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  • 1
    Publication Date: 2017-11-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 2
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 43 (12). pp. 2611-2628.
    Publication Date: 2017-10-24
    Description: The Denmark Strait Overflow (DSO) supplies about one-third of the North Atlantic Deep Water and is critical to global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its transformation there is still incomplete, however. The authors deploy over 10 000 Lagrangian particles at the Denmark Strait in a high-resolution ocean model to study these issues. First, the particle trajectories show that the mean position and potential density of dense waters cascading over the Denmark Strait sill evolve consistently with hydrographic observations. These sill particles transit the Irminger Basin to the Spill Jet section (65.25°N) in 5–7 days and to the Angmagssalik section (63.5°N) in 2–3 weeks. Second, the dense water pathways on the continental shelf are consistent with observations and particles released on the shelf in the strait constitute a significant fraction of the dense water particles recorded at the Angmagssalik section within 60 days (~25%). Some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill. Third, there are two places where the water density following particle trajectories decreases rapidly due to intense mixing: to the southwest of the sill and southwest of the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, the overflow particles exhibit a wide range of densities.
    Type: Article , PeerReviewed
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  • 3
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    In:  [Poster] In: EGU General Assembly 2017, 23.-28.04.2017, Vienna, Austria .
    Publication Date: 2017-12-11
    Description: Temperatures rise faster in the Arctic than on global average, a phenomenon known as Arctic Amplification. While this is well established from observations and model simulations, projections of future climate (here: RCP8.5) with models of the Coupled Model Intercomparison Project phase 5 (CMIP5) also indicate that the Arctic Amplification has a maximum. We show this by means of an Arctic Amplification factor (AAF), which we define as the ratio of Arctic mean to global mean surface air temperature (SAT) anomalies. The SAT anomalies are referenced to the period 1960-1980 and smoothed by a 30-year running mean. For October, the multi-model ensemble-mean AAF reaches a maximum in 2017. The maximum moves however to later years as Arctic winter progresses: for the autumn mean SAT (September to November) the maximum AAF is found in 2028 and for winter (December to February) in 2060. Arctic Amplification is driven, amongst others, by the ice-albedo feedback (IAF) as part of the more general surface albedo feedback (involving clouds, snow cover, vegetation changes) and temperature effects (Planck and lapse-rate feedbacks).We note that sea ice retreat and the associated warming of the summer Arctic Ocean are not only an integral part of the IAF but are also involved in the other drivers. In the CMIP5 simulations, the timing of the AAF maximum coincides with the period of fastest ice retreat for the respective month. Presence of at least some sea ice is crucial for the IAF to be effective because of the contrast in surface albedo between ice and open water and the need to turn ocean warming into ice melt. Once large areas of the Arctic Ocean are ice-free, the IAF should be less effective. We thus hypothesize that the ice retreat significantly affects AAF variability and forces a decline of its magnitude after at least half of the Arctic Ocean is ice-free and the ice cover becomes basically seasonal.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 47 (7). pp. 1685-1699.
    Publication Date: 2019-02-01
    Description: Seasonal variability in pathways of warm water masses toward the Kangerdlugssuaq Fjord-Glacier system (KF/KG), southeast Greenland, is investigated by backtracking Lagrangian particles seeded at the fjord mouth in a high-resolution regional ocean model simulation in the ice-free and the ice-covered seasons. The waters at KF are a mixture of Atlantic-origin water advected from the Irminger Basin (FF for Faxaflói), the deep waters from the Denmark Strait and the waters from the Arctic Ocean, both represented by the Kögur section (KO). Below 200m depth, the warm water is a mixture of FF and KO water masses, and is warmer in winter than in summer. We find that seasonal differences in pathways double the fraction of FF particles in winter, causing the seasonal warming and salinification. Seasonal temperature variations at the upstream sections (FF and KO) have a negligible impact on temperature variations near the fjord. Successful monitoring of heat flux to the fjord therefore needs to take place close to the fjord, and cannot be inferred from upstream conditions.
    Type: Article , PeerReviewed
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  • 5
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    Nature Publishing Group
    In:  Scientific Reports, 7 (1). p. 4618.
    Publication Date: 2019-02-01
    Description: The loss of Arctic sea ice is a conspicuous example of climate change. Climate models project ice-free conditions during summer this century under realistic emission scenarios, reflecting the increase in seasonality in ice cover. To quantify the increased seasonality in the Arctic-Subarctic sea ice system, we define a non-dimensional seasonality number for sea ice extent, area, and volume from satellite data and realistic coupled climate models. We show that the Arctic-Subarctic, i.e. The northern hemisphere, sea ice now exhibits similar levels of seasonality to the Antarctic, which is in a seasonal regime without significant change since satellite observations began in 1979. Realistic climate models suggest that this transition to the seasonal regime is being accompanied by a maximum in Arctic amplification, which is the faster warming of Arctic latitudes compared to the global mean, in the 2010s. The strong link points to a peak in sea-ice-related feedbacks that occurs long before the Arctic becomes ice-free in summer.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 84 (2014): 110-126, doi:10.1016/j.dsr.2013.10.007.
    Description: Data from seven moorings deployed across the East Greenland shelfbreak and slope 280 km downstream of Denmark Strait are used to investigate the characteristics and dynamics of Denmark Strait Overflow Water (DSOW) cyclones. On average, a cyclone passes the mooring array every other day near the 900 m isobath, dominating the variability of the boundary current system. There is considerable variation in both the frequency and location of the cyclones on the slope, but no apparent seasonality. Using the year-long data set from September 2007 to October 2008, we construct a composite DSOW cyclone that reveals the average scales of the features. The composite cyclone consists of a lens of dense overflow water on the bottom, up to 300 m thick, with cyclonic flow above the lens. The azimuthal flow is intensified in the middle and upper part of the water column and has the shape of a Gaussian eddy with a peak depth-mean speed of 0.22 m/s at a radius of 7.8 km. The lens is advected by the mean flow of 0.27 m/s and self propagates at 0.45 m/s, consistent with the topographic Rossby wave speed and the Nof speed. The total translation velocity along the East Greenland slope is 0.72 m/s. The self-propagation speed exceeds the cyclonic swirl speed, indicating that the azimuthal flow cannot kinematically trap fluid in the water column above the lens. This implies that the dense water anomaly and the cyclonic swirl velocity are dynamically linked, in line with previous theory. Satellite sea surface temperature (SST) data are investigated to study the surface expression of the cyclones. Disturbances to the SST field are found to propagate less quickly than the in-situ DSOW cyclones, raising the possibility that the propagation of the SST signatures is not directly associated with the cyclones.
    Description: Funding for the study was provided by National Science Foundation Grant OCE-0726640 and the Arctic Research Initiative of the Woods Hole Oceanographic Institution.
    Keywords: Denmark strait overflow water cyclone ; East greenland boundary current system ; East greenland spill jet ; Deep western boundary current
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 7
    Publication Date: 2017-06-02
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Biogeosciences 121 (2016): 675-717, doi:10.1002/2015JG003140.
    Description: The Arctic Ocean is a fundamental node in the global hydrological cycle and the ocean's thermohaline circulation. We here assess the system's key functions and processes: (1) the delivery of fresh and low-salinity waters to the Arctic Ocean by river inflow, net precipitation, distillation during the freeze/thaw cycle, and Pacific Ocean inflows; (2) the disposition (e.g., sources, pathways, and storage) of freshwater components within the Arctic Ocean; and (3) the release and export of freshwater components into the bordering convective domains of the North Atlantic. We then examine physical, chemical, or biological processes which are influenced or constrained by the local quantities and geochemical qualities of freshwater; these include stratification and vertical mixing, ocean heat flux, nutrient supply, primary production, ocean acidification, and biogeochemical cycling. Internal to the Arctic the joint effects of sea ice decline and hydrological cycle intensification have strengthened coupling between the ocean and the atmosphere (e.g., wind and ice drift stresses, solar radiation, and heat and moisture exchange), the bordering drainage basins (e.g., river discharge, sediment transport, and erosion), and terrestrial ecosystems (e.g., Arctic greening, dissolved and particulate carbon loading, and altered phenology of biotic components). External to the Arctic freshwater export acts as both a constraint to and a necessary ingredient for deep convection in the bordering subarctic gyres and thus affects the global thermohaline circulation. Geochemical fingerprints attained within the Arctic Ocean are likewise exported into the neighboring subarctic systems and beyond. Finally, we discuss observed and modeled functions and changes in this system on seasonal, annual, and decadal time scales and discuss mechanisms that link the marine system to atmospheric, terrestrial, and cryospheric systems.
    Description: World Climate Research Program-Climate and Cryosphere (WCRP-CliC); Arctic Monitoring and Assessment Program (AMAP) International Arctic Science Committee (IASC); Norwegian Ministries of Environment and of Foreign Affairs; Swedish Secretariat for Environmental Earth System Sciences (SSEESS); Swedish Polar Research Secretariat; NSF Grant Numbers: OCE 1130008, 1249133, AON-1203473, AON-1338948, OCE 1434041; Polar Research Programme of the Norwegian Research Council Grant Number: 226415
    Keywords: Arctic ; Oceans ; Circulation ; Freshwater ; Carbon cycle ; Acidification
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 8
    Publication Date: 2017-01-04
    Description: Author Posting. ©American Meteorological Society, 2008. 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 89 (2008): 1307-1324, doi:10.1175/2008BAMS2508.1.
    Description: Greenland has a major influence on the atmospheric circulation of the North Atlantic–western European region, dictating the location and strength of mesoscale weather systems around the coastal seas of Greenland and directly influencing synoptic-scale weather systems both locally and downstream over Europe. High winds associated with the local weather systems can induce large air–sea fluxes of heat, moisture, and momentum in a region that is critical to the overturning of the thermohaline circulation, and thus play a key role in controlling the coupled atmosphere–ocean climate system. The Greenland Flow Distortion Experiment (GFDex) is investigating the role of Greenland in defining the structure and predictability of both local and downstream weather systems through a program of aircraft-based observation and numerical modeling. The GFDex observational program is centered upon an aircraft-based field campaign in February and March 2007, at the dawn of the International Polar Year. Twelve missions were flown with the Facility for Airborne Atmospheric Measurements' BAe-146, based out of the Keflavik, Iceland. These included the first aircraft-based observations of a reverse tip jet event, the first aircraft-based observations of barrier winds off of southeast Greenland, two polar mesoscale cyclones, a dramatic case of lee cyclogenesis, and several targeted observation missions into areas where additional observations were predicted to improve forecasts. In this overview of GFDex the background, aims and objectives, and facilities and logistics are described. A summary of the campaign is provided, along with some of the highlights of the experiment.
    Description: The GFDex would not have been possible without the dedication and flexibility shown by all at the FAAM, DirectFlight, and Avalon. GFDex was funded by the Natural Environmental Research Council (NE/C003365/1), the Canadian Foundation for Climate and Atmospheric Sciences (GR-641), and the European Union Fleet for Airborne Research (EUFAR) and European Union Coordinated Observing System (EUCOS) schemes.
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 9
    Publication Date: 2017-01-05
    Description: Author Posting. © American Meteorological Society, 2011. 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 41 (2011): 2307–2327, doi:10.1175/JPO-D-10-05004.1.
    Description: Results from a high-resolution (~2 km) numerical simulation of the Irminger Basin during summer 2003 are presented. The focus is on the East Greenland Spill Jet, a recently discovered component of the circulation in the basin. The simulation compares well with observations of surface fields, the Denmark Strait overflow (DSO), and the hydrographic structure of typical sections in the basin. The model reveals new aspects of the circulation on scales of O(0.1–10) days and O(1–100) km. The model Spill Jet results from the cascade of dense waters over the East Greenland shelf. Spilling can occur in various locations southwest of the strait, and it is present throughout the simulation but exhibits large variations on periods of O(0.1–10) days. The Spill Jet sometimes cannot be distinguished in the velocity field from surface eddies or from the DSO. The vorticity structure of the jet confirms its unstable nature with peak relative and tilting vorticity terms reaching twice the planetary vorticity term. The average model Spill Jet transport is 4.9 ±1.7 Sv (1 Sv ≡ 106 m3 s−1) equatorward, about 2½ times larger than has been previously reported from a single ship transect in August 2001. Kinematic analysis of the model results suggests two different types of spilling events. In the first case (type I), a local perturbation results in dense waters descending over the shelf break into the Irminger Basin. In the second case (type II), surface cyclones associated with DSO deep domes initiate the spilling process. During summer 2003, more than half of the largest Spill Jet transport values are of type II.
    Description: The research is supported by the National Science Foundation Grants OCE-0726393 and OCI-0904640 (MGM and TWNH) and OCE-0726640 (RSP).
    Description: 2012-06-01
    Keywords: North Atlantic Ocean ; In situ observations ; Regional models
    Repository Name: Woods Hole Open Access Server
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  • 10
    Publication Date: 2018-06-13
    Description: Author Posting. © American Meteorological Society, 2017. 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 47 (2017): 2999-3013, doi:10.1175/JPO-D-17-0129.1.
    Description: Initial results are presented from a yearlong, high-resolution (~2 km) numerical simulation covering the east Greenland shelf and the Iceland and Irminger Seas. The model hydrography and circulation in the vicinity of Denmark Strait show good agreement with available observational datasets. This study focuses on the variability of the Denmark Strait overflow (DSO) by detecting and characterizing boluses and pulses, which are the two dominant mesoscale features in the strait. The authors estimate that the yearly mean southward volume flux of the DSO is about 30% greater in the presence of boluses and pulses. On average, boluses (pulses) are 57.1 (27.5) h long, occur every 3.2 (5.5) days, and are more frequent during the summer (winter). Boluses (pulses) increase (decrease) the overflow cross-sectional area, and temperatures around the overflow interface are colder (warmer) by about 2.6°C (1.8°C). The lateral extent of the boluses is much greater than that of the pulses. In both cases the along-strait equatorward flow of dense water is enhanced but more so for pulses. The sea surface height (SSH) rises by 4–10 cm during boluses and by up to 5 cm during pulses. The SSH anomaly contours form a bowl (dome) during boluses (pulses), and the two features cross the strait with a slightly different orientation. The cross streamflow changes direction; boluses (pulses) are associated with veering (backing) of the horizontal current. The model indicates that boluses and pulses play a major role in controlling the variability of the DSO transport into the Irminger Sea.
    Description: This work was supported by the NSF Grants OCE-1433448, OCE-1633124, and OCE- 1259618 and the Institute for Data Intensive Engineering and Science (IDIES) seed grant funding.
    Description: 2018-06-13
    Keywords: North Atlantic Ocean ; Mesoscale processes ; Ocean models ; Regional models
    Repository Name: Woods Hole Open Access Server
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