Ocean and Atmospheric SciencesOregon State UniversityCorvallis

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Ocean and Atmospheric SciencesOregon State UniversityCorvallis

Ocean and, United States
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Shroyer E.L.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Nash J.D.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Catania G.A.,The Texas Institute
Journal of Geophysical Research: Oceans | Year: 2017

The classic model of fjord renewal is complicated by tidewater glacier fjords, where submarine melt and subglacial discharge provide substantial buoyancy forcing at depth. Here we use a suite of idealized, high-resolution numerical ocean simulations to investigate how fjord circulation driven by subglacial plumes, tides, and wind stress depends on fjord width, grounding line depth, and sill height. We find that the depth of the grounding line compared to the sill is a primary control on plume-driven renewal of basin waters. In wide fjords the plume exhibits strong lateral recirculation, increasing the dilution and residence time of glacially-modified waters. Rapid drawdown of basin waters by the subglacial plume in narrow fjords allows for shelf waters to cascade deep into the basin; wide fjords result in a thin, boundary current of shelf waters that flow toward the terminus slightly below sill depth. Wind forcing amplifies the plume-driven exchange flow; however, wind-induced vertical mixing is limited to near-surface waters. Tidal mixing over the sill increases in-fjord transport of deep shelf waters and erodes basin stratification above the sill depth. These results underscore the first-order importances of fjord-glacier geometry in controlling circulation in tidewater glacier fjords and, thus, ocean heat transport to the ice. © 2017. American Geophysical Union.


Holman R.A.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Ruggiero P.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Journal of Geophysical Research: Oceans | Year: 2017

Observations on a mildly sloping beach suggest that the largest runup events are related to bore-bore capture (BBC). A numerical model based on the Reynolds-averaged Navier-Stokes equations is implemented to evaluate the effects that BBC have on runup. From simulations with realistic sea states, BBC is found to be a necessary but not sufficient condition for large runup generation. The dominant dynamics leading to BBC are amplitude dispersion and interactions with infragravity waves in the outer and inner surf zone, respectively. When the effects of BBC are isolated, it is found that the runup associated with the merging of two bores is at least 50% larger than that associated with the larger of the two waves in a monochromatic wave train. The phase difference of the incident waves with the infragravity wave can generate up to 30% variability of the runup maxima. The majority of the shoreward directed momentum flux, prior to runup initiation, is related to the interaction between the bores and the infragravity wave followed by that of the incident infragravity waves alone. © 2017. American Geophysical Union. All Rights Reserved.


Akan C.,University of California at Los Angeles | Moghimi S.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Ozkan-Haller H.T.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Osborne J.,Mississippi College | Kurapov A.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Journal of Geophysical Research: Oceans | Year: 2017

Numerical simulations were performed using a 3-D ocean circulation model (ROMS) two-way coupled to a phase-averaged wave propagation model (SWAN), to expand our understanding of the dynamics of wave-current interactions at the Mouth of the Columbia River (MCR). First, model results are compared with water elevations, currents, temperature, salinity, and wave measurements obtained by the U.S. Army Corp of Engineers during the Mega-Transect Experiment in 2005. We then discuss the effects of the currents on the waves and vice versa. Results show that wave heights are intensified notably at the entrance of the mouth in the presence of the tidal currents, especially in ebb flows. We also find nonlocal modifications to the wave field because of wave focusing processes that redirect wave energy toward the inlet mouth from adjacent areas, resulting in the presence of a tidal signatures in areas where local currents are weak. The model also suggests significant wave amplification at the edge of the expanding plume in the later stages of ebb, some tens of kilometers offshore of the inlet mouth, with potential implications for navigation safety. The effect of waves on the location of the plume is also analyzed, and results suggest that the plume is shifted in the down-wave direction when wave effects are considered, and that this shift is more pronounced for larger waves, and consistent with the presence of alongshore advection terms in the salt advection equation, which are related to the Stokes velocities associated with waves. © 2017. American Geophysical Union.


Gradoville M.R.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Crump B.C.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Letelier R.M.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Zehr J.P.,University of California at Santa Cruz | White A.E.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Limnology and Oceanography | Year: 2017

Recent observations of N2 fixation rates (NFR) and the presence of nitrogenase (nifH) genes from heterotrophic N2-fixing (diazotrophic) prokaryotes in unusual habitats challenge the paradigm that pelagic marine N2 fixation is constrained to cyanobacteria in warm, oligotrophic, surface waters. Here, we compare NFR and diazotrophic diversity (assessed via high-throughput nifH sequencing) from a region known to be dominated by cyanobacterial diazotrophs (the North Pacific Subtropical Gyre, NPSG) to two regions dominated by heterotrophic diazotrophs: the Eastern South Pacific (ESP, from the Chilean upwelling system to the subtropical gyre) and the Pacific Northwest coastal upwelling system (PNW). We observed distinct biogeographical patterns among the three regions. Diazotrophic community structure differed strongly between the NPSG, dominated by cyanobacterium UCYN-A, and the ESP, dominated by heterotrophic nifH group 1J/1K, yet surface NFR were similar in magnitude (up to 5.1 nmol N L-1 d-1). However, while diverse, predominantly heterotrophic nifH genes were recovered from the PNW and the mesopelagic of the NPSG, NFR were undetectable in both of these environments (although glucose amendments stimulated low rates in the deep NPSG). Our work suggests that while diazotrophs may be nearly omnipresent in marine waters, the activity of this functional group is regionally restricted. Further, we show that the detection limits of the 15N2 fixation assay suggest that many of the low NFR reported for the mesopelagic (often<0.1 nmol N L-1 d-1 in the literature) are not indicative of active diazotrophy, highlighting the challenges of assessing the ecosystem significance of heterotrophic diazotrophs. © 2017 Association for the Sciences of Limnology and Oceanography.


Kurapov A.L.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Kosro P.M.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Journal of Geophysical Research: Oceans | Year: 2016

The influence of varying horizontal and vertical stratification in the upper layer ( O(10) m) associated with riverine waters and seasonal atmospheric fluxes on coastal near-inertial currents is investigated with remotely sensed and in situ observations of surface and subsurface currents and realistic numerical model outputs off the coast of Oregon. Based on numerical simulations with and without the Columbia River (CR) during summer, the directly wind-forced near-inertial surface currents are enhanced by 30%-60% when the near-surface layer has a stratified condition due to riverine water inputs from the CR. Comparing model results without the CR for summer and winter conditions indicates that the directly wind-forced near-inertial surface current response to a unit wind forcing during summer are 20%-70% stronger than those during winter depending on the cross-shore location, which is in contrast to the seasonal patterns of both mixed-layer depth and amplitudes of near-inertial currents. The model simulations are used to examine aspects of coastal inhibition of near-inertial currents, manifested in their spatial coherence in the cross-shore direction, where the phase propagates upward over the continental shelf, bounces at the coast, and continues increasing upward offshore (toward surface) and then downward offshore at the surface, with magnitudes and length scales in the near-surface layer increasing offshore. This pattern exhibits a particularly well-organized structure during winter. Similarly, the raypaths of clockwise near-inertial internal waves are consistent with the phase propagation of coherence, showing the influence of upper layer stratification and coastal inhibition. © 2015. American Geophysical Union.


Chelton D.B.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Strutton P.G.,Institute for Marine and Antarctic StudiesUniversity of TasmaniaHobart
Journal of Geophysical Research C: Oceans | Year: 2014

Eddies can influence biogeochemical cycles through a variety of mechanisms, including the excitation of vertical velocities and the horizontal advection of nutrients and ecosystems, both around the eddy periphery by rotational currents and by the trapping of fluid and subsequent transport by the eddy. In this study, we present an analysis of the influence of mesoscale ocean eddies on near-surface chlorophyll (CHL) estimated from satellite measurements of ocean color. The influences of horizontal advection, trapping, and upwelling/downwelling on CHL are analyzed in an eddy-centric frame of reference by collocating satellite observations to eddy interiors, as defined by their sea surface height signatures. The influence of mesoscale eddies on CHL varies regionally. In most boundary current regions, cyclonic eddies exhibit positive CHL anomalies and anticyclonic eddies contain negative CHL anomalies. In the interior of the South Indian Ocean, however, the opposite occurs. The various mechanisms by which eddies can influence phytoplankton communities are summarized and regions where the observed CHL response to eddies is consistent with one or more of the mechanisms are discussed. This study does not attempt to link the observed regional variability definitively to any particular mechanism but provides a global overview of how eddies influence CHL anomalies. © 2014. American Geophysical Union. All Rights Reserved.


Sobarzo M.,The Interdisciplinary Center | Saldias G.S.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Tapia F.J.,The Interdisciplinary Center | Bravo L.,University of Chile | And 2 more authors.
Journal of Geophysical Research: Oceans | Year: 2016

Submarine canyons cutting across the continental shelf can modulate the cross-shelf circulation being effective pathways to bring water from the deep ocean onto the shelf. Here, we use 69 days of moored array observations of temperature and ocean currents collected during the spring of 2013 and winter-spring 2014, as well as shipboard hydrographic surveys and sea-level observations to characterize cold, oxygen poor, and nutrient-rich upwelling events along the Biobio Submarine Canyon (BbC). The BbC is located within the Gulf of Arauco at 36° 50'S in the Central Chilean Coast. The majority of subtidal temperature at 150 m depth is explained by subtidal variability in alongshore currents on the canyon with a lag of less than a day (r2=0.65). Using the vertical displacement of the 10° and 10.5°C isotherms, we identified nine upwelling events, lasting between 20 h to 4.5 days, that resulted in vertical isothermal displacements ranging from 29 to 137 m. The upwelled water likely originated below 200 m. Majority of the cooling events were related with strong northward (opposite Kelvin wave propagation) flow and low pressure at the coast. Most of these low pressure events occur during relatively weak local wind forcing conditions, and were instead related with Coastal Trapped Waves (CTWs) propagating southwards from lower latitudes. These cold, high-nutrient, low-oxygen waters may be further upwelled and advected into the Gulf of Arauco by wind forcing. Thus, canyon upwelling may be a key driver of biological productivity and oxygen conditions in this Gulf. © 2016. American Geophysical Union. All Rights Reserved.


Corson-Rikert H.A.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Wondzell S.M.,U.S. Department of Agriculture | Haggerty R.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Santelmann M.V.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Water Resources Research | Year: 2016

We investigated carbon dynamics in the hyporheic zone of a steep, forested, headwater catchment western Oregon, USA. Water samples were collected monthly from the stream and a well network during base flow periods. We examined the potential for mixing of different source waters to explain concentrations of DOC and DIC. We did not find convincing evidence that either inputs of deep groundwater or lateral inputs of shallow soil water influenced carbon dynamics. Rather, carbon dynamics appeared to be controlled by local processes in the hyporheic zone and overlying riparian soils. DOC concentrations were low in stream water (0.04-0.09 mM), and decreased with nominal travel time through the hyporheic zone (0.02-0.04 mM lost over 100 h). Conversely, stream water DIC concentrations were much greater than DOC (0.35-0.7 mM) and increased with nominal travel time through the hyporheic zone (0.2-0.4 mM gained over 100 h). DOC in stream water could only account for 10% of the observed increase in DIC. In situ metabolic processing of buried particulate organic matter as well as advection of CO2 from the vadose zone likely accounted for the remaining 90% of the increase in DIC. Overall, the hyporheic zone was a source of DIC to the stream. We suggest that, in mountain stream networks, hyporheic exchange facilitates the transformation of particulate organic carbon buried in floodplains and transports the DIC that is produced back to the stream where it can be evaded to the atmosphere. © 2016. American Geophysical Union. All Rights Reserved.


Thomas J.A.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Lerczak J.A.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Moum J.N.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Journal of Geophysical Research: Oceans | Year: 2016

A two-dimensional array of 14 seafloor pressure sensors was deployed to measure properties of tidally generated, nonlinear, high-frequency internal waves over a 14 km by 12 km area west of Stellwagen Bank in Massachusetts Bay during summer 2009. Thirteen high-frequency internal wave packets propagated through the region over 6.5 days (one packet every semidiurnal cycle). Propagation speed and direction of wave packets were determined by triangulation, using arrival times and distances between triads of sensor locations. Wavefront curvature ranged from straight to radially spreading, with wave speeds generally faster to the south. Waves propagated to the southwest, rotating to more westward with shoreward propagation. Linear theory predicts a relationship between kinetic energy and bottom pressure variance of internal waves that is sensitive to sheared background currents, water depth, and stratification. By comparison to seafloor acoustic Doppler current profiler measurements, observations nonetheless show a strong relationship between kinetic energy and bottom pressure variance. This is presumably due to phase-locking of the wave packets to the internal tide that dominates background currents and to horizontally uniform and relatively constant stratification throughout the study. This relationship was used to qualitatively describe variations in kinetic energy of the high-frequency wave packets. In general, high-frequency internal wave kinetic energy was greater near the southern extent of wavefronts and greatly decreased upon propagating shoreward of the 40 m isobath. © 2016. American Geophysical Union. All Rights Reserved.


Pujiana K.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Moum J.N.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Smyth W.D.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Warner S.J.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Journal of Geophysical Research C: Oceans | Year: 2015

Measurements of currents and turbulence beneath a geostationary ship in the equatorial Indian Ocean during a period of weak surface forcing revealed unexpectedly strong turbulence beneath the surface mixed layer. Coincident with the turbulence was a marked reduction of the current speeds registered by shipboard Doppler current profilers, and an increase in their variability. At a mooring 1 km away, measurements of turbulence and currents showed no such anomalies. Correlation with the shipboard echo sounder measurements indicate that these nighttime anomalies were associated with fish aggregations beneath the ship. The fish created turbulence by swimming against the strong zonal current in order to remain beneath the ship, and their presence affected the Doppler speed measurements. The principal characteristics of the resultant ichthyogenic turbulence are (i) low wave number roll-off of shear spectra in the inertial subrange relative to geophysical turbulence, (ii) Thorpe overturning scales that are small compared with the Ozmidov scale, and (iii) low mixing efficiency. These factors extend previous findings by Gregg and Horne (2009) to a very different biophysical regime and support the general conclusion that the biological contribution to mixing the ocean via turbulence is negligible. © 2015. American Geophysical Union. All Rights Reserved..

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