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PROJECT SUMMARY

Introduction:

The Oregon coastal ocean exhibits a strong wind-driven response, both physically and biologically and in both upwelling favorable (summer) and downwelling favorable (winter) seasons. Off northern Oregon, the region of active upwelling is narrow and the bottom topography is relatively uniform alongshore. Off central Oregon, the continental shelf broadens and alongshore uniformity is broken by Heceta Bank. The distribution of cold water and surface chlorophyll from satellite imagery mimics the width of the continental shelf, suggesting the control of bottom topography on shelf circulation and upwelling (Figure 1; Figure 2a,c).
 
 
Figure 1. (a) Oregon coastal region with bottom topography in meters. (b) Satellite sea-surface temperature image from August 13, 1995. (c) Bottom sediment type. (d) Monthly mean chlorophyll for August 1998 as measured by SeaWiFS.

Far less is known about the shelf flow and thermohaline fields during the downwelling season. Recent two-dimensional modeling results predict the formation of a strong downwelling front and jet near the mid-shelf (Figure 2b). Offshore of the mid-shelf density front there is onshore transport in a surface Ekman layer, which turns downward at the mid-shelf front, and returns offshore in a thick BBL. Inshore of the downwelling front, the water column is well mixed, the alongshore flow is considerably reduced compared with that in the mid-shelf jet, and cross-shelf flow is nearly zero. Thus, the inner shelf is a region of relatively quiescent flow.
 
 
Figure 2. (a) Schematic of upwelling circulation off Oregon north of Newport, e.g. near 45N. Isopycnals rise to meet the surface in an idealized upwelling front and arrowheads(tails) indicate southward(northward) flow. (b) Schematic of downwelling circulation for the same cross-shelf section. A well-mixed region exists inshore as indicated by vertical isopycnals. (c) Schematic upwelling season circulation for a cross-shelf section across Heceta Bank (44N). Enhanced mixing over the bank is indicated.

We will address a specific set of scientific hypotheses related to cross-shelf transport processes in a wind-driven system by conducting field experiments off the Oregon coast together with coordinated ocean circulation/ecosystem and atmospheric modeling. The hypotheses, motivated by the brief overview above, are:

(H1) the presence of upwelling and downwelling jets and fronts locally alters cross-shelf circulation in the surface and bottom boundary layers and in the interior;
(H2) alongshore topographic variations dictate the relative importance of two-dimensional versus three-dimensional cross-shelf transport processes;
(H3) patterns of turbulence on the shelf during upwelling and downwelling are influenced by fronts and jets, and the levels of turbulence can reach sufficient intensity to influence the mesoscale circulation;
(H4) the magnitude and distribution of primary production on the shelf and its subsequent transport offshore is controlled solely by the geometry of upwelling as described in H1 and H2;
(H5) (a hypothesis competing with H4) alongshore variations in turbulent mixing control the magnitude and distribution of primary production, e.g. via enhanced nutrient and/or trace chemical supply from the BBL;
(H6) the reduced cross-shelf transport implied by the presence of a downwelling front allows nutrients, trace metals and seed stocks of phytoplankton and zooplankton to accumulate in the mid- to inner shelf, thus priming the system for a strong biological response at the outset of upwelling.

During the field experiments we will make intensive observations in two regions: one north of Newport in a region of relatively simple topography and one south of Newport centered on Heceta Bank (Figure 3). High-resolution sampling will be conducted using two ships simultaneously.


 
Figure 3. COAST experimental setup. Existing observational sites include the GLOBEC mooring, the PISCO study areas, and NOAA NDBC buoy and CMAN stations. The existing OSU Coastal Radar (dark gray shading spanning Newport to Waldport) will be extended both north and south (light gray shading). Isobaths in meters.

 

We will conduct two three-week cruises during the upwelling season (June and August) 2001 (Figure 4). Sampling will be done in both the northern and southern regions during both cruises in order to directly compare results. For the downwelling study, we will conduct one three-week cruise during Jan-Feb 2003.
 
 
Figure 4. Original proposed timeline for OSU COAST project.