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At least 127 records · Page 7Linked to original sources
USGS2021

A numerical study of wave-driven mean flows and setup dynamics at a coral reef-lagoon system

Two-dimensional mean wave-driven flow and setup dynamics were investigated at a reef-lagoon system at Ningaloo Reef, Western Australia, using the numerical wave-flow model, SWASH. Phase-resolved numerical simulations of the wave and flow fields, validated with highly detailed field observations (including >10 sensors through the energetic surf zone), were used to quantify the main mechanisms that govern the mean momentum balances and resulting mean current and setup patterns, with particular attention to the role of nonlinear wave shapes. Momentum balances from the phase-resolved model indicated that onshore flows near the reef crest were primarily driven by the wave force (dominated by radiation stress gradients) due to intense breaking, whereas the flow over the reef flat and inside the lagoon and channels was primarily driven by a pressure gradient. Wave setup inside the lagoon was primarily controlled by the wave force and bottom stress. The bottom stress reduced the setup on the reef flat and inside the lagoon. Excluding the bottom stress contribution in the setup balance resulted in an over prediction of the wave-setup inside the lagoon by up to 200–370%. The bottom stress was found to be caused by the combined presence of onshore directed wave-driven currents and (nonlinear) waves. Exclusion of the bottom stress contribution from nonlinear wave shapes led to an over prediction of the setup inside the lagoon by approximately 20–40%. The inclusion of the nonlinear wave shape contribution to the bottom stress term was found to be particularly relevant in reef regions that experience a net onshore mass flux over the reef crest.

Journal of Geophysical Research-Oceans↗
USGS2012

The detection of coastal-trapped waves

We outline a simple method for estimating the cross-spectral matrix of coastal-trapped wave amplitudes,A, from a set of oceanographic observations. Specifically, we propose that A may be estimated by (M'M) -1 M'UM(M'M) -1 where a prime denote conjugate transpose, U is the sample cross-spectral matrix of observations and M is a matrix which has the spatial form of the waves for columns. In general, M will be complex and frequency-dependent. We discuss the bias of this estimator and show how to estimate the variance of the power and cross spectra of wave amplitudes. We also outline an ad hoc scheme for assessing the predictive skill of the coastal trapped wave representation and finally give some advice on how to interpret A. Although the method is presented in the context of shelf circulation and coastal trapped waves, it may be applied to any linear system where the spatial forms of the waves are known and the cross-spectral matrix of their amplitudes is required.

Journal of Geophysical Research C: Oceans↗
USGS2012

Shear stress and bed roughness estimates for combined wave and current flows over a rippled bed

High‐quality bottom boundary layer measurements and bottom photographs were obtained over a sand substrate during a 10‐day deployment of the GEOPROBE tripod at an inner shelf (35‐m water depth) location off northern California. The seafloor surrounding the tripod was composed of well‐sorted medium‐grained (mean diameter, 0.25 mm) sand which was formed into symmetrical wave ripples with heights of 3–4 cm and wavelengths of 22–30 cm. Mean velocity profiles in the region from 23 cm to 102 cm above the rippled bed were highly logarithmic ( R > 0.95) approximately 30% of the time. Nineteen profiles exhibiting R > 0.997 were analyzed to obtain the shear velocity ( U * c ) and roughness length (Z 0 c ) for the mean current. The near‐bottom flow field was composed of quasi‐steady currents (up to 12 cm s −1 at z = 102 cm) and wave‐induced, oscillatory currents (up to 14 cm s −1 ). The data‐derived estimates of U * c and Z 0 c were 0.3–0.93 cm s −1 and 0.82–1.5 cm, respectively. The mean shear estimates are 50–100% larger than those predicted using a drag coefficient ( C D ) of 3 × 10 −3 that is typical for rough boundaries, and the roughness lengths are up to an order of magnitude larger than the maximum expected values based on the observed wave‐rippled bottom. These results indicate the importance of the combined flow turbulent interaction in producing a large apparent Z 0 c . However, comparison of the shear and roughness estimates derived from the velocity profile analysis to predictions made by the combined flow model of Grant and Madsen (1979) show that direct application of the wave‐ripple roughness equation of Grant and Madsen (1982) yields large overestimates of z 0 c and U * c . Selecting the physical roughness length k b (=30 z 0 ) that produced the best agreement with the data resulted in z 0 values ranging from 0.03 to 0.43 cm. Moreover, a direct correlation exists between these physical roughness estimates and the angle (θ cr ) formed by the mean current and the trend of the wave ripple crests. A simple linear relationship between k b and θ cr is suggested by our limited data set.

California↗
USGS2012

A model for the generation of two-dimensional surf beat

A finite difference model predicting group-forced long waves in the nearshore is constructed with two interacting parts: an incident wave model providing time-varying radiation stress gradients across the nearshore, and a long-wave model which solves the equations of motion for the forcing imposed by the incident waves. Both shallow water group-bound long waves and long waves generated by a time-varying breakpoint are simulated. Model-generated time series are used to calculate the cross correlation between wave groups and long waves through the surf zone. The cross-correlation signal first observed by Tucker (1950) is well predicted. For the first time, this signal is decomposed into the contributions from the two mechanisms of leaky mode forcing. Results show that the cross-correlation signal can be explained by bound long waves which are amplified, though strongly modified, through the surf zone before reflection from the shoreline. The breakpoint-forced long waves are added to the bound long waves at a phase of pi/2 and are a secondary contribution owing to their relatively small size.

Journal of Geophysical Research C: Oceans↗
USGS2012

Seasonal fluctuations in sea level on the South Carolina shelf and their relationship to the Gulf Stream

Near-bottom pressure measurements obtained over a 9-month period in 1978 on the outer continental shelf off South Carolina document that sea level rose 35 to 50 cm between July and October. Records of coastal sea level showed a similar rise. When the pressure and coastal sea level records were corrected for the effects of wind stress and temperature, the largest portion of the rise in the two variables remained. The amplitude of the change in residual sea level was largest at the shelf break, 28 cm, and decayed to 22 cm at the coast. The relatively small magnitude of the decay suggests that alongshelf pressure gradients imposed at the shelf break have minimum alongshelf wavelengths of 2000 km. The changes in residual sea level were seasonal and associated with seasonal changes in the transport of the Gulf Stream. The 3 years of residual sea level records examined here indicate that the amplitude of a typical decrease in transport of the Gulf Stream off South Carolina between July and October has a strong interannual variability. The change in transport can be 50% lower to over 100% higher than the historical average. The residual sea level records also suggest that a change in transport of the Gulf Stream is not strongly related to the onshore/offshore position of the Gulf Stream.

South Carolina↗
USGS2012

Vertical structure of mean cross-shore currents across a barred surf zone

Mean cross-shore currents observed across a barred surf zone are compared to model predictions. The model is based on a simplified momentum balance with a turbulent boundary layer at the bed. Turbulent exchange is parameterized by an eddy viscosity formulation, with the eddy viscosity A υ independent of time and the vertical coordinate. Mean currents result from gradients due to wave breaking and shoaling, and the presence of a mean setup of the free surface. Descriptions of the wave field are provided by the wave transformation model of Thornton and Guza [1983]. The wave transformation model adequately reproduces the observed wave heights across the surf zone. The mean current model successfully reproduces the observed cross-shore flows. Both observations and predictions show predominantly offshore flow with onshore flow restricted to a relatively thin surface layer. Successful application of the mean flow model requires an eddy viscosity which varies horizontally across the surf zone. Attempts are made to parameterize this variation with some success. The data does not discriminate between alternative parameterizations proposed. The overall variability in eddy viscosity suggested by the model fitting should be resolvable by field measurements of the turbulent stresses. Consistent shortcomings of the parameterizations, and the overall modeling effort, suggest avenues for further development and data collection.

Journal of Geophysical Research C: Oceans↗
USGS2012

The M2 tide on the Amazon Shelf

As part of A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS), moored and shipboard current measurements made over the Amazon shelf during 1990–1991 have been analyzed to determine the dominant semidiurnal tidal constituent, the M 2 . These results have been combined with coastal sea level data from within the Amazon and Para Rivers, the adjacent shelf, and with satellite‐derived tidal elevation data from off the shelf to provide a more complete description of the M 2 tide in this complex river/shelf system. Near the Amazon River mouth the M 2 tide propagates across the shelf and through the mouth as a damped progressive wave, with its amplitude decreasing and phase increasing upriver. Over the adjacent shelf north of Cabo Norte, the M 2 tide approaches a damped standing wave, with large amplitudes (greater than 1.5 m) near the coast due to near resonance within the coastal embayment formed by the Cabo Norte shoal to the south and Cabo Cassipore to the north. The observed M 2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths. Comparisons between tidal observations in both the North Channel and the Cabo Norte‐Cabo Cassipore embayment and a simple variable‐width channel tidal model indicate that (1) most of the M 2 tidal energy dissipation occurs over the mid‐ and inner shelf (in water depths less than 20 m) and (2) fluid muds found there cause a significant reduction (of order 50%) in the effective bottom friction felt by the M 2 tide. The approximate resonant period of the Cabo Norte‐Cabo Cassipore embayment is 11.9 hours, and at resonance the average energy dissipation per forcing period is roughly 2.2 times the average mechanical energy in the embayment. This damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency, so that the response of the embayment to forcing over the semidiurnal band should be essentially the same. The vertical structure of the M 2 tidal current is examined at one outer shelf site located in 65‐m water depth. The observed semimajor axis increases logarithmically with height above bottom within the lowest 1–2 m and reaches a maximum in excess of 0.5 m/s at approximately 11 m above bottom. The mean ellipticity is small (less than 0.1) and positive, indicating clockwise rotation of a nearly rectilinear current, and the semimajor axis is oriented within 10° of the local cross‐isobath direction. The M 2 phase increases with height above bottom, with flood at the bottom leading flood at the surface by about 1 hour. A simple, local homogeneous tidal model with time‐ and space‐dependent eddy viscosity simulates the observed near‐bottom velocity reasonably well, however, the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site. The M 2 energy flux onto the Amazon shelf and into the Amazon and Para Rivers has been estimated using current and surface elevation data and the best fit variable‐width channel model results. The net M 2 energy flux into the mouths of the Amazon and Para Rivers is 0.47×10 10 W and 0.19×10 10 W, respectively. A net M 2 energy flux of about 3.3×10 10 W occurs onto the shelf between the North Channel of the Amazon River and Cabo Cassipore. This stretch of the Amazon shelf accounts for about 1.3% of the global dissipation of the M 2 tide.

Journal of Geophysical Research C: Oceans↗
USGS1996

The effects of snowpack grain size on satellite passive microwave observations from the Upper Colorado River Basin

Understanding the passive microwave emissions of a snowpack, as observed by satellite sensors, requires knowledge of the snowpack properties: water equivalent, grain size, density, and stratigraphy. For the snowpack in the Upper Colorado River Basin, measurements of snow depth and water equivalent are routinely available from the U.S. Department of Agriculture, but extremely limited information is available for the other properties. To provide this information, a field program from 1984 to 1995 obtained profiles of snowpack grain size, density, and temperature near the time of maximum snow accumulation, at sites distributed across the basin. A synoptic basin-wide sampling program in 1985 showed that the snowpack exhibits consistent properties across large regions. Typically, the snowpack in the Wyoming region contains large amounts of depth hoar, with grain sizes up to 5 mm, while the snowpack in Colorado and Utah is dominated by rounded snow grains less than 2 mm in diameter. In the Wyoming region, large depth hoar crystals in shallow snowpacks yield the lowest emissivities or coldest brightness temperatures observed across the entire basin. Yearly differences in the average grain sizes result primarily from variations in the relative amount of depth hoar within the snowpack. The average grain size for the Colorado and Utah regions shows much less variation than do the grain sizes from the Wyoming region. Furthermore, the greatest amounts of depth hoar occur in the Wyoming region during 1987 and 1992, years with strong El Niño Southern Oscillation, but the Colorado and Utah regions do not show this behavior.

Journal of Geophysical Research C: Oceans↗
USGS1996

Satellite imagery of the onset of streaming flow of ice streams C and D, West Antarctica

Five overlapping Landsat multispectral scanner satellite images of the interior of the West Antarctic ice sheet were enhanced with principal component analysis, high-pass filtering, and linear contrast stretching and merged into a mosaic by aligning surface features in the overlap areas. The mosaic was registered to geodetic coordinates, to an accuracy of about 1 km, using the five scene centers as control points. The onset of streaming flow of two tributaries of ice stream C and one tributary of ice stream D is visible in the mosaic. The onset appears to occur within a relatively short distance, less than the width of the ice stream, typically at a subglacial topographic feature, such as a step or ridge. The ice streams extend farther up into the interior than previously mapped. Ice stream D starts about 150 km from the ice divide, at an altitude of about 1500 m, approximately halfway up the convex-upward dome shape of the interior ice sheet. Ice stream D is relatively much longer than ice stream C, possibly because ice stream D is currently active, whereas ice stream C is currently inactive. The grounded portion of the West Antarctic ice sheet is perhaps best conceptualized as an ice sheet in which ice streams are embedded over most of its area, with slow-moving ice converging into fast-moving ice streams in a widely distributed pattern, much like that of streams and rivers in a hydrologic basin. A relic margin appears to parallel most of the south margin of the tributary of ice stream D, separated from the active shear margin by about 10 km or less for a distance of over 200 km. This means there is now evidence for recent changes having occurred in three of the five major ice streams which drain most of West Antarctica (B, C, and D), two of which (B and D) are currently active.

Journal of Geophysical Research C: Oceans↗
USGS1996

Cadmium in the California Current system: Tracer of past and present upwelling

Over 100 samples were collected off the west coast of North America during 1991–1993 to determine the relation between wind‐driven upwelling and nearshore concentrations of dissolved silicate (Si), phosphate (P), and cadmium (Cd). Highly enriched in deep water offshore, these constituents are sensitive indicators of upwelling. Coastal water was sampled from the shore in January and June 1992 at 12 sites distributed between 36° and 48°N latitude. In January the composition of nearshore water along this transect was fairly uniform: 5–15 μmol/kg for Si, 0.5 to 1.0 μmol/kg for P, and 0.1–0.3 nmol/kg for Cd. In June, elevated concentrations of Si (30 μmol/kg), P (2.0 μmol/kg), and Cd (0.6 nmol/kg) revealed a region of intense upwelling between 38° and 40°N. The pattern is broadly consistent with meridional gradients in coastal upwelling calculated from the long‐term mean of alongshore winds compiled from ship reports. Nearshore water was also collected biweekly to monthly at two sites 3 km apart near San Francisco Bay (37.5°N) during 1991–1993. The variability seen in the time series suggests that the composition of nearshore water integrates the effect of alongshore winds over timescales of several weeks. Seasonal variations in Si (5–50 μmol/kg), P (0.5–2.5 μmol/kg), and Cd (0.1–0.8 nmol/kg) concentrations were consistent with upwelling during spring and summer. Maximum Si, P, and Cd concentrations reached in May 1991 were consistent with advection to the very nearshore region from a depth of about 300 m relative to a vertical profile at a distance of 200 km from the coast. Nearshore Si, P, and Cd concentrations were reduced relative to 1991 in 1992, and, to a lesser extent, in 1993 due to weaker upwelling linked to the warm phase of the El Niño‐Southern Oscillation. During periods of weaker upwelling or downwelling, variations in P, Si, and Cd concentrations became uncoupled. There is a good correlation between the coastal Cd time series near San Francisco Bay (37.5°N) and a second order polynomial function of the upwelling index of Bakun [1975] at 36°N, filtered with a 30‐day running mean ( r 2 = 0.71, n = 39). The index is a daily estimate of coastal upwelling calculated from 6‐hourly mean atmospheric pressure distributions at 36°N. From this function and a record of daily upwelling indices, we infer a range of annually averaged coastal Cd concentrations of at least 0.3–0.5 nmol/kg since 1967. Cd/Ca ratios in shells of foraminifera from San Francisco Bay suggest that average coastal Cd concentrations 3500–4500 years ago were at the upper end of this range.

Journal of Geophysical Research C: Oceans↗
USGS1996

Using nonlinear forecasting to learn the magnitude and phasing of time-varying sediment suspension in the surf zone

The time-dependent response of sediment suspension to flow velocity was explored by modeling field measurements collected in the surf zone during a large storm. Linear and nonlinear models were created and tested using flow velocity as input and suspended-sediment concentration as output. A sequence of past velocities (velocity history), as well as velocity from the same instant as the suspended-sediment concentration, was used as input; this velocity history length was allowed to vary. The models also allowed for a lag between input (instantaneous velocity or end of velocity sequence) and output (suspended-sediment concentration). Predictions of concentration from instantaneous velocity or instantaneous velocity raised to a power (up to 8) using linear models were poor (correlation coefficients between predicted and observed concentrations were less than 0.10). Allowing a lag between velocity and concentration improved linear models (correlation coefficient of 0.30), with optimum lag time increasing with elevation above the seabed (from 1.5 s at 13 cm to 8.5 s at 60 cm). These lags are largely due to the time for an observed flow event to effect the bed and mix sediment upward. Using a velocity history further improved linear models (correlation coefficient of 0.43). The best linear model used 12.5 s of velocity history (approximately one wave period) to predict concentration. Nonlinear models gave better predictions than linear models, and, as with linear models, nonlinear models using a velocity history performed better than models using only instantaneous velocity as input. Including a lag time between the velocity and concentration also improved the predictions. The best model (correlation coefficient of 0.58) used 3 s (approximately a quarter wave period) of the cross-shore velocity squared, starting at 4.5 s before the observed concentration, to predict concentration. Using a velocity history increases the performance of the models by specifying a more complete description of the dynamical forcing of the flow (including accelerations and wave phase and shape) responsible for sediment suspension. Incorporating such a velocity history and a lag time into the formulation of the forcing for time-dependent models for sediment suspension in the surf zone will greatly increase our ability to predict suspended-sediment transport.

Journal of Geophysical Research C: Oceans↗
USGS1996

Factors affecting suspended-solids concentrations in South San Francisco Bay, California

Measurements of suspended-solids concentration (SSC) were made at two depths at three sites in South San Francisco Bay (South Bay) to determine the factors that affect SSC. Twenty-eight segments of reliable and continuous SSC time series data longer than 14 days were collected from late 1991 or 1992 through September 1993. Spectral analysis and singular spectrum analysis were used to relate these data segments to time series of several potential forcing factors, including diurnal and semidiurnal tides, the spring-neap tidal cycle, wind shear, freshwater runoff, and longitudinal density differences. SSC is greatest during summer, when a landward wind shear is applied to South Bay by the afternoon sea breeze. About one half the variance of SSC is caused by the spring-neap cycle, and SSC lags the spring-neap cycle by about 2 days. Relatively short duration of slack water limits the duration of deposition of suspended solids and consolidation of newly deposited bed sediment during the tidal cycle, so suspended solids accumulate in the water column as a spring tide is approached and slowly deposit as a neap tide is approached. Perturbations in SSC caused by wind and local runoff from winter storms during the study period were usually much smaller than SSC variations caused by the spring-neap cycle. Variations of SSC at the study sites at tidal timescales are tidally forced, and nonlinear physical processes are significant. Advective transport dominates during spring tides, when water with higher SSC due to wind wave resuspension is advected to the main channel from shallow water, but, during neap tides, advective transport is less significant. The findings of this and other studies indicate that the tidally averaged transport of suspended solids responds to seasonal variations of wind shear in South Bay.

Journal of Geophysical Research C: Oceans↗
USGS1996

Subtidal circulation patterns in a shallow, highly stratified estuary: Mobile Bay, Alabama

Mobile Bay is a wide (25-50 km), shallow (3 m), highly stratified estuary on the Gulf coast of the United States. In May 1991 a series of instruments that measure near-surface and near-bed current, temperature, salinity, and middepth pressure were deployed for a year-long study of the bay. A full set of measurements were obtained at one site in the lower bay; all but current measurements were obtained at a midbay site. These observations show that the subtidal currents in the lower bay are highly sheared, despite the shallow depth of the estuary. The sheared flow patterns are partly caused by differential forcing from wind stress and river discharge. Two wind-driven flow patterns actually exist in lower Mobile Bay. A barotropic response develops when the difference between near-surface and near-bottom salinity is less than 5 parts per thousand. For stronger salinity gradients the wind-driven currents are larger and the response resembles a baroclinic flow pattern. Currents driven by river flows are sheared and also have a nonlinear response pattern. Only near-surface currents are driven seaward by discharges below 3000 m3/s. At higher discharge rates, surface current variability uncouples from the river flow and the increased discharge rates drive near-bed current seaward. This change in the river-forced flow pattern may be associated with a hydraulic jump in the mouth of the estuary. Copyright 1996 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗
USGS1998

Greenland Sea Odden sea ice feature: Intra-annual and interannual variability

The “Odden” is a large sea ice feature that forms in the east Greenland Sea that may protrude eastward to 5°E from the main sea ice pack (at about 8°W) between 73° and 77°N. It generally forms at the beginning of the winter season and can cover 300,000 km 2 . Throughout the winter the outer edge of the Odden may advance and retreat by several hundred kilometers on timescales of a few days to weeks. Satellite passive microwave observations from 1978 through 1995 provide a continuous record of the spatial and temporal variations of this extremely dynamic phenomenon. Aircraft synthetic aperture radar, satellite passive microwave, and ship observations in the Odden show that the Odden consists of new ice types, rather than older ice types advected eastward from the main pack. The 17-year record shows both strong interannual and intra-annual variations in Odden extent and temporal behavior. For example, in 1983 the Odden was weak, in 1984 the Odden did not occur, and in 1985 the Odden returned late in the season. An analysis of the ice area and extent time series derived from the satellite passive microwave observations along with meteorological data from the International Arctic Buoy Program (IABP) determined the meteorological forcing associated with Odden growth, maintenance, and decay. The key meteorological parameters that are related to the rapid ice formation and decay associated with the Odden are, in order of importance, air temperature, wind speed, and wind direction. Oceanographic parameters must play an important role in controlling Odden formation, but it is not yet possible to quantify this role because of a lack of long-term oceanographic observations.

Journal of Geophysical Research C: Oceans↗
USGS2012

Storm‐dominated bottom boundary layer dynamics on the Northern California Continental Shelf: Measurements and predictions

Measurements of near‐bottom velocity profiles in 85 m water depth during a storm on the continental shelf off northern California using the GEOPROBE tripod in December 1979 provided estimates of shear velocities, , and roughness lengths, , when the near‐bottom velocity profiles were logarithmic. These estimates agree within 90% confidence intervals with values computed from a simple near‐bottom combined wave‐current model that includes movable bed effects. The reasonably good comparison between model and profile estimates of suggests that such models can be used to predict bed shear stresses (or shear velocities) under combined flows of waves and currents typical of stormy conditions on continental shelves if stratification corrections to the velocity profile resulting from suspended sediment are small. The repeated occurrences of storms of similar intensities and wind velocities during the winter months off northern California suggest that the resultant high bottom stresses due to the combined effects of waves and currents are major factors in controlling the distribution of surficial sediment on the central portion of the northern California shelf.

California↗
USGS2012

Anchor ice, seabed freezing, and sediment dynamics in shallow arctic seas

Diving investigations confirm previous circumstantial evidence of seafloor freezing and anchor ice accretion during freeze-up storms in the Alaskan Beaufort Sea. These related bottom types were found to be continuous from shore to 2-m depth and spotty to 4.5-m depth. Spotty anchor ice occurred as pillow-shaped crystal aggregates on buried slabs of frozen sand surrounded by unfrozen sand. Considerations of required conditions for ice bonding and anchor ice growth allows regional extrapolation and suggests the possibility of anchor ice growth out to 20-m depth, the estimated maximum depth of supercooling during fall storms. Anchor ice and seabed freezing apparently do not develop during a calm freeze-up. Because of the abrupt growth of anchor ice during a freezing storm and its release soon after formation of a surface ice cover, this ice type has not been documented before. The concretelike nature of frozen bottom, where present, should prohibit sediment transport by any conceivable wave or current regime during the freezing storm. But elsewhere, particularly where the bonded crust is broken by grounded ice, anchor ice lifts coarse material off the bottom and incorporates it into the ice canopy, thereby leading to significant ice rafting of shallow shelf sediment and likely sediment loss to the deep sea.

Journal of Geophysical Research-Oceans↗
USGS2012

Effect of wave-current interaction on wind-driven circulation in narrow, shallow embayments

The effect of wind waves on the steady wind-driven circulation in a narrow, shallow bay is investigated with a two-dimensional ( y , z ) circulation model and the Grant and Madsen [1979] bottom-boundary layer model, which includes wave-current interaction. A constant wind stress is applied in the along-channel x direction to a channel with a constant cross-sectional profile h ( y ). The wind-induced flushing of shallow bays is shown to be sensitive to both the shape of the cross section and the effects of surface waves. The flushing increases with increasing , where h ′ is the standard deviation of cross-channel depth and is the mean depth. This is consistent with the findings of Hearn et al. [1987]. The flushing decreases, however, with the inclusion of surface wave effects which act to increase the bottom drag felt by the currents. Increasing effective bottom friction reduces the strength of the circulation, while the along-bay surface slope, bottom stress and the structure of current profiles remain nearly unchanged. An implication of the circulation dependence on wave-current interaction is that low-frequency oscillatory winds may drive a mean circulation when the wave field changes with wind direction. x

Journal of Geophysical Research C: Oceans↗
USGS2012

Stress origins and earthquake potentials in Cascadia

Focal mechanism solutions for shallow earthquakes throughout the Cascadia plate system indicate that the primary regional stress is northerly compression, even though the Juan de Fuca plate generally is thought to be subducting N50°E. This compressional stress is pervasive throughout the Gorda-Juan de Fuca-Explorer plate system and much of the adjoining section of North American plate. Modeling, using a discrete element code, shows that this stress primarily is due to the Pacific plate being driven into the Gorda block and Juan de Fuca plate (at the Mendocino and Blanco fracture zones). This plate collision causes compression of the offshore plate system northward through the 45°W trending Vancouver Island and leads to transfer of northerly compression into the overriding plate. The northerly compression in the Cascadia plate system has caused large, crustal earthquakes in the regions of Vancouver and northern California and may be capable of causing large earthquakes also in and offshore of Washington and Oregon. Several independent lines of evidence confirm that the Cascadia subduction interface is coupled strongly and that subduction primarily must be accompanied by significant earthquakes, rather than occurring aseismically. For numerous earthquakes within the subducted plate, focal mechanisms indicate extensional stresses that trend downdip. These earthquakes reflect plate extension that is due to sinking of the more deeply subducted plate and to plate motion being resisted at the shallow subduction interface. The slab pull force of the sinking plate also can lead to earthquakes at the shallow subduction interface and the potential exists for M 7.5–8.0 subduction earthquakes to occur at segments of the subduction boundary at Washington and Vancouver and less frequently so at segments of the subduction boundary at Oregon. The in-plate driving forces of the offshore Cascadia plate system do not appear to contribute significantly to regional stresses. Stresses and earthquakes at the Cascadia province primarily result from the action of the Pacific plate on the Gorda-Juan de Fuca-Explorer plate system, a transfer of this action into the overriding North American plate, and the independent sinking and slab pull force of the subducted oceanic lithosphere.

Journal of Geophysical Research Solid Earth↗