Bower Amy S.

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Bower
First Name
Amy S.
ORCID
0000-0003-0902-4984

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  • Preprint
    Mesoscale eddies in the Gulf of Aden and their impact on the spreading of Red Sea Outflow Water
    ( 2010-11) Bower, Amy S. ; Furey, Heather H.
    The Gulf of Aden (GOA) in the northwestern Indian Ocean is the receiving basin for Red Sea Outflow Water (RSOW), one of the World’s few high-salinity dense overflows, but relatively little is known about spreading pathways and transformation of RSOW through the gulf. Here we combine historical data, satellite altimetry, new synoptic hydrographic surveys and the first in situ direct observations of subsurface currents in the GOA to identify the most important processes in the spreading of RSOW. The new in situ data sets were collected in 2001-2003 as part of the Red Sea Outflow Experiment (REDSOX) and consist of two CTD/LADCP Surveys and 49 one-year trajectories from acoustically tracked floats released at the depth of RSOW. The results indicate that the prominent positive and negative sea level anomalies frequently observed in the GOA with satellite altimetry are associated with anticyclonic and cyclonic eddies that often reach to at least 1000 m depth, i.e., through the depth range of equilibrated RSOW. The eddies dominate RSOW spreading pathways and help to rapidly mix the outflow water with the background. Eddies in the central and eastern gulf are basin-scale (~250-km diameter) and have maximum azimuthal speeds of about 30 cm/s at the RSOW level. In the western gulf, smaller eddies not detectable with satellite altimetry appear to form as the larger westward-propagating eddies impale themselves on the high ridges flanking the Tadjura Rift. Both the hydrographic and Lagrangian observations show that eddies originating outside the gulf often transport a core of much cooler, fresher water from the Arabian Sea all the way to the western end of the GOA, where the highest-salinity outflow water is found. This generates large vertical and horizontal gradients of temperature and salinity, setting up favorable conditions for salt fingering and diffusive convection. Both of these mixing processes were observed to be active in the gulf. Two new annually appearing anticyclonic eddies are added to the previously identified Gulf of Aden Eddy (GAE; Prasad and Ikeda, 2001) and Somali Current Ring (SCR; Fratantoni et al., 2006). These are the Summer Eddy (SE) and the Lee Eddy (LE), both of which form at the beginning of the summer monsoon when strong southwest winds blowing through Socotra Passage effectively split the GAE into two smaller eddies. The SE strengthens as it propagates westward deeper in the GOA, while the Lee Eddy remains stationary in the lee of Socotra Island. Both eddies are strengthened or sustained by Ekman convergence associated with negative wind stress curl patches caused by wind jets through or around high orography. The annual cycle in the appearance, propagation and demise of these new eddies and those described in earlier work is documented to provide a comprehensive view of the most energetic circulation features in the GOA. The observations contain little evidence of features that have been shown previously to be important in the spreading of Mediterranean Outflow Water (MOW) in the North Atlantic, namely a wall-bounded subsurface jet (the Mediterranean Undercurrent ) and submesoscale coherent lenses containing a core of MOW (‘meddies’). This is attributed to the fact that the RSOW enters the open ocean on a western boundary. High background eddy kinetic energy typical of western boundary regimes will tend to shear apart submesoscale eddies and boundary undercurrents. Even if a submesoscale lens of RSOW did form in the GOA, westward self-propagation would transport the eddy and its cargo of outflow water back toward, rather than away from, its source.
  • Article
    Observed deep cyclonic eddies around Southern Greenland
    (American Meteorological Society, 2021-10-01) Zou, Sijia ; Bower, Amy S. ; Furey, Heather H. ; Pickart, Robert S. ; Houpert, Loïc ; Holliday, Naomi Penny
    Recent mooring measurements from the Overturning in the Subpolar North Atlantic Program have revealed abundant cyclonic eddies at both sides of Cape Farewell, the southern tip of Greenland. In this study, we present further observational evidence, from both Eulerian and Lagrangian perspectives, of deep cyclonic eddies with intense rotation (ζ/f > 1) around southern Greenland and into the Labrador Sea. Most of the observed cyclones exhibit strongest rotation below the surface at 700–1000 dbar, where maximum azimuthal velocities are ~30 cm s−1 at radii of ~10 km, with rotational periods of 2–3 days. The cyclonic rotation can extend to the deep overflow water layer (below 1800 dbar), albeit with weaker azimuthal velocities (~10 cm s−1) and longer rotational periods of about one week. Within the middepth rotation cores, the cyclones are in near solid-body rotation and have the potential to trap and transport water. The first high-resolution hydrographic transect across such a cyclone indicates that it is characterized by a local (both vertically and horizontally) potential vorticity maximum in its middepth core and cold, fresh anomalies in the deep overflow water layer, suggesting its source as the Denmark Strait outflow. Additionally, the propagation and evolution of the cyclonic eddies are illustrated with deep Lagrangian floats, including their detachments from the boundary currents to the basin interior. Taken together, the combined Eulerian and Lagrangian observations have provided new insights on the boundary current variability and boundary–interior exchange over a geographically large scale near southern Greenland, calling for further investigations on the (sub)mesoscale dynamics in the region.
  • Article
    Dominant circulation patterns of the deep Gulf of Mexico
    (American Meteorological Society, 2018-03-01) Perez-Brunius, Paula ; Furey, Heather H. ; Bower, Amy S. ; Hamilton, Peter ; Candela, Julio ; García-Carrillo, Paula ; Leben, Robert
    The large-scale circulation of the bottom layer of the Gulf of Mexico is analyzed, with special attention to the historically least studied western basin. The analysis is based on 4 years of data collected by 158 subsurface floats parked at 1500 and 2500 m and is complemented with data collected by current meter moorings in the western basin during the same period. Three main circulation patterns stand out: a cyclonic boundary current, a cyclonic gyre in the abyssal plain, and the very high eddy kinetic energy observed in the eastern Gulf. The boundary current and the cyclonic gyre appear as distinct features, which interact in the western tip of the Yucatan shelf. The persistence and continuity of the boundary current is addressed. Although high variability is observed, the boundary flow serves as a pathway for water to travel around the western basin in approximately 2 years. An interesting discovery is the separation of the boundary current over the northwestern slope of the Yucatan shelf. The separation and retroflection of the along-slope current appears to be a persistent feature and is associated with anticyclonic eddies whose genesis mechanism remains to be understood. As the boundary flow separates, it feeds into the westward flow of the deep cyclonic gyre. The location of this gyre—named the Sigsbee Abyssal Gyre—coincides with closed geostrophic contours, so eddy–topography interaction via bottom form stresses may drive this mean flow. The contribution to the cyclonic vorticity of the gyre by modons traveling under Loop Current eddies is discussed.
  • Article
    A Deep Water Dispersion Experiment in the Gulf of Mexico
    (American Geophysical Union, 2021-09-18) Meunier, Thomas ; Pérez-Brunius, Paula ; Rodríguez Outerelo, Javier ; García-Carrillo, Paula ; Ronquillo-Mendez, Argelia ; Furey, Heather H. ; Ramsey, Andree L. ; Bower, Amy S.
    The Deep Water Horizon oil spill dramatically impacted the Gulf of Mexico from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied, little is known about deep water dispersion properties. This study describes the results of the Deep Water Dispersion Experiment (DWDE), which consisted of the release of surface drifters and acoustically tracked RAFOS floats drifting at 300 and 1,500 dbar in the Gulf of Mexico. We show that surface diffusivity is elevated and decreases with depth: on average, diffusivity at 1,500 dbar is 5 times smaller than at the surface, suggesting that the dispersion of contaminants at depth is a significantly slower process than at the surface. This study also examines the turbulent regimes driving the dispersion, although conflicting evidences and large uncertainties do not allow definitive conclusions. At all depths, while the growth of dispersion and kurtosis with time supports the possibility of an exponential regime at very short time scales, indicating that early dispersion is nonlocal, finite size Lyapunov exponents support the hypothesis of local dispersion, suggesting that eddies of size comparable to the initial separation (6 km), may dominate the early dispersion. At longer time scales, the quadratic growth of dispersion is indicative of a ballistic regime, where a mean shear flow would be the dominating process. Examination of the along- and across-bathymetry components of float velocities supports the idea that boundary currents could be the source for this shear dispersion.
  • Technical Report
    Warm water pathways in the northeastern North Atlantic ACCE RAFOS float data report November 1996 - November 1999
    (Woods Hole Oceanographic Institution, 2001-11) Furey, Heather H. ; Bower, Amy S. ; Richardson, Philip L.
    This is the final data report of all acoustically tracked RAFOS float data collected by the Woods Hole Oceanographic Institution in 1996-1999 during the Atlantic Climate Change Experiment (ACCE). The RAFOS float component of ACCE, entitled "Warm Water Pathways and Intergyre Exchange in the Northeastern North Atlantic," was designed to measure the warm water currents entering the northeastern North Atlantic which become the source of intermediate and deep waters in the subpolar region. The experiment was comprised of three RAFOS float deployments on the R/V Knorr: the first in fall 1996 along the continental slope seaward of Porcupine Bank, the second in spring 1997 along the mid-Atlantic Ridge, and the final deployment in fall 1997 along both the Ridge and the Bank. Seventy floats were deployed, 13 RAFOS and 2 ALFOS in fall 1996, 14 RAFOS in spring 1997, and 41 RAFOS in fall 1997. The isobaric ALFOS floats were ballasted for 800 decibars and were launched to monitor the regions' sound sources during the experiment. The RAFOS floats were isopycnal and ballasted for the 27.5 sigma-t surface to target the intermediate-depth North Atlantic and Poleward Eastern Boundary Currents. The objectives of the Lagrangian float study were (1) to provide a quantitative description of the bifurcation of the North Atlantic Current east of the Mid-Atlantic Ridge, (2) to assess the importance of meridional eddy fluxes, compared to large-scale advection, in the northward flux of heat and salt in the northeastern North Atlantic, and (3) to establish the degree of continuity of the Poleward Eastern Boundary Current as it flows to the entrance of the Norwegian Sea and the fate of the Mediterranean Outflow Water carried by this current.
  • Article
    Two years of observations of warm-core anticyclones in the Labrador Sea and their seasonal cycle in heat and salt stratification
    (American Meteorological Society, 2014-02) de Jong, Marieke F. ; Bower, Amy S. ; Furey, Heather H.
    Between 25 September 2007 and 28 September 2009, a heavily instrumented mooring was deployed in the Labrador Sea, offshore of the location where warm-core, anticyclonic Irminger rings are formed. The 2-year time series offers insight into the vertical and horizontal structure of newly formed Irminger rings and their heat and salt transport into the interior basin. In 2 years, 12 Irminger rings passed by the mooring. Of these, 11 had distinct properties, while 1 anticyclone likely passed the mooring twice. Eddy radii (11–35 km) were estimated using the dynamic height signal of the anticyclones (8–18 cm) together with the observed velocities. The anticyclones show a seasonal cycle in core properties when observed (1.9°C in temperature and 0.07 in salinity at middepth) that has not been described before. The temperature and salinity are highest in fall and lowest in spring. Cold, fresh caps, suggested to be an important source of freshwater, were seen in spring but were almost nonexistent in fall. The heat and freshwater contributions by the Irminger rings show a large spread (from 12 to 108 MJ m−2 and from −0.5 to −4.7 cm, respectively) for two reasons. First, the large range of radii leads to large differences in transported volume. Second, the seasonal cycle leads to changes in heat and salt content per unit volume. This implies that estimates of heat and freshwater transport by eddies should take the distribution of eddy properties into account in order to accurately assess their contribution to the restratification.
  • Article
    Iceland-Scotland Overflow Water transport variability through the Charlie-Gibbs Fracture Zone and the impact of the North Atlantic Current
    (John Wiley & Sons, 2017-09-01) Bower, Amy S. ; Furey, Heather H.
    The Charlie-Gibbs Fracture Zone (CGFZ), a deep and wide gap in the Mid-Atlantic Ridge near 52°N, is a gateway between the eastern and western subpolar regions for the Atlantic Meridional Overturning Circulation (AMOC). In 2010–2012, an eight-mooring array of current meters and temperature/salinity sensors was installed across the CGFZ between 500 m and the sea floor to measure the mean transport of westward-flowing Iceland-Scotland Overflow Water (ISOW) and investigate the impact of the eastward-flowing North Atlantic Current (NAC) on ISOW transport variability. The 22 month record mean ISOW transport through the CGFZ, −1.7 ± 0.5 Sv (95% confidence interval), is 30% lower than the previously published estimate based on 13 months of current-only measurements, −2.4 ± 1.2 Sv. The latter mean estimate may have been biased high due to the lack of continuous salinity measurements, although the two estimates are not statistically different due to strong mesoscale variability in both data sets. Empirical Orthogonal Function analysis and maps of satellite-derived absolute dynamic topography show that weak westward ISOW transport events and eastward reversals are caused by northward meanders of the NAC, with its deep-reaching eastward velocities. These results add to growing evidence that a significant fraction of ISOW exits the Iceland Basin by routes other than the CGFZ.
  • Article
    Overflow Water pathways in the North Atlantic
    (Elsevier, 2022-09-09) Lozier, M. Susan ; Bower, Amy S. ; Furey, Heather H. ; Drouin, Kimberley L. ; Xu, Xiaobiao ; Zou, Sijia
    As part of the international Overturning in the Subpolar North Atlantic Program (OSNAP), 135 acoustically-tracked deep floats were deployed to track the spreading pathways of Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) from 2014 to 2018. These water masses, which originate in the Nordic Seas, are transported by the deepest branch of the Atlantic Meridional Overturning Circulation (AMOC). The OSNAP floats provide the first directly-observed, comprehensive Lagrangian view of ISOW and DSOW spreading pathways throughout the subpolar North Atlantic. The collection of OSNAP float trajectories, complemented by model simulations, reveals that their pathways are (a) not restricted to western boundary currents, and (b) remarkably different from each other in character. The spread of DSOW from the Irminger Sea is primarily via the swift deep boundary currents of the Irminger and Labrador Seas, whereas the spread of ISOW out of the Iceland Basin is slower and along multiple export pathways. The characterization of these Overflow Water pathways has important implications for our understanding of the AMOC and its variability. Finally, reconstructions of AMOC variability from proxy data, involving either the strength of boundary currents and/or the property variability of deep waters, should account for the myriad pathways of DSOW and ISOW, but particularly so for the latter.
  • Article
    Redrawing the Iceland−Scotland overflow water pathways in the North Atlantic
    (Nature Research, 2020-04-20) Zou, Sijia ; Bower, Amy S. ; Furey, Heather H. ; Lozier, M. Susan ; Xu, Xiaobiao
    Iceland-Scotland Overflow Water (ISOW) is a primary deep water mass exported from the Norwegian Sea into the North Atlantic as part of the global Meridional Overturning Circulation. ISOW has historically been depicted as flowing counter-clockwise in a deep boundary current around the subpolar North Atlantic, but this single-boundary-following pathway is being challenged by new Lagrangian observations and model simulations. We show here that ISOW leaves the boundary and spreads into the interior towards the central Labrador and Irminger basins after flowing through the Charlie-Gibbs Fracture Zone. We also describe a newly observed southward pathway of ISOW along the western flank of the Mid-Atlantic Ridge. The partitioning of these pathways is shown to be influenced by deep-reaching eddies and meanders of the North Atlantic Current. Our results, in tandem with previous studies, call for a revision in the historical depiction of ISOW pathways throughout the North Atlantic.
  • Article
    The Loop Current: Observations of deep eddies and topographic waves.
    (American Meteorological Society, 2019-05-29) Hamilton, Peter ; Bower, Amy S. ; Furey, Heather H. ; Leben, Robert ; Pérez-Brunius, Paula
    A set of float trajectories, deployed at 1500- and 2500-m depths throughout the deep Gulf of Mexico from 2011 to 2015, are analyzed for mesoscale processes under the Loop Current (LC). In the eastern basin, December 2012–June 2014 had >40 floats per month, which was of sufficient density to allow capturing detailed flow patterns of deep eddies and topographic Rossby waves (TRWs), while two LC eddies formed and separated. A northward advance of the LC front compresses the lower water column and generates an anticyclone. For an extended LC, baroclinic instability eddies (of both signs) develop under the southward-propagating large-scale meanders of the upper-layer jet, resulting in a transfer of eddy kinetic energy (EKE) to the lower layer. The increase in lower-layer EKE occurs only over a few months during meander activity and LC eddy detachment events, a relatively short interval compared with the LC intrusion cycle. Deep EKE of these eddies is dispersed to the west and northwest through radiating TRWs, of which examples were found to the west of the LC. Because of this radiation of EKE, the lower layer of the eastern basin becomes relatively quiescent, particularly in the northeastern basin, when the LC is retracted and a LC eddy has departed. A mean west-to-east, anticyclone–cyclone dipole flow under a mean LC was directly comparable to similar results from a previous moored LC array and also showed connections to an anticlockwise boundary current in the southeastern basin.
  • Article
    Hydrography of the Gulf of Mexico using autonomous floats
    (American Meteorological Society, 2018-04-04) Hamilton, Peter ; Leben, Robert ; Bower, Amy S. ; Furey, Heather H. ; Perez-Brunius, Paula
    Fourteen autonomous profiling floats, equipped with CTDs, were deployed in the deep eastern and western basins of the Gulf of Mexico over a four-year interval (July 2011–August 2015), producing a total of 706 casts. This is the first time since the early 1970s that there has been a comprehensive survey of water masses in the deep basins of the Gulf, with better vertical resolution than available from older ship-based surveys. Seven floats had 14-day cycles with parking depths of 1500 m, and the other half from the U.S. Argo program had varying cycle times. Maps of characteristic water masses, including Subtropical Underwater, Antarctic Intermediate Water (AAIW), and North Atlantic Deep Water, showed gradients from east to west, consistent with their sources being within the Loop Current (LC) and the Yucatan Channel waters. Altimeter SSH was used to characterize profiles being in LC or LC eddy water or in cold eddies. The two-layer nature of the deep Gulf shows isotherms being deeper in the warm anticyclonic LC and LC eddies and shallower in the cold cyclones. Mixed layer depths have an average seasonal signal that shows maximum depths (~60 m) in January and a minimum in June–July (~20 m). Basin-mean steric heights from 0–50-m dynamic heights and altimeter SSH show a seasonal range of ~12 cm, with significant interannual variability. The translation of LC eddies across the western basin produces a region of low homogeneous potential vorticity centered over the deepest part of the western basin.
  • Article
    Seasonal and interannual variations of Irminger ring formation and boundary–interior heat exchange in FLAME
    (American Meteorological Society, 2016-05-23) de Jong, Marieke F. ; Bower, Amy S. ; Furey, Heather H.
    The contribution of warm-core anticyclones shed by the Irminger Current off West Greenland, known as Irminger rings, to the restratification of the upper layers of the Labrador Sea is investigated in the 1/12° Family of Linked Atlantic Models Experiment (FLAME) model. The model output, covering the 1990–2004 period, shows strong similarities to observations of the Irminger Current as well as ring observations at a mooring located offshore of the eddy formation region in 2007–09. An analysis of fluxes in the model shows that while the majority of heat exchange with the interior indeed occurs at the site of the Irminger Current instability, the contribution of the coherent Irminger rings is modest (18%). Heat is provided to the convective region mainly through noncoherent anomalies and enhanced local mixing by the rings facilitating further exchange between the boundary and interior. The time variability of the eddy kinetic energy and the boundary to interior heat flux in the model are strongly correlated to the density gradient between the dense convective region and the more buoyant boundary current. In FLAME, the density variations of the boundary current are larger than those of the convective region, thereby largely controlling changes in lateral fluxes. Synchronous long-term trends in temperature in the boundary and the interior over the 15-yr simulation suggest that the heat flux relative to the temperature of the interior is largely steady on these time scales.
  • Article
    Lagrangian perspective on the origins of Denmark Strait Overflow
    (American Meteorological Society, 2020-08-01) Saberi, Atousa ; Haine, Thomas W. N. ; Gelderloos, Renske ; de Jong, Marieke Femke ; Furey, Heather H. ; Bower, Amy S.
    The Denmark Strait Overflow (DSO) is an important contributor to the lower limb of the Atlantic meridional overturning circulation (AMOC). Determining DSO formation and its pathways is not only important for local oceanography but also critical to estimating the state and variability of the AMOC. Despite prior attempts to understand the DSO sources, its upstream pathways and circulation remain uncertain due to short-term (3–5 days) variability. This makes it challenging to study the DSO from observations. Given this complexity, this study maps the upstream pathways and along-pathway changes in its water properties, using Lagrangian backtracking of the DSO sources in a realistic numerical ocean simulation. The Lagrangian pathways confirm that several branches contribute to the DSO from the north such as the East Greenland Current (EGC), the separated EGC (sEGC), and the North Icelandic Jet (NIJ). Moreover, the model results reveal additional pathways from south of Iceland, which supplied over 16% of the DSO annually and over 25% of the DSO during winter of 2008, when the NAO index was positive. The southern contribution is about 34% by the end of March. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the North Atlantic Deep Water (NADW), and needs to be explored further, with in situ observations.
  • Article
    Variability of the Iceland‐Scotland overflow water transport through the Charlie‐Gibbs fracture zone : results from an eddying simulation and observations
    (John Wiley & Sons, 2018-08-20) Xu, Xiaobiao ; Bower, Amy S. ; Furey, Heather H. ; Chassignet, Eric P.
    Observations show that the westward transport of the Iceland‐Scotland overflow water (ISOW) through the Charlie‐Gibbs Fracture Zone (CGFZ) is highly variable. This study examines (a) where this variability comes from and (b) how it is related to the variability of ISOW transport at upstream locations in the Iceland Basin and other ISOW flow pathways. The analyses are based on a 35‐year 1/12° eddying Atlantic simulation that represents well the main features of the observed ISOW in the area of interest, in particular, the transport variability through the CGFZ. The results show that (a) the variability of the ISOW transport is closely correlated with that of the barotropic transports in the CGFZ associated with the meridional displacement of the North Atlantic Current front and is possibly induced by fluctuations of large‐scale zonal wind stress in the Western European Basin east of the CGFZ; (b) the variability of the ISOW transport is increased by a factor of 3 from the northern part of the Iceland Basin to the CGFZ region and transport time series at these two locations are not correlated, further suggesting that the variability at the CGFZ does not come from the upstream source; and (c) the variability of the ISOW transport at the CGFZ is strongly anticorrelated to that of the southward ISOW transport along the eastern flank of the Mid‐Atlantic Ridge, suggesting an out‐of‐phase covarying transport between these two ISOW pathways.
  • Technical Report
    Red Sea Outflow Experiment (REDSOX) : DLD2 RAFOS float data report February 2001 - March 2003
    (Woods Hole Oceanographic Institution, 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.
  • Technical Report
    A crossroads of the Atlantic Meridional Overturning Circulation : the Charlie-Gibbs Fracture Zone data report August 2010 – June 2012
    (Woods Hole Oceanographic Institution, 2014-08) Furey, Heather H. ; Trafford, Leah ; Bower, Amy S.
    This is the final data report of all mooring data collected by the Woods Hole Oceanographic Institution in 2010-2012 during the experiment A Crossroads of the Atlantic Meridional Overturning Circulation: The Charlie-Gibbs Fracture Zone. The objectives of this experiment were (1) to obtain an improved direct estimate of the mean and low-frequency variability of the deep westward transport of the Iceland-Scotland Overflow Water through the Charlie-Gibbs Fracture Zone (CGFZ), and (2) to gain a better understanding of the causes of the low-frequency variability in the transport of overflow waters through the CGFZ, especially of the role of the North Atlantic Current in generating this variability. The mooring deployment and recovery cruises were on German research vessels, courtesy of Drs. Monika Rhein and Dagmar Kieke: the R/V Meteor cruise M82/2 in August 2010 and R/V Maria S. Merian cruise MSM 21/2 in June 2012, respectively. The CGFZ moored array complemented other moored arrays being maintained by German scientists just west of the CGFZ (Pressure Inverted Echo Sounders, or PIES) and the Faraday Fracture Zone (current meter and microcat moorings). A set of eight moorings were set up across the CGFZ to measure the intermediate and deep water variability for a two-year period, from a depth of 500 m to the ocean floor. The moorings held a total of three McClane Moored Profilers (MMPs), 10 Nortek and 18 Aanderaa current meters, and 36 Seabird MicroCATs, deployed from 18-20 August 2010 through 28-30 June 2012. This yielded a nearly two-year record of velocity, temperature, salinity and pressure. The MMPs profiled every five days, and resulted in a high-resolution time series of temperature, salinity, pressure and velocity data across the interface between the generally eastward flowing Labrador Sea Water carried underneath the North Atlantic Current, and the westward flowing deep Iceland-Scotland Overflow Water.
  • Technical Report
    Overturning of the Subpolar North Atlantic Program (OSNAP): RAFOS Float Data Report June 2014 - January 2019
    (Woods Hole Oceanographic Institution, 2020-12) Ramsey, Andree L. ; Furey, Heather H. ; Bower, Amy S.
    The Overturning in the Subpolar North Atlantic Program (OSNAP) is an international effort started in 2014 dedicated to achieving a better understanding of the link between dense-water formation and the meridional overturning circulation in the high-latitude North Atlantic. Moorings, gliders, and subsurface acoustically-tracked RAFOS floats have been used to collect temperature, salinity, and current data across the Labrador Sea, Irminger Sea, Reykjanes Ridge, Iceland Basin, Rockall-Hatton Plateau, and Rockall Trough. The specific objective of the OSNAP float program is to gather information on the pathways of the dense overflow waters transported by the deep limb of the overturning circulation and assess the connection of those pathways with currents observed crossing the OSNAP mooring line. This data report details the observations collected by 148 floats that were deployed for OSNAP during the summers of 2014, 2015, 2016 and 2017. Deployment locations were in the Iceland Basin, Irminger Sea, and in the Charlie-Gibbs Fracture Zone. Mission lengths ranged from 540-730 days, and the floats were ballasted to passively drift at a fixed pressure of either 1800, 2000, 2200, 2500, or 2800 dbar to tag the deep overflow water masses of the subpolar North Atlantic (Iceland-Scotland and Denmark Strait Overflow Waters).
  • Technical Report
    Impact of Irminger Rings on Deep Convection in the Labrador Sea : mooring instrument, cruise CTD, and APEX data report September 2007 – September 2009
    (Woods Hole Oceanographic Institution, 2013-05) Furey, Heather H. ; McKee, Theresa K. ; de Jong, Marieke F. ; Robbins, Paul E. ; Bower, Amy S.
    This is the final data report of all hydrographic station, mooring, and subsurface float data collected by the Woods Hole Oceanographic Institution in 2007-2009 during the Impact of Irminger Rings on Deep Convection in the Labrador Sea experiment (IRINGS). The objectives of IRINGS were to (1) to determine the full water column hydrographic and velocity structure of newlyformed Irminger Rings that have entered the interior Labrador Sea; (2) to observe how Irminger Ring core properties are modified by atmospheric forcing over their lifetime; and (3) to improve the interpretation of sea surface height (SSH) anomalies in terms of newly formed coherent heat containing Irminger Rings. The mooring deployment and recovery cruises were both on the R/V Knorr: KN192-01 in September 2007 and KN196-01 in September 2009, respectively. The single mooring held eight Aanderaa current meters (RCM-11), two Submerged Autonomous Launch Platforms (SALPs), and nine Seabird microcats (SBE37), deployed from 26 September 2007 through 27 September 2009, yeilding full water column (100-3000 meters) records of temperature, salinity, pressure, and velocity data for the two year period. The two SALP cages contained eleven APEX floats, and released some of these floats according to local oceanographic conditions, so as to seed the floats in passing Irminger Rings, and the remainder of floats as timed releases. Thirteen conductivity-temperature-depth (CTD) stations were taken on the mooring recovery cruise, creating a boundary current cross-section from the mooring site to Nuuk, Greenland.
  • Article
    Lagrangian views of the pathways of the Atlantic meridional overturning circulation
    (American Geophysical Union, 2019-07-19) Bower, Amy S. ; Lozier, M. Susan ; Biastoch, Arne ; Drouin, Kimberley L. ; Foukal, Nicholas P. ; Furey, Heather H. ; Lankhorst, Matthias ; Rühs, Siren ; Zou, Sijia
    The Lagrangian method—where current location and intensity are determined by tracking the movement of flow along its path—is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice‐free oceans and begin to construct basin‐scale, and eventually global‐scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC.
  • Article
    Labrador Sea Water transport across the Charlie-Gibbs Fracture Zone
    (American Geophysical Union, 2020-07-03) Gonçalves Neto, Afonso ; Palter, Jaime B. ; Bower, Amy S. ; Furey, Heather H. ; Xu, Xiaobiao
    Labrador Sea Water (LSW) is a major component of the deep limb of the Atlantic Meridional Overturning Circulation, yet LSW transport pathways and their variability lack a complete description. A portion of the LSW exported from the subpolar gyre is advected eastward along the North Atlantic Current and must contend with the Mid‐Atlantic Ridge before reaching the eastern basins of the North Atlantic. Here, we analyze observations from a mooring array and satellite altimetry, together with outputs from a hindcast ocean model simulation, to estimate the mean transport of LSW across the Charlie‐Gibbs Fracture Zone (CGFZ), a primary gateway for the eastward transport of the water mass. The LSW transport estimated from the 25‐year altimetry record is 5.3 ± 2.9 Sv, where the error represents the combination of observational variability and the uncertainty in the projection of the surface velocities to the LSW layer. Current velocities modulate the interannual to higher‐frequency variability of the LSW transport at the CGFZ, while the LSW thickness becomes important on longer time scales. The modeled mean LSW transport for 1993–2012 is higher than the estimate from altimetry, at 8.2 ± 4.1 Sv. The modeled LSW thickness decreases substantially at the CGFZ between 1996 and 2009, consistent with an observed decline in LSW volume in the Labrador Sea after 1994. We suggest that satellite altimetry and continuous hydrographic measurements in the central Labrador Sea, supplemented by profiles from Argo floats, could be sufficient to quantify the LSW transport at the CGFZ.