1,349 research outputs found
Persistent Transport Barrier on the West Florida Shelf
Analysis of drifter trajectories in the Gulf of Mexico has revealed the
existence of a region on the southern portion of the West Florida Shelf (WFS)
that is not visited by drifters that are released outside of the region. This
so-called ``forbidden zone'' (FZ) suggests the existence of a persistent
cross-shelf transport barrier on the southern portion of the WFS. In this
letter a year-long record of surface currents produced by a Hybrid-Coordinate
Ocean Model simulation of the WFS is used to identify Lagrangian coherent
structures (LCSs), which reveal the presence of a robust and persistent
cross-shelf transport barrier in approximately the same location as the
boundary of the FZ. The location of the cross-shelf transport barrier undergoes
a seasonal oscillation, being closer to the coast in the summer than in the
winter. A month-long record of surface currents inferred from high-frequency
(HF) radar measurements in a roughly 60 km 80 km region on the WFS off
Tampa Bay is also used to identify LCSs, which reveal the presence of robust
transient transport barriers. While the HF-radar-derived transport barriers
cannot be unambiguously linked to the boundary of the FZ, this analysis does
demonstrate the feasibility of monitoring transport barriers on the WFS using a
HF-radar-based measurement system. The implications of a persistent cross-shelf
transport barrier on the WFS for the development of harmful algal blooms on the
shoreward side of the barrier are considered.Comment: Submitted to Geophysical Research Letter
Nonhydrostatic Modeling of West Florida Shelf Flow
LONG-TERM GOALS: The long range scientific goal of the Oceanic Planetary Boundary Layer (OPBL) Laboratory is to understand the role of the OPBL in the coupled exchange of energy, momentum and mass between the upper ocean and the atmosphere (and the cryosphere).Document Number: N0001401WR2039
Comparison of the X-TRACK altimetry estimated currents with moored ADCP and HF radar observations on the West Florida Shelf
The performance of coastal altimetry over a wide continental shelf is
assessed using multiple-year ocean current observations by moored Acoustic
Doppler Current Profilers (ADCP) and high frequency (HF) radar on the West
Florida Shelf. Across track, surface geostrophic velocity anomalies, derived
from the XTRACK along-track sea level anomalies are compared with the near
surface current vector components from moored ADCP observations at mid shelf.
The altimeter derived velocity anomalies are also directly compared with the HF
radar surface current vector radial components that are aligned perpendicular
to the satellite track. Preliminary results indicate the potential usefulness
of the along-track altimetry data in contributing to descriptions of the
surface circulation on the West Florida Shelf and the challenges of such
applications. On subtidal time scales, the root mean square difference (rmsd)
between the estimated and the observed near surface velocity component
anomalies is 8 11 cms-1, which is about the same magnitude as the standard
deviations of the velocity components themselves. Adding a wind-driven Ekman
velocity component generally helps to reduce the rmsd values
Shallow- water hardbottom communities support the separation of biogeographic provinces on the west- central Florida Gulf Coast
Several studies have found separation of biogeographic provinces on the West Florida Shelf (WFS), but the location of this separation differs depending on different organisms with faunal boundaries proposed at Apalachicola, Cedar Key, Anclote Key. Tampa Bay, Charlotte Harbor, Cape Romano, or Cape Sable. Biogeographic boundaries can be gradual over a given space and are often species-specific. Analyses of marine benthic mapping and community characterization of Florida’s West-central coast shallow water (depth) hardbottom habitats indicate a major shift in the benthos across Tampa Bay. Quantitative benthic surveys of 29 sites yielded a total of 4,079 individuals of nine stony coral species and 1,918 soft coral colonies. Populations were dominated by four species of corals: Siderastrea radians, Oculina robusta, Solenastrea hyades, and Cladacora arbuscula. Most corals were less than 10 cm in diameter. Cluster analyses of coral density and major functional group percent cover showed distinct differences in hard and soft coral densities and species demographics from south to north with clear spatial patterns between regions. These benthic hardbottom coral communities change over a relatively small spatial scale (10’s of km), indicating a biogeographical province or ecosystem region boundary in marine benthic communities at, or very near, the mouth of Tampa Bay. Broader studies are needed to identify the shifts in benthic community biogeography along the West Florida Shelf
Tracing the early development of harmful algal blooms with the aid of Lagrangian coherent structures
Several theories have been proposed to explain the development of harmful
algal blooms (HABs) produced by the toxic dinoflagellate \emph{Karenia brevis}
on the West Florida Shelf. However, because the early stages of HAB development
are usually not detected, these theories have been so far very difficult to
verify. In this paper we employ simulated \emph{Lagrangian coherent structures}
(LCSs) to trace the early location of a HAB in late 2004 before it was
transported to an area where it could be detected by satellite imagery, and
then we make use of a population dynamics model to infer the factors that may
have led to its development. The LCSs, which are computed based on a surface
flow description provided by an ocean circulation model, delineate past and
future histories of boundaries of passively advected fluid domains. The
population dynamics model determines nitrogen in two components, nutrients and
phytoplankton, which are assumed to be passively advected by the simulated
surface currents. Two nearshore nutrient sources are identified for the HAB
whose evolution is found to be strongly tied to the simulated LCSs. While one
nutrient source can be associated with a coastal upwelling event, the other is
seen to be produced by river runoff, which provides support to a theory of HAB
development that considers nutrient loading into coastal waters produced by
human activities as a critical element. Our results show that the use of
simulated LCSs and a population dynamics model can greatly enhance our
understanding of the early stages of the development of HABs.Comment: Submitted to JGR-Ocean
Synopsis of biological data on the porgies, Calamus arctifrons and C. proridens (Pisces: Sparidae)
Information on the biology and fishery resources of two common species of western Atlantic porgies, Calamus areli/rons and C. proridens, is compiled, reviewed, and
analyzed in the FAO species synopsis style. (PDF file contains 25 pages.
Dynamically Constrained Ensemble Perturbations: Application to Tides on the West Florida Shelf
Abstract. A method is presented to create an ensemble of perturbations that satisfies linear dynamical constraints. A cost function is formulated defining the probability of each perturbation. It is shown that the perturbations created with this approach take the land-sea mask into account in a similar way as variational analysis techniques. The impact of the land-sea mask is illustrated with an idealized configuration of a barrier island. Perturbations with a spatially variable correlation length can be also created by this approach. The method is applied to a realistic configuration of the West Florida Shelf to create perturbations of the M2 tidal parameters for elevation and depth-averaged currents. The perturbations are weakly constrained to satisfy the linear shallow-water equations. Despite that the constraint is derived from an idealized assumption, it is shown that this approach is applicable to a non-linear and baroclinic model. The amplitude of spurious transient motions created by constrained perturbations of initial and boundary conditions is significantly lower compared to perturbing the variables independently or to using only the momentum equation to compute the velocity perturbations from the elevation
Lagrangian dynamical geography of the Gulf of Mexico
We construct a Markov-chain representation of the surface-ocean Lagrangian
dynamics in a region occupied by the Gulf of Mexico (GoM) and adjacent portions
of the Caribbean Sea and North Atlantic using satellite-tracked drifter
trajectory data, the largest collection so far considered. From the analysis of
the eigenvectors of the transition matrix associated with the chain, we
identify almost-invariant attracting sets and their basins of attraction. With
this information we decompose the GoM's geography into weakly dynamically
interacting provinces, which constrain the connectivity between distant
locations within the GoM. Offshore oil exploration, oil spill contingency
planning, and fish larval connectivity assessment are among the many activities
that can benefit from the dynamical information carried in the geography
constructed here.Comment: Submitted to Scientific Report
Extracting quasi-steady Lagrangian transport patterns from the ocean circulation: An application to the Gulf of Mexico
We construct a climatology of Lagrangian coherent structures (LCSs), the
concealed skeleton that shapes transport, with a twelve-year-long
data-assimilative simulation of the sea-surface circulation in the Gulf of
Mexico (GoM). Computed as time-mean Cauchy-Green strain tensorlines of the
climatological velocity, the climatological LCSs (cLCSs) unveil recurrent
Lagrangian circulation patterns. cLCSs strongly constrain the ensemble-mean
Lagrangian circulation of the instantaneous model velocity, thus we show that a
climatological velocity may preserve meaningful transport information. Also,
the climatological transport patterns we report agree well with GoM kinematics
and dynamics, as described in several previous observational and numerical
studies. For example, cLCSs identify regions of persistent isolation, and
suggest that coastal regions previously identified as high-risk for pollution
impact, are regions of maximal attraction. Also, we show examples where cLCSs
are remarkably similar to transport patterns observed during the Deepwater
Horizon and Ixtoc oil spills, and during the Grand LAgrangian Deployment (GLAD)
experiment. Thus, it is shown that cLCSs are an efficient way of synthesizing
vast amounts of Lagrangian information. The cLCS method confirms previous GoM
studies, and contributes to our understanding by revealing the persistent
nature of the dynamics and kinematics treated therein.Comment: To be submitte
Seafloor ripples created by waves from Hurricane Ivan on the west Florida shelf
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2006Recent studies have shown that the presence of sand ripples on the seabed
improves sonar detection of buried mines at sub-critical angles. Sidescan sonar data of
ripples on the west Florida shelf were collected as part of ONR's Ripples
Departmental Research Initiative (DRI) September 26-29th and November 7-9th, 2004.
Hurricane Ivan, the strongest storm of the 2004 hurricane season, passed over the
experiment site a week before the first data collection. This study focuses on the ripples
created by Ivan. Average relict ripple wavelengths left after the storm were found to
increase with water depth (50 cm, 62 cm, and 83 cm in 20, 30, and 50 meter water
depths) despite the fact that orbital diameter decreases with water depth.
Ripple prediction requires information about surface gravity waves and sediment
grain size. The most reliable offshore wave field available was created with Wavewatch
III by Naval Postgraduate School scientists. These waves were inputted into Delft3D
WAVE, incorporating the nearshore wave model SWAN to predict waves at the locations
where ripples were measured. Orbital motions at the seabed and grain size were inputted
into a time-dependent ripple model with varying dissipation parameters to estimate sand
ripples created by Hurricane Ivan. Ripple wavelength was found to be more strongly
dependent on grain size than wave dissipation.Funding from the Oceanographer of the Navy and
the Rear Admiral Richard F. Pittenger, USN (Ret) Fellowship
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