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  • 1
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    Sears Foundation for Marine Research
    Publication Date: 2017-01-07
    Description: Author Posting. © Sears Foundation for Marine Research, 2009. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 67 (2009): 273-303, doi:10.1357/002224009789954757.
    Description: An idealized model for a convective basin is used to investigate the mechanisms of variability of the formation and export of dense water. In this model, which consists of two isopycnic layers, dense water formation is induced by surface buoyancy loss in the interior, which is at rest. Newly formed dense water is transmitted to the surrounding boundary current through parameterized eddy fluxes. Variability in the formation and export of dense water is due to changes in the two main drivers: variations in the surface buoyancy fluxes and variations in the large-scale wind via a barotropic boundary current. Numerical integrations of the nonlinear model, with parameters and forcings corresponding to the Labrador Sea, show that the rate of dense water formation in the interior of the basin is strongly affected by changes in the buoyancy forcing, but not significantly affected by seasonal to interannual changes in the wind-driven barotropic boundary current. The basin tends to integrate the buoyancy forcing variability with a memory time scale set by eddies, which is decadal for the Labrador Sea. Variability in dense water export, on the contrary, is strongly affected by changes in the wind-driven barotropic boundary current but hardly affected by changes in buoyancy forcing. Indeed changes in the transport of dense water at the basin outflow are dominated by those at the basin inflow, which, in this model, are directly related to fluctuations in the wind-driven barotropic boundary current. These results, which are consistent with analytical solutions of the linear model, suggest that fluctuations in the surface buoyancy fluxes in the interior Labrador Sea have little impact on the interannual variability of the dense water transport by the Deep Western Boundary Current at the outflow of the Labrador Sea, which is dominated by fluctuations in the wind-driven North Atlantic subpolar gyre, but influence the formation and export of recently ventilated waters.
    Description: Support for JD from the NOAA Office of Hydrologic Development through a scientific appointment administered by UCAR is gratefully acknowledged. Support for FS was provided by NSF grant OCE−0525929. Support for MAS was provided by NSF grant OCE−0423975.
    Repository Name: Woods Hole Open Access Server
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  • 2
    Publication Date: 2017-01-04
    Description: Author Posting. © The Authors, 2009. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 3 (2010): 182-186, doi:10.1038/ngeo764.
    Description: The recent rapid increase in mass loss from the Greenland Ice Sheet is primarily attributed to an acceleration of outlet glaciers. One possible cause is increased melting at the ice/ocean interface driven by the synchronous warming of subtropical waters offshore of Greenland. This hypothesis is largely untested, however, because of the lack of observations from Greenland’s glacial fjords and our limited understanding of their dynamics. Here, we present new ship-based and moored oceanographic data, collected in Sermilik Fjord, a large glacial fjord in East Greenland, showing that subtropical waters are present throughout the fjord and are continuously replenished via a wind-driven exchange with the shelf, where they occur year-round. The temperature and rapid renewal of these waters suggest that, at present, they drive enhanced submarine melting at the terminus. Key controls on the melting rate are the volume and properties of subtropical waters on the shelf and the patterns of the along-shore winds, suggesting the glaciers’ acceleration was triggered by a combination of atmospheric and oceanic changes. These measurements provide evidence of rapid advective pathway for the transmission of oceanic variability to the ice-sheet margins and highlight an important process that is missing from prognostic ice-sheet models.
    Description: F.S. acknowledges support from WHOI’s Ocean and Climate Change Institute’s Arctic Research Initiative and from NSF OCE 0751896, and G.S.H and L.A.S from NASA’s Cryospheric Sciences Program. Funding for the hooded seal deployments was obtained from the International Governance and Atlantic Seal Research Program, Fisheries and Oceans, Canada, to G. B. S. and to the Greenland Institute of Natural Resources to A. R. A.
    Repository Name: Woods Hole Open Access Server
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  • 3
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2006. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 36 (2006): 1822-1840, doi:10.1175/JPO2932.1.
    Description: An isopycnal, two-layer, idealized model for a convective basin is proposed, consisting of a convecting, interior region and a surrounding boundary current (buoyancy and wind-driven). Parameterized eddy fluxes govern the exchange between the two. To balance the interior buoyancy loss, the boundary current becomes denser as it flows around the basin. Geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation. The poleward heat transport, associated with convection in the basin, can thus be viewed as a result of both an overturning and a horizontal circulation. When adapted to the Labrador Sea, the model is able to reproduce the bulk features of the mean state, the seasonal cycle, and even the shutdown of convection from 1969 to 1972. According to the model, only 40% of the poleward heat (buoyancy) transport of the Labrador Sea is associated with the overturning circulation. An exact solution is presented for the linearized equations when changes in the boundary current are small. Numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin. These results highlight how the overturning circulation is not simply related to the amount of dense water formed. A speeding up of the circulation around the basin due to wind forcing, for example, will decrease the intensity of the overturning circulation while the dense water formation remains unvaried. In general, it is shown that the fraction of poleward buoyancy (or heat) transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors.
    Description: This work was supported by NSF OCE 02-40978 and by the Climate Institute at the Woods Hole Oceanographic Institution (WHOI).
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  • 4
    Publication Date: 2017-01-05
    Description: Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 37-41, doi:10.1038/ngeo1349.
    Description: During the early 2000s the Greenland Ice Sheet experienced the largest ice mass loss observed on the instrumental record1, largely as a result of the acceleration, thinning and retreat of major outlet glaciers in West and Southeast Greenland2-5. The quasi-simultaneous change in the glaciers suggests a common climate forcing and increasing air6 and ocean7-8 temperatures have been indicated as potential triggers. Here, we present a new record of calving activity of Helheim Glacier, East Greenland, extending back to c. 1890 AD. This record was obtained by analysing sedimentary deposits from Sermilik Fjord, where Helheim Glacier terminates, and uses the annual deposition of sand grains as a proxy for iceberg discharge. The 120 year long record reveals large fluctuations in calving rates, but that the present high rate was reproduced only in the 1930s. A comparison with climate indices indicates that high calving activity coincides with increased Atlantic Water and decreased Polar Water influence on the shelf, warm summers and a negative phase of the North Atlantic Oscillation. Our analysis provides evidence that Helheim Glacier responds to short-term (3-10 years) large-scale oceanic and atmospheric fluctuations.
    Description: This study has been supported by Geocenter Denmark in financial support to the SEDIMICE project. CSA was supported by the Danish Council for Independent Research│Nature and Universe (Grant no. 09-064954/FNU). FSt was supported by NSF ARC 0909373 and by WHOI’s Ocean and Climate Change Institute and MHRI was supported by the Danish Agency for Science, Technology and Innovation.
    Description: 2012-06-11
    Repository Name: Woods Hole Open Access Server
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  • 5
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    American Geophysical Union
    Publication Date: 2017-01-05
    Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): C06010, doi:10.1029/2011JC007652.
    Description: We propose a conceptual model for an Arctic sea that is driven by river runoff, atmospheric fluxes, sea ice melt/growth, and winds. The model domain is divided into two areas, the interior and boundary regions, that are coupled through Ekman and eddy fluxes of buoyancy. The model is applied to Hudson and James Bays (HJB, a large inland basin in northeastern Canada) for the period 1979–2007. Several yearlong records from instruments moored within HJB show that the model results are consistent with the real system. The model notably reproduces the seasonal migration of the halocline, the baroclinic boundary current, spatial variability of freshwater content, and the fall maximum in freshwater export. The simulations clarify the important differences in the freshwater balance of the western and eastern sides of HJB. The significant role played by the boundary current in the freshwater budget of the system, and its sensitivity to the wind-forcing, are also highlighted by the simulations and new data analyses. We conclude that the model proposed is useful for the interpretation of observed data from Arctic seas and model outputs from more complex coupled/climate models.
    Description: We thank NSERC and the Canada Research Chairs program for funding. FS acknowledges support from NSF OCE–0927797 and ONR N00014-08-10490.
    Description: 2012-12-20
    Keywords: Arctic ; Models ; Sea ice
    Repository Name: Woods Hole Open Access Server
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © International Glaciological Society, 2012. This article is posted here by permission of International Glaciological Society for personal use, not for redistribution. The definitive version was published in Annals of Glaciology 53 (2012): 50-58, doi:10.3189/2012AoG60A050.
    Description: Submarine melting at the ice-ocean interface is a significant term in the mass balance of marine-terminating outlet glaciers. However, obtaining direct measurements of the submarine melt rate, or the ocean heat transport towards the glacier that drives this melting, has been difficult due to the scarcity of observations, as well as the complexity of oceanic flows. Here we present a method that uses synoptic velocity and temperature profiles, but accounts for the dominant mode of velocity variability, to obtain representative heat transport estimates. We apply this method to the Sermilik Fjord-Helheim Glacier system in southeastern Greenland. Using lowered acoustic Doppler current profiler (LADCP) and hydrographic data collected in summer 2009, we find a mean heat transport towards the glacier of 29 × 109 W, implying a submarine melt rate at the glacier face of 650 m a-1. The resulting adjusted velocity profile is indicative of a multilayer residual circulation, where the meltwater mixture flows out of the fjord at the surface and at the stratification maximum.
    Description: Funding for this work came from US National Science Foundation (NSF) Office of Polar Programs (OPP) grant 0909373 and the WHOI Arctic Research Initiative.
    Description: 2013-05-01
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2018-02-02
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 94 (2013): 1131–1144, doi:10.1175/BAMS-D-12-00100.1.
    Description: The recent retreat and speedup of outlet glaciers, as well as enhanced surface melting around the ice sheet margin, have increased Greenland's contribution to sea level rise to 0.6 ± 0.1 mm yr−1 and its discharge of freshwater into the North Atlantic. The widespread, near-synchronous glacier retreat, and its coincidence with a period of oceanic and atmospheric warming, suggests a common climate driver. Evidence points to the marine margins of these glaciers as the region from which changes propagated inland. Yet, the forcings and mechanisms behind these dynamic responses are poorly understood and are either missing or crudely parameterized in climate and ice sheet models. Resulting projected sea level rise contributions from Greenland by 2100 remain highly uncertain. This paper summarizes the current state of knowledge and highlights key physical aspects of Greenland's coupled ice sheet–ocean–atmosphere system. Three research thrusts are identified to yield fundamental insights into ice sheet, ocean, sea ice, and atmosphere interactions, their role in Earth's climate system, and probable trajectories of future changes: 1) focused process studies addressing critical glacier, ocean, atmosphere, and coupled dynamics; 2) sustained observations at key sites; and 3) inclusion of relevant dynamics in Earth system models. Understanding the dynamic response of Greenland's glaciers to climate forcing constitutes both a scientific and technological frontier, given the challenges of obtaining the appropriate measurements from the glaciers' marine termini and the complexity of the dynamics involved, including the coupling of the ocean, atmosphere, glacier, and sea ice systems. Interdisciplinary and international cooperation are crucial to making progress on this novel and complex problem.
    Description: 2014-02-01
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  • 8
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7705–7713, doi:10.1002/2015GL065003.
    Description: We present the first noble gas observations in a proglacial fjord in Greenland, providing an unprecedented view of surface and submarine melt pathways into the ocean. Using Optimum Multiparameter Analysis, noble gas concentrations remove large uncertainties inherent in previous studies of meltwater in Greenland fjords. We find glacially modified waters with submarine melt concentrations up to 0.66 ± 0.09% and runoff 3.9 ± 0.29%. Radiogenic enrichment of Helium enables identification of ice sheet near-bed melt (0.48 ± 0.08%). We identify distinct regions of meltwater export reflecting heterogeneous melt processes: a surface layer of both runoff and submarine melt and an intermediate layer composed primarily of submarine melt. Intermediate ocean waters carry the majority of heat to the fjords' glaciers, and warmer deep waters are isolated from the ice edge. The average entrainment ratio implies that ocean water masses are upwelled at a rate 30 times the combined glacial meltwater volume flux.
    Description: We gratefully acknowledge funding from WHOI's Ocean and Climate Change Institute, the Doherty Postdoctoral Scholarship, and ship time from the Advanced Climate Dynamics Summer School (SiU grant NNA-2012/10151).
    Description: 2016-03-30
    Keywords: Glacial melt ; Noble gases ; Tracers ; Meltwater ; Greenland ; Fjord
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  • 9
    Publication Date: 2017-01-07
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7648–7654, doi:10.1002/2015GL064944.
    Description: The mass loss at Nioghalvfjerdsbræ is primarily due to rapid submarine melting. Ocean data obtained from beneath the Nioghalvfjerdsbræ ice tongue show that melting is driven by the presence of warm (1°C) Atlantic Intermediate Water (AIW). A sill prevents AIW from entering the cavity from Dijmphna Sund, requiring that it flow into the cavity via bathymetric channels to the south at a pinned ice front. Comparison of water properties from the cavity, Dijmphna Sund, and the continental shelf support this conclusion. Overturning circulation rates inferred from observed melt rates and cavity stratification suggest an exchange flow between the cavity and the continental shelf of 38mSv, sufficient to flush cavity waters in under 1 year. These results place upper bounds on the timescales of external variability that can be transmitted to the glacier via the ice tongue cavity.
    Description: NASA Grant Number: NNX13AK88G, NSF Grant Number: OCE-1434041
    Description: 2016-03-22
    Keywords: 79North ; Ice tongue ; NEGIS ; Nioghalvfjerdsfjorden ; Circulation ; Ice-ocean
    Repository Name: Woods Hole Open Access Server
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  • 10
    Publication Date: 2017-01-25
    Description: Author Posting. © The Oceanography Society, 2016. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 29, no. 4 (2016): 34–45, doi:10.5670/oceanog.2016.97.
    Description: The rapid ice loss from the Greenland Ice Sheet that began in the late 1990s sparked an interest in glacier/ocean exchanges both because an increase in submarine melting of the glacier is a potential trigger of glacier retreat and because the increasing freshwater discharge can affect the regional ocean’s circulation and ecosystems. An interdisciplinary field project focused on the Helheim Glacier-Sermilik Fjord system began in 2008 and has continued to date. We found that warm, Atlantic Water flows into the fjord, drives melting of the glacier, and is regularly replenished through shelf-forced and glacier-driven circulations. In summer, the release of surface melt at the base of the glacier has a pronounced impact on local ocean circulation, the properties of the glacier, and its melt rate. Measurements taken in the fjord indicate that it is virtually impossible to derive submarine melt rates from hydrographic (including moored) data due to the fjord’s pronounced water mass variability and uncertain contribution from iceberg melt. Efforts to correlate glacier behavior with ocean forcing on seasonal and interannual time scales yield no straightforward connections, likely because of a dependence on a wider range of parameters, including subglacial discharge and bedrock geometry. This project emphasizes the need for sustained long-term measurements of multiple glacier/ocean/atmosphere systems to understand the different dynamics that control their evolution.
    Description: This work has been supported directly or indirectly by the National Science Foundation; NASA; the Woods Hole Oceanographic Institution; the universities of Kansas, Maine, and Oregon; the Kerr, Clark, and Haas Foundations; and Greenpeace.
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