Fratantoni David M.

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Fratantoni
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David M.
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Now showing 1 - 14 of 14
  • Article
    Equilibration and circulation of Red Sea Outflow water in the western Gulf of Aden
    (American Meteorological Society, 2005-11) Bower, Amy S. ; Johns, William E. ; Fratantoni, David M. ; Peters, Hartmut
    Hydrographic, direct velocity, and subsurface float observations from the 2001 Red Sea Outflow Experiment (REDSOX) are analyzed to investigate the gravitational and dynamical adjustment of the Red Sea Outflow Water (RSOW) where it is injected into the open ocean in the western Gulf of Aden. During the winter REDSOX cruise, when outflow transport was large, several intermediate-depth salinity maxima (product waters) were formed from various bathymetrically confined branches of the outflow plume, ranging in depth from 400 to 800 m and in potential density from 27.0 to 27.5 σθ, a result of different mixing intensity along each branch. The outflow product waters were not dense enough to sink to the seafloor during either the summer or winter REDSOX cruises, but analysis of previous hydrographic and mooring data and results from a one-dimensional plume model suggest that they may be so during wintertime surges of strong outflow currents, or about 20% of the time during winter. Once vertically equilibrated in the Gulf of Aden, the shallowest RSOW was strongly influenced by mesoscale eddies that swept it farther into the gulf. The deeper RSOW was initially more confined by the walls of the Tadjura Rift, but eventually it escaped from the rift and was advected mainly southward along the continental slope. There was no evidence of a continuous boundary undercurrent of RSOW similar to the Mediterranean Undercurrent in the Gulf of Cadiz. This is explained by considering 1) the variability in outflow transport and 2) several different criteria for separation of a jet at a sharp corner, which indicate that the outflow currents should separate from the boundary where they are injected into the gulf.
  • Article
    Real-time reporting of baleen whale passive acoustic detections from ocean gliders
    (Acoustical Society of America, 2013-08) Baumgartner, Mark F. ; Fratantoni, David M. ; Hurst, Thomas P. ; Brown, Moira W. ; Cole, Timothy V. N. ; Van Parijs, Sofie M. ; Johnson, Mark P.
    In the past decade, much progress has been made in real-time passive acoustic monitoring of marine mammal occurrence and distribution from autonomous platforms (e.g., gliders, floats, buoys), but current systems focus primarily on a single call type produced by a single species, often from a single location. A hardware and software system was developed to detect, classify, and report 14 call types produced by 4 species of baleen whales in real time from ocean gliders. During a 3-week deployment in the central Gulf of Maine in late November and early December 2012, two gliders reported over 25 000 acoustic detections attributed to fin, humpback, sei, and right whales. The overall false detection rate for individual calls was 14%, and for right, humpback, and fin whales, false predictions of occurrence during 15-min reporting periods were 5% or less. Transmitted pitch tracks—compact representations of sounds—allowed unambiguous identification of both humpback and fin whale song. Of the ten cases when whales were sighted during aerial or shipboard surveys and a glider was within 20 km of the sighting location, nine were accompanied by real-time acoustic detections of the same species by the glider within ±12 h of the sighting time.
  • Article
    Collective motion, sensor networks, and ocean sampling
    (IEEE, 2007-01) Leonard, Naomi Ehrich ; Paley, Derek A. ; Lekien, Francois ; Sepulchre, Rodolphe ; Fratantoni, David M. ; Davis, Russ E.
    This paper addresses the design of mobile sensor networks for optimal data collection. The development is strongly motivated by the application to adaptive ocean sampling for an autonomous ocean observing and prediction system. A performance metric, used to derive optimal paths for the network of mobile sensors, defines the optimal data set as one which minimizes error in a model estimate of the sampled field. Feedback control laws are presented that stably coordinate sensors on structured tracks that have been optimized over a minimal set of parameters. Optimal, closed-loop solutions are computed in a number of low-dimensional cases to illustrate the methodology. Robustness of the performance to the influence of a steady flow field on relatively slow-moving mobile sensors is also explored.
  • Article
    Mixing and entrainment in the Red Sea outflow plume. Part I : plume structure
    (American Meteorological Society, 2005-05) Peters, Hartmut ; Johns, William E. ; Bower, Amy S. ; Fratantoni, David M.
    When the salty and heavy water of the Red Sea exits from the Strait of Bab el Mandeb, it continues downslope into the Gulf of Aden mainly along two channels. The 130-km-long “Northern Channel” (NC) is topographically confined and is typically only 5 km wide. In it, the Red Sea plume shows unanticipated patterns of vertical structure, turbulent mixing, and entrainment. Above the seafloor a 25–120-m-thick weakly stratified layer shows little dilution along the channel. Hence this bottom layer undergoes only weak entrainment. In contrast, a 35–285-m-thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing. It is thus the interface that exhibits the bulk of entrainment of the Red Sea plume in the NC. The interfacial layer also carries most of the overall plume transport, increasingly so with downstream distance. The “Southern Channel” (SC) is wider than the NC and is accessed from the latter by a sill about 33 m above the floor of the NC. Entrainment into the bottom layer of the SC is diagnosed to be strong near the entry into the SC such that the near-bottom density and salinity are smaller in the SC than in the NC at the same distance from Bab el Mandeb. In comparison with winter conditions, the authors encountered weaker outflow with shallower equilibration depths during the summer cruise. Bulk Froude numbers computed for the whole plume varied within the range 0.2–1. Local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment.
  • Article
    Laboratory experiments on mesoscale vortices interacting with two islands
    (American Geophysical Union, 2005-09-30) Cenedese, Claudia ; Adduce, Claudia ; Fratantoni, David M.
    The present study investigates the interaction between a self-propagating cyclonic vortex with two right vertical cylinders and determines the conditions for a vortex to bifurcate into two or more vortices. As in previous studies, after the cyclonic vortex came in contact with a cylinder, fluid peeled off the outer edge of the vortex and a so-called “streamer” went around the cylinder in a counterclockwise direction. Under the right conditions, this fluid formed a new cyclonic vortex in the wake of the cylinder, causing bifurcation of the original vortex into two vortices. In some cases, two streamers formed and went around the two cylinders, each forming a new cyclonic vortex. During the experiments, three parameters were varied: G, the separation between the cylinders; d, the diameter of the incident vortex; and y, the distance of the center of the vortex from an axis passing through the center of the gap between the cylinders. The number of vortices generated by the interaction depends on the ratio G/d and on the geometry of the encounter, which is given by the ratio y/g, where g = G/2. An unexpected and revealing result was the formation of a dipole vortex downstream of the two islands for values of −2 < y/g < 0, 0.25 ≤ G/d ≤ 0.4, and Re G > 200, where Re G = U G G/ν is the Reynolds number and U G is the maximum velocity of the vortex fluid in the gap. A possible mechanism is that the flow within the vortex was funneled between the two islands, and provided it had a sufficiently high velocity, a dipole formed, much like water ejected from a circular nozzle generates a dipole ring. The formation of a vortex of opposite sign to the incident vortex (i.e., anticyclonic) is in agreement with recent observations of North Brazil Current (NBC) rings interacting with the islands of Saint Vincent and Barbados in the eastern Caribbean. The passage between the islands of Saint Vincent and Barbados has values of G/d of approximately 0.5; hence the laboratory result suggests that both cyclonic and anticyclonic vortices could form downstream of them.
  • Article
    The CLIMODE field campaign : observing the cycle of convection and restratification over the Gulf Stream
    (American Meteorological Society, 2009-09) Marshall, John C. ; Ferrari, Raffaele ; Forget, Gael ; Andersson, A. ; Bates, Nicholas R. ; Dewar, William K. ; Doney, Scott C. ; Fratantoni, David M. ; Joyce, Terrence M. ; Straneo, Fiamma ; Toole, John M. ; Weller, Robert A. ; Edson, James B. ; Gregg, M. C. ; Kelly, Kathryn A. ; Lozier, M. Susan ; Palter, Jaime B. ; Lumpkin, Rick ; Samelson, Roger M. ; Skyllingstad, Eric D. ; Silverthorne, Katherine E. ; Talley, Lynne D. ; Thomas, Leif N.
    A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as “mode water” in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air–sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.
  • Article
    AXIS—an Autonomous Expendable Instrument System
    (American Meteorological Society, 2017-12-28) Fratantoni, David M. ; O’Brien, Jeff ; Flagg, Charles Noel ; Rossby, H. Thomas
    Expendable bathythermographs (XBT) to profile upper-ocean temperatures from vessels in motion have been in use for some 50 years now. Developed originally for navy use, they were soon adapted by oceanographers to map out upper-ocean thermal structure and its space–-time variability from both research vessels and merchant marine vessels in regular traffic. These activities continue today. This paper describes a new technology—the Autonomous Expendable Instrument System (AXIS)—that has been developed to provide the capability to deploy XBT probes on a predefined schedule, or adaptively in response to specific events without the presence of an observer on board. AXIS is a completely self-contained system that can hold up to 12 expendable probes [XBTs, XCTDs, expendable sound velocimeter (XSV)] in any combination. A single-board Linux computer keeps track of what probes are available, takes commands from ashore via Iridium satellite on what deployment schedule to follow, and records and forwards the probe data immediately with a time stamp and the GPS position. This paper provides a brief overview of its operation, capabilities, and some examples of how it is improving coverage along two lines in the Atlantic.
  • Article
    AUV observations of the diurnal surface layer in the North Atlantic salinity maximum
    (American Meteorological Society, 2014-06) Hodges, Benjamin A. ; Fratantoni, David M.
    Autonomous underwater vehicle (AUV) surveys of temperature, salinity, and velocity in the upper 10 m of the ocean were carried out in low-wind conditions near the North Atlantic surface salinity maximum as part of the Salinity Processes in the Upper Ocean Regional Study (SPURS) project. Starting from a well-mixed state, the development, deepening, and decay of a warm salty diurnal surface layer was observed at <1-h resolution. The evaporation rate deduced from the freshwater anomaly of the layer corroborates measurements at a nearby flux mooring. Profiles within a few hundred meters of the stationary research vessel showed evidence of mixing, highlighting the effectiveness of AUVs for collecting uncontaminated time series of near-surface thermohaline structure. A two-dimensional horizontal subsurface survey within the diurnal warm layer revealed coherent warm and cool bands, which are interpreted as internal waves on the diurnal thermocline.
  • Article
    Salinity and temperature balances at the SPURS central mooring during fall and winter
    (The Oceanography Society, 2015-03) Farrar, J. Thomas ; Rainville, Luc ; Plueddemann, Albert J. ; Kessler, William S. ; Lee, Craig M. ; Hodges, Benjamin A. ; Schmitt, Raymond W. ; Edson, James B. ; Riser, Stephen C. ; Eriksen, Charles C. ; Fratantoni, David M.
    One part of the Salinity Processes in the Upper-ocean Regional Study (SPURS) field campaign focused on understanding the physical processes affecting the evolution of upper-ocean salinity in the region of climatological maximum sea surface salinity in the subtropical North Atlantic (SPURS-1). An upper-ocean salinity budget provides a useful framework for increasing this understanding. The SPURS-1 program included a central heavily instrumented mooring for making accurate measurements of air-sea surface fluxes, as well as other moorings, Argo floats, and gliders that together formed a dense observational array. Data from this array are used to estimate terms in the upper-ocean salinity and heat budgets during the SPURS-1 campaign, with a focus on the first several months (October 2012 to February 2013) when the surface mixed layer was becoming deeper, fresher, and cooler. Specifically, we examine the salinity and temperature balances for an upper-ocean mixed layer, defined as the layer where the density is within 0.4 kg m–3 of its surface value. The gross features of the evolution of upper-ocean salinity and temperature during this fall/winter season are explained by a combination of evaporation and precipitation at the sea surface, horizontal transport of heat and salt by mixed-layer currents, and vertical entrainment of fresher, cooler fluid into the layer as it deepened. While all of these processes were important in the observed seasonal (fall) freshening at this location in the salinity-maximum region, the variability of salinity on monthly-to-intraseasonal time scales resulted primarily from horizontal advection.
  • Article
    The evolution and demise of North Brazil Current rings
    (American Meteorological Society, 2006-07) Fratantoni, David M. ; Richardson, Philip L.
    Subsurface float and surface drifter observations illustrate the structure, evolution, and eventual demise of 10 North Brazil Current (NBC) rings as they approached and collided with the Lesser Antilles in the western tropical Atlantic Ocean. Upon encountering the shoaling topography east of the Lesser Antilles, most of the rings were deflected abruptly northward and several were observed to completely engulf the island of Barbados. The near-surface and subthermocline layers of two rings were observed to cleave or separate upon encountering shoaling bathymetry between Tobago and Barbados, with the resulting portions each retaining an independent and coherent ringlike vortical circulation. Surface drifters and shallow (250 m) subsurface floats that looped within NBC rings were more likely to enter the Caribbean through the passages of the Lesser Antilles than were deeper (500 or 900 m) floats, indicating that the regional bathymetry preferentially inhibits transport of intermediate-depth ring components. No evidence was found for the wholesale passage of rings through the island chain.
  • Article
    Satellites to seafloor : toward fully autonomous ocean sampling
    (Oceanography Society, 2017-06) Thompson, Andrew F. ; Chao, Yi ; Chien, Steve ; Kinsey, James C. ; Flexas, M. Mar ; Erickson, Zachary K. ; Farrara, John ; Fratantoni, David M. ; Branch, Andrew ; Chu, Selina ; Troesch, Martina ; Claus, Brian ; Kepper, James
    Future ocean observing systems will rely heavily on autonomous vehicles to achieve the persistent and heterogeneous measurements needed to understand the ocean’s impact on the climate system. The day-to-day maintenance of these arrays will become increasingly challenging if significant human resources, such as manual piloting, are required. For this reason, techniques need to be developed that permit autonomous determination of sampling directives based on science goals and responses to in situ, remote-sensing, and model-derived information. Techniques that can accommodate large arrays of assets and permit sustained observations of rapidly evolving ocean properties are especially needed for capturing interactions between physical circulation and biogeochemical cycling. Here we document the first field program of the Satellites to Seafloor project, designed to enable a closed loop of numerical model prediction, vehicle path-planning, in situ path implementation, data collection, and data assimilation for future model predictions. We present results from the first of two field programs carried out in Monterey Bay, California, over a period of three months in 2016. While relatively modest in scope, this approach provides a step toward an observing array that makes use of multiple information streams to update and improve sampling strategies without human intervention.
  • Article
    Autonomous multi-platform observations during the Salinity Processes in the Upper-ocean Regional Study
    (Oceanography Society, 2017-06) Lindstrom, Eric ; Shcherbina, Andrey Y. ; Rainville, Luc ; Farrar, J. Thomas ; Centurioni, Luca R. ; Dong, Shenfu ; D'Asaro, Eric A. ; Eriksen, Charles C. ; Fratantoni, David M. ; Hodges, Benjamin A. ; Hormann, Verena ; Kessler, William S. ; Lee, Craig M. ; Riser, Stephen C. ; St. Laurent, Louis C. ; Volkov, Denis L.
    The Salinity Processes in the Upper-ocean Regional Study (SPURS) aims to understand the patterns and variability of sea surface salinity. In order to capture the wide range of spatial and temporal scales associated with processes controlling salinity in the upper ocean, research vessels delivered autonomous instruments to remote sites, one in the North Atlantic and one in the Eastern Pacific. Instruments sampled for one complete annual cycle at each of these two sites, which are subject to contrasting atmospheric forcing. The SPURS field programs coordinated sampling from many different platforms, using a mix of Lagrangian and Eulerian approaches. This article discusses the motivations, implementation, and first results of the SPURS-1 and SPURS-2 programs.
  • Article
    A thin layer of phytoplankton observed in the Philippine Sea with a synthetic moored array of autonomous gliders
    (American Geophysical Union, 2009-10-23) Hodges, Benjamin A. ; Fratantoni, David M.
    A synthetic moored array composed of five buoyancy-propelled autonomous underwater gliders was used to characterize mesoscale variability and phytoplankton distribution in a 100 km × 100 km domain in the Philippine Sea east of Luzon Strait for 10 days in May 2004. The study area, located east of the Kuroshio near the subtropical front, is dominated by strong internal tides, by energetic westward-propagating mesoscale eddies with azimuthal velocities exceeding 50 cm/s, and by a deep (130 m) maximum in chlorophyll fluorescence. Each glider in the array was instructed to maintain geographic position while repeatedly profiling to 200-m depth. Good station-keeping performance enabled the resulting series of vertical profiles to be interpreted in the same manner as a physically moored chain of instruments. Although organized primarily as a demonstration of glider capabilities, this field exercise provides a unique data set for examining biological-physical interactions in the open ocean. Here we report on the evolution of a thin layer of phytoplankton observed near the deep chlorophyll maximum. Coincident observations of fine structure in temperature and salinity suggest that the thinning process of this layer was driven primarily by physical forcing, most probably vertical shear associated with energetic diurnal internal waves, as opposed to a biological mechanism, such as convergent swimming, grazing, or spatial variation in growth rate.
  • Article
    Somali Current rings in the eastern Gulf of Aden
    (American Geophysical Union, 2006-09-30) Fratantoni, David M. ; Bower, Amy S. ; Johns, William E. ; Peters, Hartmut
    New satellite-based observations reveal that westward translating anticyclonic rings are generated as a portion of the Somali Current accelerates northward through the Socotra Passage near the mouth of the Gulf of Aden. Rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm/s, are comparable in overall diameter to the width of the Gulf of Aden (250 km), and translate westward into the gulf at 5–8 cm/s. Ring generation is most notable in satellite ocean color imagery in November immediately following the transition between southwest (boreal summer) and northeast (winter) monsoon regimes. The observed rings contain anomalous fluid within their core which reflects their origin in the equator-crossing Somali Current system. Estimates of Socotra Passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in May following the winter-to-summer monsoon transition. Cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together. Recent shipboard and Lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern Gulf of Aden.