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Northwest Coastal Oceanography

posted Jul 6, 2011, 1:32 PM by mikutas   [ updated Dec 19, 2012, 9:20 PM ]
Embedded in the California Current System (CCS), the low-frequency oceanography of the Washington (WA) continental shelf (Figure 1) has been studied in a great detail over the last several decades owing in part to its high productivity, and lately owing to its sensitivity to harmful alga blooms, hypoxic and anoxic events, and ocean acidification. Overall biological productivity in the CCS, which flow along the western boundary of the United States and southern Canadian Pacific coast, is generally attributed to seasonal upwelling of nutrient-rich deep waters to the continental shelf (Hickey and Banas, 2003). This upwelling is usually caused by the stress of winds blowing equatorward on the ocean's surface along the coastal boundary. It might be expected that overall productivity along the boundary coast would be correlated with the strength of the upwelling-favorable wind stress at a given location. However, in the CCS, this relationship does not hold in a way that the seasonal average coastal chlorophyll concentrations increase fivefold from northern California to southern Vancouver Island, counter to
the magnitude of alongshelf wind stress, which decreases by a factor of eight over this region (Hickey and Banas, 2008).

Figure 1: Map showing the location of the NEMO moorings(star) and gliderline (blackline), as well as the NDBC Cape Elizabeth meteorological buoy 46021.
Observed barotropic tidal flow and time-mean vector of the California Current are shown with  a black ellipse and arrow, respectively, with scaleas indicated.
The red and blue arrows are the semidiurnal energy flux computed from observations and model, respectively. The colormap shows conversion of barotropic
 tidal energy to baroclinic tides. The black and yellow dashed line is the boundary of the Olympic Coast National Marine Sanctuary. Contour levels are each
100m until 500m, then each 200m thereafter.

Previous studies suggested that the northern CCS has several mechanisms that can produce upwelled nutrient concentrations comparable to those in regions with much greater wind stress. Top candidates include a persistent nutrient supply through the dynamics of the Strait of Juan de Fuca (Crawford and Dewey, 1988), freshwater input from the Columbia river plume (MacCready et al., 2009), the presence of the Juan de Fuca eddy, and enhanced upwelling by the Juan de Fuca canyon (MacFadyen and Hickey, 2010). In ways that are not yet fully understood, the unique aspects of the Washington continental shelf may conspire to boost its productivity (Hickey and Banas, 2008).

Internal waves and mixing are likely key players in transportation of nutrients and DO from the deep layer to the eutrophic zone. Limited by the observation data on the WA continental shelf, no previous studies have reported a detailed study on the internal waves or mixing in this regions. We have recently designed and deployed a new three-component observing system consisting of two moorings and a glider line off the Washington coast. A rich, variable internal wave field is seen, appearing to feature some of the stronger nonlinear internal waves (NLIW) yet reported on continental shelves, when viewed
as a fraction of the water depth. I propose to study the characteristics of internal waves on the WA continental shelf, the generation mechanism and dissipation of NLIWs and their potential influence on the nutrient supply in this region.

We're interested in the oceanography off our coast.  As part of the NANOOS program, we now maintain the Cha-Ba surface buoy, its subsurface analogue, and a glider line.  We collectively refer to these systems as NEMO.

NEMO schematic

Figure 2: Schematic of the NEMO (Northwest Enhanced Moored Observatory) system, with Seaglider, NEMO-subsurface, and the surface mooring Cha' Ba.

Figure 3: Large, ubiquitous nonlinear internal waves (NLIW) have been observed at the NEMO mooring location---associated with rapid vertical movement of density surfaces and strong velocities and velocity shear.  These waves may be an important key to the increased productivity on the Washington coast, with prior research showing that they may be important to supplying nutrients to the shelf.

Figure 4:  Top: Synthetic Aperture Radar (SAR) satellite image showing sea-surface roughness patterns off the Washington Coast offshore of Olympic National Park (Lake Ozette and Point of Arches). The image shows two regions ("a" and "b" in black boxes) of nonlinear internal waves (NLIW), as indicated by coherent bands of rough and smooth water associated with wave troughs (convergence) and crests (divergence) respectively.  White bathymetry contours suggest topographic steering of the waves in box "b" similar to the refraction of surface waves as they approach the beach.  Bottom: A time series of temperature at a depth of 15 m on the NEMO surface mooring (Cha' Ba), verifying that this is a propagating NLIW.