Graduate Thesis Or Dissertation
 

The effect of river discharge and wind forcing on the Oregon coastal ocean during fall and winter

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s1784p40g

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  • Freshwater provided from river discharge influences the dynamics and circulation of most continental shelves around the world. It has profound effects on the transport and fate of materials and substances originated from rivers and estuaries, as well as on the ocean biogeochemistry and marine ecosystems. The effect of buoyancy forcing and its modification by windstress off the Oregon coast are studied here, with an emphasis on the downwelling season (fall/winter). Six years of data are used in this study, from 2006 to 2012, obtained by a network of coastal oceanographic observations. During the downwelling season, buoyancy-forcing resulting from significant freshwater input from multiple river sources along the coast, together with the predominantly downwelling-favorable windstress and the large-scale Davidson Current, drives what we have named the Oregon Coastal Current (OCC). Based on a 2-layer model, the dominant forcing mechanism of the OCC is buoyancy, followed by the Davidson Current, and then the wind stress, accounting for 61% (±22.6%), 26% (±18.6%) and 13% (±11.7%) of the along-shore transports, respectively. The OCC is a surface-trapped coastal current, with transports comparable to the summertime upwelling jet off the Oregon coast. Offshore of the OCC, the seasonal evolution of the salinity field is controlled by different mechanisms at two distinct times: prior and after the remotely forced spring transition (RFST). After the RFST, along-shelf advection of the Columbia River Plume by remotely forced southward currents dominate the salinity variability. Prior to the RFST, this variability is dominated by cross-shelf freshwater fluxes from the OCC, influencing an offshore distance of approximately 33 km from the OCC's edge. The rate-of-change of salinity over this region can be explained in terms of eddy and wind-driven Ekman cross-shelf freshwater fluxes, however it was not possible to distinguish their relative contributions. Based on the estimated freshwater loss from the OCC, a leaking pipe model was developed, and it was estimated that the along-shelf freshwater fluxes through a cross-shelf section off Newport can be explained by the summed discharges from 3-4 rivers upstream, reaching as far as the Siuslaw of Umpqua rivers. Salinity off Oregon is also variable at interannual time scales. Low salinities during the upwelling season (spring/summer), produced by increased river discharges from the Columbia River are correlated to El Niño/La Niña. The lowest salinity recorded off Newport, was registered during an extreme La Niña event of 2011. For the first time the Columbia River Plume was tracked from mid-shelf all the way into the Yaquina Bay estuary. Finally, the effect of wind-forcing and flow-topography interaction are investigated, in a continental shelf in the absence of freshwater input, off the Brazilian coast. Our results demonstrate that on larger scales, the sea surface temperature variability along the coast is mainly controlled by wind-driven upwelling, while upwelling due to flow-topography interaction is responsible for the smaller scale sea surface temperature variability.
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