Edward P. Dever

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.

Coastal Perturbations of Marine-Layer Winds, Wind Stress, and Wind Stress Curl along California and Baja California in June 1999

Abstract Month-long simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with a horizontal resolution of 9 km have been used to investigate perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along the California and Baja California coasts during June 1999. The dominant spatial inhomogeneity of the wind stress and wind stress curl is near the coast. Wind and wind stress maxima are found in the lees of major capes near the coastline. Positive wind stress curl occurs in a narrow band near the coast, while the region farther offshore is characterized by a broad band of weak negative curl. Curvature of the coastline, such as along the Southern California Bight, forces the northerly flow toward the east and generates positive wind stress curl even if the magnitude of the stress is constant. The largest wind stress curl is simulated in the lees of Point Conception and the Santa Ba...

Statistical aspects of surface drifter observations of circulation in the Santa Barbara Channel

Argos-tracked drifters are used to study the near-surface circulation in the Santa Barbara Channel. The mean consists of a cyclonic cell in the western Santa Barbara Channel with weaker flow in the eastern Channel. Drifter mean velocities agree well with record means from near-surface current meters. At the eastern entrance to the channel, drifter velocities are biased toward outflow (eastward velocity) conditions. Drifter variability at synoptic and seasonal scales shows a tendency for upwelling and eastward flow in spring, a strong cyclonic circulation in summer, poleward relaxation in fall, and weak, variable circulation in winter. Drifter estimates of eddy stress divergence indicate advective terms play a secondary role in the mean surface momentum balance. Lagrangian time and space scales are about 1 day and under 10 km, respectively. The mismatch between Lagrangian and Eulerian timescales indicates advective terms are important to the fluctuating circulation.

Near‐surface trajectories off central and southern California

The near-surface circulation in the Santa Barbara Channel and off the coast of central and southern California is described based on 20 releases of drifters drogued 1 m beneath the surface from 12 sites within the channel at bimonthly intervals. This description includes small-scale features of the circulation which are not part of descriptions based on moored observations or of the statistics of the drifter releases. The eventual fate of drifters at long time intervals compared to the residence time in the channel (about 7 days) is also included. In the channel the trajectories document a persistent cyclonic circulation with a typical recirculation period between 3 and 5 days. In the spring, currents near the mainland are weaker than near the Channel Islands, and the overall flow is toward the southeast. Trajectories document the possibility for water parcels to leave the channel through the interisland passes. In the late fall and winter a poleward flow with velocities often exceeding 0.5 m s−1 is confined within 20 km of the mainland. Between these two seasons the cyclonic tendency is enhanced, although most of the drifters eventually migrate westward. The trajectories of drifters released at the same time from sites only 20 km apart can be remarkably different. Once the drifters migrate out of the channel, their trajectories can be grouped into a few patterns. In spring and summer, drifters tend to remain in the Southern California Bight. Their trajectories often remain close over extended periods, as if they were caught in convergence zones. In fall the drifters often are caught in a poleward current.

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).

Subtidal velocity correlation scales on the northern California shelf

Along- and cross-shelf correlation scales of subtidal cross-shelf (u) and along-shelf (v) velocities are estimated using moored records from several field programs over the northern California shelf. Over record lengths of 4–6 months, along-shelf correlation scales of v are greater than maximum mooring separations (60 km). In the cross-shelf direction, v is generally correlated between the 60 and 130 m isobaths (10–15 km separation). Along-shelf correlation scales of u are much smaller than those of v and are often not resolved by minimum mooring separations. Time series between November 1988 and May 1989 do resolve along-shelf correlation scales of near-surface u and indicate that they are 15–20 km. During this time the along-shelf correlation scale of near-surface u shows variability on a monthly scale. It is generally long (30 km or more) when correlation of u with wind stress is high and short (15 km or less) when correlation with wind stress is low. On at least one occasion, short along-shelf correlation scales coincide with the intrusion of an offshore mesoscale feature onto the shelf. Cross-shelf correlation scales of u are resolved for typical mooring separations. In general, u is correlated between the 90 and 130 m isobaths (7–13 km separation) and between the 60 and 90 m isobaths (∼5 km).

An evaluation of the thermal properties and albedo of a macrotidal flat

estimated heat capacity of the upper 0.1 m is 3.65 � 10 6 Jm � 3 K � 1 with a water content of 70%. Heat capacity decreases with depth to 2.96 � 10 6 Jm � 3 K � 1 at 0.4 m depth. Estimated thermal diffusivities are 0.47–0.63 � 10 � 6 m 2 s � 1 and 0.38–0.64 � 10 � 6 m 2 s � 1 in spring and summer, respectively. The calculated albedo is a strong function of the solar altitude and the atmospheric transmittance. Atmospheric transmittance is especially important to the albedo when the solar altitude is low. Seasonal mean albedos are 0.13 and 0.15 in spring and summer, respectively. The heat capacity and albedo values obtained above were verified by using them to make independent heat flux estimates at other stations. Estimates based on heat capacity were correlated to albedo-based heat flux estimates with an r 2 greater than 0.7.

Seasonal Cross-Shelf Flow Structure, Upwelling Relaxation, and the Alongshelf Pressure Gradient in the Northern California Current System*

AbstractMoored observations are used to investigate the seasonal change in vertical structure of the cross-shelf circulation at a midshelf location in the northern California Current System. A streamwise–normal coordinate system is employed to eliminate meander- and eddy-induced biases in the cross-shelf flow that are unaccounted for with an alternative, commonly applied approach. The resulting flow structure develops an organized pattern midway through the upwelling season. In particular, under upwelling-favorable conditions an onshore return layer occurs just beneath the offshore surface flow, and a third offshore-directed layer exists at depth that does not appear to satisfy Ekman dynamics (to within 9 m of the bottom). Both subsurface layers strengthen in time over the upwelling season. Mechanisms to explain the mean structure are evaluated, and it is suggested that the timing of the development and strengthening of both the interior onshore return flow and the offshore near-bottom layer are consisten...

Anomalous Near-Surface Low-Salinity Pulses off the Central Oregon Coast

From mid-May to August 2011, extreme runoff in the Columbia River ranged from 14,000 to over 17,000 m3/s, more than two standard deviations above the mean for this period. The extreme runoff was the direct result of both melting of anomalously high snowpack and rainfall associated with the 2010–2011 La Niña. The effects of this increased freshwater discharge were observed off Newport, Oregon, 180 km south of the Columbia River mouth. Salinity values as low as 22, nine standard deviations below the climatological value for this period, were registered at the mid-shelf. Using a network of ocean observing sensors and platforms, it was possible to capture the onshore advection of the Columbia River plume from the mid-shelf, 20 km offshore, to the coast and eventually into Yaquina Bay (Newport) during a sustained wind reversal event. Increased freshwater delivery can influence coastal ocean ecosystems and delivery of offshore, river-influenced water may influence estuarine biogeochemistry.

Response Characteristics of the VACM Compass and Vane Follower

Abstract Several simple laboratory experiments have been conducted to study the dynamic behavior of the vector-averaging current meter (VACM) compass and vane follower. They demonstrate that the behavior of the compass and vane follower can be modeled as a damped linear harmonic oscillator for small-amplitude forcing. The combined eddy-current and bearing-friction torque nearly critically damps the free oscillation of the compass and vane follower. This frictional torque is proportional to the angular-velocity difference between the instrument magnet assembly and housing. Dynamic experiments on five compasses indicate a mean (undamped) resonant period of 3–5 s at 41°N. Similar experiments on two vane followers indicate a resonant period of 2–3 s. For the VACM dynamic compass experiments, frictional torque allowed an angular oscillation of the compass housing to drive an oscillation of the compass magnet, and at resonant forcing, the compass magnet oscillates exactly in phase with its housing. For the VACM...