Robert Chant - Academia.edu (original) (raw)

Papers by Robert Chant

Journal of Geophysical Research: Oceans, 2019

A numerical model with a vortex force formalism is used to study the role of wind waves in the mo... more A numerical model with a vortex force formalism is used to study the role of wind waves in the momentum budget and subtidal exchange of a shallow coastal plain estuary, Delaware Bay. Wave height and age in the bay have a spatial distribution that is controlled by bathymetry and fetch, with implications for the surface drag coefficient in young, underdeveloped seas. Inclusion of waves in the model leads to increases in the surface drag coefficient by up to 30% with respect to parameterizations in which surface drag is only a function of wind speed, in agreement with recent observations of air-sea fluxes in estuaries. The model was modified to prevent whitecapping wave dissipation from generating breaking forces since that contribution is integrally equivalent to the wind stress. The proposed adjustment is consistent with previous studies of wave-induced nearshore currents and with additional parameterizations for breaking forces in the model. The mean momentum balance during a simulated wind event was mainly between the pressure gradient force and surface stress, with negligible contributions by vortex, wave breaking (i.e., depth-induced), and Stokes-Coriolis forces. Modeled scenarios with realistic Delaware bathymetry suggest that the subtidal bay-ocean exchange at storm time scales is sensitive to wave-induced surface drag coefficient, wind direction, and mass transport due to the Stokes drift. Results herein are applicable to shallow coastal systems where the typical wave field is young (i.e., wind seas) and modulated by bathymetry. Plain Language Summary Water circulation and pollutant flushing in estuaries, bays, and similar coastal environments depend mainly on tidal currents, winds, and density differences between fresh and saltwater. However, winds normally coexist with waves, and the impact of waves on circulation patterns is often overlooked in most numerical models. In this study we used a numerical model to explore how Delaware Bay responds to a typical storm event. The model was configured to consider or ignore the effects of waves on circulation, and we used both options to contrast and compare the simulated results. The model that ignored waves could not reproduce the mechanisms that have been shown to control the energy transfer from the atmosphere to the water column. In agreement with recent observations in estuaries, the model indicates that wind waves in shallow water have an important regulating effect on the roughness of the sea surface, which is key for surface drag and circulation. We also report that even when the wind speed is the same above the water surface, the spatial distribution of waves determines the effective amount of energy that gets transferred from the air to the water column.

Journal of Geophysical Research: Oceans, 2018

The Mid-Atlantic Bight (MAB) Cold Pool is a distinctive cold (lower than 10 ∘ C) and relatively f... more The Mid-Atlantic Bight (MAB) Cold Pool is a distinctive cold (lower than 10 ∘ C) and relatively fresh (lower than 34 practical salinity unit) water mass. It is located over the middle and outer shelf of the MAB, below the seasonal thermocline, and is attached to the bottom. Following this definition, we put forward a method that includes three criteria to capture and quantify Cold Pool characteristics, based on a 50-year (1958-2007) high-resolution regional ocean model hindcast. The seasonal climatology of the Cold Pool and its properties are investigated during its onset-peak-decline cycle. Three stages of the Cold Pool event are defined according to its evolution and characteristics. The Cold Pool cores travel along the 60-m isobath starting south of the New England shelf to the Hudson Shelf Valley at a speed of 2-3 cm/s. Furthermore, the northern extent of the Cold Pool retreats about 2.6 times faster than the southern extent during the summer progression. The heat balance of near-bottom waters over the MAB and Georges Bank is computed and it is found that the heat advection, rather than vertical diffusion, dominates the resulting spatial patterns of warming. Possible origins of the Cold Pool are investigated by performing a lead-lag correlation analysis. Results suggest that the Cold Pool originates not only from local remnants of winter water near the Nantucket Shoals, but has an upstream source traveling in the spring time from the southwestern flank of the Georges Bank along the 80-m isobath. Plain Language Summary The Mid-Atlantic Bight (MAB) Cold Pool is a bottom-trapped relatively cold and fresh water mass over the middle and outer continental shelf. It is a seasonal thermal feature that emerges during spring and summer when the water column is warmer at the surface. We use three criteria to quantify the Cold Pool and investigate its seasonal evolution of properties. We find the Cold Pool typically experiences an onset-peak-decline cycle during a year. Its coldest part travels along the 60-m isobath at a speed of 2-3 cm/s from the south of the Nantucket Shoals to the Hudson Shelf Valley. During its seasonal collapse, the northern and southern boundaries retreat at different speeds, with the northern boundary retreating 2.6 times faster. In order to investigate relative importance of vertical mixing and advection of heat, we build a heat balance of the water column below 30 m depth. Results suggest that vertical mixing is the main reason for the warming in the Cold Pool, however, horizontal and vertical advection of heat that regulates the spatial difference in warming rates of the Cold Pool. We further discuss the possible origins of the Cold Pool.

Nature microbiology, Jan 12, 2018

Marine phytoplankton account for approximately half of global primary productivity, making their ... more Marine phytoplankton account for approximately half of global primary productivity, making their fate an important driver of the marine carbon cycle. Viruses are thought to recycle more than one-quarter of oceanic photosynthetically fixed organic carbon, which can stimulate nutrient regeneration, primary production and upper ocean respirationvia lytic infection and the 'virus shunt'. Ultimately, this limits the trophic transfer of carbon and energy to both higher food webs and the deep ocean. Using imagery taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite, along with a suite of diagnostic lipid- and gene-based molecular biomarkers, in situ optical sensors and sediment traps, we show that Coccolithovirus infections of mesoscale (~100 km) Emiliania huxleyi blooms in the North Atlantic are coupled with particle aggregation, high zooplankton grazing and greater downward vertical fluxes of both particulate organic and particulate inorga...

Marine Biology, 2006

Previous time-series studies of meroplankton abundances in the LEO-15 research area off Tuckerton... more Previous time-series studies of meroplankton abundances in the LEO-15 research area off Tuckerton, New Jersey, USA (39°28¢N, 74°15¢W) indicated shortlived (6-12 h) pulses in larval surfclam (Spisula solidissima Dillwyn) concentration often associated with the initiation of downwelling. To examine possible larval surfclam (and other bivalve) concentrating mechanisms during upwelling and downwelling, six sets of adaptive mobile zooplankton pump samples were taken in July 1998 at different depths at five to six stations along a 25-km transect perpendicular to the coastline and crossing Beach Haven Ridge at LEO-15. Sampling was guided by near real-time, satellite imagery of sea surface temperature overlain by sea surface currents from a shore-based ocean surface current radar (OSCR) unit. A Seabird CTD on the mobile pump frame near the intake provided information on thermocline depth, and sampling depths were adjusted according to the temperature profiles. Near shore, the thermocline was tilted down during downwelling, and up during upwelling. The highest concentrations of surfclam larvae occurred near the bottom at a station near Beach Haven Ridge during downwelling, and just above the thermocline 3 km further offshore during well-developed upwelling. For other bivalve taxa, the larvae were concentrated near the thermocline (Anomia simplex Orbigny and Pholadidae spp.) or concentrated upslope near the bottom (Mytilidae spp.) during upwelling, and the larvae were concentrated near the bottom or were moved downslope during downwelling. Donax fossor Say larvae were found near the surface or above the thermocline during upwelling and downwelling. The general patterns of larval bivalve distribution appear to be influenced by water mass movement during upwelling and downwelling. The larval concentration patterns of individual species are likely a consequence of advection due to upwelling and downwelling circulation, vertical shear in the front region, species-specific larval behaviors, and larval sources. Communicated by R.J.

Limnology and Oceanography: Methods, 2008

For decades, optical instruments measuring light scattering and/or light transmission in situ hav... more For decades, optical instruments measuring light scattering and/or light transmission in situ have been used to derive information about the nature of the underwater light field (

Journal of Physical Oceanography, 2000

Subinertial currents on a wide (ϳ100 km), shallow (ϳ20 m), but nevertheless vertically stratified... more Subinertial currents on a wide (ϳ100 km), shallow (ϳ20 m), but nevertheless vertically stratified shelf off the Atlantic seaboard of the United States are investigated at spatial scales of about 20 km in the alongshore and 10 km in the across-shore direction. During the summer of 1996 the inner shelf off New Jersey was stratified due to both temperature and salinity that varied vertically by more than 12ЊC and 4 psu, respectively. Upwelling favorable winds and an intermittent buoyancy-driven Hudson coastal current impact this stratification inshore of the 15-m isobath. Waters offshore were always stratified except during the passage of Tropical Storm Bertha. Mean currents are weak because wind-forced upwelling and buoyancy-forced downwelling events occurred about evenly during the observational study period. At monthly to daily timescales currents always veered counterclockwise with depth in a bottom Ekman-layer sense by more than 30Њ inshore and 50Њ offshore. Complex empirical orthogonal function (CEOF) analyses revealed that these veering angles are contained in the first mode that explains 80%-85% of the total variance at individual locations. It also explains 72% of the variance of 44 current time series of an across-shore section. The veering constitutes a robust feature that cannot be rationalized by Ekman dynamics in shallow water alone. The authors hypothesize that the veering represents a frictional response common to both upwelling and downwelling events. The CEOF analysis does not separate wind from buoyancy forcing. The two forcing mechanisms thus appear to be dynamically coupled. Nevertheless, the first two CEOFs suggest distinctly different circulation regimes: For positive and negative temporal amplitudes mode 1 represents a wind-forced upwelling and a buoyancy-forced downwelling circulation while mode 2 represents the lateral shear of the flow field. Synoptic maps of surface currents from ocean surface current radar reveal realizations of these event types.

Journal of Marine Research, 2001

Observations in the Navesink River estuary in northern New Jersey demonstrate that buoyancy augme... more Observations in the Navesink River estuary in northern New Jersey demonstrate that buoyancy augments the particle trapping tendencies of flood-dominated systems because these estuaries heighten tidal period asymmetries in stratification. During the long and slow ebb which typifies flood-dominated systems, a positive feedback between tidal straining and weak vertical mixing stratifies the estuary. In contrast, during flood, turbulence generated by the stronger tidal currents augments overstraining of the density field and the water column becomes well mixed. The tidal period asymmetries in stratification have profound effects on the vertical structure and transport of suspended matter. During ebb, weak vertical mixing allows suspended material to settle downward. In contrast, strong turbulence during flood mixes suspended matter into the water column where it is transported up estuary. Furthermore, observations reveal that resuspension events are marked by multiple turbidity spikes, suggestive of multiple, limited layers of erodible material. The transport of the turbid waters is consistent with horizontal advection modified by horizontal dispersion. Periods of enhanced stratification are also marked by relatively low levels of turbidity during the ebb, consistent with more complete settling of suspended material following times of high river discharge. The interplay between buoyancy and tidal asymmetries are further elucidated with a onedimensional numerical model featuring a turbulent closure scheme and a passively settling tracer. Model results are generally consistent with the field observations, both emphasizing the robust particle trapping tendencies of a stratified flood-dominated estuary. We speculate that enhanced particle trapping following times of high river discharge may have important biological consequences.

Journal of Geophysical Research, 2007

A series of underway current velocity profiles and near-surface temperature and salinity measurem... more A series of underway current velocity profiles and near-surface temperature and salinity measurements were combined with temperature and salinity profiles to characterize the subtidal flow structure at the turning region of a wide plume, the Chesapeake Bay outflow plume. In this context, ''wide'' refers to the ratio of lateral plume expansion to internal radius of deformation being greater than one. Observations were obtained in September and November of 1996 and in February and May of 1997 with the idea of capturing the variability in forcing conditions typically associated with these seasons. However, regional precipitation patterns yielded similar buoyancy forcing conditions for the four surveys and among the wettest years on record. This buoyancy forcing produced a well-delineated outflow plume that separated from the coast on its way out the estuary. The plume separation acted in conjunction with frictional effects to delineate an inshore front, in addition to the customarily described offshore front. The outflow plume was markedly constrained by the Chesapeake Channel, which was also the main conduit of shelf waters toward the estuary. The bathymetric influence was also evident in the surface salinity field, the mean flows and the volume fluxes. The offshore extent of the plume was found between the scale predicted by geostrophic dynamics (internal Rossby radius) and that predicted by cyclostrophic dynamics. Such offshore extent was most likely linked to the plume interactions with the bathymetrically steered up-estuary flow. This was corroborated by an analytical solution that explored the dynamical balance among pressure gradient, Coriolis accelerations and friction. In addition to being influenced by bathymetry, the Chesapeake Bay outflow plume was modified by local and remote effects related to atmospheric forcing.

Journal of Geophysical Research: Oceans, 2000

Shipboard acoustic Doppler current profiler and conductivity-temperaturedepth data obtained durin... more Shipboard acoustic Doppler current profiler and conductivity-temperaturedepth data obtained during highly stratified conditions in the Hudson River estuary along a section of variable width and breadth are presented. The observations emphasize tidal period asymmetries in the vertical structure of current and salinity. However, these asymmetries exhibit significant along-channel structure which is determined by channel morphology. During the ebb the flow is linearly sheared, and steep halocline slopes in the vicinity of channel contractions are maintained by momentum advection. A minimum in vertical shear across the pycnocline occurs in channel contractions. During food the pycnocline sharpens and flattens with a middepth velocity maximum embedded in the pycnocline which separates a stratified surface layer from a bottom mixed layer. The along-channel structure in vertical shear is consistent with a lateral vorticity equation. Estimates of Richardson numbers suggest that vertical mixing across the pycnocline is enhanced downstream of channel contractions.

Journal of Geophysical Research, 2004

The temporal evolution of a flow reversal during upwelling conditions along New Jersey's inner sh... more The temporal evolution of a flow reversal during upwelling conditions along New Jersey's inner shelf is characterized with shipboard, moored, and remote observations. The flow reversal occurs nearshore in the form of a subsurface jet with maximum velocities exceeding 30 cm/s. The jet is most intense in the thermocline, commences during maximum alongshore wind stress, and has a spin-up time approximately equal to the local inertial period. The jet also has a surface signature apparent in ocean current radar data that shows the jet veering offshore and feeding an upwelling center that drifts southward at 5 cm/s. Moored instrumentation within the upwelling center indicates that cross-shelf transport in the warm surface layer is consistent with the predicted Ekman transport prior to the spin-up of the jet, but exceeds Ekman transport thereafter. However, onshore transport in the lower layer never compensates for offshore flow in the surface layer, suggestive that the mass balance requires a three-dimensional closure. Finally, we suggest that the flow reversal provides a significant fraction of cool water to the evolving upwelling center, and that the offshore veering is due to enhanced friction over a shoaling and rougher topography.

Geophysical Research Letters, 2008

The first sustained glider observations in the Middle Atlantic Bight are used to describe the sea... more The first sustained glider observations in the Middle Atlantic Bight are used to describe the seasonal evolution of hydrographic fields off New Jersey. Near-surface temperatures respond to the seasonal cycle of surface heating, while waters at depth are primarily influenced by advection of cold waters from the north in the cold-pool during spring/summer, and warming due to mixing during fall. The thermocline thickness increases in the offshore direction. Salinity presents seasonal variability due to river discharge and wind variations, with low-salinity waters spanning 100kmacrosstheshelffromMaytoSeptemberina100 km across the shelf from May to September in a 100kmacrosstheshelffromMaytoSeptemberina10 m thick surface layer. Stratification intensifies from April/May to late summer, especially within 80 km from the coast. The pycnocline deepens in the water column during late summer, while the passage of storms during fall rapidly reduces the stratification. The glider high-resolution observations allowed for unprecedented detailed characterization of the spatial scales of variability.

Estuaries, 2004

Seasonal and interannual patterns in the spatial distribution of bluefish (Pomatomus saltatrix) w... more Seasonal and interannual patterns in the spatial distribution of bluefish (Pomatomus saltatrix) within a Middle Atlantic Bight estuary were examined using multipanel gillnets fished biweekly at 14 fixed stations in the Sandy Hook Bay-Navesink River estuary during May-November of 1998 and 1999. To characterize habitats along the estuarine gradient, we measured several abiotic and biotic variables concurrently with gillnet sampling. Juvenile (age-0 and age-1؉) bluefish were captured regularly during both years along with large numbers of Atlantic menhaden (Brevoortia tyrannus), which were confirmed by diet analyses to be bluefish's primary forage species. The date of initial appearance of age-0 bluefish and menhaden in the estuary varied between years and may have been related to interannual differences in seawater temperatures on the continental shelf during spring. Delayed estuarine arrival of prey fishes may have contributed to variability in bluefish diets between years. Within the estuary, bluefish spatial distributions were consistent across seasons and years: bluefish were most common in areas associated with high concentrations of suspended materials and the presence of menhaden. Community analyses also indicated habitat overlap between bluefish and menhaden. Spatial distribution patterns revealed the consistent occurrence of piscivorous bluefish in shallow estuarine habitats that retained suspended materials and aggregated prey fishes. Foraging success of bluefish and other estuarine piscivores may be closely linked with the availability of these productive habitats, highlighting the need for future study of biological interactions and the governing physical processes.

Continental Shelf Research, 2013

The complex dynamics associated with coastal buoyant plumes make it difficult to document the int... more The complex dynamics associated with coastal buoyant plumes make it difficult to document the interactions between light availability, phytoplankton carbon fixation, and biomass accumulation. Using real-time data, provided by satellites and high frequency radar, we adaptively sampled a low salinity parcel of water that was exported from the Hudson river estuary in April 2005. The water was characterized by high nutrients and high chlorophyll concentrations. The majority of the low salinity water was re-circulated within a nearshore surface feature for 5 days during which nitrate concentrations dropped 7-fold, the maximum quantum yield for photosynthesis dropped 10-fold, and primary productivity rates decreased 5-fold. Associated with the decline in nitrate was an increase in phytoplankton biomass. The phytoplankton combined with the Colored Dissolved Organic Matter (CDOM) and non-algal particles attenuated the light so the 1% light level ranged between 3 and 10 m depending on the age of the plume water. As the plume was 10-15 m thick, the majority of the phytoplankton were light-limited. Vertical mixing within the plume was high as indicated by the dispersion of injected of rhodamine dye. The mixing within the buoyant plume was more rapid than phytoplankton photoacclimation processes. Mixing rates within the plume was the critical factor determining overall productivity rates within the turbid plume.

Continental Shelf Research, 2005

A LISST-100 in situ laser particle sizer was deployed together with a digital floc camera during ... more A LISST-100 in situ laser particle sizer was deployed together with a digital floc camera during field work in the Newark Bay area (USA) and along the Apennine margin (the Adriatic Sea, Italy). The purpose of these simultaneous deployments was to investigate how well in situ particle (floc) sizes and volume concentrations from the two different instruments compared. In the Adriatic Sea the two instruments displayed the same temporal variation, but the LISST provided lower estimates of floc size by a factor of 2-3, compared to the DFC. In the Newark Bay area, the LISST provided higher values of floc size by up to a factor of 2. When floc size was computed using only the overlapping size bins from the two instruments the discrepancy disappeared. The reason for the discrepancy in size was found to be related to several issues: First, the LISST measured particles in the 2.5-500 mm range, whereas the camera measured particles in the 135-9900 mm range, so generally the LISST should provide lower estimates of floc size, as it measures the smaller particles. Second, in the Newark Bay area scattering from particles 4500 mm generally caused the LISST to overestimate the volume of particles in its largest size bin, thereby increasing apparent floc size. Relative to the camera, the LISST generally provided estimates of total floc volume that were lower by a factor of 3. Factors that could explain this discrepancy are errors arising from the accuracy of the LISST volume conversion coefficient and image processing. Regardless of these discrepancies, the shapes of the size spectra from the instruments were similar in the regions of overlap and could be matched by multiplying with an appropriate correction coefficient. This facilitated merging of the size spectra from the LISST and the DFC, yielding size spectra in the 2.5-9900 mm range. The merged size spectra generally had one or more peaks in the coarse end of the spectrum, presumably due to the presence of flocs. The fine end (o100 mm) of the spectrum displayed a flat tail with equal concentration of particles in all size classes. Size spectra with this shape indicate that the classical Junge model for description of in situ particle size spectra is reasonable for particles

Continental Shelf Research, 2011

For this manuscript we use a 9-year time series of Sea-viewing Wide Field of view Sensor (SeaWiFS... more For this manuscript we use a 9-year time series of Sea-viewing Wide Field of view Sensor (SeaWiFS), HF radar, and Webb Glider data to assess the physical forcing of the seasonal and inter-annual variability of the spatial distribution in phytoplankton. Using Empirical Orthogonal Function (EOF) analysis, based on 4-day average chlorophyll composites, we characterized the two major periods of enhanced chlorophyll biomass for the MAB in the fall-winter and the spring. Monthly averaged data showed a recurrent chlorophyll biomass in the fall-winter months, which represented 58% of the annual surface chlorophyll for the MAB. The first EOF mode explained $ 33% of the chlorophyll variance and was associated with the enhanced phytoplankton biomass in the fall-winter found between the 20 and 60 m isobaths. Variability in the magnitude of the enhanced chlorophyll in fall-winter was associated with buoyant plumes and the frequency of storms. The second EOF mode accounted for 8% of the variance and was associated with the spring time enhancements in chlorophyll at the shelf-break/slope (water depths greater than 80 m), which was influenced by factors determining the overall water column stability. Therefore the timing and the inter-annual magnitude of both events are regulated by factors influencing the stability of the water column, which determines the degree that phytoplankton are light-limited. Decadal changes observed in atmospheric forcing and ocean conditions on the MAB have the potential to influence these phytoplankton dynamics.

Journal of Geophysical Research, 2002

On the basis of a 301 day acoustic Doppler current profiler mooring in an estuarine tidal strait ... more On the basis of a 301 day acoustic Doppler current profiler mooring in an estuarine tidal strait the strength and structure of secondary circulation in a region of flow curvature is related to variations in tidal forcing and river discharge. During low-flow conditions the structure of secondary flow is consistent with a centrifugally forced helical flow, with bottom flow toward the inside of the bend and surface flow toward the outside of the bend. The strength of secondary flow increases linearly with tidal range and is consistent with a vertical eddy viscosity that is linearly dependent on tidal current speed. During times of high river discharge the strength of secondary flow is significantly reduced, and its vertical structure undergoes a fundamental change over the spring/neap cycle. During spring tides the classic helical flow pattern is evident, albeit weaker than during low-flow conditions. However, during neap tides a more complex two-cell structure is evident. The change between these two states occurs with a spring/neap transition in the subtidal flow, indicating that it is also accompanied by changing stratification. Simple scaling analysis suggests that during weakly stratified conditions, secondary circulation will influence stream-wise dynamics and dispersion for channels with widths on order or less than 0.1 H/C d , where H is the water column depth and C d is a quadratic bottom drag coefficient. In contrast, during highly stratified conditions, lateral excursions due to secondary flows are limited to approximately one tenth of the channel's width and are an ineffective lateral mixing agent.

2006 IEEE US/EU Baltic International Symposium, 2006

The Lagrangian Transport and Transformation Experiment (LaTTE) study of the Hudson River Plume ha... more The Lagrangian Transport and Transformation Experiment (LaTTE) study of the Hudson River Plume has now completed 2 of its 3 field seasons. The interdisciplinary study is being conducted in a sustained coastal research observatory that provides a spatial and temporal context for adaptive shipboard sampling. Observations from the second LaTTE field season are used here to describe the processes responsible for a previously unexplained recurrent hypoxia region along the New Jersey coast.

Operational Oceanography in the 21st Century, 2011

In coastal oceanography, simulation models are used to a variety of ends. Idealized studies may a... more In coastal oceanography, simulation models are used to a variety of ends. Idealized studies may address particular dynamical processes or features of coastline and bathymetry; reproducing the circulation in a geographical region can compliment studies of ecosystems and geomorphology; and models may be employed to simulate observing systems and to forecast oceanic conditions for practical operational needs. Frequently, the interplay between multiple forcing mechanisms, geographic detail, stratification, and nonlinear dynamics, is significant, and this demands that ocean models for coastal applications are capable of representing a comprehensive suite of dynamical processes. Drawing on a series of recent modelbased studies of the inner to mid-shelf region of the Middle Atlantic Bight (MAB) we illustrate, by example, these methodologies and the breadth of dynamical processes that influence coastal ocean circulation. We demonstrate that the recent introduction of variational methods into coastal ocean simulation is a development that greatly enhances our ability to integrate models with data from the evolving coastal ocean observatories for the purposes of improved ocean prediction, adaptive sampling and observing system design.