Fratantoni David M.

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Fratantoni
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David M.
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Article

Equilibration and circulation of Red Sea Outflow water in the western Gulf of Aden

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.

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Technical Report

EcoMapper operations---KN209-1

2012-12 , Hodges, Benjamin A. , Fratantoni, David M.

This report describes the collection of water property data from EcoMapper AUVs during the R/V Knorr 209-1 cruise as part of the SPURS (Salinity Processes Upper-ocean Regional Study) project. Post-processing was required to improve the quality of the raw data, particularly salinity, and is documented herein. Initial results from temperature and salinity records are presented. The measurements are concentrated in the upper 10 meters of the mixed layer during calm conditions, and reveal significant diurnal warming (up to 3°C) and salinification (up to 0.1 psu) of the surface (< 1 meter) layer. The mixing promoted by the motion of the research vessel destroys this shallow stratification, so the ability of the AUVs to sample undisturbed water hundreds of meters from the ship was critical to the effort of accurately resolving it.

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Technical Report

North Brazil Current Rings Experiment : mooring S1 data report, November 1998 - June 2000

2001-06 , Glickson, Deborah A. , Fratantoni, David M.

Nineteen months of temperature and salinity data were recovered from North Brazil Current (NBC) Rings Experiment Mooring S1. The mooring, located east of Barbados at 13º 00’N, 57º 53’W between November 1998 and June 2000, consisted of a vertical array of five temperature/conductivity recorders, five temperature recorders, one 150 kHz acoustic Doppler current profiler (ADCP), and one 260 Hz RAFOS sound source. This instrumentation was distributed over a depth interval (500-1100m) coincident with the low-salinity core of Antarctic Intermediate Water. Due to low concentration of scattering particles at 1000 m, the ADCP failed to return useful velocity data. Heading, pitch, and roll data were successfully recorded, however, and provide coarse measurement of current intensity. Four anomalously low temperature, low salinity, and (inferred) high-velocity events appear toward the end of the record. The temperature and salinity fluctuations observed during these events are most likely due to a combination of vertical instrument excursions due to current-induced mooring tilt and advection of anomalous NBC ring-core water past the mooring site. Anomalous conditions persist for a period of 2-3 weeks and appear, based on simultaneous surface drifter trajectories and satellite ocean color observations, to be associated with the passage of NBC Rings near Barbados.

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Article

The CLIMODE field campaign : observing the cycle of convection and restratification over the Gulf Stream

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.

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Article

AUV observations of the diurnal surface layer in the North Atlantic salinity maximum

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.

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Article

The evolution and demise of North Brazil Current rings

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.

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Technical Report

CLIMODE Subsurface Mooring Report : November 2005 - November 2007

2013-03 , Lund, John M. , Davis, Xujing Jia , Ramsey, Andree L. , Straneo, Fiamma , Torres, Daniel J. , Palter, Jaime B. , Gary, Stefan F. , Fratantoni, David M.

Two years of temperature, salinity, current, and nutrient data were collected on four subsurface moorings as part of the 2 year field component of the CLIMODE experiment. The moorings were located in North Atlantic’s subtropical gyre, south-east of the Gulf Stream. Two moorings, the most heavily instrumented, were close to the Gulf Stream, in the region where cold air outbreaks force large air-sea fluxes and where Eighteen Degree Water outcrops. Two other moorings were located farther south and carried more limited instrumentation. The moorings were initially deployed in November of 2005, turned around in November of 2006 and finally recovered in November of 2007. During the first year, the moorings close to the Gulf Stream suffered considerable blow down, and some of the instruments failed. During the second year, the blow down was greatly reduced and most instruments collected a full year worth of data.

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Technical Report

CLIVAR Mode Water Dynamics Experiment (CLIMODE) fall 2005, R/V Oceanus voyage 419, November 9, 2005–November 27, 2005

2006-02 , Hutto, Lara , Weller, Robert A. , Fratantoni, David M. , Lord, Jeffrey , Kemp, John N. , Lund, John M. , Brambilla, Elena , Bigorre, Sebastien P.

CLIMODE (CLIVAR Mode Water Dynamic Experiment) is a program designed to understand and quantify the processes responsible for the formation and dissipation of North Atlantic subtropical mode water, also called Eighteen Degree Water (EDW). Among these processes, the amount of buoyancy loss at the ocean-atmosphere interface is still uncertain and needs to be accurately quantified. In November 2005, a cruise was made aboard R/V Oceanus in the region of the separated Gulf Stream, where intense oceanic heat loss to the atmosphere is believed to trigger the formation of EDW. During that cruise, one surface mooring with IMET meteorological instruments was anchored in the core of the Gulf Stream as well as two moored profilers on its southeastern edge. Surface drifters, APEX floats and bobby RAFOS floats were also deployed along with two other moorings with sound sources. CTD profiles and water samples were also carried out. This array of instruments will permit a characterization of EDW with high spatial and temporal resolutions, and accurate in-situ measurements of air-sea fluxes in the formation region. The present report documents this cruise, the instruments that were deployed and the array of measurements that was set in place.

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Article

Collective motion, sensor networks, and ocean sampling

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.

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Article

Mixing and entrainment in the Red Sea outflow plume. Part I : plume structure

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.

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Technical Report

Red Sea Outflow Experiment (REDSOX) : DLD2 RAFOS float data report February 2001 - March 2003

2005-01 , Furey, Heather H. , Bower, Amy S. , Fratantoni, David M.

This is the final data report of all acoustically tracked second-generation Deep Lagrangian Drifter (DLD2) RAFOS float data collected by the Woods Hole Oceanographic Institution in 2001-2003 during the Red Sea Outflow Experiment (REDSOX). The float component of REDSOX was comprised of two deployments on the R/V Knorr and R/V Ewing: the first in February-March 2001, with 26 floats, and the second in August-September 2001, with 27 floats. The isobaric floats were ballasted for 650 decibars to target the intermediate-depth, high-salinity outflow waters from the Red Sea. The objectives of the Lagrangian float study were (1) to identify the spreading pathways of the equilibrated Red Sea outflow, and to quantify the velocities and eddy variability typical of this outflow and of the background oceanic environment in the Gulf of Aden, and (2) to identify and describe the mesoscale processes which contribute to the seaward transport of Red Sea Overflow Water properties through the Gulf of Aden and into the western Indian Ocean. In addition to floats activated and launched during the two cruises, four time-series sites were chosen for dual-release float moorings. The dual-release floats were released every two months between cruises and every two months after the second cruise, with the final release in March 2002. A pirate attack on the R/V Ewing forced some modification of the float deployment plan during the second cruise.

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Technical Report

OC449-09 Data Report : St. Thomas, USVI to Bermuda, December 1-10, 2008

2009-10 , Kirincich, Anthony R. , Hodges, Benjamin A. , Fratantoni, David M. , Bahr, Frank B.

Data collected during multiple surveys of hydrography, velocity, and biological quantities are presented from a 9-day cruise aboard the R/V Oceanus near the island of St. Thomas, USVI and a subsequent transit to Bermuda during December, 2008. This cruise (OC449-09) was undertaken primarily to field test a newly acquired towed-undulating body, the Scanfish. The Scanfish and a second towed body, the Video Plankton Recorder (VPR), were used to survey hydrographic, optical, and biological properties north and south of St. Thomas. Conductivity-Temperature-Depth (CTD) casts and plankton net-tows were made at locations along the survey transects for inter-comparison. The VPR was also used to profile conditions between St. Thomas and Bermuda during transit. An overview of the cruise is given along with descriptions of the data collection methods, processing steps taken, and data products available for distribution.

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Technical Report

CLIMODE bobber data report : July 2005 - May 2009

2010-03 , Fratantoni, David M. , McKee, Theresa K. , Hodges, Benjamin A. , Furey, Heather H. , Lund, John M.

This report summarizes direct observations of Eighteen Degree Water (EDW) subduction and dispersal within the subtropical gyre of the North Atlantic Ocean. Forty acoustically-tracked bobbing, profiling floats (“bobbers”) were deployed to study the formation and dispersal of EDW in the western North Atlantic. The unique bobber dataset described herein provides insight into the evolution of EDW by means of direct, eddy-resolving measurement of EDW Lagrangian dispersal pathways and stratification. Bobbers are modified Autonomous Profiling Explorer (APEX) profiling floats which actively servo their buoyancy control mechanism to follow a particular isothermal surface. The CLIVAR Mode Water Dynamics Experiment (CLIMODE) bobbers tracked the 18.5°C temperature surface for 3 days, then bobbed quickly between the 17°C and 19°C isotherms. This cycle was repeated for one month, after which each bobber profiled to 1000 m before ascending to the surface to transmit data. The resulting dataset (37/40 tracked bobbers; more than half still profiling as of January 2010) yields well-resolved trajectories, unprecedented velocity statistics in the core of the subducting and spreading EDW, and detailed information about the Lagrangian evolution of EDW thickness and vertical structure. This report provides an overview of the experimental procedure employed and summarizes the initial processing of the bobber dataset.

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Technical Report

North Brazil Current Rings Experiment : surface drifter data report, November 1998-June 2000

2000-07 , Glickson, Deborah A. , Fratantoni, David M. , Wooding, Christine M. , Richardson, Philip L.

This data report summarzes 45 surface drifter trajectories collected between November 1998 and June 2000 as part of the North Brazil Current (NBC) Rings Experiment. NBC rings have been proposed as one of several important mechanisms for the transport of South Atlantic upper-ocean water across the equatorial-tropical gyre boundary and into the North Atlantic subtropical gyre. Such transport is required to complete the meridional overturning cell in the Atlantic forced by the high-latitude production and southward export of North Atlantic Deep Water. The goal of this program is to obtain, for the first time, comprehensive observations of the NBC retroflection, the NBC ring formation process, and the physical structure and properties of NBC rings as they translate northwestward along the low-latitude western boundary. A total of 45 drifters were deployed. Twenty-four of these looped anticyclonically within the five rings identified during this experiment. Seven of the looping ring drifters entered the Caribbean, while the rest moved northward along the eastern flank of the Lesser Antiles.

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Article

Real-time reporting of baleen whale passive acoustic detections from ocean gliders

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.

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Technical Report

North Brazil Current Rings Experiment : RAFOS float data report : November 1998 – June 2000

2002-07 , Wooding, Christine M. , Richardson, Philip L. , Pacheco, Marguerite A. , Glickson, Deborah A. , Fratantoni, David M.

Twenty-one RAFOS floats were tracked at depths of 200-1000 meters in and around several North Brazil Current Rings between November 1998 and June 2000. This was part of an experiment to study the role of these current rings in transporting upper level South Atlantic water across the equatorial-tropical gyre boundary into the North Atlantic subtropical gyre. The float trajectories in combination with surface drifters and satellite imagery reveal the sometimes complex life histories of several rings and their fate as they collide with the Lesser Antilles Islands. This report describes the float trajectories, the velocity, temperature, and depth time series, and a preliminary analysis of the float data.

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Article

Laboratory experiments on mesoscale vortices interacting with two islands

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.

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Article

AXIS—an Autonomous Expendable Instrument System

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.

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Article

Salinity and temperature balances at the SPURS central mooring during fall and winter

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.

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Preprint

The mid-depth circulation of the northwestern tropical Atlantic observed by floats

2009-05-29 , Lankhorst, Matthias , Fratantoni, David M. , Ollitrault, Michel , Richardson, Philip L. , Send, Uwe , Zenk, Walter

A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600–1050 dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200–2050 dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e. g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7 Sv across 16° N between the Antilles islands and the Mid- Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADWeast of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow.