Lozier M. Susan

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M. Susan

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  • Article
    Estimating the predictability of an oceanic time series using linear and nonlinear methods
    (American Geophysical Union, 2004-08-03) Yuan, Guo-Cheng ; Lozier, M. Susan ; Pratt, Lawrence J. ; Jones, C. K. R. T. ; Helfrich, Karl R.
    This study establishes a series of tests to examine the relative utility of nonlinear time series analysis for oceanic data. The performance of linear autoregressive models and nonlinear delay coordinate embedding methods are compared for three numerical and two observational data sets. The two observational data sets are (1) an hourly near-bottom pressure time series from the South Atlantic Bight and (2) an hourly current-meter time series from the Middle Atlantic Bight (MAB). The nonlinear methods give significantly better predictions than the linear methods when the underlying dynamics have low dimensionality. When the dimensionality is high, the utility of nonlinear methods is limited by the length and quality of the time series. On the application side we mainly focus on the MAB data set. We find that the slope velocities are much less predictable than shelf velocities. Predictability on the slope after several hours is no better than the statistical mean. On the other hand, significant predictability of shelf velocities can be obtained for up to at least 12 hours.
  • Article
    Quantifying the impact of submesoscale processes on the spring phytoplankton bloom in a turbulent upper ocean using a Lagrangian approach
    (John Wiley & Sons, 2016-05-18) Brody, Sarah R. ; Lozier, M. Susan ; Mahadevan, Amala
    The spring phytoplankton bloom in the subpolar North Atlantic and the mechanisms controlling its evolution and onset have important consequences for marine ecosystems and carbon cycling. Submesoscale mixed layer eddies (MLEs) play a role in the onset of the bloom by creating localized stratification and alleviating phytoplankton light limitation; however, the importance of MLEs for phytoplankton in a turbulent surface mixed layer has not yet been examined. We explore the effect of MLEs on phytoplankton by simulating their trajectories with Lagrangian particles subject to turbulent vertical displacements in an MLE-resolving model. By tracking the light exposure of the simulated phytoplankton, we find that MLEs can advance the timing of the spring bloom by 1 to 2 weeks, depending on surface forcing conditions. The onset of the bloom is linked with the onset of positive heat fluxes, whether or not MLEs are present.
  • Article
    Observation-based estimates of heat and freshwater exchanges from the subtropical North Atlantic to the Arctic
    (Elsevier, 2021-07-06) Li, Feili ; Lozier, M. Susan ; Holliday, Naomi Penny ; Johns, William E. ; Le Bras, Isabela A. ; Moat, Bengamin I. ; Cunningham, Stuart A. ; de Jong, Marieke Femke
    Continuous measurements from the OSNAP (Overturning in the Subpolar North Atlantic Program) array yield the first estimates of trans-basin heat and salinity transports in the subpolar latitudes. For the period from August 2014 to May 2018, there is a poleward heat transport of 0.50 ± 0.05 PW and a poleward salinity transport of 12.5 ± 1.0 Sv across the OSNAP section. Based on the mass and salt budget analyses, we estimate that a surface freshwater input of 0.36 ± 0.05 Sv over the broad subpolar-Arctic region is needed to balance the ocean salinity change created by the OSNAP transports. The overturning circulation is largely responsible for setting these heat and salinity transports (and the derived surface freshwater input) derived from the OSNAP array, while the gyre (isopycnal) circulation contributes to a lesser, but still significant, extent. Despite its relatively weak overturning and heat transport, the Labrador Sea is a strong contributor to salinity and freshwater changes in the subpolar region. Combined with trans-basin transport estimates at other locations, we provide new estimates for the time-mean surface heat and freshwater divergences over a wide domain of the Arctic-North Atlantic region to the north and south of the OSNAP line. Furthermore, we estimate the total heat and freshwater exchanges across the surface area of the extratropical North Atlantic between the OSNAP and the RAPID-MOCHA (RAPID Meridional Overturning Circulation and Heat-flux Array) arrays, by combining the cross-sectional transports with vertically-integrated ocean heat and salinity content. Comparisons with the air-sea heat and freshwater fluxes from atmospheric reanalysis products show an overall consistency, yet with notable differences in the magnitudes during the observation time period.
  • Article
    The fate of North Atlantic Subtropical Mode Water in the FLAME model
    (American Meteorological Society, 2014-05) Gary, Stefan F. ; Lozier, M. Susan ; Kwon, Young-Oh ; Park, Jong Jin
    North Atlantic Subtropical Mode Water, also known as Eighteen Degree Water (EDW), has the potential to store heat anomalies through its seasonal cycle: the water mass is in contact with the atmosphere in winter, isolated from the surface for the rest of the year, and reexposed the following winter. Though there has been recent progress in understanding EDW formation processes, an understanding of the fate of EDW following formation remains nascent. Here, particles are launched within the EDW of an eddy-resolving model, and their fate is tracked as they move away from the formation region. Particles in EDW have an average residence time of ~10 months, they follow the large-scale circulation around the subtropical gyre, and stratification is the dominant criteria governing the exit of particles from EDW. After sinking into the layers beneath EDW, particles are eventually exported to the subpolar gyre. The spreading of particles is consistent with the large-scale potential vorticity field, and there are signs of a possible eddy-driven mean flow in the southern portion of the EDW domain. The authors also show that property anomalies along particle trajectories have an average integral time scale of ~3 months for particles that are in EDW and ~2 months for particles out of EDW. Finally, it is shown that the EDW turnover time for the model in an Eulerian frame (~3 yr) is consistent with the turnover time computed from the Lagrangian particles provided that the effects of exchange between EDW and the surrounding waters are included.
  • Article
    Overturning in the Subpolar North Atlantic Program : a new international ocean observing system
    (American Meteorological Society, 2017-04-24) Lozier, M. Susan ; Bacon, Sheldon ; Bower, Amy S. ; Cunningham, Stuart A. ; de Jong, Marieke Femke ; de Steur, Laura ; deYoung, Brad ; Fischer, Jürgen ; Gary, Stefan F. ; Greenan, Blair J. W. ; Heimbach, Patrick ; Holliday, Naomi Penny ; Houpert, Loïc ; Inall, Mark E. ; Johns, William E. ; Johnson, Helen L. ; Karstensen, Johannes ; Li, Feili ; Lin, Xiaopei ; Mackay, Neill ; Marshall, David P. ; Mercier, Herlé ; Myers, Paul G. ; Pickart, Robert S. ; Pillar, Helen R. ; Straneo, Fiamma ; Thierry, Virginie ; Weller, Robert A. ; Williams, Richard G. ; Wilson, Christopher G. ; Yang, Jiayan ; Zhao, Jian ; Zika, Jan D.
    For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.
  • Article
    Near-surface transport pathways in the north Atlantic Ocean : looking for throughput from the subtropical to the subpolar gyre
    (American Meteorological Society, 2011-05) Rypina, Irina I. ; Pratt, Lawrence J. ; Lozier, M. Susan
    Motivated by discrepancies between Eulerian transport estimates and the behavior of Lagrangian surface drifters, near-surface transport pathways and processes in the North Atlantic are studied using a combination of data, altimetric surface heights, statistical analysis of trajectories, and dynamical systems techniques. Particular attention is paid to the issue of the subtropical-to-subpolar intergyre fluid exchange. The velocity field used in this study is composed of a steady drifter-derived background flow, upon which a time-dependent altimeter-based perturbation is superimposed. This analysis suggests that most of the fluid entering the subpolar gyre from the subtropical gyre within two years comes from a narrow region lying inshore of the Gulf Stream core, whereas fluid on the offshore side of the Gulf Stream is largely prevented from doing so by the Gulf Stream core, which acts as a strong transport barrier, in agreement with past studies. The transport barrier near the Gulf Stream core is robust and persistent from 1992 until 2008. The qualitative behavior is found to be largely independent of the Ekman drift.
  • 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
    Latitudinal structure of the meridional overturning circulation variability on interannual to decadal time scales in the North Atlantic Ocean
    (American Meteorological Society, 2020-04-06) Zou, Sijia ; Lozier, M. Susan ; Xu, Xiaobiao
    The latitudinal structure of the Atlantic meridional overturning circulation (AMOC) variability in the North Atlantic is investigated using numerical results from three ocean circulation simulations over the past four to five decades. We show that AMOC variability south of the Labrador Sea (53°N) to 25°N can be decomposed into a latitudinally coherent component and a gyre-opposing component. The latitudinally coherent component contains both decadal and interannual variabilities. The coherent decadal AMOC variability originates in the subpolar region and is reflected by the zonal density gradient in that basin. It is further shown to be linked to persistent North Atlantic Oscillation (NAO) conditions in all three models. The interannual AMOC variability contained in the latitudinally coherent component is shown to be driven by westerlies in the transition region between the subpolar and the subtropical gyre (40°–50°N), through significant responses in Ekman transport. Finally, the gyre-opposing component principally varies on interannual time scales and responds to local wind variability related to the annual NAO. The contribution of these components to the total AMOC variability is latitude-dependent: 1) in the subpolar region, all models show that the latitudinally coherent component dominates AMOC variability on interannual to decadal time scales, with little contribution from the gyre-opposing component, and 2) in the subtropical region, the gyre-opposing component explains a majority of the interannual AMOC variability in two models, while in the other model, the contributions from the coherent and the gyre-opposing components are comparable. These results provide a quantitative decomposition of AMOC variability across latitudes and shed light on the linkage between different AMOC variability components and atmospheric forcing mechanisms.
  • 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
    Local and downstream relationships between Labrador Sea Water volume and North Atlantic meridional overturning circulation variability
    (American Meteorological Society, 2019-07-11) Li, Feili ; Lozier, M. Susan ; Danabasoglu, Gokhan ; Holliday, Naomi Penny ; Kwon, Young-Oh ; Romanou, Anastasia ; Yeager, Stephen G. ; Zhang, Rong
    While it has generally been understood that the production of Labrador Sea Water (LSW) impacts the Atlantic meridional overturning circulation (MOC), this relationship has not been explored extensively or validated against observations. To explore this relationship, a suite of global ocean–sea ice models forced by the same interannually varying atmospheric dataset, varying in resolution from non-eddy-permitting to eddy-permitting (1°–1/4°), is analyzed to investigate the local and downstream relationships between LSW formation and the MOC on interannual to decadal time scales. While all models display a strong relationship between changes in the LSW volume and the MOC in the Labrador Sea, this relationship degrades considerably downstream of the Labrador Sea. In particular, there is no consistent pattern among the models in the North Atlantic subtropical basin over interannual to decadal time scales. Furthermore, the strong response of the MOC in the Labrador Sea to LSW volume changes in that basin may be biased by the overproduction of LSW in many models compared to observations. This analysis shows that changes in LSW volume in the Labrador Sea cannot be clearly and consistently linked to a coherent MOC response across latitudes over interannual to decadal time scales in ocean hindcast simulations of the last half century. Similarly, no coherent relationships are identified between the MOC and the Labrador Sea mixed layer depth or the density of newly formed LSW across latitudes or across models over interannual to decadal time scales.
  • Article
    Linking Southern Ocean mixed-layer dynamics to net community production on various timescales
    (American Geophysical Union, 2021-09-21) Li, Zuchuan ; Lozier, M. Susan ; Cassar, Nicolas
    Mixed-layer dynamics exert a first order control on nutrient and light availability for phytoplankton. In this study, we examine the influence of mixed-layer dynamics on net community production (NCP) in the Southern Ocean on intra-seasonal, seasonal, interannual, and decadal timescales, using biogeochemical Argo floats and satellite-derived NCP estimates during the period from 1997 to 2020. On intraseasonal timescales, the shoaling of the mixed layer is more likely to enhance NCP in austral spring and winter, suggesting an alleviation of light limitation. As expected, NCP generally increases with light availability on seasonal timescales. On interannual timescales, NCP is correlated with mixed layer depth (MLD) and mixed-layer-averaged photosynthetically active radiation (PAR) in austral spring and winter, especially in regions with deeper mixed layers. Though recent studies have argued that winter MLD controls the subsequent growing season's iron and light availability, the limited number of Argo float observations contemporaneous with our satellite observations do not show a significant correlation between NCP and the previous-winter's MLD on interannual timescales. Over the 1997–2020 period, we observe regional trends in NCP (e.g., increasing around S. America), but no trend for the entire Southern Ocean. Overall, our results show that the dependence of NCP on MLD is a complex function of timescales.
  • Article
    Year-to-year reoutcropping of Eighteen Degree Water in an eddy-resolving ocean simulation
    (American Meteorological Society, 2015-04) Kwon, Young-Oh ; Park, Jong Jin ; Gary, Stefan F. ; Lozier, M. Susan
    Winter outcropping of the Eighteen Degree Water (EDW) and its subsequent dispersion are studied using a ° eddy-resolving simulation of the Family of Linked Atlantic Modeling Experiments (FLAME). Outcropped EDW columns in the model simulations are detected in each winter from 1990 to 1999, and particles are deployed in the center of each outcropped EDW column. Subsequently, the trajectories of these particles are calculated for the following 5 yr. The particles slowly spread away from the outcropping region into the nonoutcropping/subducted EDW region south of ~30°N and eventually to the non-EDW region in the greater subtropical gyre. Approximately 30% of the particles are found in non-EDW waters 1 yr after deployment; after 5 yr, only 25% of the particles are found within EDW. The reoutcropping time is defined as the number of years between when a particle is originally deployed in an outcropping EDW column and when that particle is next found in an outcropping EDW column. Of the particles, 66% are found to reoutcrop as EDW in 1 yr, and less than 5% of the particles outcrop in each of the subsequent 4 yr. While the individual trajectories exhibit significant eddy-like motions, the time scale of reoutcropping is primarily set by the mean circulation. The dominance of reoutcropping in 1 yr suggests that EDW outcropping contributes considerably to the persistence of surface temperature anomalies from one winter to the next, that is, the reemergence of winter sea surface temperature anomalies.
  • 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
    Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation
    (Nature Research, 2021-05-24) Li, Feili ; Lozier, M. Susan ; Bacon, Sheldon ; Bower, Amy S. ; Cunningham, Stuart A. ; de Jong, Marieke F. ; deYoung, Brad ; Fraser, Neil ; Fried, Nora ; Han, Guoqi ; Holliday, Naomi Penny ; Holte, James W. ; Houpert, Loïc ; Inall, Mark E. ; Johns, William E. ; Jones, Sam ; Johnson, Clare ; Karstensen, Johannes ; Le Bras, Isabela A. ; Lherminier, Pascale ; Lin, Xiaopei ; Mercier, Herlé ; Oltmanns, Marilena ; Pacini, Astrid ; Petit, Tillys ; Pickart, Robert S. ; Rayner, Darren ; Straneo, Fiamma ; Thierry, Virginie ; Visbeck, Martin ; Yashayaev, Igor ; Zhou, Chun
    Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.
  • Article
    How much Arctic fresh water participates in the subpolar overturning circulation?
    (American Meteorological Society, 2021-03-01) Le Bras, Isabela A. ; Straneo, Fiamma ; Muilwijk, Morven ; Smedsrud, Lars H. ; Li, Feili ; Lozier, M. Susan ; Holliday, Naomi Penny
    Fresh Arctic waters flowing into the Atlantic are thought to have two primary fates. They may be mixed into the deep ocean as part of the overturning circulation, or flow alongside regions of deep water formation without impacting overturning. Climate models suggest that as increasing amounts of freshwater enter the Atlantic, the overturning circulation will be disrupted, yet we lack an understanding of how much freshwater is mixed into the overturning circulation’s deep limb in the present day. To constrain these freshwater pathways, we build steady-state volume, salt, and heat budgets east of Greenland that are initialized with observations and closed using inverse methods. Freshwater sources are split into oceanic Polar Waters from the Arctic and surface freshwater fluxes, which include net precipitation, runoff, and ice melt, to examine how they imprint the circulation differently. We find that 65 mSv (1 Sv ≡ 106 m3 s−1) of the total 110 mSv of surface freshwater fluxes that enter our domain participate in the overturning circulation, as do 0.6 Sv of the total 1.2 Sv of Polar Waters that flow through Fram Strait. Based on these results, we hypothesize that the overturning circulation is more sensitive to future changes in Arctic freshwater outflow and precipitation, while Greenland runoff and iceberg melt are more likely to stay along the coast of Greenland.
  • Article
    Slope water, Gulf Stream, and seasonal influences on southern Mid-Atlantic Bight circulation during the fall-winter transition
    (American Geophysical Union, 2005-02-16) Rasmussen, Linda L. ; Gawarkiewicz, Glen G. ; Owens, W. Brechner ; Lozier, M. Susan
    Observations from autumn, 2000, near the shelfbreak front in the Middle Atlantic Bight are used to describe the transition from stratified summer conditions to well-mixed winter conditions over the shelf. During the observational period, the front differed dramatically from climatological conditions, with buoyant Gulf Stream water found shoreward over the sub-surface shelfbreak front. Water mass analysis shows a large number of separate water masses with shelf, slope and Gulf Stream origins. The coolest shelf water was located at the shelfbreak and may be related to “cold pool” water masses observed to the north during summer. Shoreward of this shelfbreak water mass, a mid-shelf front was present which intersected the bottom at the 50 m isobath. High volume transports were associated with both the shelfbreak and mid-shelf fronts. Transport estimates from the cross-shelf sections were approximately 1 Sverdrup, which is large relative to previous estimates of shelf transport. The foot of the front was near the 130 m isobath, much deeper than the climatological position near the 75 m isobath, however this is consistent with a recent theory relating the magnitude of alongshelf transport to the depth at which the front intersects the bottom.
  • Article
    Influence of ocean circulation changes on the inter-annual variability of American eel larval dispersal
    (John Wiley & Sons, 2016-06-24) Rypina, Irina I. ; Pratt, Lawrence J. ; Lozier, M. Susan
    American eel (Anguilla rostrata) complete their life cycle by migrating from the east coast of North America to Sargasso Sea, where they spawn planktonic eggs and dye. Larvae that develop from eggs need to return to North American coastal waters within the first year of life and are influenced by the oceanic currents during this journey. A coupled physical–biological model is used to investigate the extent to which inter-annual changes in the ocean circulation affect the success rates of larvae in reaching coastal nursery habitats. Our results suggest that natural oceanic variability can lead to changes in larval success rates by a factor of 2. Interannual variation in success rates are strongly affected by the Gulf Stream inertial overshoot events, with the largest success in years with an inertial overshoot and the smallest in years with a straighter and more southern configuration of the Gulf Stream downstream of Cape Hatteras. The mean Gulf Stream length and latitude between 75W and 70W longitude can be used as proxies for characterizing the overshoot events and can be converted into success rates using linear regression.
  • Article
    Examining the origins of ocean heat content variability in the eastern North Atlantic subpolar gyre
    (John Wiley & Sons, 2018-10-27) Foukal, Nicholas P. ; Lozier, M. Susan
    We analyze sources of ocean heat content (OHC) variability in the eastern North Atlantic subpolar gyre from both Eulerian and Lagrangian perspectives within two ocean simulations from 1990 to 2015. Heat budgets reveal that while the OHC seasonal cycle is driven by air‐sea fluxes, interannual OHC variability is driven by both air‐sea fluxes and the divergence of ocean heat transport, the latter of which is dominated by the oceanic flux through the southern face of the study area. Lagrangian trajectories initialized along the southern face and run backward in time indicate that interannual variability in the subtropical‐origin volume flux (i.e., the upper limb of the overturning circulation) drives variability in the temperature flux through the southern face. As such, the heat carried by the imported subtropical waters is an important component of the eastern subpolar gyre heat budget on interannual time scales.
  • Article
    Atlantic meridional overturning circulation: Observed transport and variability
    (Frontiers Media, 2019-06-07) Frajka-Williams, Eleanor ; Ansorge, Isabelle ; Baehr, Johanna ; Bryden, Harry L. ; Chidichimo, Maria Paz ; Cunningham, Stuart A. ; Danabasoglu, Gokhan ; Dong, Shenfu ; Donohue, Kathleen A. ; Elipot, Shane ; Heimbach, Patrick ; Holliday, Naomi Penny ; Hummels, Rebecca ; Jackson, Laura C. ; Karstensen, Johannes ; Lankhorst, Matthias ; Le Bras, Isabela A. ; Lozier, M. Susan ; McDonagh, Elaine L. ; Meinen, Christopher S. ; Mercier, Herlé ; Moat, Bengamin I. ; Perez, Renellys ; Piecuch, Christopher G. ; Rhein, Monika ; Srokosz, Meric ; Trenberth, Kevin E. ; Bacon, Sheldon ; Forget, Gael ; Goni, Gustavo J. ; Kieke, Dagmar ; Koelling, Jannes ; Lamont, Tarron ; McCarthy, Gerard D. ; Mertens, Christian ; Send, Uwe ; Smeed, David A. ; Speich, Sabrina ; van den Berg, Marcel ; Volkov, Denis L. ; Wilson, Christopher G.
    The Atlantic Meridional Overturning Circulation (AMOC) extends from the Southern Ocean to the northern North Atlantic, transporting heat northwards throughout the South and North Atlantic, and sinking carbon and nutrients into the deep ocean. Climate models indicate that changes to the AMOC both herald and drive climate shifts. Intensive trans-basin AMOC observational systems have been put in place to continuously monitor meridional volume transport variability, and in some cases, heat, freshwater and carbon transport. These observational programs have been used to diagnose the magnitude and origins of transport variability, and to investigate impacts of variability on essential climate variables such as sea surface temperature, ocean heat content and coastal sea level. AMOC observing approaches vary between the different systems, ranging from trans-basin arrays (OSNAP, RAPID 26°N, 11°S, SAMBA 34.5°S) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16°N). In this paper, we outline the different approaches (aims, strengths and limitations) and summarize the key results to date. We also discuss alternate approaches for capturing AMOC variability including direct estimates (e.g., using sea level, bottom pressure, and hydrography from autonomous profiling floats), indirect estimates applying budgetary approaches, state estimates or ocean reanalyses, and proxies. Based on the existing observations and their results, and the potential of new observational and formal synthesis approaches, we make suggestions as to how to evaluate a comprehensive, future-proof observational network of the AMOC to deepen our understanding of the AMOC and its role in global climate.