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Strub P.T.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | James C.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Combes V.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Matano R.P.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | And 5 more authors.
Journal of Geophysical Research C: Oceans | Year: 2015

Altimeter sea surface height (SSH) fields are analyzed to define and discuss the seasonal circulation over the wide continental shelf in the SW Atlantic Ocean (27°-43°S) during 2001-2012. Seasonal variability is low south of the Rio de la Plata (RdlP), where winds and currents remain equatorward for most of the year. Winds and currents in the central and northern parts of our domain are also equatorward during autumn and winter but reverse to become poleward during spring and summer. Transports of shelf water to the deep ocean are strongest during summer offshore and to the southeast of the RdlP. Details of the flow are discussed using mean monthly seasonal cycles of winds, heights, and currents, along with analyses of Empirical Orthogonal Functions. Principle Estimator Patterns bring out the patterns of wind forcing and ocean response. The largest part of the seasonal variability in SSH signals is due to changes in the wind forcing (described above) and changes in the strong boundary currents that flow along the eastern boundary of the shelf. The rest of the variability contains a smaller component due to heating and expansion of the water column, concentrated in the southern part of the region next to the coast. Our results compare well to previous studies using in situ data and to results from realistic numerical models of the regional circulation. © 2015 The Authors.


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.


Letelier R.M.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | White A.E.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis
Journal of Geophysical Research C: Oceans | Year: 2015

During the summer of 2012, we used laser diffractometry to investigate the temporal and vertical variability of the particle size spectrum (1.25-100 μm in equivalent diameter) in the euphotic zone of the North Pacific Subtropical Gyre. Particles measured with this optical method accounted for ∼40% of the particulate carbon stocks (<202 μm) in the upper euphotic zone (25-75 m), as estimated using an empirical formula to transform particle volume to carbon concentrations. Over the entire vertical layer considered (20-180 m), the largest contribution to particle volume corresponded to particles between 3 and 10 μm in diameter. Although the exponent of a power law parameterization suggested that larger particles had a lower relative abundance than in other regions of the global ocean, this parameter and hence conclusions about relative particle abundance are sensitive to the shape of the size distribution and to the curve fitting method. Results on the vertical distribution of particles indicate that different size fractions varied independently with depth. Particles between 1.25 and 2 μm reached maximal abundances coincident with the depth of the chlorophyll a maximum (averaging 121±10 m), where eukaryotic phytoplankton abundances increased. In contrast, particles between 2 and 20 μm tended to accumulate just below the base of the mixed layer (41±14 m). Variability in particle size tracked changes in the abundance of specific photoautotrophic organisms (measured with flow cytometry and pigment concentration), suggesting that phytoplankton population dynamics are an important control of the spatiotemporal variability in particle concentration in this ecosystem. © 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.


Vivier F.,University Pierre and Marie Curie | Hutchings J.K.,Ocean and Atmospheric SciencesOregon State UniversityCorvallis | Kawaguchi Y.,Research Institute of Global Change | Kikuchi T.,Research Institute of Global Change | And 3 more authors.
Journal of Geophysical Research: Oceans | Year: 2016

In the central Arctic Ocean, autonomous observations of the ocean mixed layer and ice documented the transition from cold spring to early summer in 2011. Ice-motion measurements using GPS drifters captured three events of lead opening and ice ridge formation in May and June. Satellite sea ice concentration observations suggest that locally observed lead openings were part of a larger-scale pattern. We clarify how these ice deformation events are linked with the onset of basal sea ice melt, which preceded surface melt by 20 days. Observed basal melt and ocean warming are consistent with the available input of solar radiation into leads, once the advent of mild atmospheric conditions prevents lead refreezing. We use a one-dimensional numerical simulation incorporating a Local Turbulence Closure scheme to investigate the mechanisms controlling basal melt and upper ocean warming. According to the simulation, a combination of rapid ice motion and increased solar energy input at leads promotes basal ice melt, through enhanced mixing in the upper mixed layer, while slow ice motion during a large lead opening in mid-June produced a thin, low-density surface layer. This enhanced stratification near the surface facilitates storage of solar radiation within the thin layer, instead of exchange with deeper layers, leading to further basal ice melt preceding the upper surface melt. © 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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