3,740 research outputs found
Stability of a coastal upwelling front over topography
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October 1987A two-layer shallow water equation model is used to investigate the linear
stability of a coastal upwelling front. The model features a surface front near a
coastal boundary and bottom topography which is an arbitrary function of the
cross-shelf coordinate. By combining the various conservation statements for the
global properties of the system, a general stability theorem is established which
allows the a priori determination of the stability of a coastal upwelling front.
Unstable waves are found for the modelled coastal upwelling front. The unstable
wave motions are frontally-trapped and dominant in the upper layer. The wave
propagates phase in the direction of the basic state flow and the primary energy
conversion is via baroclinic instability. The effect of varying the model parameters
is presented. Moving the front closer than ~ 2 Rossby radii to the coastal boundary
results in a decrease in the growth rate of the fastest growing wave. Increasing
the overall vertical shear of the basic state flow, by either decreasing the lower
layer depth or increasing the steepness of the interface, results in an increase in
the growth of the fastest growing wave.
A bottom sloping in the same sense as the interface results in a decrease of
the growth rates and alongfront wavenumbers of the unstable waves in the system.
Linearized bottom friction is included in the stability model and results in a
decrease in the growth rates of the unstable waves by extracting energy from the
system. Since the unstable mode is strongest in the upper layer, bottom friction
will not stabilize the upwelling front.
A comparison between the predictions from the simple two-layer model and
observed alongfront variability for three areas of active upwelling is presented.
Reasonable agreement is found, suggesting that observed alongfront variability
can be interpreted in terms of the instability of a coastal upwelling front.This study was supported by the National Science Foundation Grant
OCE 84-08563 and the Office of Naval Research Coastal Ocean Sciences
Program 10/1984.37
Variation in the Position of the Upwelling Front on the Oregon Shelf
As part of an experiment to study wind-driven coastal circulation, 17 hydrographic surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999 with a small, towed, undulating vehicle. The cross-shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field. A measure of upwelling intensity based on the position of the permanent pycnocline is developed. This measure is designed so as to be insensitive to density-modifying surface processes such as heating, cooling, buoyancy plumes, and wind mixing. It is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress. This analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic observations. This relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling
Dispersion and connectivity estimates along the U.S. west coast from a realistic numerical model
Near-surface particle dispersion, larval dispersal and connectivity along the U.S west coast were explored using a realistic numerical model of the California Current System. Seasonal model velocities were qualitatively and quantitatively evaluated using Global Drifter Program data. The model displayed a clear seasonal cycle of eddy energy near the coast with energy maxima southwest of major headlands. Eddy speeds were correlated with drifter-based estimates during summer and fall when compared spatially. Over six million passive, Lagrangian particles were released in the upper 20 m of the water column within 10 km of the California and Oregon coasts and tracked for 7 years. The effect of subgridscale vertical turbulence was parameterized with a random walk model. Resulting trajectories yielded climatological maps of particle dispersion. Particle densities varied with release region, release season and time-since-release. Dispersal distances and coastal connectivity varied with season of release, release location, release depth and pelagic larval duration (PLD). Connectivity was clearly influenced by major geographic features such as the Gulf of the Farallones and Cape Mendocino. Given a moderate (30–60 day) PLD, mean dispersal distances varied from ∼10–230 km, with standard deviations of ∼130–220 km. For release locations from Palos Verdes to Point Sur, the primary direction of dispersal was northward for a moderate PLD, regardless of season. For long PLDs (120–180 day), mean dispersal distances were larger (∼40–440 km), with standard deviations of ∼330–540 km. In winter given a long PLD, dispersal was primarily southward for release locations north of Point Arena. Increasing release depths to 40–60 m altered mean dispersal distances by 50–250 km polewards, but had little effect on standard deviations. Point Conception did not act as a barrier to dispersal for source regions in the Southern California Bight
Defect tolerance in as-deposited selenium-alloyed cadmium telluride solar cells
The efficiency of cadmium telluride (CdTe) solar cells is limited primarily by voltage, which is known to depend on the carrier concentration and carrier lifetimes within the absorber layer of the cell. Here, cathodoluminescence measurements are made on an as-deposited CdSeTe/CdTe solar cell that show that selenium alloyed CdTe material luminesces much more strongly than non-alloyed CdTe. This reduction in non-radiative recombination in the CdSeTe suggests that the selenium gives it a certain defect tolerance. This has implications for carrier lifetimes and voltages in cadmium telluride solar cells
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Anomalous southward advection during 2002 in the Northern California current : evidence from Lagrangian Surface Drifters
Equatorward velocities in the upwelling jet of the northern California Current were 0.05–0.06 m s¯¹ faster in spring and summer 2002 than on average over 1998–2002. This result is based on a five-year data set of surface drifters released across the continental margin off central Oregon (44.65°N) during April and July of each year. At this speed, anomalous water displacements of over a degree of latitude can occur in 20–25 days. Given a source of cold, Subarctic water to the north, this anomalous southward displacement is a plausible explanation for the cold, nutrient-rich halocline water observed off Oregon during the summer of 2002. This interannual variability in the northern California Current and its implications for the ecosystem response, i.e., increased primary productivity, may be contrasted with interannual variability of the opposite sign - increased poleward velocity, warmer temperatures and decreased productivity - observed in this same region during El Niño years
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Short-wavelength instabilities on coastal jets and fronts
The stability of a coastal jet and front is investigated using the primitive
equations applied to a continuously stratified flow in geostrophic balance. A linear
stability analysis successfully explains the growth of two modes of instability with
distinctly different horizontal scales. A long-wavelength mode (fastest-growing
wavelength of 0(100 km)) is found which is a modified version of a traditional
baroclinic instability. A second, rapidly growing frontal instability also exists. For
a realistic basic state density and flow structure, this mode has its fastest growth
at short wavelengths (0(20 km)), e-folds in less than 1.5 days and propagates
rapidly in the direction of the mean flow. The frontal instability grows primarily
by extracting the available potential energy of the mean flow via a baroclinic
instability mechanism. A small contribution from vertical Reynolds stress is also
found, but the transfer via horizontal Reynolds stress is from the eddy to the mean
kinetic energy. Further evidence shows that the frontal instability is not a result
of horizontal shear instability nor is it an inertial instability. The frontal mode is
trapped to the surface front and its influence is confined to the upper water column
(z≤70m). A significant subsurface vertical velocity maximum (20 m d¯¹ at 30 m)
is associated with a frontal instability with a reasonable, as judged by satellite
sea surface temperature observations, surface temperature perturbation of 0.35°C.
The linear stability predictions are verified by and compared with results from a
time-dependent, three-dimensional, nonlinear ocean circulation model. Finally, the
frontal instability is discussed in the context of other recent stability analyses that
yield high-wave number modes
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Variation in the position of the upwelling front on the Oregon shelf
As part of an experiment to study wind-driven coastal circulation, 17 hydrographic
surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from
roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999
with a small, towed, undulating vehicle. The cross-shelf survey data were combined with
data from several other surveys at the same latitude to study the relationship between
upwelling intensity and wind stress field. A measure of upwelling intensity based on the
position of the permanent pycnocline is developed. This measure is designed so as to be
insensitive to density-modifying surface processes such as heating, cooling, buoyancy
plumes, and wind mixing. It is highly correlated with an upwelling index formed by
taking an exponentially weighted running mean of the alongshore wind stress. This
analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a
timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic
observations. This relationship allows the amount of time the pycnocline is outcropped
to be estimated and could be used with historical wind records to better quantify
interannual cycles in upwelling
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Drifter Behavior on the Oregon–Washington Shelf during Downwelling-Favorable Winds
Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results
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Connectivity and larval dispersal along the Oregon coast estimated by numerical simulations
Connectivity and larval dispersal is explored off the Oregon coast during the summer upwelling season of 2001 using numerical ocean circulation simulations. The study region, with strong wind-driven currents and variable topography, is modeled using the Regional Ocean Modeling System (ROMS) forced by the Coupled Ocean Atmosphere Mesoscale Prediction System. A large number of passive particles as models of planktonic larvae are released daily for 120 days from 1 May to 28 August at depths of 1, 7, 15, 20, 50, and 75 m at every grid point shoreward of the 200 m isobath (on average 32 km offshore). The particles are transported by the three-dimensional currents of the model simulation. The competency time window for larval settlement is assumed to be in between days 15 and 35 after larvae are released. Larval settlement occurs at the shallowest location during the competency time window. Connectivity matrices reveal that some of the places of highest retention are similar to the proposed Oregon marine reserve sites, especially Cape Perpetua. The Heceta Bank region has high probabilities as both a source and a destination for settled larvae. Larvae released in the Heceta Bank region often settle at higher latitudes than their release location. There are strong correlations between the number of settled larvae shallower than the 50 m isobath and a 6 to 8 day running mean of the alongshore wind stress. Larvae are retained near the shore when the winds, averaged over the previous 6 to 8 days, are relaxed or downwelling favorable.This is the publisher's final pdf. The published article is copyrighted by American Geophysical Union and can be found at: http://www.agu.org/journals/jc/index.shtm
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Introduction to special section : Coastal Advances in Shelf Transport
The Coastal Ocean Advances in Shelf Transport (COAST) program conducted an
interdisciplinary study of coastal upwelling off central Oregon during summer 2001.
Two intensive field efforts during May–June and August 2001 were coordinated with
ocean circulation, ecosystem, and atmospheric modeling of the region. A primary
goal was to contrast the coastal ocean response to wind forcing in a region of relatively
simple alongshore bottom topography versus that associated with a substantial submarine
bank. In this overview we provide background motivation for the COAST project
and summarize the major research findings
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