Barbara M. Hickey

The California current system—hypotheses and facts☆

The primary purpose of this paper is to describe the seasonal variation of the various currents which comprise the California Current System—the California Current, the California Undercurrent, the Davidson Current and the Southern California Countercurrent—and to investigate qualitatively the dynamical relationships among these currents. Although the majority of information was derived from existing literature, previously unpublished data are introduced to provide direct evidence for the existence of a jet-like Undercurrent over the continental slope off Washington, to illustrate ‘event’-scale fluctuations in the Undercurrent and to investigate the existence of the Undercurrent during the winter season. The existing literature is thoroughly reviewed and synthesized. In addition, and more important, geostrophic velocities are computed along several sections from the Columbia River to Cape San Lazaro from dynamic heights given by Wyllie (1966), Budinger, Coachman and Barnes (1964), and Reed and Halpern (1976). From these data and from long-term monthly wind stress data and vertical component of wind stress curl data (denoted curl τ) given by Nelson (1977), interesting new conclusions are made. 1. The flow that has been denoted the California Current generally has both an offshore and a nearshore maximum in its alongshore coponent. 2. The seasonal variation of the nearshore region of strong flow appears to be related to the seasonal variation of the alongshore component of wind stress at the coast, τyN, at all latitudes. Curl τ near the coast may also contribute to the seasonal signal, accounting for the lead of maximum current over maximum wind stress from about 40°N northward. Large-scale flow separation and fall countercurrents that of headlands may account for the sudden occurrence of late summer and fall countercurrents that appear as large anomalies from the wind-driven coastal flow south of 40°N. 3. From Cape Mendocino southward a northward mean is imposed on the nearshore current distribution. The mean is largest where curl τ is locally strongest, in particular, off and south of San Francisco and in the California Bight. It may be responsible for the portion of the Davidson Current that occurs off California, for the San Francisco Eddy and for the Southern California Eddy or Countercurrent. When southward wind stress weakens in these regions, the northward mean dominates the flow. Flow separation in the vicinity of headlands may also be responsible for these northward flows. There is some evidence that during periods of northward flow a mean monthly τyN-driven southward current occurs inshore of the mean northward flow. At all latitudes, wind-driven ‘event’-scale fluctuations are expected to be superimposed on the seasonal nearshore flow. 4. The spatial distribution and seasonal variation oftthe offshore region of southward flow appear to be related to the spatial distribution and seasonal variation of curl τ. The seasonal variation of curl τ in these areas, curl τl, is roughly in phase with the seasonal variation of τy near the coast and roughly 180° out of phase with the seasonal variation of curl τ near the coast. Southward flow lags negative curl τ by from two to four months. The offshore region of southward flow is strongest during the summer and early fall. The mean annual location of the maximum flow is at about 250–350 km from shore off Washington and Oregon, and at 430 km off Cape Mendocino, 270 km off Point Conception and 240 km off northern Baja. The offshore branch of the flow bends shoreward near 30°N, which is consistent with the shoreward extension of the region of negative curl τ, so that by Cape San Lazaro (25°N), a single region of strong flow is observed within 200 km of the coast. 5. A third region of strong southward flow occurs at distances exceeding 500 km from the coast. The spatial distribution of this flow appears to be related to the spatial distribution of curl τ. 6. The mean northward flow known as the Davidson Current consists of two regions in which the forcing may be dynamically different—seaward of the continental slope off Washington and Oregon and between Cape Mendocino and Point Conception, the mean monthly northward currents appear to be related to the occurrence of positive curl τ; along the coast of Oregon and Washington the northward currents are not related to the occurrence of positive curl τ but are consistent with forcing by the mean monthly northward wind stress at the coast. 7. A region of southward flow that is continuous with the California Current to the south is generally maintained off Oregon and parts of Washington during the winter. This southward flow appears to separate the northward-flowing Davidson and Alaskan Currents in some time-dependent region south of Vancouver Island. The banded current structure is consistent with the distribution of curl τ, if southward flow is related to negative curl τ. 8. The seasonal progression of the California Undercurrent may be related both to the seasonal variation of the offshore region of strong flow (hence to curl τl) and to the alongshore component of wind stress at the coast. South of Cape Mendocino a northward mean also seems to be superimposed on the flow. This mean may be related to the occurrence of strong positive curl τ near the coast. Velocities at Undercurrent depths have two maxima, one in late summer and one in winter. The slope Undercurrent is indistinguishable, except by location, from the undercurrent that is observed on the Oregon-Washington continental shelf.

Oceanography of the U.S. Pacific Northwest Coastal Ocean and Estuaries with Application to Coastal Ecology

Ocean processes are generally large scale on the U.S. Pacific Northwest coast; this is true of both seasonal variations and event-scale upwelling-downwelling fluctuations., which are highly energetic. Coastal upwelling supplies most of the macronutrients available for production, although the intensity of upwelling-favorable wind forcing increases southward while primary production and chlorophyll are higher in the north, off the Washington coast. This discrepancy could be related to several mesoscale features: the wider, more gently sloping shelf to the north, the existence of numerous submarine canyons to the north, the availability of Columbia River plume water and sediment north of the river mouth, and the existence of a semi-permanent eddy offshore of the Strait of Juan de Fuca. We suggest that these features have important effects on the magnitude and timing of macronutrient or micronutrient delivery to the plankton. These features are potentially important as well to transport pathways and residence times of planktonic larvae and to the development of harmful algal blooms. The coastal plain estuaries, with the exception of the Columbia River, are relatively small, with large tidal forcing and highly seasonal direct river inputs that are low to negligible during the growing season. Primary production in these estuaries is likely controlled not by river-driven stratification but by coastal upwelling and exchange with the ocean. Both baroclinic mechanisms (the gravitational circulation) and barotropic ones (lateral stirring by tide and, possibly, wind) contribute to this exchange. Because estuarine hydrography and ecology are so dominated by ocean signals, the coastal estuaries, like the coastal ocean, are largely synchronous on seasonal and event time scales, though, intrusions of the Columbia River plume can cause strong asymmetries between Washington and Oregon estuaries especially during spring downwelling conditions. Water property correlation increases between spring and summer as wind forcing becomes more spatially coherent along the coast. Estuarine habitat is structure not only, by large scale forcing but also by fine scale processes in the extensive intertidal zone, such as by solar heating or differential advection by tidal, curents.

The Columbia River Plume Study: Subtidal variability in the velocity and salinity fields

A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonstrates that the plume has two basic structures during the fall/winter season, namely, a thin (∼5–15 m), strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker (∼10–40 m), weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind fluctuations have timescales of 2–10 days. Frictional wind-driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds, respectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In particular, during periods when the plume is separated from the coast, currents tend to flow around the mound of fresher water. At distances exceeding about 20 km from the river mouth, the along-shelf depth-averaged flow over the inner to midshelf is linear, and depth-averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along-shelf geostrophic buoyancy-driven currents at ∼5 m (calculated from surface density) and along-shelf geostrophic wind-driven currents (computed from a depth-averaged linear model) are comparable in magnitude (∼10–25 cm s−1).

Circulation over the Santa Monica-San Pedro Basin and Shelf

Abstract The spatial and temporal structure of the circulation in Santa Monica-San Pedro basin and over the adjacent mainland shelf has been described in a series of experiments that included moored arrays of currents meters, hydrographic surveys, satellite-derived sea surface temperature maps and Lagrangian drifter deployments. This basin, located in the Southern California Bight adjacent to the coast, is roughly 100km long, 40km wide and 900m deep. From the sea surface to a depth of about 250m, the basin is open to the San Diego Trough to the southeast, the Santa Barbara Channel to the northwest, and the Santa Cruz basin to the west. The resulting data set is the first spatially comprehensive data set over the continental slope of the US west coast, the first comprehensive data set in a coastal region of such complicated topography, and the first that includes detailed measurements over a semi-enclosed shelf, namely, Santa Monica bay. The most significant contributions of the research based on this data set are: (1) demonstration of the alteration of a low mode coastal-trapped wave by a sharp bend in topography; (2) demonstration that fluctuating currents on a semi-enclosed shelf can be driven by the flow along its open boundary; (3) demonstration that topographic waves exist in the completely enclosed portion of coastal basins. In addition, the research allowed an order of magnitude improvements in regional knowledge of both the seasonal and subtidal scale fluctuations within the Southern California Bight. For example, on seasonal scales, the winter surfacing of the California undercurrent (the dominant feature of the seasonal mean flow field) as well as the continuity of the undercurrent in a spatially limited region were addressed. The existence of an equatorward undercurrent at mid water column depths (300–500m) over the continental slope during the late summer to fall season was also documented. On subtidal scales, the data demonstrated that the velocity field is far from “quiescent” over the basin, even below the depth of the deepest basin sill. Spatially-organized subtidal fluctuations occur at all depths and the time scale of the dominant fluctuations (∼ 20–30d) is a factor of two or more longer than that typical of most coastal shelves. The fluctuations have a subsarface maximum during most of the year and the maximum is best developed in late summer and over the mainland slope of the basin. Propagation characteristics south of the basin and within the basin along its coastal perimeter are consistent with those of first mode freely propagating coastal-trapped waves at these long periods. The long period waves enter the basin from southeast and then appear to pivot conterclockwise around the basin, so that at least some porion of the waves exits the basin over its western sill. The phase speed of the waves is reduced by an order of magnitude over all but the coastal slope of the basin. Below sill-depth velocity and temperature fluctuations appear to be the signature of freely propagating basin-scale topographic waves driven by the long period upper water column fluctuations. The waves travel counterclockwise around the lower basin at speeds about 25cm s −1 . Current fluctuations over the adjacent inner Santa Monica shelf have significant variance at shorter periods as well as at the longer periods observed over the basin and its slopes. These fluctuations are driven predominantly by boundary forcing over the outer shelf and only secondary by local wind stress. Wind data suggests that although local upwelling within the Bight is extremely limited from late spring through fall, strong upwelling events occur routinely in winter early spring. During the summer to fall months effects of upwelling may be experienced within the Bight, but these effects are primarily the results of upwelling outside the Bight in combination with lateral advection. During periods of strong upwelling outside the Bigth, outflow from Santa Monica basin appears to be preferentially directed south of the Channel Islands rather than through the Santa Barbara Channel.

The Response of a Steep-Sided, Narrow Canyon to Time-Variable Wind Forcing

Abstract The response of a relatively narrow (∼7 km wide) and deep (∼450 m deep) steep-sided (up to 45° bottom slope) submarine canyon to strong wind forcing is explored using data from an 18-element moored array as well as CTD surveys in the vicinity of Astoria submarine canyon. The data are used to describe spatial patterns and phase relationships between lateral velocity, vertical velocity, temperature, relative and stretching vorticity, alongshelf wind, and the flow incident on the canyon. Upwelling within the canyon is simultaneous and spatially uniform to zero order, and vertical velocity is highly correlated and in phase with alongshelf wind. Vertical velocity within the canyon is not related to flow incident on the canyon except during strong upwelling. Above the canyon, temperature, rather than vertical velocity (time rate of change of temperature), is in phase with wind. Estimated vertical velocities within the canyon were as great as 50 m d−1 (upward) during upwelling and 90 m d−1 (downward) du...

River Influences on Shelf Ecosystems: Introduction and synthesis

[1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ‘‘shadow zone’’ off northern Oregon.

Suspended particle movement in and around Quinault submarine canyon

Abstract Time- and space-dependent light transmission, temperature, salinity, and velocity data obtained during two winter experiments in the vicinity of Quinault submarine canyon are used to examine processes controlling subtidal ( cpd ) fluctuations in suspended particulate concentration in the region; in particular, local resuspension of particles by waves and currents, horizontal advection and vertical advection. Data include time series from four to eight mooring locations (as many as 16 current meters and 10 transmissometers) on the shelf, over the open slope, and in the canyon during each experiment, spatial distributions from five CTD/transmission surveys, and gravity wave information. Particle concentration in the bottom boundary layer at the shelf edge was dominated by resuspension events. The mechanism for resuspension had a large inter-annual variation; in the first year, wave-induced resuspension dominated, whereas, in the second year, current-induced resuspension dominated. The cut-off depth for storm-related resuspension was generally m . Subtidal advection across isobaths (upwelling/downwelling) accounted for ∼20% of the variance in concentration fluctuations at the shelf edge in the bottom boundary layer. Horizontal advection of material suspended in the bottom nepheloid layer on the shelf resulted in the formation of shelf break depth intermediate nepheloid layers over the canyon in regions where the flow crosses isobaths. The existence, strength, onshore—offshore extent, and vertical structure of such nepheloid layers at a given time in this asymmetrical canyon appear to be a function of the direction of flow over the outer shelf and over the upper slope in the canyon; whether active resuspension is occurring over the shelf and the maximum resuspension depth and magnitude of the active resuspension event; the frequency, timing, and magnitude of previous resuspension events; spatial and temporal continuity, magnitude and direction of flow over the open slope north and south of the canyon; vertical settling; and upwelling or downwelling related to the along-isobath flow over the upper slope. Fluctuations in particle concentration below the shelf break depth over the canyon slope were controlled by vertical, rather than horizontal, advection. Vertical excursions by the water mass of ±100 m occurred both as a result of isopycnal adjustment to the quasi-geostrophic along-isobath flow and as a result of semidiurnal tidal oscillations. When an overhead nepheloid layer was present, such water mass excursions produced concentration fluctuations at depths below the maximum depth of storm-related local resuspension. In open slope regions, where strong INLs (= intermediate depth nepheloid layers ) were absent, the portion of concentration variance that could be explained by advective models with constant spatial gradients was almost negligible. Only one resuspension event was documented in the bottom boundary layer within the deep canyon (bottom depth ⩾1000 m ), suggesting that advection must account for the observed variance in particle concentration. However, the large-scale gradients at these depths are sufficiently weak, cf. the smaller-scale gradients or “noise”, that advective control could only be demonstrated during the second winter experiment when velocities along the canyon axis were relatively large. Finally, the data demonstrate that subtidal concentration fluctuations within 50 m of the canyon floor are correlated to those at the shelf edge. This correlation reflects a correlation in the velocity field rather than a transfer of particles, as might occur with a turbidity current.

Application of Remote Wind-Forced Coastal Trapped Wave Theory to the Oregon and Washington Coasts

Abstract The theory of coastal trapped waves generated by remote wind forcing (Clarke) is used to calculate coastal subsurface pressure (SSP) and longshore velocity along the Oregon and Washington coasts for three two-month periods: summer of 1972, summer of 1978 and winter of 1977. The response in SSP and longshore velocity is assumed to be dominated by the mode one wave. In every case, coherence squared between observed and modeled SSP is significant at the 95% level over the entire low frequency band (≤0.2 cpd) with an average phase difference less than ±30°. Greater than 80% of the variance in coastal SSP is accounted for by the mode one coastal trapped wave (CTW). The SSP response off Washington and Oregon during summer is primarily (∼35% of the variance) a result of wind forcing between San Francisco and Cape Mendocino, California. Wind stress in this region during summer is significantly larger than that off Oregon and Washington at low frequencies so that the CTW generated off California propagate...

Dynamics of Willapa Bay, Washington: A Highly Unsteady, Partially Mixed Estuary

Results from 3 yr of hydrographic time series are shown for Willapa Bay, Washington, a macrotidal, partially mixed estuary whose river and ocean end members are both highly variable. Fluctuating ocean conditions— alternations between wind-driven upwelling and downwelling, and intrusions of the buoyant Columbia River plume—are shown to force order-of-magnitude changes in salinity gradients on the event (2‐10 day) scale. An effective horizontal diffusivity parameterizing all up-estuary salt flux is calculated as a function of riverflow: results show that Willapa’s volume-integrated salt balance is almost always far from equilibrium. At very high riverflows (the top 15% of observations) the estuary loses salt, on average, while at all other riverflow levels it gains salt. Under summer, low-riverflow conditions, in fact, the effective diffusivity K is large enough to drive a net increase in salinity that is 3‐6 times the seaward, river-driven salt flux. This diffusion process is amplified, not damped, by increased tidal forcing, contrary to the expectation for baroclinic exchange. Furthermore, K varies along the length of the estuary as ;5% of the rms tidal velocity times channel width, a scaling consistent with density-independent stirring by tidal residuals. To summarize Willapa’s event- and seasonal-scale variability, a simple diagnostic parameter space for unsteady estuarine salt balances is presented, a generalization from the Hansen and Rattray steady-state scheme.

Sedimentation dynamics in the Santa Monica-San Pedro Basin off Los Angeles: radiochemical, sediment trap and transmissometer studies

Abstract A large number of sediment cores and sediment trap samples collected from different parts of the Santa Monica-San Pedro (SM-SP) Basin during 1985–1988 were studied for radionuclides, trace metals and other sedimentary components. The radiochemical data are presented here to give a basin-wide view of the sedimentation dynamics. 210 Pb stratigraphy indicated that sedimentation rates were higher and more variable (30–80 mg cm −2 y −1 ) in the more dynamic slope region, but were uniformly low (13.4–18.8 mg cm −2 y −1 ) in the flat, deep basin. From the sediment record, it was suggested that sedimentation rates were decreasing and the area of anoxia had been expanding, at least during the past one to two centuries. Turbidite layers found in the sediment cores suggested higher frequency and more recent occurrence toward the basin margins. 210 Pb geochronologies indicated that the recent turbidites might be related to major storms which occurred during the past two decades. Sediment traps deployed in the basin recorded very large short-term spatial and temporal variabilities of mass flux, with unusually high fluxes corresponding to recorded large wave events. Trap-measured near-bottom mass fluxes averaged over all collection periods (622 days) were consistent with 210 Pb-based sediment accumulation rates. With few exceptions, trap-measured fluxes decreased offshore but increased with depth at any location, strongly suggesting lateral input of materials. Transmissometer data demonstrated the existence of nepheloid plumes off the eastern slope of the SM-SP Basin. The offshore decrease of sedimentation rate in the eastern part of the SM Basin was consistent with the fact that nepheloid plumes were confined to the basin slopes and that the major transport pathway of suspended particles, as indicated via progressive vector analysis, was alongshore from the SP Basin toward the SM Basin. Concentrations and fluxes of radionuclides measured in the near-bottom trap in the deep basin compared favorably with those registered in bottom sediments. Based on water column, sediment trap and sediment core data, self-consistent flux balances can be constructed for 228 Th and 210 Pb. Flux balances for 234 Th were less well defined. The cyclic pattern of uranium profiles in deep basin sediments appeared to be in phase with the sedimentary record of CaCO 3 and the historical record of primary production and anchovy biomass. It is suggested that the removal of uranium from the water might be regulated by longterm regional changes in biological processes and sedimentation environments.