Northern California Shelf Research Articles

Modeling the dispersal of the San Francisco Bay plume over the northern and central California shelf

High-resolution simulations by the Regional Ocean Modeling System (ROMS) were used to investigate the dispersal of the San Francisco Bay (SFB) plume over the northern-central California continental shelf during the period of 2011 to 2012. The modeled bulk dynamics of surface currents and state variables showed many similarities to corresponding observations. After entering the Pacific Ocean through the Golden Gate, the SFB plume is dispersed across the shelf via three pathways: (i) along the southern coast towards Monterey Bay, (ii) along the northern coast towards Point Arena, and (iii) an offshore pathway restricted within the shelf break. On the two-year mean timescale, the along-shore zone of impact of the northward-dispersed plume is about 1.5 times longer than that of the southern branch. Due to the opposite surface Ekman transports induced by the northerly or southerly winds, the southern plume branch occupies a broader cross-shore extent, roughly twice as wide as the northern branch which extends roughly two times deeper due to coastal downwelling. Besides these mean characteristics, the SFB plume dispersal also shows considerable temporal variability in response to various forcings, with wind and surface-current forcing most strongly related to the dispersing direction. Applying constituent-oriented age theory, we determine that it can be as long as 50 days since the SFB plume was last in contact with SFB before being flushed away from the Gulf of the Farallones. This study sheds light on the transport and fate of SFB plume and its impact zone with implications for California’s marine ecosystems.

Spatial patterns in aragonite saturation horizon over the northern California shelf

Ocean acidification along the California Shelf is controlled by the upwelling of deep CO2-rich waters. This coastal upwelling is driven by along-shore winds, which vary seasonally. We report aragonite undersaturation based on proxy values derived from temperature, salinity, and dissolved oxygen data and show the relationship between the spatial extent of low aragonite saturation state values and upwelling-favorable along-shore winds. Our data were collected on seasonal hydrographic surveys off northern California that span several years. Transect data show a persistent spatial pattern with shoaling of the aragonite saturation horizon over the shelf (during May and July surveys, mean depth of horizon is less than 20 m within ∼20 km of the coast), and surfacing of the saturation horizon during times of strong upwelling events. Our model shows that a higher proportion of the water column is undersaturated in aragonite when northerly winds are stronger. In addition, we found that pteropod abundance was positively correlated with aragonite saturation state, with lower densities associated with lower saturation values. If upwelling-favorable winds increase with global warming, as anticipated, more shelf water habitat will become undersaturated in aragonite, negatively impacting populations of calcifying organisms like pteropods.

Reconstructing Aragonite Saturation State Based on an Empirical Relationship for Northern California

Ocean acidification is a global phenomenon with highly regional spatial and temporal patterns. In order to address the challenges of future ocean acidification at a regional scale, it is necessary to increase the resolution of spatial and temporal monitoring of the inorganic carbon system beyond what is currently available. One approach is to develop empirical regional models that enable aragonite saturation state to be estimated from existing hydrographic measurements, for which greater spatial coverage and longer time series exist in addition to higher spatial and temporal resolution. We present such a relationship for aragonite saturation state for waters off Northern California based on in situ bottle sampling and instrumental measurements of temperature, salinity, and dissolved oxygen. Application of this relationship to existing datasets (5 to 200 m depth) demonstrates both seasonal and interannual variability in aragonite saturation state. We document a deeper aragonite saturation horizon and higher near surface aragonite saturation state in the summers of 2014 and 2015 (compared with 2010–2013), associated with anomalous warm conditions and decadal scale oscillations. Application of this model to time series data reiterates the direct association between low aragonite saturation state and upwelled waters and highlights the extent to which benthic communities on the Northern California shelf are already exposed to aragonite undersaturated waters.

Benthic flux of oxygen and nutrients across Oregon/California shelf sediments

A free vehicle benthic chamber has been used to define benthic nutrient fluxes on the Southern Oregon and Northern California shelf (90–200m). We measured fluxes on two occasions, in the spring and fall of 2007, to assess variability in fluxes and bioirrigation rates. Chambers were also employed to assess the impact of changing oxygen content on benthic fluxes. Oxygen uptake rates are similar to within 30% between sites in Oregon, Northern California and Central California, suggesting that a large portion of the Western US continental shelf experiences the same oxygen demand. Oxygen uptake (−2 to −10mmolm−2d−1) at S. Oregon/N. California sites is likely responsible for <50% of the organic carbon (Corg) oxidized in these sediments as denitrification and other electron transport processes generate much of the observed TCO2 flux (4 to 31mmolCm−2d−1). Much like other coastal regions, these sediments are generally a net sink of water column fixed N as nitrate uptake rates (−0.6 to −2.2mmolNm−2d−1) typically exceed ammonium effluxes (0 to 1.9mmolNm−2d−1). As chamber oxygen concentrations decline during incubation, the concentration vs. time trends in phosphate, nitrate, and silicate remain linear. We interpret this to signify that there is no short-term change in the flux of these species as a response to changing oxygen concentration. However, there is a non-linear response for ammonium, with some ammonium fluxes increasing non-linearly by as much as a factor of 14. This occurs after an incubation period of 5–15h when oxygen levels are 50%–20% of their ambient values. This change in flux may be related to rather small changes in chamber oxygen concentration, suggesting that under hypoxic conditions, there could be an enhanced efflux of ammonia to the water column. This would provide a positive feedback leading to enhanced primary productivity and increasing hypoxia.

Calculating wave-generated bottom orbital velocities from surface-wave parameters

Near-bed wave orbital velocities and shear stresses are important parameters in many sediment-transport and hydrodynamic models of the coastal ocean, estuaries, and lakes. Simple methods for estimating bottom orbital velocities from surface-wave statistics such as significant wave height and peak period often are inaccurate except in very shallow water. This paper briefly reviews approaches for estimating wave-generated bottom orbital velocities from near-bed velocity data, surface-wave spectra, and surface-wave parameters; MATLAB code for each approach is provided. Aspects of this problem have been discussed elsewhere. We add to this work by providing a method for using a general form of the parametric surface-wave spectrum to estimate bottom orbital velocity from significant wave height and peak period, investigating effects of spectral shape on bottom orbital velocity, comparing methods for calculating bottom orbital velocity against values determined from near-bed velocity measurements at two sites on the US east and west coasts, and considering the optimal representation of bottom orbital velocity for calculations of near-bed processes. Bottom orbital velocities calculated using near-bed velocity data, measured wave spectra, and parametric spectra for a site on the northern California shelf and one in the mid-Atlantic Bight compare quite well and are relatively insensitive to spectral shape except when bimodal waves are present with maximum energy at the higher-frequency peak. These conditions, which are most likely to occur at times when bottom orbital velocities are small, can be identified with our method as cases where the measured wave statistics are inconsistent with Donelan's modified form of the Joint North Sea Wave Project (JONSWAP) spectrum. We define the “effective” forcing for wave-driven, near-bed processes as the product of the magnitude of forcing times its probability of occurrence, and conclude that different bottom orbital velocity statistics may be appropriate for different problems.

Late Holocene sediment accumulation on the northern California shelf: Oceanic, fluvial, and anthropogenic influences

The late Holocene sedimentary record of the northern California continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux, and provides perspective on the cumulative impacts of major floods and land-use change in the drainage basin during historical time. Piston cores collected on the shelf (50−150 m water depths) off the Eel and Klamath Rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k.y., and dated using 137Cs and 14C methods to resolve spatial patterns in sediment accumulation rates. Sediment accumulation rates averaged over the past several millennia display along- and across-shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography. The highest rates (3−6 mm/yr; 0.4−0.8 g/cm2/yr) are associated with structural lows on the middle shelf, whereas the lowest rates (0.2−0.6 mm/yr; 0.03−0.05 g/cm2/yr) occur at structural highs and at the shelf edge. This accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf, and with depocenters that form on shorter time scales in response to oceanic flood events. The sediment column exhibits an upward-fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the Holocene transgression. Sedimentary event beds indicative of major river floods, including two probable hyperpycnal discharges of the Eel River within the past 1 k.y., are preserved. Stratigraphic evidence of accelerated sediment delivery to the shelf during historical time, in the form of increased burial of fluvial mud relative to fine-grained sand, is superimposed on this natural record. This change is concordant with the regional history of forest clearing and related land-use practices in the watershed after the early 1800s, and appears to record the combined impacts of timber harvesting and several extreme floods after ca. A.D. 1950.

Surface boundary-layer variability off Northern California, USA, during upwelling

Abstract A five-element mooring array is used to study surface boundary-layer transport over the Northern California shelf from May to August 2001. In this region, upwelling favorable winds increase in strength offshore, leading to a strong positive wind stress curl. We examine the cross-shelf variation in surface Ekman transport calculated from the wind stress and the actual surface boundary-layer transport estimated from oceanic observations. The two quantities are highly correlated with a regression slope near one. Both the Ekman transport and surface boundary layer transport imply curl-driven upwelling rates of about 3×10 −4 m s −1 between the 40 and 90 m isobaths (1.5 and 11.0 km from the coast, respectively) and curl-driven upwelling rates about 1.5×10 −4 m s −1 between the 90 and 130 m isobaths (11.0 and 28.4 km from the coast, respectively). Thus curl-driven upwelling extends to at least 25 km from the coast. In contrast, upwelling driven by the adjustment to the coastal boundary condition occurs primarily inshore of the 40-m isobath. The upwelling rates implied by the differentiating the 40-m transport observations with the coastal boundary condition are up to 8×10 −4 m s −1 . The estimated upwelling rates and the temperature–nitrate relationship imply curl-driven vertical nitrate flux divergences are about half of those driven by coastal boundary upwelling.

On sediment accumulation rates and stratigraphic completeness: Lessons from Holocene ocean margins

Stratigraphic completeness is a fundamental consideration when deciphering the mass accumulation history of sediments and the geologic record of earth and ocean processes. In this study, stratigraphic completeness was examined in the context of late Holocene sedimentary successions using published sediment accumulation rates for five ocean margin systems (Amazon shelf, Hudson estuary, northern California shelf, Mid-Atlantic slope, Santa Monica Bay). Plots of mass accumulation rate versus time span of averaging were used to determine how rates scale with measurement period, and to estimate levels of stratigraphic completeness for comparison within and among margin systems. Statistically significant inverse correlations between accumulation rate and time span of averaging are indicated for all but one of these systems—most of the sedimentary records examined are stratigraphically incomplete. At the 10 3-yr level of resolution, completeness is 20–48% for strongly tidal estuarine (Hudson estuary) and deltaic shelf (Amazon shelf) sites, 51–91% for accretionary shelves (northern California shelf) and slopes (Mid-Atlantic slope), and 85–100% for a sediment-starved slope (Santa Monica bay). Mass accumulation rates converge to a relatively narrow range (0.01–0.1 g cm −2 yr −1) at the 10 4-yr level of resolution, consistent with the notion that there are universal controls on sediment accumulation rate, i.e., rate of sea-level rise and sediment supply. Among sites on the Amazon and northern California shelves, within-system completeness varies by ∼10–20% on account of site-specific sedimentary processes that preferentially trap or disperse suspended sediment. Overall, stratigraphic completeness increases with water depth shelf-to-slope, yet depth is not a robust predictor of completeness in general owing to differences in strata-forming processes among shallow-marine environments. Significantly, completeness varies inversely with instantaneous deposition rate as the most sediment-rich systems tend to exhibit the most incomplete sedimentary records. The findings of this study emphasize the importance of considering time span and fidelity when interpreting the accumulation history of modern and Holocene sedimentary strata.

River flooding, storm resuspension, and event stratigraphy on the northern California shelf: observations compared with simulations

Coast-hugging surface flood plumes occur on the inner shelf of northern California during the winter season, generating dense, near-bottom suspensions which may attain fluid mud concentrations as particles settle. The period of storm-heightened waves may continue into the flood period, leading to gravity-driven seaward displacement of the bottom suspension; or the wave regime may ameliorate, leaving the suspension to consolidate as a short-lived, inner-shelf flood bed. Such beds tend to be resuspended within days or weeks by subsequent storm events that may recreate the original high concentrations. The sediment is thus dispersed seaward by gravity flows, to be deposited as a muddy flood bed on the central shelf. The locus of deposition of these “high-concentration regimes” is a function of the relative intensities of river discharge and storm wave height. Greater discharge piles thicker storm beds nearer shore, while intense wave regimes allow deposition of the fluid mud further seaward. During events with high values of both parameters, large amount of fluid mud may bypass over the shelf edge. In contrast, “low-concentration regimes” occur during storm periods when there has been no recent flood deposition on the inner shelf. The shelf floor is better consolidated than in the previous case, and the resulting suspended sediment concentrations are lower. As a consequence, low-concentration regimes are winnowing and bypassing regimes, and the beds deposited are thinner and sandier. Algorithms describing deposition by high and low-concentration regimes have been embedded in a probabilistic model. A simulation of a 400-year sequence of beds deposited by winter storms and floods suggests that on the Eel shelf, the Holocene transgressive systems tract consists of back-stepping, seaward-fining event beds, whose timelines (bedding planes) dip more gently than do their gradational facies boundaries. At these longer time scales, flood beds dominate over storm beds.

Sediment transport on the Palos Verdes shelf over seasonal to decadal time scales

We combine direct observations, longer-term wave data, and model calculations to characterize resuspension and transport of fine-grained, effluent-affected sediment on the Palos Verdes shelf. Near-bed waves, currents, and suspended sediment concentrations were monitored during the winter of 1992–93 with a bottom tripod and current-meter mooring at a 63-m-deep site. Wave conditions that winter were moderate (∼2 year recurrence interval), and mean current was alongshelf to the northwest; currents were not significantly correlated with wave conditions. Seven wave events during the winter (December–March) produced near-bed wave orbital velocities at the study site in excess of 14 cm s −1, the observed threshold for significant resuspension. Three of these events occurred during the bottom tripod deployment and are characterized by the highest persistent suspended sediment concentrations in the tripod record. Suspended sediment flux was alongshelf to the northwest for 5 of the 6 wave events for which current data were available; one event occurred during low southeast currents. Measured suspended sediment concentration and grain size generally agree with values that were calculated using a shelf sediment transport model with no adjustment of parameters from values determined for two muddy sites on the northern California shelf. We extend our seasonal observations to a period of almost 2 decades by applying the observed thresholds for wave-driven resuspension to near-bed wave conditions calculated from NDBC Buoy 46025 surface wave data. An average of 10 resuspension events per year, with an average duration of 1.6 days, were identified at a water depth of 60 m; the number of events dropped to 3 per year at 90 m, beyond the shelf break. For the majority of these events, calculated net suspended sediment flux is toward the northwest (alongshelf) at an average rate of 140 kg m −1 h −1; about a third of the events have net southeastward flux at an average rate of 30 kg m −1 h −1. The calculated thickness of the resuspended surface layer of the bed was less than 1 cm for all events at 60 m.

Effects of ambient currents and waves on gravity-driven sediment transport on continental shelves

Observations from several shelf environments show that down-slope gravity-driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity-induced sediment flux. The phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment-laden rivers. The field sites examined are the Gulf of Bohai off the mouth of the Yellow River (Huanghe), the northern California shelf off the mouth of the Eel River, and the Louisiana shelf west of the mouths of the Mississippi River. Off the Yellow River, rapid down-slope transport over distances of a few km occurred when frictional resistance, induced by strong along shelf currents, was temporarily relaxed. More prolonged down-slope motion over longer distances occurred following floods of the Eel River, when wave and current agitation provided turbulence to sustain gravity-driven transport of fluid mud. On the Louisiana inner shelf, the down-slope gravity force was much weaker, but observations suggest that thin gravity flows may still have occurred in the presence of waves. A simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity-driven transport of suspended sediment. This theory is quantitatively consistent with the observations from the three sites. If the supply of easily suspended sediment is less than the capacity of ambient currents (including waves) to carry sediment, then intense turbulence limits gravity-induced sediment transport by increasing the drag at the base of the flow. When ambient currents abruptly cease, rapid down-slope transport can then occur over short distances until the sediment settles. Such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow. The maximum sustained rate of gravity-induced sediment transport occurs when ambient currents are strong, but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents. Feedback then leads to values of the gradient Richardson number (Ri) within the flow that are near the critical value of 1/4. This partially damps bottom drag, but still allows the generation of sufficient turbulence to maintain sediment in suspension. Observations also indicate systematic relationships among Ri, the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow.

A Dynamical Description of Fall and Winter Mean Current Profiles over the Northern California Shelf*

Abstract Fall and winter mean current profiles from a midshelf (water depth ∼90 m) northern California site exhibit a similar vertical structure for several different years. The alongshelf flow is poleward with a maximum velocity of 5–10 cm s−1 in the middle or upper water column. There is an offshore flow of about 2 cm s−1 in the upper 20–30 m, an onshore flow of about 2 cm s−1 in the interior (depths 35–65 m), and an offshore flow of about 1 cm s−1 within 20 m of the bottom. Profiles are similar for averages over timescales from weeks to months. Mean current profiles at other midshelf sites along northern California and two sites off Peru also have a similar vertical structure. The vertical shear in the mean alongshelf flow is geostrophic throughout the water column, that is, in thermal wind balance with the cross-shelf density gradient. For timescales of a week or longer the thermal wind balance extends to within 1 m of the bottom and reduces the mean near-bottom alongshelf flow to 1 cm s−1 or less. Th...

Oceanic flood deposits on the northern California shelf: large-scale distribution and small-scale physical properties

Flooding by northern California's Eel River in January 1995, March 1995 and January 1997 produced sedimentologically and geochemically distinct deposits on the adjacent continental shelf. X-radiographs collected during an extensive suite of box coring cruises revealed that these layers extended along-shelf for up to 50 km, were in water depths of 50–110 m and were up to 8 cm thick. The areal distribution of the three flood layers was remarkably similar, with <8 km separating their respective centers of mass. Hourly measurements of river discharge made by the US Geological Survey were coupled with an empirically determined rating curve to predict the mass of fine-grained (<63 μm) suspended sediment that entered the ocean during each flood. Comparison of these estimates to independent estimates of flood layer mass suggests that <25% of the fine-grained sediment delivered to the ocean resides in the flood layers. The remaining flood sediment is temporarily sequestered within sand deposits of the inner shelf or transported along- and off-shelf by buoyant plumes and density-driven fluid-mud flows. The Eel River dispersal system therefore has two seemingly contradictory attributes: widespread dispersal of flood sediment (most likely off shelf), but a consistent locus of deposition of flood layers on the shelf. Explanations for this paradox involve the small size of the drainage basin and the extreme suspended-sediment concentrations during floods. The former results in a rapid throughput of sediment in the basin, whereby sediment is delivered to the ocean during storms, when energy levels are high (leading to widespread dispersal), but repeatable (leading to a consistent locus of deposition). In addition, high suspended-sediment concentrations result in fluid muds that may be a necessary condition for the formation of the flood deposits. Other West Coast river systems (e.g., Columbia, Russian) either have longer sediment throughput times or lower riverine suspended-sediment concentrations, thereby resulting in a greater fraction of sediment deposited on the shelf, but not as thick, recognizable layers (i.e. flood deposits).

Bottom-boundary-layer processes associated with fine sediment accumulation in coastal seas and bays

Bottom-boundary-layer velocity profiles, bed stresses and suspended sediment concentration profiles were measured with instrumented tripods in four contrasting shelf and semi-enclosed bay environments that are presently accumulating fine sediments. The sites were: the northern California shelf off the mouth of the Eel River; Eckernförde Bay, southern Baltic Sea; the York River estuary, lower Chesapeake Bay; and the Louisiana shelf to the west of the Mississippi River mouth. Bed micromorphology was often biogenic and generally smooth. Suspended sediment concentration within the mean current boundary layer was attributable to local resuspension from underlying, very easily eroded pools and to the horizontal advection of near bottom turbid layers (benthic plumes). The presence of density stratification caused simple fits of log profiles to velocity observations over the lowest meter to overestimate bottom stress. Observed semi-log velocity profiles were (1) concave downward, (2) straight, or (3) concave upward depending on whether the density anomaly decreased with height above the bed (1) much more slowly than, (2) at roughly the same rate as, or (3) much more rapidly than z −1. These patterns were shown to be consistent with analytical relations between velocity and density stratification within the near-bed constant stress layer. Stable stratification was attributable to a combination of locally resuspended sediment and thermohaline effects, with the former and latter dominating under high- and low-energy conditions, respectively. Near-bed thermohaline stratification was observed to increase z oc , the elevation of the zero intercept of the logarithmic velocity profile. The effects of sediment-induced stratification on z oc were less clear. At all sites the near-bed gradient Richardson number ( Ri) approached or exceeded the critical value of 1 4 implying that turbulence was damped by stable stratification. Results point to the likelihood that stable near-bed stratification, which is often associated with fine sediment accumulation, plays a role in accelerating accumulation by reducing near-bed turbulence. Results further indicate that the benthic sediment plumes and/or mud pools which result in Ri⩾ 1 4 cannot be maintained simply by vertical suspension within the current boundary layer. Rather, horizontal advection, flux convergence and/or high concentration sediment suspension within the wave boundary layer is also required.

Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon

The 1995 flood of the Eel River in northern California provided an opportunity to follow the short-term history of riverine particulate matter on a continental shelf. Particulate organic carbon, including both vascular plant debris and soil carbon, was utilized as a tracer of the distribution and modification of the shelf flood deposit. These components were sorted during initial emplacement of the deposit, both in the river plume and in the benthic boundary layer. During the 8 months following the flood, lateral transport, shallow burial, and bioturbation contributed to spatial changes in the amount and character of carbon in the flood deposit. Study of the deposit suggests that this and previous flood layers may be preserved in the shelf and upper slope stratigraphic record as clay-rich beds with relatively high carbon to nitrogen ratios and more negative δ 13C values than background, non-flood sediment. Concentrations of vascular plant debris are locally characteristic of such layers, and may serve as distinctive stratigraphic markers. Investigation of organic carbon in the 1995 flood layer on the Eel River shelf provides insight into the fate of terrestrial particulate carbon on continental margins in general. Flooding of the Eel River, and of similar river systems draining mountainous terrains, introduces large quantities of terrestrial carbon to the marine environment and may favor the preservation of both terrestrial and marine carbon. The abundant woody vascular plant debris discharged during floods is especially likely to survive oxidation and sulfate reduction in the shallower zones of the seabed. This study reveals the dominant role that physical and biological processes may play in attenuating the terrestrial organic-carbon signal on shelves over monthly to yearly time scales.

Across-shelf variations in bed roughness, bed stress and sediment suspension on the northern California shelf

Field measurements of bed micromorphology (roughness), benthic flow, bed stress, and suspended-sediment flux were made on the northern California continental shelf in connection with the STRATAFORM program. A sediment profiling camera and side-scan sonar were used to observe bed features in December 1995. Biogenic roughness with ∼2 cm relief prevailed at both sites. Bottom-boundary-layer tripods were deployed on the 60-m and 70-m isobaths over the period 5 January to 7 March 1996, during which time two high-energy events occurred. Skin-friction shear stresses were subcritical under `average' conditions but appreciably exceeded the threshold for sediment suspension during storms. For those high-energy events, near-bed suspended-sediment concentrations reached 2 g/l at 60 m (15 cm above bed) and 1g/l at 70 m (27 cm above bed) and suspended-sediment-induced stratification significantly affected bed stress estimates. At those times, increases in current shear were accompanied by increases in suspended-sediment-concentration gradients, causing the gradient Richardson number within the log layer to remain near the critical value of 1/4. This suggests suppression of turbulence by sediment-induced stratification. The abundance of under-consolidated fine sediment on the shelf to the north of the Eel River mouth presumably allows increases in stress to be accompanied by progressive increases in suspended-sediment concentration within the log layer to maintain the critical Richardson number. This contrasts to situations where bed armoring limits the total amount of fine sediment in suspension. Applications of a wave–current boundary-layer model, with stratification effects included, reduces estimates of current friction velocities by (u ∗ c ) about 24% relative to results from fitting the von Karman–Prantdl equation without stratification. The model suggests that the total wave–current friction velocity (u ∗ cw ) reached 3.0 cm/s at 70 m and 3.5 cm/s at 60 m. Depth-integrated across-shelf suspended-sediment fluxes were seaward at the 60-m isobath and near zero to weakly landward at the 70-m isobath during high-energy periods, implying flux convergence. This is consistent with the conclusions of other STRATAFORM investigators that rapid long-term accumulation of mud is occurring on the mid-shelf.

Two-dimensional numerical modeling of storm deposition on the northern California shelf

A two-dimensional, across-shelf sediment-transport model has been developed to simulate storm deposition on continental shelves. This paper describes the structure of the model and related algorithms. In the model, the time-dependent sediment resuspension, transportation and deposition, and across-shelf transport gradients are simulated as responses to storm waves on the Eel shelf, northern California. The simulations show that the pattern of storm deposition on the sandy inner shelf is different from that on the muddy middle and outer shelf. Storm-bed thickness decreases seaward across the inner-shelf sandy zone as far as the 50-m isobath. Farther seaward, storm-bed thickness increases as the mud component in the sea floor becomes abundant, then decreases again, as the weakening of bottom-wave motion with greater depth becomes the dominant control. Storm-bed thickness in the mud zone thus has a nearly symmetrical cross-shelf pattern, with the maximum thickness at a water depth of 70 m. The pattern suggests that while the sequence of new beds found in February 1995 on the Eel shelf may have had a flood source, their geometry reflects storm resuspension and transport. The new muds found in these beds may have been reworked, transported and redeposited several times by storm waves and other oceanic forces. Numerical simulations illustrate that the critical bottom shear stress for mud erosion, τ e, and the coefficient for sand resuspension, γ 0, are more important than other parameters in determining storm depositional patterns on the Eel shelf.

Dynamics of the Bottom Boundary Layer on the Northern California Shelf*

Abstract Time-series measurements of velocity, temperature, and conductivity on the northern California shelf during two winter seasons permit an observational test, in vertically integrated form, of a simple set of subinertial momentum and heat balances for the bottom boundary layer, which have resulted from recent theoretical work. These are 1) an along-isobath momentum equation that reduces to a classic Ekman balance; 2) a cross-isobath momentum equation in which the Ekman balance is modified by a buoyancy force caused by distortion of the isopycnal surfaces within the boundary layer; and 3) a heat balance in which variability of temperature is produced by cross-isobath advection. The measurements confirm the importance of buoyancy in the cross-isobath momentum equation, and, as has recently been predicted theoretically, they indicate that buoyancy is a dominant effect when the boundary layer is thick, which typically occurs during downwelling-favorable flows. An Ekman balance describes subinertial flu...

Surface heat flux variability over the northern California shelf

Surface heat flux components are estimated at a midshelf site over the northern California shelf using moored measurements from the 1981–1982 Coastal Ocean Dynamics Experiment (CODE) and the 1988–1989 Shelf Mixed Layer Experiment (SMILE). Time series of estimated fluxes extend from early winter through summer upwelling conditions, allowing examination of seasonal variations as well as synoptic events. On a seasonal timescale, the surface heat flux is strongly influenced net surface heat flux are the annual variation in incident shortwave solar radiation (insolation) and the atmospheric spring transition. Between mid‐November 1988 and late February 1989, insolation is weak and the mean daily averaged heat flux is nearly zero (absolute value less than 10 W m−2), with a standard deviation of ∼50 W m−2. Beginning in March, insolation increases markedly, and typical daily‐averaged heat fluxes increase to greater than 100 W m−2 by the spring transition in April or May. In June and July, the average heat flux is near 200 W m−2, with a standard deviation of ∼90 W m−2. In winter, the daily‐averaged heat flux varies on periods of several days. Net heat flux losses can range up to 130 W m−2. These losses are not identified with any one type of event. For example, comparable heat flux losses can occur for very low relative humidities (RHs), moderate winds, and clear skies, and for high RHs, high winds, and cloudy skies. In summer, surface heat flux variability is strongly influenced by upwelling and relaxation events. Upwelling is characterized by clear skies and high equatorward winds, while relaxation is characterized by the presence of clouds and low or northward winds. These conditions lead to opposing changes in insolation and in longwave radiative cooling and latent heat flux. Variability in insolation dominates, and the daily‐averaged heat flux into the ocean is greatest during upwelling events (up to 350 W m−2 or more) and least during relaxation events (sometimes less than 100 W m−2).

Wind-Forced Cross-Shelf Circulation on the Northern California Shelf*

Velocity time series are used to study cross-shelf circulation on the northern California shelf and to examine classical ideas of locally wind-forced cross-shelf circulation. A simple linear two-dimensional model of crossshelf transport is compared to estimates of cross-shelf transport in the near surface, interior, and near bottom. In winter, when wind forcing is brief and episodic, model transports are highly correlated to the total surface flow and show some skill in predicting subsurface cross-shelf flow. The same model does not work well below the surface in summer when persistent upwelling is observed. This suggests a two-dimensional wind-forced model of cross-shelf circulation may have more applicability to the brief wind events observed in winter than to the persistent wind events observed in summer. The reason for this is unclear. Numerous factors not included in the simple linear wind-forced model such as mesoscale features, upwelling fronts, the interaction of flow with topography, baroclinic pressure gradients, remote forcing, and small-scale wind stress all affect cross-shelf circulation. It is possible some of these are more pronounced on the northern California shelf in summer.