California Shelf Research Articles

A Coupled Hydrodynamic–Bottom Boundary Layer Model of Ekman Flow on Stratified Continental Shelves

Abstract This paper describes a hydrodynamic model with turbulent energy closure that uses a simplified wave-current interaction model of the bottom boundary layer to compute bed drag coefficients. The coupled model is used to investigate the interaction of the upper and lower boundary layers with the geostrophic core flow for simple shelf geometry and forcing, and to evaluate the effects of increased bottom friction on coastal hydrodynamics for summer and winter stratification. The thickness of the bottom boundary layer predicted by the model ranges from 10 to 35 m and is consistent with observations from the California shelf. The increased bottom friction calculated by the coupled model in intermediate water depths increases bottom Ekman veering (leftward in the Northern Hemisphere) by as much as 10° if stratification is strong, thus enhancing downwelling and upwelling. Currents along isobaths in shallow water are uniformly decreased by as much as 25% in the coupled model for both summer and winter init...

Laser diffraction particle sizing in STRESS

An autonomous instrument system for measuring particle size spectra in the sea is described. The instrument records the small-angle scattering characteristics of the particulate ensemble present in water. The small-angle scattering distribution is inverted into size spectra. The discussion of the instrument in this paper is included with a review of the information content of the data. It is noted that the inverse problem is sensitive to the forward model for light scattering employed in the construction of the matrix. The instrument system is validated using monodisperse polystyrene and NIST standard distributions of glass spheres. Data from a long-term deployment on the California shelf during the field experiment Sediment Transport Events on Shelves and Slopes (STRESS) are included. The size distribution in STRESS, measured at a fixed height-above-bed 1.2 m, showed significant variability over time. In particular, the volume distribution sometimes changed from mono-modal to bi-modal during the experiment. The data on particle-size distribution are combined with friction velocity measurements in the current boundary layer to produce a size-dependent estimate of the suspended mass at 10 cm above bottom. It is argued that these concentrations represent the reference concentration at the bed for the smaller size classes. The suspended mass at all sizes shows a strong correlation with wave variance. Using the size distribution, corrections in the optical transmissometry calibration factor are estimated for the duration of the experiment. The change in calibration at 1.2 m above bed (mab) is shown to have a standard error of 30% over the duration of the experiment with a range of 1.8-0.8.

Temporal variation in bed configuration and one-dimensional bottom roughness at the mid-shelf STRESS site

Time-lapse stereophotographs were taken over a 90-day period from mid-November 1990 to late-February 1991 at a 90-m silt-bottom site on the central California shelf as part of the STRESS (Sediment Transport Events on Shelves and Slopes) project. Five distinct bed configurations were observed, in order of decreasing abundance, these are: (1) bioturbated bed; (2) smoothed bed; (3) current-rippled bed; (4) scour-pitted bed; and (5) wave-rippled bed. Concurrent measurements of the flow field implicate along-shelf currents, rather than waves, as the primary agent forming the physical bed configurations. The presence of a wave-induced cross-shelf gradient in near-bottom suspended sediment during storm events and the redistribution of this sediment by upwelling or downwelling currents is postulated to control the appearance of depositional current-ripples (northwest poleward flow, downwelling) and erosional scour-pits (southeast equatorward flow, upwelling). All physical bed forms are destroyed by bioturbation processes in periods of hours to days. Analytical photogrammetric techniques were used to extract high-resolution sea floor height data from the stereophotographs. Results indicate maximal relief over a 0.25-m 2 area at this site never exceeded 5 cm. Root-mean-square (rms) height varied by a factor of 3 (3.2–9.2 mm) and is a weak function of bed configuration. Current ripples have the largest rms-height, smoothed and scour-pitted beds the smallest. Rms-heights of bioturbated beds are variable and appear to depend on the previously produced physical bed configuration. Changes in rms-height can be abrupt with factor of 2 changes observed over a 12-h period. Horizontal descriptors of roughness such as peak spacing or peak width cannot separate bed configurations. Results from surface slope distributions are broadly coherent with the rms-height data, in that surfaces with large rms-heights have broad slope distributions and vice versa. Slope distribution data also indicate that all bed configurations except the current-rippled bed are isotropic. These preliminary data suggest that time series information is needed to adequately resolve both the micro-scale roughness of the sea floor on continental shelves and the presence of short lived, but potentially flow-diagnostic bed configurations.

Residual Circulations Due to Bottom Roughness Variability under Tidal Flows

Abstract Tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth. Using the depth-averaged vorticity equations, the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined. The authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity. Specific cases are considered on the northern California shelf, Georges Bank, and the U.S. South Atlantic Bight. The generation of residual vorticity is a strong function of the length scales at which roughness or depth vary. Length scales of bottom roughness variation are commonly within the range of greatest effect (e.g., sand patchiness, cobbly outcrops, etc.). The site-specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability. The implication for numerical modeling studies ...

Subtidal circulation over the northern California shelf

The circulation over the shelf and upper slope off northern California, between 38°N and 42°N, was observed using moored current and temperature recorders deployed as part of the Northern California Coastal Circulation Study (NCCCS), from March 1987 through October 1989. The results of this study provide a larger‐scale view of the wind‐driven circulation than that described through the 1981–1982 Coastal Ocean Dynamics Experiment (CODE), particularly with regard to alongshore and temporal variations. From an improved description of the frequency structure of wind and current, a very low frequency (VLF) signal is identified in the observed currents at frequencies below those typical of wind forcing. The majority of this VLF variance appears to be accounted for by persistent flow events associated with the presence of mesoscale circulation in the adjacent ocean. The longer duration of the NCCCS also allows an improved description of the seasonality of flow regimes off northern California. Three oceanic seasons are identified: an upwelling season (April‐July), a relaxation season (August‐November), and a storm season (December‐March). Alongshore variations in the strength of upwelling, in the strength of the alongshore flow, both near‐surface and undercurrent, and in water temperature not only are a function of latitude, as is the wind, but they also correspond to location relative to promontories, notably Cape Mendocino. Immediately south of Cape Mendocino, the near‐surface flow exhibits an equatorward minimum and a temperature minimum, whereas the undercurrent exhibits a poleward maximum. Conversely, at the moorings immediately north of the cape, temperatures are a maximum and the undercurrent exhibits a minimum. The maximum in near‐surface temperature relates to a minimum in upwelling; no significant correlation was found between local wind and current immediately north of Cape Mendocino. This upwelling minimum and the upwelling maximum south of the cape were also observed as persistent sea surface temperature patterns in satellite imagery.

Long‐Term Temporal and Spatial Fluctuations of Soft Bottom Infaunal Invertebrates Associated with an Ocean Outfall from the San Pedro Shelf, California

AbstractTemporal (annual) and spatial fluctuations of soft‐bottom invertebrates from 1977 to 1988 were reviewed and analyzed from an ocean outfall area on the San Pedro Shelf, California. Several dominant species which occurred throughout the monitoring period provide continuity between early sampling efforts and recent ones. Annual fluctuations in the mean number of species, abundance, diversity and biomass were generally conservative compared to spatial fluctuations. Temporal changes of note include a marked reduction in abundance and biomass between 1981 and 1984 and another decline in biomass in 1988. The former was associated with conditions of the 1982–1983 El Niño, and the latter with a shift in population dominance from a bivalve to a polychaete.Mean abundance and biomass were enhanced by the outfall without a significant reduction in number of species. The response of the biota to the outfall extended further upcoast (northwest) than downcoast.Several hypotheses were posed and briefly examined. These include the effects of total suspended solids generated from the outfall, and the role of the “Three Little Pigs” (bivalve‐Parvilucina tenuisculpta CARPENTER, ostracod‐Euphilomedes carcharodonta (SMITH), polychaete‐Capitella capitata (FABRICIUS)) in influencing benthic community structure. Finally, measures of infauna from the San Pedro Shelf were, except for abundance, very similar to those reported from other California shelf areas. Through time and space the outfall has had limited negative effect on infaunal community structure.

Taxonomic Review of Pea Crabs in the Genus Pinnixa (Decapoda: Brachyura: Pinnotheridae) Occurring on the California Shelf, with Descriptions of Two New Species

Taxonomic Review of Pea Crabs in the Genus Pinnixa (Decapoda: Brachyura: Pinnotheridae) Occurring on the California Shelf, with Descriptions of Two New Species

Estimation of stress and bed roughness during storms on the Northern California Shelf

Turbulent boundary shear stress depends on a roughness length scale which characterizes momentum transfer to the seabed. The effective roughness length is due to physical roughness geometry such as sediment ripples, grain size and processes affecting momentum transfer, such as bedload transport and wave motions. During storms on the California shelf, wave motions dominate the turbulent boundary layer, although feedback through wave-induced bed forms and sediment transport are important. Several intense storms on the Northern California Shelf were monitored with pressure sensors, acoustic current meters and an optical backscatter sensor within 5 m of the bed at 90 m depth. Wave spectra, velocity profiles, turbulent kinetic energy and suspended sediment concentrations were obtained. At 90 m depth, waves were measured in the band of 0.05-0.08 Hz, with nearbed wave velocities of 5–30 cm s −1. In spite of wave-induced currents of up to three times the mean speed, 30 min average velocities yielded typical logarithmic profiles. Roughness, as indicated by the zero intercept of the logarithmic profiles, z 0, varied by a factor of 25 throughout the storms, with a maximum of 18 cm, when mean currents were above 5 cm s −1 and the wave amplitude was maximal. Such large increases in z 0 are predicted by models of wave-current interaction. Effects of sediment transport and bed forms are not easily extracted from the data during storms. But, the fairly large non-storm values of ∼0.5cm indicate the effect of bed ripples.

TAXONOMIC REVIEW OF PEA CRABS IN THE GENUS PINNIXA (DECAPODA: BRACHYURA: PINNOTHERIDAE) OCCURRING ON THE CALIFORNIA SHELF, WITH DESCRIPTIONS OF TWO NEW SPECIES

ABSTRACT Many species in the genus Pinnixa are not well differentiated, both in life and in the literature. Evolution within the genus has resulted in pairs and groups of very similar species which are frequently difficult to distinguish with existing keys and descriptions. In this paper, the 11 species of Pinnixa previously known from the California shelf are redescribed, with new illustrations. Two species, Pinnixa forficulimanus, new species, and P. minuscula, new species, from soft-bottom shelf habitats, are also described and illustrated, bringing the total number of species present in California coastal waters to 13. Host and distributional data are summarized for each species, and a key for differentiating the species is provided.

The Bottom Boundary Layer Over the Northern California Shelf

Moored temperature and shipboard CTD observations from a northern California coastal upwelling region reveal variable bottom mixed-layer heights that are typically 5–15 m, but occasionally exceed 50 m. Observations from Oregon, northern California, and Peru, indicate that in coastal upwelling regions, maximum bottom mixed-layer heights tend to increase with water depth over the shelf, but rarely exceed half the water depth. Over the northern California shelf the bottom mixed-layer height is shown to depend on the stratification, the current magnitude, and the current direction. The dependence on current direction tends to dominate the response, with thicker bottom mixed layers during poleward flows and thinner bottom mixed layers during equatorward flows. This asymmetric response to poleward and equatorward currents is consistent with model results which indicate that the asymmetric response is due to the up- or downslope Ekman transport of buoyancy along the bottom.

Marine deep seismic reflection profiles off central California

A strong reflection horizon at two‐way travel time of approximately 6 s is observed in a deep seismic profile across the outer continental shelf of central California. It is interpreted as the seismic image of subducted oceanic crust emplaced prior to the change from principally convergent to principally transcurrent motion between the Pacific and North American plates during the late Paleogene. The reflector dips landward at a very shallow angle and is at a depth of 14–15 km under Santa Lucia Bank. The reflection is not observed, or at best is very discontinuous, under the inner shelf (Santa Maria Basin). This suggests that tectonic or other processes have produced significantly different structural styles or compositions on the two sides of the Santa Lucia Bank fault. Under the outer shelf a prominent, apparently deeper (later arrival time) horizon dips more steeply and diverges from the 6‐s reflector. The deep horizon is at least partially composed of diffracted energy but is nearly linear after migration. Possible interpretations are that the horizon indicates crustal imbrication or out‐of‐plane diffractions. Alternatively, it is a relict feature imparted to the crust at the now inactive Pacific‐Farallon spreading ridge. Reflective zones at intermediate depths are observed in apparently accreted sediments in parts of the Santa Lucia and Santa Maria basins. These features could represent technically induced fabrics within the accretionary complex, or they could be coherent depositional sequences.

Coefficient of pollution (p): the southern California shelf and some ocean outfalls

The coefficient of pollution (p) was applied to the southern California shelf including three major ocean outfalls (Whites Point, Orange County, San Diego City). P values from the shelf were considerably lower than those reported for portions of the Gulf of Saronikos for which the coefficient was originally developed. Corrections to an earlier application of p to Whites Point did not change major conclusions, but did change the magnitude of values, reducing them in this case. Within the California shelf there were regional differences in p at various outfalls. There was a significant reduction in p at Whites Point between 1973 and 1984. P values at the Orange County outfall were generally lower than at the other outfalls. In our opinion, the development of an integrated (abiotic- biotic) measure of pollution incorporating granulometry and depth and basic community parameters (number of species and number of individuals) is highly desirable.

Vorticity and Instability during Flow Reversals on the Continental Shelf

Abstract Flow reversals during relaxation of the equatorward wind on the northern California shelf are studied with observations and a simple numerical model. Data from the CODE experiment are used to document the changes in the cross-shelf profiles of alongshelf currents and potential vorticity when the wind forcing changes from upwelling-favorable to near-zero. Upwelling vorticity dynamics are considered using a simple equation, where also the importance of stress curls is discussed. During the wind relaxation events, the usually equatorward flow reveres to poleward on the inner shelf, resulting in a region of high shear and occasionally a localized maximum in potential vorticity on the shelf. In the course of such an episode, a single eddylike structure on the shelf was observed with satellite imagery. In order to simulate the basic dynamics of the flow reversals and instability, a contour dynamics model is developed for barotropic flow on an exponentially sloping bottom, in which potential vorticity i...

Seasonal differences in the current and temperature variability over the northern California Shelf during the Coastal Ocean Dynamics Experiment

A long‐term moored array was maintained over the northern California continental shelf from April 1981 to April 1983 as part of the Coastal Ocean Dynamics Experiment to determine the seasonal differences in the current and water temperature characteristics. The array consisted of two midshelf moorings and one upper slope mooring from April 1981 through July 1982 and one midshelf mooring from August 1982 to April 1983. The two summers were characterized by southeastward mean wind stresses (upwelling favorable), cold water, isotherms sloping upward toward the coast, and a sheared equatorward mean shelf current. The intervening fall and winter were characterized by a weak mean wind stress, relatively warm shelf water, level isotherms, and a more barotropic poleward mean shelf current. In contrast to the differences in the characteristics of the means many of the characteristics of the current and temperature variability are the same for the summer and winter. The vertical structure of the largest current empirical orthogonal functions (EOFs) for each of the shelf moorings, which account for more than 75% of the subtidal variance, do not vary with season. The current is vertically sheared, oriented 15°–20°clockwise relative to local isobaths near the surface and along isobaths near the bottom. Currents at the two shelf moorings (alongshore separation 29–37 km) are correlated with each other and with the local wind stress, and these relationships do not exhibit a seasonal variation. Denser arrays of wind and current sensors during the two summers indicated that there were strong, persistent variations in the wind field over relatively small scales (tens of kilometers) with corresponding variations in the shelf current field during the summer of 1981 but not during the summer of 1982. Over the slope, the largest EOFs account for 56–72% of the current variance and have a vertical structure which is relatively uniform with depth during the summer, fall, and winter of 1981 but is strongly sheared in the upper 100 m during the summer of 1982. Current observations over the slope are not correlated with either the shelf currents or the local wind stress during any season. In contrast, the temperature variability, as described by EOFs, does exhibit a seasonal variation in vertical structure, with the variability concentrated in the upper 20–30 m in summer and more uniform with depth in winter. The near‐surface temperature variability over the slope is correlated with the shelf temperature variability. Estimates of the cross‐shelf heat flux due to the wind‐driven Ekman transport accounts for a large part of the shelf temperature variability. Observations from the midshelf mooring maintained through the fall and winter of 1982–1983 showed evidence of the 1982–1983 El Niño with persistent onshore and poleward currents and water temperatures typically above 12°C.

Coefficient of pollution: Palos Verdes California Shelf 1973 and 1984

The coefficient of pollution (P) proposed by researchers from Greece working in the Mediterranean was applied to an ocean outfall area on the Palos Verdes California Shelf. P values were lower on the California Shelf than in the Gulf of Saronikos. Contour maps of P values revealed that there has been significant sediment transport from 1973 to 1984. In addition, the coefficient yielded patterns consistent with other local studies. Pending additional application of the coefficient to other Southern California ocean outfalls, this methodology may become a viable monitoring tool.

Variability on the California shelf forced by local and remote winds during the Coastal Ocean Dynamics Experiment

The coastal response to local and remote wind forcing is examined using observations from the Coastal Dynamics Experiment (CODE) which took place on the northern California coast during 1981 and 1982. The responses to local and remote forcing are determined from multivariate statistical cross‐spectral analysis covering periods from 12 to 2.5 days. Remote forcing is deduced from coastal sea level. This analysis leads to discovery of a mode of sea level variability, apparently a barotropic Kelvin wave, with much greater alongshore and across‐shelf scales than those of coastally trapped waves; there is no measurable response to this mode in coastal temperature or velocity. Coastally trapped waves are also evident and have responses over the shelf much as predicted by theories which account for bottom friction. The local response to wind differs markedly from coastally trapped waves, showing the across‐shelf flow responsible for upwelling, surface‐intensified temperature fluctuations caused by advection and mixing and even the alongshore pressure gradient partly responsible for the across‐shelf return flow which, on the largest scale, balances Ekman transport. Unfortunately, even the extensive CODE observations are inadequate to diagnose the dynamics of the across‐shelf flow or accurately determine the cross‐shelf heat flux supported by eddies and the fluctuating responses to local wind forcing.

Estimates of the suspended sediment reference concentration ( C a) and resuspension coefficient (γ 0) from near-bottom observations on the California shelf

Near-bottom current and suspended sediment measurements above a sandy clayey silt bed on the California continental shelf are used to estimate the near-bed suspended sediment reference concentration, C a , and the bed shear stress, τ 0. A proposed model contends that C a should be proportional to the normalized excess shear stress acting on the sediment grains and to the availability of erodible grains in the bed. The coefficient of proportionality (γ 0) in this model is estimated for 10 erosion events and is found to range from about 1.5 × 10 −5 to 3 × 10 −4. Moreover, the estimated γ 0 values appear to be inversely proportional to the excess bed shear stress. This result suggests that other factors, such as bed armoring and grain cohesion, inhibited the resuspension of particles at the relatively higher shear stresses.

The morphology of shelfbreak eddies

We used a combination of buoy tracking, intensive hydrography, satellite thermal imagery, and moored current meters to resolve the structure of eddies at the shelfbreak front in the Middle Atlantic Bight south of New England. Eddylike features were always present at the front in our study area throughout the 15‐day period of observations in June 1984. We found that hydrographic features in our across‐shelf hydrographic transects that appeared to represent the detached parcels of shelf water often reported in the literature were, in fact, part of the three‐dimensional structure of shelfbreak eddies. Adequate alongshelf resolution, in particular, enabled us to determine that no detached parcels were present. The two prominent features of the eddy groups we found were plumes of lighter shelf water that protruded into slope water, curling “backward” opposite the direction of mean shelf flow, and neighboring cyclones with warmer, saltier slope water in their cores, partly or wholly encircled by the plumes. The plumes have the potential especially for producing vigorous across‐front exchange of heat, salt, and nutrients and may play roles analogous to the “squirts” found on the California shelf.

Sedimentologic Approaches to Paleobathymetry, with Applications to the Merced Formation of Central California

Sedimentologic evidence for paleobathymetty, like paleontologic evidence, is largely indirect in that it mostly reflects processes and conditions that are themselves partly depth-dependent. Sedimentologic approaches can be a useful adjunct to paleontology for interpreting paleodepths, particularly in relatively shallow water. Pleistocene deposits of the Merced Formation on the San Francisco Peninsula, California, provide an excellent opportunity to demonstrate the methods, value, and difficulties of the sedimentologic approach. Distinction between subaerial and subaqueous deposition can be drawn on the basis of eolian structure, paleosols, and other sedimentary features. The spacing of symmetric ripples relative to their grain size and lenticular laminae of heavy minerals may be useful for identifying deposition in very shallow water. The boundary between intertidal and subtidal deposits serves to establish ancient sea-level positions. The boundary is reflected by the upward transitionsfrom cross-bedded sand (nearshore facies) to planar-bedded sand (foreshore facies) in open-coast deposits, and from laminated sand andlor mud (subtidal channel bank facies) to bioturbated sand andlor mud (intertidal flat facies) in the deposits of coastal embayments. Where the physical setting (i.e., wave energy, sediment input, etc.) can be reasonably reconstructed, paleodepths in shelffacies are indicated by the maximum depth of cross-bedding and gravel deposits generated by the effects of shoaling waves, the contact between shelf sand and mud, and the deepest occurrence of shell lags. In Pleistocene shelf deposits of the Merced Formation on the San Francisco Peninsula, California, these depths are placed at 10 + 5 m, 60 + 10 m, and 100 + 20 m, respectively, based on their inferred depth on the modern central California shelf. At greater depths the sedimentologic approach loses resolution and reliability. Interpretations based on the distribution of turbidites, evidence of anoxic conditions, and calcium carbonate compensation depths and lysoclines are probably most useful for establishing paleodepth in a relative sense only. Integration of the sedimentologic and paleontologic approaches carries a high potential for improving our ability to determine paleobathymetry. INTRODUCTION

A heat budget for the Northern California Shelf during CODE 2

Moored current, water temperature, and meteorological observations made during the summer of 1982 as part of the Coastal Ocean Dynamics Experiment (CODE 2) are used to study both the mean and fluctuating heat budget for the northern California shelf. The volume considered extends from the sea surface to the bottom, from the coast to near the shelf break (13 to 23 km offshore), and 56 km alongshore. The largest terms in the mean heat budget over a 105‐day period spanning the summer season are a surface heat flux of 1.6 × 1011 W into the ocean and an offshore heat flux of 2.4 × 1011 W due primarily to an offshore flow of relatively warm water in the upper 30 m. Cooling due to an alongshore heat flux divergence is 0.6 × 1011 W. Fluctuations with time scales from days to weeks are large relative to the means. The correlation between observed changes in the heat content of the volume and the changes predicted by estimating the heat fluxes into and out of the volume is 0.82. The primary balance in the fluctuating heat budget is heating and cooling of the volume in response to on/offshore fluxes of heat in the upper 30 m. A simple two‐dimensional Ekman model reproduces the major features of the observed offshore heat flux fluctuations, correlation 0.82, indicating that this offshore heat flux is consistent with wind‐driven upwelling. However, at alongshore scales less than 56 km there is considerable alongshore variability in the cross‐shelf heat flux, particularly over the outer shelf. This spatial variability is presumably due to eddy motions. Standard deviations of the alongshore heat flux are about 50% of the cross‐shelf heat flux. Two modes of alongshore heat flux variability are observed. The dominant mode is characterized by large equatorward heat fluxes over the outer shelf during the early part of the upwelling season. This mode is associated with strong vertical shears in alongshore velocity in the vicinity of the upwelling front which results in equatorward flow of the warm surface water relative to the colder deep water. The second mode of alongshore heat flux variability is concentrated over the inner half of the shelf and is related to the wind such that poleward heat fluxes occur in the northern portion of the volume during wind relaxations (periods of weak winds). The presence of an anomalous lens of warm water over the shelf during CODE 2 is also examined.