Daniel L. Codiga

Networked Acoustic Modems for Real-Time Data Delivery from Distributed Subsurface Instruments in the Coastal Ocean: Initial System Development and Performance

Abstract Results are reported from field tests of networked acoustic modems used for wireless real-time delivery of oceanographic measurements from a distributed array of subsurface instruments in coastal waters. The network demonstrated consists of sensor nodes, repeater nodes, gateway nodes, and a shore-based control center. Sensors are oceanographic instruments interfaced with acoustic modems, deployed in trawl-resistant bottom frames with azimuthally omnidirectional acoustic signaling needed for flexible network rerouting. Repeaters are individual acoustic modems to relay data so the array covers a larger area; only these relatively low-cost nodes are suited for deployment unprotected from trawlers. Gateways are buoys with acoustic modems interfaced to cellular telephone modems for communication between the underwater network and the shore. The experiment site is the inner continental shelf off Montauk Point, New York, and Block Island, Rhode Island, with a U.S. Coast Guard navigation buoy equipped as...

Observations of low‐frequency circulation and amplified subinertial tidal currents at Cobb Seamount

We present acoustic Doppler current profiler (ADCP) survey results resolving three-dimensional current structure in thermocline stratification atop Cobb Seamount (130.8°W, 46.8°N). Mean flow includes clockwise circulation near the bottom and an east-northeastward background current. Clockwise flow is not strong enough to isolate fluid from the background current as in a Taylor cap unless this occurs in the deepest 50 m, outside ADCP coverage. Diurnal currents (subinertial, 0.69ƒ) are amplified within a few kilometers radially and about 100 m vertically of the seamount by up to 5.3 times ambient K1 tidal currents. Their diagnostics include clockwise propagation around the seamount with first azimuthal wavenumber, clockwise rotation in time, narrow current ellipses oriented nearly along bathymetric contours with positively correlated radial and azimuthal components, counterclockwise turning with depth, and downward propagation. They are compared to the stratified seamount-trapped wave of first azimuthal wavenumber and third-highest subinertial resonant frequency (0.70ƒ) calculated numerically using measured bathymetric and stratification profiles. Inviscid free wave currents exhibit symmetric patterns not consistent with the measurements; in a forced damped wave maintained by ambient tidal currents against parameterized friction of 2 day timescale, as appropriate to dissipation timescales estimated from microstructure measurements, symmetries are broken such that each observed current diagnostic is matched. When the wave is linearized about a steady, baroclinic, clockwise current representing the measured mean flow, its frequency and structure are very weakly modified.

Interplay of Wind Forcing and Buoyant Discharge off Montauk Point: Seasonal Changes to Velocity Structure and a Coastal Front

Abstract Seasonal-mean currents in fall, winter, and spring on the bathymetrically complex continental shelf 15–65 m deep off Montauk Point, outside Block Island Sound, are analyzed using moored profiling current-meter records from a 2.5-yr period. A sharp boundary, or coastal front, occurs where strong, shallow, generally southwestward flow that weakens nearly linearly with increasing depth meets deeper flow nearly opposite in direction (fall/winter) or markedly weaker (spring). Velocities veer clockwise (counterclockwise) with increasing depth inshore (offshore) of the front. Evidence is presented that thermal wind balance holds without major frictional modification: it accounts for the veering, and the seasonal-mean horizontal density gradients it implies are generally toward the southeast quadrant in agreement with limited hydrographic measurements. Substantial seasonal changes in flow and frontal attributes occur because of the interplay of annual cycles in wind forcing and buoyant discharge. In fall...

Laboratory Realizations of Stratified Seamount-trapped Waves

Abstract The response of a uniformly rotating, uniformly stratified fluid surrounding an axisymmetric Gaussian seamount to barotropic, rectilinear tidal forcing at a range of subinertial frequencies is examined in a rotating table laboratory experiment. Flow is compared to numerical solutions for linear, stratified, seamount-trapped waves. As is appropriate to model the oceanic system at Fieberling Guyot, laboratory forcing is weak and motions have small Rossby number and small wave steepness, the ratio between particle orbit length and seamount circumference. The numerical solutions are used to explore properties of the gravest first azimuthal seamount-trapped wave mode, which is most likely to be observed in the ocean. Particles execute nearly circular orbits in a bottom-trapped region above the seamount summit. On the sloping flanks below, particles follow narrow elliptical paths oriented along seamount contours, and the azimuthal velocity is reversed relative to that above the summit. Force balances a...

Response of a Large Stratified Estuary to Wind Events: Observations, Simulations, and Theory for Long Island Sound

AbstractThe response to wind events in the Long Island Sound (LIS), a large macrotidal estuary influenced by rotation and stratification, is studied using long-term ferry-based current observations near the mouth, unstratified and stratified numerical simulations forced with along-estuary winds, and analytic solutions based on linear barotropic theory. The observed wind-event velocity anomalies for down-estuary winds have surface-intensified downwind flows flanking a deeper central upwind flow. Response to up-estuary wind events has a weaker magnitude and a broader and thicker downwind flow. The downwind and upwind flows are more laterally aligned than vertically layered, as determined by a newly defined dimensionless lateral alignment index. Simulation results and analytic solutions share the gross spatial patterns of the observed response, though statistical measures indicate weak agreement. Along-estuary variations in the simulation results and analytic solutions follow similar trends and are strongly ...

The Physical Oceanography of Long Island Sound

Coastal ocean ecosystems are strongly influenced by circulation, tides, waves, and the rates of mixing of the water. Many shoreline communities are increasingly threatened by the same phenomena, most notably through flooding and coastal erosion. In this review we summarize the observations that have been acquired in LIS to describe and explain the magnitude and variability of these physical processes. We also comment on the status of our theoretical understanding of the links between them and some of their consequences. Analysis of available buoy and coastal wind observations suggests that the shear stress due to wind over LIS is under-predicted by a factor of between 2 and 3 if shore station winds are used to make the estimates. This difference is significant to both water quality and wave forecasting. We describe the magnitude of seasonal variations in wind and waves and use long-term records from coastal stations to show that there are decadal-scale variations in both wind speed and directions. Available wave data from two buoys suggest that the wave field is consistent with that predicted by fetch- limited wind forcing. Semi-diurnal tidal sea level variations and vertically averaged currents are well described by theoretical models, however, recent observations show high amplitude over tides in the western LIS that remain to be explained. The vertical structure of the tidal currents is much more complex and a closer examination of model predictions, particularly of the across Sound velocity components, should be conducted. The interaction of the vertical structure of tidal currents and the salinity and temperature distributions may lead to significant heat and salt transport vertically and horizontally. Observations of the mean density, temperature, and salt distributions and the mean circulation in LIS are qualitatively consistent with several models and we summarize the recent work. A more critical evaluation is now appropriate. We also discuss evidence of long-term trends. The role of shorter time-scale meteorological forcing and the bathymetry of the Sound on the structure and variability of the circulation is summarized using observations and simulation. Long-term observations of both hypoxia duration and hypoxia areal extent in western and west central Long Island Sound are analyzed to determine the directional response to wind forcing. We show that a substantial fraction of the inter-annual variability in area and duration can be explained by the directional statistics of wind. Using simulation, we demonstrate that the geometry of the basin and across isobath flow may be significant. The response of the Sound to severe storms is outlined and the technical developments in simulation and observation that are necessary to the improvement of model predictions are suggested.

Real-time delivery of subsurface coastal circulation measurements from distributed instruments using networked acoustic modems

A novel system to use networked acoustic modems for real-time wireless delivery of oceanographic measurements from a distributed array of subsurface instruments in coastal waters is presented. The network consists of sensor nodes, repeater nodes, gateway nodes, and a server. Gateway nodes are buoys equipped with acoustic modems interfaced to Internet-accessible cellular modems for two-way communication between server and subsurface network. Network data flow is remotely re-configurable with multiple paths to the server from each sensor so loss or failure of an individual node can be accommodated. Oceanographic instruments are housed in bottom-mounted frames to limit biofouling and minimize fishing and shipping interference. A trawl-resistant frame enables azimuthally omnidirectional acoustic signaling as required for the network. Gateways serve multiple sensors distributed across wide areas, minimizing the number of instruments with surface exposure. A low-bandwidth data format for acoustic Doppler current profilers is devised to reduce network throughput and power consumption. Gateway equipment is engineered for a Coast Guard buoy. Tests in winter and spring indicate wind limits acoustic communications range by roughening the sea surface. A preliminary network deployment demonstrates gateway control, data delivery across multiple repeaters, and route reconfiguration.

A Marine Autonomous Surface Craft for Long-Duration, Spatially Explicit, Multidisciplinary Water Column Sampling in Coastal and Estuarine Systems

The Surveying Coastal Ocean Autonomous Profiler (SCOAP) is a large catamaran marine autonomous surface craft (MASC) for unattended weeks-long, spatially explicit, multidisciplinary oceanographic water column profile sampling in coastal/estuarine waterbodies. Material transport rates/pathways, crucial to understanding these ecosystems, are typically poorly known. SCOAP addresses demanding spatiotemporal sampling needs and operational challenges (strong currents, open coastal sea states, complex bathymetry, heavy vessel traffic). Its large size (11-m length, 5-m beam) provides seaworthiness/stability. The average speed of 2.5ms 21 meets the representative goal to traverse an 18-km transect, sampling 10min at each of 10 stations 2km apart, nominally 4 times daily. Efficient hulls and a diesel‐electric energy system can provide the needed endurance. The U.S. Coast Guard guidelines are followed: lighting, code flags, the Automatic Identification System (AIS), and collision avoidance regulations (COLREGs)-based collision avoidance (CA) by onboard autonomy software. Large energy reserves obviate low-power optimization of sensors, enabling truly multidisciplinary sampling, and provide on-demand propulsion for effective CA. Vessel stability facilitates high-quality current profile observations and will aid engineering/operation of the planned winched profiling system, performance of an anticipated radar system to detect/track non-AIS vessels, and potentialresearch-qualitymeteorologicalsensoroperation.ANarragansettBaytestdeployment,attendedby anescortvessel,metdesigngoals;anunattendedopencoastaldeploymentisplannedforRhodeIslandSound. Scientific and operational strengths of large catamaran MASCs suggest they could be an important costeffective complement to other sampling platforms (e.g., improved spatiotemporal coverage and resolution, extending farther inshore, with a broader range of sensors, compared to underwater gliders) in coastal/ estuarine waters.

Tidal Cycles in Stratification and Shear and Their Relationship to Gradient Richardson Number and Eddy Viscosity Variations in Estuaries

AbstractTidal cycles in stratification and shear lead to tidal variations in mixing in many estuaries. This study 1) defines readily observable dimensionless parameters for establishing the sense and magnitude of gradient Richardson number Ri and eddy viscosity K changes from maximum to minimum stratification during a tidal cycle and 2) calculates where representative estuaries fit in this parameter space. The dimensionless parameters are Ri calculated with tidal-averaged stratification and shear, scaled stratification amplitude, and a scaled shear parameter. The scaled stratification amplitude is approximately the tidal amplitude of stratification divided by the tidal-averaged stratification. The scaled shear parameter depends on the scaled tidal amplitude of shear and the phase difference between the tidal cycles of stratification and shear. Over most of the parameter space, Ri is larger at maximum stratification. If the scaled stratification amplitude falls below a threshold value defined in terms of t...

Density stratification in an estuary with complex geometry: Driving processes and relationship to hypoxia on monthly to inter‐annual timescales

[1] The density field in Narragansett Bay (NB), a northeast U.S. estuary with complex geometry that suffers hypoxia, is described and related to driving factors using monthly means from time series observations at 9 sites during late spring to early fall 2001–2009. Stratification (deep-shallow density difference) is dominated by salinity and strongest (4– 7k g m � 3 in late spring) near rivers in the north and east. Shallow horizontal density gradients are about 0.2 kg m � 3 km � 1 ; deep densities have minor spatial and seasonal variations. Geographic structure in density, and its inter-annual anomalies, is weaker than expected based on the complex geometry and large size relative to the internal deformation radius. Inter-annual variability is primarily driven by river flow and weakly influenced by winds, contrasting nearby systems (Chesapeake Bay, Long Island Sound), likely due to reduced fetch and/or unfavorable alignment with prevailing winds. Stratification response to river flow follows 2/3 power scaling despite that the theory omits important NB attributes (complex geometry, depth-varying horizontal gradients). Contrasting other systems (Delaware Bay, San Francisco Bay), horizontal gradients are at least as responsive to river forcing as theoretical 1/3 power scaling; depth-dependent horizontal gradients or finite basin constraint of intrusion length may be responsible. Bay-wide inter-annual variations in seasonal hypoxia correlate with late spring stratification, though stratification peaks in the north and east with hypoxia most severe in the north and west. Long-term response of stratification, and thus its role in hypoxia, to climate-driven increases in river flow and temperatures will be dominated by the former.