge of coastal ocean dynamics which is presently based on two- or highly simplified three-dimensional models and observations heavily biased toward the spring-summer season. We will use glider observations together with non-dimensional measures of the inertial and geometric effects to make our results on the dynamics of interaction between alongshore bathymetric variations and both wind-driven upwelling jets and wind- and/or buoyancy-driven right-bounded flows broadly applicable. In addition, these complex time-dependent, three-dimensional processes have a profound affect on cross-margin exchange and coastal ecosystems.

Broader Impact: Real-time glider observations will provide essential in situ data over the shelf and slope to complement remotely sensed surface observations (satellite SST and ocean color; velocity from land-based radar). The subsurface glider observations are important for data-assimilative modeling efforts. The subsurface frontal structure, chlorophyll and dissolved oxygen concentrations are of particular interest to other scientists, local fishermen and other Oregon ocean users.