Northern California Continental Shelf Research Articles

Environmental variability and krill abundance in the central California current: Implications for ecosystem monitoring

Krill are a direct conduit between primary productivity and recreationally and commercially important higher trophic level species globally. Determining how krill abundance varies with temporal environmental variation is key to understanding their function in coastal-pelagic food webs, as well as applications in fisheries management. We used nine years (2012–19 and 2021) of late spring/early summer hydroacoustic-trawl survey data in the California Current Ecosystem (CCE), coupled with new target strength models of two krill species (Euphausia pacifica and Thysanoessa spinifera), to investigate how adult krill biomass varied during a decade of unusual ocean climate variability. We estimate a mean biomass of 1.75–2.0 million metric tons on the central and northern California continental shelf. Overall, relative krill biomass was ~30% lower during 2015 and 2016, corresponding to a major warming event, and ~30% higher in 2013 and 2018, years of exceptionally strong upwelling. Variation in biomass was related to the prior year’s environmental conditions derived from our seasonal Multivariate Ocean Climate Index (MOCI), and E. pacifica and T. spinifera showed similar covariation during the study period. Biomass co-varied at different spatial scales and across sampling devices, suggesting that multiple indicators of abundance (and dispersion) are available and should be applied in ecosystem monitoring and modeling of krill and krill-dependent predators in the California Current ecosystem.

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.

Sedimentation and carbon burial on the northern California continental shelf: the signatures of land-use change

The burial of organic carbon (OC) on continental margins is strongly coupled to the supply and accumulation of inorganic mineral particles. It follows that carbon burial on the margins should be impacted by changes in riverine sediment delivery, yet these impacts have not been well documented. In this study, an ∼2000-year record of sedimentation and carbon burial on the continental shelf offshore from the Eel River in northern California was examined. The record reveals a 6–11-fold increase in the rate of sediment accumulation on the mid-shelf beginning about 1955, and a concomitant decrease in grain size and increase in flood-layer preservation. At the same time, the age of buried wood fragments abruptly decreased and their stable carbon isotopic composition became enriched in 13C. We argue that these changes can be explained largely as the result of altered land use in the Eel watershed during the past century and its impacts on shelf sediment dispersal processes. Sedimentary OC on the Eel shelf consists primarily of discrete wood fragments associated with coarse-silt- and sand-sized particles, and of organic matter strongly bound to clay-sized mineral grains. The clay fraction is a particularly sensitive recorder of environmental change in the Eel system. Above the 1955 horizon, the clay fraction shows an abrupt decrease in OC concentration and loading (OC content normalized to particle surface area) attendant with the increased accumulation rate. Kerogen carbon constitutes a relatively constant proportion of the clay-associated OC throughout the ∼2000-year record. Increases in mass wasting and input of bedrock material following the onset of intensive industrial logging in the Eel watershed may have resulted in a lower loading of terrestrial plant OC in the clay fraction deposited after 1955 as suggested by isotopic mass balance calculations. The Eel River is representative of small mountainous watersheds worldwide that deliver a major portion of the sediment and carbon flux to the margins and that have been strongly impacted by land-use change during the past century. Our results suggest that such changes leave a distinctive mark in both the sedimentological and geochemical records preserved offshore.

Observations of storm and river flood-driven sediment transport on the northern California continental shelf

In the winter of 1996–1997, three bottom-boundary layer tripods were placed in an alongshelf array on the northern California continental shelf off Eureka, CA in 60–65-m water depth. During the observation period, multiple storms and river discharge events occurred, as well as the largest flood on record since 1964. Suspended-sediment concentration at all three sites fluctuated in response to both wave resuspension and advection of river-derived sediments. However, considerable spatial differences in low-frequency currents and suspended-sediment concentration were observed at the three sites. Sediment flux vectors calculated during periods of high suspended-sediment concentration suggest a convergence of sediment flux coincident with the center of recent flood deposits. Suspended-sediment concentrations observed at the two northern tripod sites following the large flood reached magnitudes typical of fluid mud (>10 g/l) in a thin near-bed layer. The net sediment flux during the single three-day event was two orders of magnitude larger than any other event during the winter, and accounted for seven times the flux observed over an entire year (1995–1996). A conceptual model for the advection of sediment to the mid shelf is proposed in which river plume sediments are trapped on the inner shelf either due to a weak front or the rapid input of sediment from a confined plume, and form a thin layer of fluid mud. The fluid mud subsequently is transported seaward due to gravitational forcing. The measured sediment concentration and velocity profiles on the shelf provide strong evidence to support this conclusion.

Sediment-transport events on the northern California continental shelf

A long-term monitoring tripod has been maintained in 60-m water depth at the northern end of the STRATAFORM study site on the northern California continental shelf. As part of this ongoing study, tripod data for 1 year beginning 24 September 1995 are used to provide a sediment-transport analysis on an event-by-event basis. The objective of this paper is to highlight the energetic nature of this shelf region in terms of the frequency, duration, and magnitude of sediment-suspension events and the associated particle flux. Analyses are based on measurements from two current meters and two optical backscatterance sensors located at 30 and 100 cm above the bed. Data from these instruments and a pressure sensor were averaged over 7.5 min every hour. During the year of record, 41 distinct sediment-suspension events occurred (i.e., sustained suspended-sediment concentrations greater than 20 mg/l above background level at 100 cm above bed). Suspended-sediment events were associated with significant wave activity, tidal currents, and river discharge. The average duration of a sediment-suspension event was 3.1 days and varied from 0.7 to 8 days. During events, mean suspended-sediment concentrations of 110 mg/l were observed with peak hourly observations exceeding 1000 mg/l. Concentrations between events averaged 35 mg/l. The overall sediment flux for the period of record was directed seaward and southward. The distribution and magnitude of sediment-suspension events and particle flux showed a strong seasonality. For example, of the sediment transport during events, 98% of the net along-shelf, and 73% of the net across-shelf sediment transport occurred in the winter. Three major winter storms could account for 72% of the total along-shelf transport but only 10% of the across-shelf transport. The across-shelf flux was more evenly distributed over the year and occurred as a result of mean currents and very low-frequency seaward flows associated with mesoscale circulation patterns.

Measurement of size and settling velocity of suspended aggregates on the northern California continental shelf

A high-resolution video camera attached to a miniature sediment trap was used to make time-lapse images of suspended aggregates settling through the water column. The video-trap system was placed 2 m above the seabed in the STRATAFORM study area off Eureka, California, and was programmed to take a 7-second time sequence of settling particles every 6 h over a 4-month period. The camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 μm. Data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size, shape, and settling velocity. Results of the first deployment of this system (September 1995) show that particle sizes (elliptical nominal diameter) determined from the video images ranged from 130 to 740 μm with a modal size of 300 μm. The average form factor (ratio of short to long semi-axis) of the aggregates was 0.71. Measured settling velocities varied from 0.09 to 8.13 mm/s. In terms of weight-percent, or mass, the median size of the aggregate distribution is 600 μm. Analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1.0 to 63 μm with a modal size of 26 μm and a median size of 11 μm. Additionally, an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity, and these results were compared to observations. Using the equations for settling velocity of spheres and prolate spheroids, and substituting published relationships for size versus excess density into these equations, the settling velocity for combinations of shape and excess density has been calculated. Results show that differences between sphere and prolate spheroid (with major axis both parallel and perpendicular to flow) approximations caused less than a 13% difference in computed settling velocity. In contrast, the various size/excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values. In all cases, however, the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution.

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.

Approaches to quantifying long-term continental shelf sediment transport with an example from the Northern California STRESS mid-shelf site

Modeling shelf sediment transport rates and bed reworking depths is problematic when the wave and current forcing conditions are not precisely known, as is usually the case when long-term sedimentation patterns are of interest. Two approaches to modeling sediment transport under such circumstances are considered. The first relies on measured or simulated time series of flow conditions to drive model calculations. The second approach uses as model input probability distribution functions of bottom boundary layer flow conditions developed from wave and current measurements. Sediment transport rates, frequency of bed resuspension by waves and currents, and bed reworking calculated using the two methods are compared at the mid-shelf STRESS (Sediment TRansport on Shelves and Slopes) site on the northern California continental shelf. Current, wave and resuspension measurements at the site are used to generate model inputs and test model results. An 11-year record of bottom wave orbital velocity, calculated from surface wave spectra measured by the National Data Buoy Center (NDBC) Buoy 46013 and verified against bottom tripod measurements, is used to characterize the frequency and duration of wave-driven transport events and to estimate the joint probability distribution of wave orbital velocity and period. A 109-day record of hourly current measurements 10 m above bottom is used to estimate the probability distribution of bottom boundary layer current velocity at this site and to develop an auto-regressive model to simulate current velocities for times when direct measurements of currents are not available. Frequency of transport, the maximum volume of suspended sediment, and average flux calculated using measured wave and simulated current time series agree well with values calculated using measured time series. A probabilistic approach is more amenable to calculations over time scales longer than existing wave records, but it tends to underestimate net transport because it does not capture the episodic nature of transport events. Both methods enable estimates to be made of the uncertainty in transport quantities that arise from an incomplete knowledge of the specific timing of wave and current conditions. © 1997 Elsevier Science Ltd

Spatial and temporal variability in aggregated grain-size distributions, with implications for sediment dynamics

The grain-size distribution of bottom sediments has important implications for diverse aspects of sediment dynamics, including prediction of the critical boundary shear stress and calibration of suspended sediment sensors. Past sampling strategies to obtain estimates of the seabed grain-size distribution typically have not considered spatial and temporal variability, and have been insufficient to resolve potential millimeter-scale vertical variations in grain size. Moreover, laboratory analyses have been predicated on chemically and/or ultrasonically disaggregating the sediments before resolving particle diameter, therefore the more dynamically relevant in situ grain-size spectrum is not measured. To test for such effects, three sites on the northern California continental shelf comprising a cross-shelf transect from a sandy, inner-shelf (60 m) site, to a muddy, mid-shelf (90 m) site and a relict, outer-shelf (130 m) site were studied. Replicate box cores were collected over two winter field seasons, and multiple subcores from each box core were vertically sectioned at 2-mm intervals. Gentle wet sieving techniques were used to determine the mass fraction in the <20, 20–44, 45–62, 63–89, 90–179, 180–300 and >300,um size classes. In addition, a lesser number of standard disaggregated grain-size analyses were performed using a Coulter Counter. Results from the sandy, inner-shelf site indicate the presence of an ephemeral fine-grained (<20 μm) surface layer (0–2 mm) that substantively alters the grain-size distribution “seen” by the flow. In addition, there is evidence for a progressive and substantial winnowing of fine-grained sediment from the surface layer over the course of a winter storm season. At the deeper sites, the upper 2 mm of the bed contained 5–20% more material <20 μm than deeper portions of the sediment column. At all three sites, the level of small- (10s of centimeters) and intermediate- (10s of meters) scale spatial variability is modest. In addition, the disaggregated grain-size distribution at the two muddy sites is, in all cases, markedly finer than the in situ grain-size distribution. Therefore, calibrations and predictions based on a knowledge of the size distribution of the primary (i.e. disaggregated) particles could be in serious error.

On the effect of the alongshore pressure gradient on numerical simulations over the Northern California Continental Shelf

Chen and Wang's (1990) two‐dimensional shelf circulation model has been extended to include the alongshore (the dimension absent in the model) pressure gradient. The alongshore pressure gradient, calculated independently from a linear coastal‐trapped wave model, is included as an external “forcing”. The extended model has been used to simulate the temperature and velocity fields observed during the Coastal Ocean Dynamics Experiment 2 (CODE 2) in the spring and summer of 1982 on the Northern California Continental Shelf. The results were compared to the CODE 2 observations and to the results of the purely two‐dimensional and coastal trapped wave models. Results using the extended model show some improvement in the cross‐shore and alongshore velocity fluctuations. The differences between the models are explained in terms of the opposition between the wind stress and the pressure gradient in the alongshore direction, which reduces the magnitude of the alongshore velocity. The reduced magnitude of the alongshore flow decreases the alongshore bottom stress which, in turn, reduces the offshore transport in the bottom boundary layer.

In situ measurements of particle settling velocity on the northern California continental shelf

As part of the Sediment TRansport Events on Shelves and Slopes (STRESS) program, a remote optical settling ☐ was deployed on the northern California continental shelf. The device operates by isolating a volume of sediment-laden fluid from the environment and then monitoring its sedimentation behavior with a transmissometer. Results show a bimodal distribution of suspended sediment during low-energy periods on the shelf that reflects the size distribution of bottom sediments. The coarse mode sinks at 0.026 cm s −1 (22 m day −1) and the fine mode settles at 0.0025 cm s −1 (2 m day −1). Between one-quarter and two-thirds of the total mass resides in the coarse mode. Roughly one-quarter is in the fine mode. The remainder sinks too slowly (<0.0015cm s −1 or <1.3m day −1) to be resolved during the 18-h measurement cycles. Greatest uncertainty in assigning mass to the various settling velocity classes comes from sensitivity to ill-constrained particle geometry of the conversion from light attenuation to mass. The device failed during higher energy periods, probably due to penetration of fluid into the ☐. Complete isolation of the fluid from the environment would improve the performance of settling ☐es in energetic settings.

Sediment-transport events on the northern California continental shelf during the 1990–1991 STRESS experiment

Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) “C” transect as part of the 1990–1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.

Baroclinic semidiurnal tidal currents over the continental shelf off northern California

Time‐varying depth‐dependent currents of semidiurnal frequency are identified in data collected over the northern California continental shelf during the 1982 up welling season as part of the Coastal Ocean Dynamics Experiment. An increase in semidiurnal band kinetic energy is seen during wind relaxation periods when stratification over the shelf and upper slope increases. Comparison with internal wave theory shows the vertical and horizontal structure of the semidiurnal band horizontal current and temperature fluctuations to be consistent with a first baroclinic mode internal wave with a horizontal wavelength of 20–30 km. Over the continental shelf the semidiurnal band temperature fluctuations show a phase shift which may be evidence of the influence of the subcritical bottom slope and/or the effects of bottom friction. The bottom slope is supercritical over the upper continental slope, suggesting that generation of the semidiurnal internal waves observed over the shelf may take place near the shelf break. Application of Baines' (1982) theory for the generation of internal tides underpredicts the observed baroclinic tidal currents.

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.

The Marine Layer off Northern California: An Example of Supercritical Channel Flow

Abstract During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which o...

Mass and heat budgets on the northern California continental shelf

The mass and heat budgets on the northern California continental shelf between Point Arena and Point Reyes are examined using moored measurements of horizontal water velocity and temperature made during the second Coastal Ocean Dynamics Experiment (CODE 2). Empirical orthogonal functions (EOFs) are employed to identify components of current and temperature variability with scales that are resolved by the observations. A constrained least squares fit to the EOF‐filtered data is used to find maps of velocity which conserve mass and maps of temperature which conserve heat integrated over control volumes. The wind during CODE 2 is characterized by strong southward “events” separated by “relaxations” of the wind. Currents are southward and offshore during events and northward and only slightly onshore during relaxations. Vertical velocity is estimated from the divergence of the massconserving horizontal velocity field. Upwelling occurs during wind events, with the strongest upwelling occurring nearshore and just to the south of Point Arena. Downwelling is observed during each relaxation but is too weak to be statistically significant. Water temperature rises gradually during relaxations and drops quickly near the beginning of events. The vertical turbulent heat flux at a depth z is estimated as the residual in the heat budget of a volume bounded by the surface and z after accounting for heat advection and surface heat flux. Statistically significant mixing is found at the beginning of two events and is followed by strong advective cooling. Close to shore, heat is advected primarily in the cross‐shore direction, while farther offshore, alongshore advection is also important. The pattern of vertical mixing followed by offshore advection, as observed near shore, is that predicted by two‐dimensional models of upwelling which include mixed‐layer dynamics.

Sea-floor gouges caused by migrating gray whales off northern California

Side-scan sonar records collected during March and April 1981 and 1982 off northern California contain elongate depressions whose sizes and shapes are similar to sea-floor gouges made by feeding gray whales ( Eschrichtius robustus) in the northern Bering Sea and in shallow embayments off British Columbia. The discovery of the whale gouges in the sonar records was unexpected, and supports some of the previous speculation that gray whales feed opportunistically during migration. Gouges occupy about 0.032% of the 7.6 km 2 of sea floor that was surveyed, which represents about 575 metric tons of excavated material. Although seemingly minor in amount, the total amount of bottom sediment removed from the central and northern California continental shelf by gray whale activities year after year represents macroscale biologically induced erosion and could have significant geological implications in shelf erosion and depositional schemes. This is the only published evidence of benthic feeding by gray whales along their migration route off northern California.

A description of subtidal pressure field observations on the Northern California Continental Shelf during the Coastal Ocean Dynamics Experiment

During the Coastal Ocean Dynamics Experiment (CODE) along the northern California continental shelf in the spring and summer of 1981 and 1982, the first comprehensive effort to obtain all of the observations necessary to infer pressure fluctuations throughout the water column was made. This is a description of the subtidal fluctuations in measured bottom pressure, internal pressures inferred from moored conductivity and temperature observations, and the water column total pressure anomalies computed from a combination of the two components. The pressure field during CODE 2 (April‐July 1982) is found to be more energetic than that in CODE 1 (April‐July 1981), with more than 70% total pressure variance concentrated at frequencies of less than (15 days)−1. The spatial structure of the pressure fluctuation field is characterized by coastal intensification and a tendency for poleward phase propagation. Like the pressures, most of the CODE across‐shelf pressure difference variance is found at the lower frequencies, with more than 60% of the total across‐shelf pressure difference variance at frequencies of less than (15 days)−1 The CODE across‐shelf pressure differences are found to be in geostrophic balance with both alongshelf currents and the total alongshelf transport. The alongshelf pressure difference variance is generally found to be concentrated at the lower frequencies much like that of the across‐shelf pressure differences. Most of the pressure and pressure difference variance is found to be highly coherent with wind stress. The 20‐day band pressure variability is apparently related to the response of the pressure field during so‐called “relaxation events.” The barotropic component of the alongshelf geostrophic transport (i.e., across‐shelf pressure difference) has been found to be almost perfectly coherent with regional wind stress τy and to propagate poleward within the CODE region at the same rate as τy and the alongshelf pressure gradient is very important in accelerating the alongshelf flow. The 5‐day band pressure variability in the CODE region appears to result from local wind stress fluctuations. The alongshelf pressure gradient was found to be of secondary importance relative to the wind in accelerating the alongshelf flow.

Moored wind, temperature, and current observations made during Coastal Ocean Dynamics Experiments 1 and 2 over the Northern California Continental Shelf and upper slope

Moored observations of winds, currents, and temperature made over the northern California shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the Coastal Ocean Dynamics Experiment (CODE) are described. In both field experiments (CODE 1 and CODE 2), the onset of the upwelling season is triggered by the strengthening of the North Pacific high off northern California and signalled by the presence of water colder than 9°C near the bottom over the shelf. Vertical temperature stratification over the shelf is reduced during active upwelling. Local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction, and the mean wind stress is directed equatorward. Response of alongshelf currents to wind forcing is similar at all depths over the shelf. The response of the cross‐shelf current is more complex. In the surface layer, offshelf flow is significantly and positively correlated with the upwelling‐favorable alongshelf wind stress. Beneath this layer a weak return flow exists. The thickness of the upper mixed layer varies greatly, reaching 60 m during upwelling events. Thus direct wind forcing affects a substantial fraction of the water over the shelf. Low‐frequency wind stress, current, and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure. The most energetic mode typically accounts for 75% or more of the variance of wind stress and temperature. Over the middle and inner shelf, the most energetic current mode accounts for 90% or more of the variance, is dominated by he alongshelf component, and veers to the left with depth. The low‐frequency alongshelf flow fluctuations are quasi‐barotropic, with some vertical shear but no reversal with depth; vertical shear in alongshelf current and the cross‐shelf density gradient obey the thermal wind relation. The simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross‐shelf transects in CODE 2. Volume transport over the shelf is often nearly two‐dimensional, but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope. Correlation of low‐frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf, where a 15 cm−1 alongshelf flow lags a 1 dyn cm−2 alongshelf wind stress by the order of 10 hours. Currents over the upper slope are not significantly correlated with local wind. Alongshelf transport over the shelf is highly correlated with local wind stress, explaining 80% of the variance in the low‐frequency transport during CODE 2. Volume transport is best correlated with wind stress approximately 30 km to the south. Winds in the CODE area during spring and summer are characterized by strong and persistent upwelling‐favorable winds interrupted by shorter periods of weak winds. Regression analysis indicates that in the absence of wind, a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the CODE area. An upwelling‐favorable wind counters this jet over the shelf, so that the alongshore flow is everywhere equator‐ward during strong wind events. Since the maximum response to wind forcing is found at midshelf and near surface, the mean current over the upwelling season is strongly sheared in both the vertical and cross‐shelf directions. Mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward, while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward.

Tidal currents seaward of the Northern California Continental Shelf

Current measurements obtained from a sparse array of moorings deployed over the slope and the adjacent deep‐sea basin off northern California for 4 to 14 months have been analyzed for their tidal constituents. The M2 tidal currents were the strongest tidal constituent, containing 50% to 75% of the total current variance in the four major tidal bands. The semimajor axis of the M2 constituent ranged from 2 to 4 cm s−1 and tended to be aligned with the local topography. Internal tides, which were usually present in the M2 band, were often the dominant cross‐slope current component on the middle slope. At least a portion of the internal tidal current field had a steady phase that slowly changed over 1 to 2 months. The semimajor axes of the diurnal barotropic ellipses had amplitudes ranging from 0.7 to 1.2 cm s−1 and were oriented both parallel and perpendicular to the local topography. These tidal current measurements suggest that the barotropic diurnal currents propagate poleward as a combination of Kelvin and continental shelf waves and that the barotropic M2 tides propagate poleward primarily as a Kelvinlike wave. Battisti and Clarke's (1982) model, which uses coastal sea level data to predict M2 tidal currents, agrees reasonably well with the observations presented here, indicating that the model can be used to predict the M2 currents seaward of the middle slope off Northern California.