Implementation of The South Atlantic Bight Synoptic Offshore Observational Network

Continuous information on coastal ocean conditions in the South Atlantic Bight (SAB), provided by an interdisciplinary real-time coastal ocean observing network, is now accessible. Many' applications are envisioned. Academic researchers will use the network for targeted research and as a test bed for sensor development. The National Weather Service will use the continuously available meteorological data to predict, more accurately, weather conditions affecting the eastern seaboard of the U.S. Resource managers will be able to manage, more effectively, the use of the SAB coastal waters. The network can also serve as the nucleus of an observing system that will provide in situ observations for a nowcast/ forecast regional coupled ocean/atmosphere model of the SAB

The importance of aspiration and channel curvature in producing strong vertical mixing over a sill

On the basis of observations of the time-dependent, tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing. Aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast. Channel curvature induces a cross-channel circulation that can rapidly overturn a stratified flow. Eighteen along-channel sections of density, velocity, and dissipation rate of turbulent kinetic energy c were collected in and around the Tacoma Narrows of Puget Sound, a site suspected of driving a strong vertical circulation in adjoining Main Basin. Rapid inflow to the Narrows on flood from one channel of a triple junction reduces dynamic pressure, allowing dense water from below sill depth to be uplifted, or aspirated, into the Narrows. We estimate water from below 150 m, 3 times the sill depth, is drawn into the Narrows on a 3-m flood tide. Once in the Narrows the flow remains stratified until it passes a 50° bend where a strong secondary circulation overturns the 50-m-deep water column and generates intense turbulent mixing. Cross-channel velocities of up to 0.4 m s-1are observed, and maximum values of e exceed 10∼3 W kg-1. Upon leaving the sill, stratification is reestablished, and turbulence decays. A similar set of sequences occurs on ebb, except that the outflow bypasses the flood inflow channel and instead discharges into Colvos Passage, the third branch of the triple junction. Colvos Passage ultimately discharges the ebb effluent back into Main Basin, enhancing the impact of mixing at the Narrows by discharging the mixed product far from the source. Scaling of the cross-channel momentum equation suggests that, below a threshold value of along-channel velocity, stratification should suppress secondary circulation for a given vertical shear, radius of curvature and channel width. Above the threshold velocity the magnitude of the cross-channel velocity is roughly consistent with predictions for unstratified flow. We estimate the maximum effective eddy diffusivity that aspiration and mixing in the Narrows can produce in Main Basin to be 10"3 m2 s-1

Detailed observations of a naturally occurring shear instability

Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow description of a naturally occurring shear instability. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer-long set of 20-m-tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum mixing across a shear layer with a Reynolds number of 3 × 106. The billows generated large density overturns and were fully turbulent early in their development. As the billows grew, a well-mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods

Energetics of a naturally occurring shear instability

Using observations of an energetic shear instability (Seim and Gregg, 1994), examines the energy budget of the mixing event by comparing microstructure measurements of the dissipation rates of turbulent kinetic energy ε and turbulent potential energy χpe with changes in fine-scale velocity and density. Two sets of observations are used. The first set sampled the shear instability early in its evolution, when overturns occurred in strong stratification. The second set of observations found the same water vertically homogenized by turbulent mixing

Mixing in a coastal environment: 1. A view from dye dispersion

Dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of New England. The experiments were done in 1996 and 1997 as part of the Coastal Mixing and Optics Experiment. During the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km 2 to more than 50 km2 [Sundermeyer and Ledwell, 2001]. Diapycnal diffusivities inferred from dye dispersion range from 10-6 to 10-5 m2/s at buoyancy frequencies from 9 to 28 cycles/hour. Diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by Oakey and Greenan [2004] agree closely in most cases. Estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by Duda and Rehmann [2002] and Rehmann and Duda [2000] are fairly consistent with the dye diffusivities. The dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by MacKinnon and Gregg [2003] from profiling microstructure measurements obtained at the same site in August 1996. All of the measurements support the general conclusion that the diffusivity, averaged over several days, is seldom greater than 10-5 m2/s in the stratified waters at the site, and usually not much greater than 10-6 m 2/s. Severe storms, such as a hurricane that passed over the CMO site in 1996, can dramatically increase the mixing at the site, however

Monthly climatology of the continental shelf waters of the South Atlantic Bight

Monthly circulation of the South Atlantic Bight is diagnosed using a 3-D, shallow water, finite element model forced with monthly wind stress and hydrographic climatology. Temperature and salinity observations from the period 1950-1999 are objectively interpolated onto the model domain, and Comprehensive Ocean-Atmosphere Data Set (COADS) wind velocities from 1975-1999 are used to prescribe the model surface wind stress. The resulting monthly temperature and salinity fields compare favorably to existing shelf climatology. River discharge maxima are evident in the spring temperature and salinity fields, and the rapid heating and cooling of the shelf are captured. The diagnostic circulation is largely wind-driven in the inner and mid-shelf, and the Gulf Stream is apparent in the solutions on the outer shelf. We present the monthly fields, including the temporal and spatial distribution of available hydrographic data, the regional COADS data that provide surface wind stress forcing, the objective analysis, and the model response to these forcings. The hydrographic and velocity fields provide best-prior-estimates of the circulation for data assimilation studies in the region, as well as initial conditions for process-oriented prognostic model studies in the Georgia coastal region

Cold event in the South Atlantic Bight during summer of 2003: Model simulations and implications

A set of model simulations are used to determine the principal forcing mechanisms that resulted in anomalously cold water in the South Atlantic Bight (SAB) in the summer of 2003. Updated mass field and elevation boundary conditions from basin-scale Hybrid Coordinate Ocean Model (HYCOM) simulations are compared to climatological forcing to provide offshore and upstream influences in a one-way nesting sense. Model skill is evaluated by comparing model results with observations of velocity, water level, and surface and bottom temperature. Inclusion of realistic atmospheric forcing, river discharge, and improved model dynamics produced good skill on the inner shelf and midshelf. The intrusion of cold water onto the shelf occurred predominantly along the shelf-break associated with onshore flow in the southern part of the domain north of Cape Canaveral (29° to 31.5°). The atmospheric forcing (anomalously strong and persistent upwelling-favorable winds) was the principal mechanism driving the cold event. Elevated river discharge increased the level of stratification across the inner shelf and midshelf and contributed to additional input of cold water into the shelf. The resulting pool of anomalously cold water constituted more than 50% of the water on the shelf in late July and early August. The excess nutrient flux onto the shelf associated with the upwelling was approximated using published nitrate-temperature proxies, suggesting increased primary production during the summer over most of the SAB shelf

Barotropic tides in the South Atlantic Bight

The characteristics of the principal barotropic diurnal and semidiurnal tides are examined for the South Atlantic Bight (SAB) of the eastern United States coast. We combine recent observations from pressure gauges and ADCPs on fixed platforms and additional short-term deployments off the Georgia and South Carolina coasts together with National Ocean Service coastal tidal elevation harmonics. These data have shed light on the regional tidal propagation, particularly off the Georgia/South Carolina coast, which is perforated by a dense estuary/tidal inlet complex (ETIC). We have computed tidal solutions for the western North Atlantic Ocean on two model domains. One includes a first-order representation of the ETIC in the SAB, and the other does not include the ETIC. We find that the ETIC is highly dissipative and affects the regional energy balance of the semidiurnal tides. Nearshore, inner, and midshelf model skill at semidiurnal frequencies is sensitive to the inclusion of the ETIC. The numerical solution that includes the ETIC shows significantly improved skill compared to the solution that does not include the ETIC. For the M2 constituent, the largest tidal frequency in the SAB, overall amplitude and phase error is reduced from 0.25 m to 0.03 m and 13.8° to 2.8° for coastal observation stations. Similar improvement is shown for midshelf stations. Diurnal tides are relatively unaffected by the ETIC

Global perspectives on observing ocean boundary current systems

Ocean boundary current systems are key components of the climate system, are home to highly productive ecosystems, and have numerous societal impacts. Establishment of a global network of boundary current observing systems is a critical part of ongoing development of the Global Ocean Observing System. The characteristics of boundary current systems are reviewed, focusing on scientific and societal motivations for sustained observing. Techniques currently used to observe boundary current systems are reviewed, followed by a census of the current state of boundary current observing systems globally. Next steps in the development of boundary current observing systems are considered, leading to several specific recommendations

A global research priority agenda to advance public health responses to fatty liver disease

Background & aims An estimated 38% of adults worldwide have non-alcoholic fatty liver disease (NAFLD). From individual impacts to widespread public health and economic consequences, the implications of this disease are profound. This study aimed to develop an aligned, prioritised fatty liver disease research agenda for the global health community. Methods Nine co-chairs drafted initial research priorities, subsequently reviewed by 40 core authors and debated during a three-day in-person meeting. Following a Delphi methodology, over two rounds, a large panel (R1 n = 344, R2 n = 288) reviewed the priorities, via Qualtrics XM, indicating agreement using a four-point Likert-scale and providing written feedback. The core group revised the draft priorities between rounds. In R2, panellists also ranked the priorities within six domains: epidemiology, models of care, treatment and care, education and awareness, patient and community perspectives, and leadership and public health policy. Results The consensus-built fatty liver disease research agenda encompasses 28 priorities. The mean percentage of ‘agree’ responses increased from 78.3 in R1 to 81.1 in R2. Five priorities received unanimous combined agreement (‘agree’ + ‘somewhat agree’); the remaining 23 priorities had >90% combined agreement. While all but one of the priorities exhibited at least a super-majority of agreement (>66.7% ‘agree’), 13 priorities had 90% combined agreement. Conclusions Adopting this multidisciplinary consensus-built research priorities agenda can deliver a step-change in addressing fatty liver disease, mitigating against its individual and societal harms and proactively altering its natural history through prevention, identification, treatment, and care. This agenda should catalyse the global health community’s efforts to advance and accelerate responses to this widespread and fast-growing public health threat. Impact and implications An estimated 38% of adults and 13% of children and adolescents worldwide have fatty liver disease, making it the most prevalent liver disease in history. Despite substantial scientific progress in the past three decades, the burden continues to grow, with an urgent need to advance understanding of how to prevent, manage, and treat the disease. Through a global consensus process, a multidisciplinary group agreed on 28 research priorities covering a broad range of themes, from disease burden, treatment, and health system responses to awareness and policy. The findings have relevance for clinical and non-clinical researchers as well as funders working on fatty liver disease and non-communicable diseases more broadly, setting out a prioritised, ranked research agenda for turning the tide on this fast-growing public health threat