Results are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (10(-6) s(-1)), the second in a region of intermediate horizontal strain rate (10(-5) s(-1)). Both experiments lasted 6 days, covering spatial scales of 1-10 and 1-50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were kappa(z) = (2-5) x 10(-6) m(2) s(-1), while isopycnal diffusivities were kappa(H) = (0.2-3) m(2) s(-1), with the range in kappa(H) being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of 5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3-5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.

Abstract. Numerical solutions of the Korteweg-de Vries (KdV) and extended Korteweg-de Vries (eKdV) equations are used to model the transformation of a sinusoidal internal tide as it propagates across the continental shelf. The ocean is idealized as being a two-layer fluid, justified by the fact that most of the oceanic internal wave signal is contained in the gravest mode. The model accounts for nonlinear and dispersive effects but neglects friction, rotation, and mean shear. The KdV model is run for a variety of idealized stratifications and unique realistic topographies to study the role of the nonlinear and dispersive effects. In all model solutions the internal tide steepens forming a sharp front from which a packet of nonlinear solitary-like waves evolves. Comparisons between KdV and eKdV solutions is explored. The model results for realistic topography and stratification are compared with observations made at moorings off Massachusetts in the Mid Atlantic Bight. Some features of the observations compare well with the model. The leading face of the internal tide steepens to form a shock like front, while nonlinear high frequency waves evolve shortly after the appearance of the jump. Although not rank ordered, the wave of maximum amplitude is always close to the jump. Some features of the observations are not found in the model. Nonlinear waves can be very widely spaced and persist over a tidal period.

Numerical solutions of the Korteweg–de Vries (KdV) and extended Korteweg–de Vries (eKdV) equations are used to model the transformation of a sinusoidal internal tide as it propagates across the continental shelf. The ocean is idealized as being a two-layer fluid, justified by the fact that most of the oceanic internal wave signal is contained in the gravest mode. The model accounts for nonlinear and dispersive effects but neglects friction, rotation and mean shear. The KdV model is run for a number of idealized stratifications and unique realistic topographies to study the role of the nonlinear and dispersive effects. In all model solutions the internal tide steepens forming a sharp front from which a packet of nonlinear solitary-like waves evolve. Comparisons between KdV and eKdV solutions are made. The model results for realistic topography and stratification are compared with observations made at moorings off Massachusetts in the Middle Atlantic Bight. Some features of the observations compare well with the model. The leading face of the internal tide steepens to form a shock-like front, while nonlinear high-frequency waves evolve shortly after the appearance of the jump. Although not rank ordered, the wave of maximum amplitude is always close to the jump. Some features of the observations are not found in the model. Nonlinear waves can be very widely spaced and persist over a tidal period.

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.

Examining fluxes of biogeochemical constituents at the mouth of an estuary is necessary for assessing the modification of terrigenous‐source materials in the estuary prior to reaching the ocean. In many rivers and estuaries, including the Columbia River estuary (CRE), methane is highly enriched with respect to oceanic concentrations and the equilibrium solubility of the atmospheric gas. We developed a methane budget for the CRE to examine the potential for significant modification of the estuarine methane budget by lateral exchange with peripheral tide flats. We accomplished the challenging task of constraining the net transfer through the estuary‐ocean interface using novel instrumentation: a rapid methane analyzer combined with a membrane‐contactor interfaced with a pumped‐sampling undulating towed vehicle. Transport of riverine methane into the CRE was essentially balanced by losses due to flux to the atmosphere (42%), microbial oxidation in the water column (21%), and transport to the ocean (32%), suggesting limited net effect of lateral tide flat processes on the CRE methane budget. Estimated uncertainty bounds constrained lateral sink/source terms within −30% to +20% of the primary river input. This result contrasts with a number of prior studies of methane cycling in estuaries that reported dominant contributions from lateral sources and relatively minor export to the coastal ocean. The magnitude of lateral supply of methane is a useful indicator of the hydrologic source potential of other related signals of organic matter remineralization from anoxic or suboxic settings in the estuary.

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1-10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m(2) s(-1) as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

Surface tides are the heartbeat of the ocean. Because they are controlled by Earth's motion relative to other astronomical objects in our solar system, surface tides act like clockwork and generate highly deterministic ebb and flow familiar to all mariners. In contrast, baroclinic motions at tidal frequencies are much more stochastic, owing to complexities in how these internal motions are generated and propagate. Here, we present analysis of current records from continental margins worldwide to illustrate that coastal internal tides are largely unpredictable. This conclusion has numerous implications for coastal processes, as across-shelf exchange and vertical mixing are, in many cases, strongly influenced by the internal wave field.

Moored current, temperature, and conductivity measurements are used to study the temporal variability of M-2 internal tide generation above the Kaena Ridge, between the Hawaiian islands of Oahu and Kauai. The energy conversion from the barotropic to baroclinic tide measured near the ridge crest varies by a factor of 2 over the 6-month mooring deployment (0.5-1.1 W m(-2)). The energy flux measured just off the ridge undergoes a similar modulation as the ridge conversion. The energy conversion varies largely because of changes in the phase of the perturbation pressure, suggesting variable work done on remotely generated internal tides. During the mooring deployment, low-frequency current and stratification fluctuations occur on and off the ridge. Model simulations suggest that these variations are due to two mesoscale eddies that passed through the region. The impact of these eddies on low-mode internal tide propagation over the ridge crest is considered. It appears that eddy-related changes in stratification and perhaps cross-ridge current speed contribute to the observed phase variations in perturbation pressure and hence the variable conversion over the ridge.

The capability to perform autonomous, long-term, reliable, multi-parameter vertical profiling of the coastal oceans has been clearly stated as a high priority need to support the oceanographic research and ocean observatory communities. Through a collaborative effort bringing together the scientific and technical expertise of the College of Oceanic and Atmospheric Sciences at Oregon State University with the engineering and manufacturing expertise of WET Labs, we have developed the Coastal Autonomous Profiling and Boundary Layer System (CAPABLE). CAPABLE consists of a compact, hydrodynamic, multi-instrumented, self-contained profiler termed the Extended Endurance Autonomous Moored Profiler (X10 AMP), a docking station/bottom boundary layer sampling system termed the Shallow Coastal Upward Looking Profiler Integration Node (SCULPIN), and a nearby Surface Boundary Buoy (SBB) for data transmission. The X10 AMP includes a suite of physical, biological, chemical and optical sensors with integrated antifouling devices, a platform control system, a rechargeable power system, and a remote telemetry unit. The X10 AMP design is based on a winch-on-board concept, where all components of the profiler are contained within the platform. The X10 AMP design combines vertical hydrofoil shape and a spar buoy weight arrangement to consistent vertical orientation for the platform while profiling. These two features provide a stable profiling platform from which to conduct fine scale vertical resolution measurements. The primary control and power electronics for the X10 AMP, winch system, winch motor control, telemetry system, and inductive charging/communication system are all located within a single pressure housing located on the leading edge of the platform. The primary package control electronics of the X10 AMP provides for power distribution, regulation and fault monitoring, instrument power and data acquisition, platform command and control, telemetry system interface and power control, winch system interface, and data storage and transfer. The SCULPIN includes a large capacity power supply, a recharging system for the profiler, an upward-looking acoustic Doppler current profiler (ADCP), and an acoustic release mechanism. The inductive charging/communications docking system is comprised of two aluminum components: a docking stinger mounted on the X10 AMP and a docking drogue mounted on the SCULPIN. When the X10 AMP is docked, and inductive link is established between the X10 AMP and the SCUPLIN through which the on-board battery pack on the X10 AMP are recharged. The design also provides for a communication link to monitor the charging system and allow communication between the X10 AMP and SCULPIN when docked. The SBB provides an acoustic communication link to the SCULPIN, and also communicates via cellular phone to shore. Several communication pathways exist within the CAPABLE system to insure remote control and confi 1 guration of the system as well as data offload. The CAPABLE system has been deployed and tested in the coastal waters off of Oregon, and we present a description of the system as well as data collected during these field deployments. CAPABLE provides the capability to sample the physical (pressure, temperature, salinity, currents), biological (chlorophyll fluorescence), chemical (dissolved oxygen, nitrate concentrations, and colored dissolved organic material fluorescence), and optical (backscattering) properties over the entire water column which are critical for assessing the state of the coastal ecosystem and for monitoring changes due to natural and anthropogenic forcing.

Abstract : Horizontal dispersion of momentum and scalar quantities by submesoscale processes in the ocean remains an under-studied topic. The physical forcings that potentially contribute to horizontal dispersion are many, such as, wind stress, frontal dynamics, river input and internal waves. Typically, regional numerical models use simple horizontal diffusion to parameterize the complex horizontal dispersion, producing variability at horizontal scales of 100 m to 1 km and time scales of hours to days. To improve model parameterization of horizontal dispersion, we need additional observations and a better understanding of the processes involved. The focus is on tasks that are intended to help in the planning of a larger coordinated effort. The specific components of the proposed research include: (1) participation in a planning study group to develop a coordinated multi-year program. (2) testing of our dye tracking equipment and instrumentation and assessment of their condition.

A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M-2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01 degrees-horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model's sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M-2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and south of Niihau and Kauai. The majority of the baroclinic energy (1.7 GW) is radiated out of the model domain, while 0.45 GW is dissipated close to the generation regions. The modeled baroclinic dissipation within the 1000-m isobath for the Kaena Ridge agrees to within a factor of 2 with the area-weighted dissipation from 313 microstructure profiles. Topographic resolution is important, with the present 0.01 degrees resolution model resulting in 20% more barotropic-to-baroclinic conversion compared to when the same analysis is performed on a 4-km resolution simulation. A simple extrapolation of these results to the entire Hawaiian Ridge is in qualitative agreement with recent estimates based on satellite altimetry data.

Observations, from the Oregon continental shelf, describe the slumping of a coastal upwelling front in response to a reversal of winds from upwelling‐ to downwelling‐favorable. Initially, the front outcropped in a surface mixed layer of depth 10–20 m with a pronounced cross‐shelf density gradient. Following wind reversal, both the unbalanced cross‐shelf pressure gradient and wind‐driven Ekman transport drove a rapid onshore movement of near‐surface water, causing the mixed layer to restratify. At the surface, the cross‐shelf density gradient steepened to become a discontinuous front, which propagated onshore at 0.43–0.60 m s −1 as the head of a buoyant gravity current. Internal waves were driven ahead of the front on the base of the former mixed layer. An injected dye tracer revealed that surface water from inshore of the strongest frontal gradient detached from the surface as the gravity current passed over the top of it. This water largely retained the low potential vorticity signature that it had taken on in the mixed layer as it spread across the shelf in a weakly stratified, subsurface layer. Restratification and frontogenesis were most likely gravity‐driven in the first hours following wind reversal, with the contribution from Ekman transport becoming increasingly significant.

The upwelling‐driven coastal jet off Oregon is in geostrophic balance to first order. The accompanying thermal wind shear is stable to shear instability. Yet enhanced turbulence is observed in the upwelling jet, typically as long, thin patches with horizontal to vertical aspect ratios of 102 to 103 (median value ∼300). These patches are clearly defined by regions of low Richardson number and occur where and when the linear superposition of the three dominant shear constituents (near‐inertial, M2, and thermal wind) interferes constructively. This is most pronounced at the base of the coastal jet, where the thermal wind shear is largest. While the effect of the turbulence stress divergence on the jet is small compared to geostrophy (∼1%), it is significant in the second‐order force balance governing secondary circulation. The timescale associated with the decay of the thermal wind shear via turbulence stress is O(10) days. We confirm that the vertical salt flux due to mixing is comparable to the net Ekman transport of salt onto the shelf within the bottom boundary layer. Because numerical models of coastal circulation lack turbulence in midwater column, any vertical transport of scalars, including salt and heat, must be achieved inshore of the 40‐m isobath. This is inconsistent with the observations presented in this study, in which significant vertical turbulent salt transport is found to exist across the entire shelf.

Detailed observations of the Ekman spiral in the stratified bottom boundary layer during a 3‐month period in an upwelling season over the Oregon shelf suggest a systematic organization. Counter‐clockwise veering in the bottom boundary layer is constrained to the weakly stratified layer below the pycnocline, and its height is nearly identical to the turbulent boundary layer height. Veering reaches 13+/−4 degrees near the bottom and exhibits a very weak dependence on the speed and direction of the interior flow and the thickness of the veering layer. A simple Ekman balance model with turbulent viscosity consistent with the law‐of‐the‐wall parameterization modified to account for stratification at the top of the mixed layer is used to demonstrate the importance of stratification on the Ekman veering. The model agrees reasonably well with observations in the lower 60–70% of the bottom mixed layer, above which it diverges from the data due to the unaccounted physics in the interior. Neglect of stratification in an otherwise identical model results in far worse agreement with the data yielding veering in the bottom Ekman layer which is much smaller than measured, but distributed over a much thicker layer.

Tidal mixing over a slope was explored using moored time series observations on Kaena Ridge extending northwest from Oahu, Hawaii, during the Survey component of the Hawaii Ocean Mixing Experiment (HOME). A mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453-m water depth for 2 months beginning in November 2000. The semidiurnal barotropic tidal currents at this site have a significant cross-ridge component, favorable for exciting an internal tidal response. A large-amplitude response is expected, given that the slope of the topography (4.5) is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency. Density overturns were inferred from temperature profiles measured every 2 min. The number and strength of the overturns are greater in the 200 m nearest the bottom, with overturns exceeding 24 m present at any depth nearly 10% of the time. Estimates of turbulent dissipation rate epsilon were made for each overturn by associating the measured Thorpe scale with the Ozmidov scale. The average epsilon between 1300 and 1450 m for the entire experiment is about 10(-8) m(2) s(-3), corresponding to an average K-rho of 10(-3) m(2) s(-1). Both epsilon and K-rho decrease by about an order of magnitude by 1200 m. The occurrence of overturns and the magnitude of epsilon are both highly correlated with the tide: both with the spring-neap cycle as well as the phase of the semidiurnal tide itself. Dissipation rate varies by at least an order of magnitude over the spring-neap cycle. It appears that tidal frequency vertical shear within 200 m of the boundary leads to significant strain (vertical divergence). Most of the overturns occur during the few hours when the vertical strain is greatest. The buoyancy frequency N calculated from reordering these overturns is a factor of 3 lower than the background (N) over barN. This is consistent with the following kinematic description: the internal tide first strains the mean density field, leading to regions of low N that subsequently overturn. Less regularly, overturns also occur when the internal tide strain has created relatively high stratification within 200 m of the bottom.

A fluorescent dye tracer was injected into the pycnocline on the Oregon shelf at a depth of 9–10 m. It spread rapidly cross‐shelf as two distinct layers, one above the other in the water column, split by interleaving dye‐free water. The vertical scale of these layers, and associated density steps, was 1–2 m, and the horizontal extent of interleaving exceeded 1.6 km after an inertial period. The upper dye layer was sharply peaked and embedded in a strong vertical density gradient. The lower layer was slab‐like and associated with weak stratification. Both layers were inclined slightly in density space. It is proposed that internal wave‐induced mixing and the lateral collapse of mixing patches were important mechanisms. Analogies can be drawn between these dye structures and frequently‐observed thin planktonic layers. By approximating the dye dispersion as a Fickian process, estimated isopycnal and diapycnal eddy diffusivities of κ x = 4.1 m 2 s −1 and κ z = 1.4 × 10 −5 m 2 s −1 are obtained.