Graduate Thesis Or Dissertation
 

Optics, structure and variability of the offshore Columbia River plume

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  • The Columbia River delivers the greatest amount of freshwater to the coastal ocean along the U.S. Pacific coast. This freshwater forms the Columbia River plume, a mesoscale plume with significant implications on coastal ocean physical, biological, chemical, and geological processes. The plume is transported south and offshore during the upwelling season (The offshore Columbia River plume) in response to the dominant southward upwelling-favorable winds and associated surface circulation. Here a detailed investigation of the optics, structure and variability of the offshore plume is presented. A study of the interannual variability of river plumes off central-southern Chile is also included for comparison with another eastern boundary upwelling system with significant freshwater river outflows.Chapter 2 presents the main optical characterization of the offshore Columbia River plume, combining in situ observations from underwater gliders and MODIS satellite imagery. The cross-shore variability of relevant optical properties are described for the offshore Columbia River plume in comparison with ambient waters in absence of the plume off Newport, central Oregon. The plume thickens in the offshore direction, through a fresh and warm surface layer, which is concordant with the deepening of peaks in optical properties. This pattern has implications for the detection of the plume from ocean color remote sensing as the plume is practically undetectable from MODIS imagery by itself farther than ∼154 km from shore. Empirical algorithms, based on multivariate regression analyses of normalized-water leaving radiance (nLw(λ)) plus sea surface temperature (SST), are presented with more accurate results detecting offshore plume waters than previous studies using single visible bands.Chapter 3 contains a study of the structure and variability of the offshore Columbia River plume, with emphasis on the cross-shore structure off central Oregon, and its variability in response to wind forcing and river discharge. On average, the plume presents marked seasonal variability off central Oregon – early in spring it is located nearshore (inshore of 126◦W) whereas later in the upwelling season (July and August) its offshore extension increases to about 128◦W. On the interannual scale, anomalously fresh plume events occurred during the highest river discharges associated with spring snowmelt (i.e. 2008, 2011, 2012). Additional glider observations from the Ocean Observatories Initiative Endurance array and along-track Aquarius measurements provide new insights about the alongshore plume structure and its response to wind variability. Idealized numerical experiments of a large river plume during coastal upwelling, resembling the offshore Columbia River plume, are also presented with the objective of isolating the effects of differences in the strength of upwelling-favorable winds and discharge rates on the offshore plume.Chapter 4 inspects the interannual variability of freshwater plumes along the Oregon coast, including not only the offshore Columbia River plume but also the small coastal rivers along the entire Oregon coast with high accumulated winter discharges. Empirical Orthogonal Function (EOF) analysis reveals two dominant modes associated with (i) the winter plumes of coastal rivers which are merged along the entire Oregon shelf (EOF1) due to their dominant downstream direction of propagation as coastal-attached buoyancy-driven flows, and (ii) the offshore Columbia River plume occupying most of the coastal ocean off Oregon (EOF2) as it is transported south and offshore during spring-summer upwelling. Major plumes are found to correspond mainly with El Niño Southern Oscillation cycles, but longer time series are needed to better evaluate the influence of Pacific Decadal Oscillation and the North Pacific Gyre Oscillation because of their dominant decadal variability.Finally, Chapter 5 includes a comparative study of the influence of climate variability on the development of anomalous turbid river plumes off central-southern Chile, another eastern boundary upwelling system with significant freshwater input during the winter and spring seasons. Major turbid river plume events occur primarily during warm phases of the El Niño Southern Oscillation and Pacific Decadal Oscillation, and negative phases of the Antarctic Oscillation, when storm tracks are further north off central Chile. Anomalously large turbid plumes extend long distances offshore (∼70-80 km), and individual plumes coalesce into a continuous plume along the coast that covers the entire continental shelf.
  • Keywords: Columbia River plume, coastal ocean, upwelling
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