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  • 1
    Publication Date: 2016-11-29
    Description: The Arctic freshwater (FW) has been the focus of many modeling studies, due to the potential impact of Arctic FW on the deep water formation in the North Atlantic. A comparison of the hindcasts from ten ocean-sea ice models shows that the simulation of the Arctic FW budget is quite different in the investigated models. While they agree on the general sink and source terms of the Arctic FW budget, the long-term means as well as the variability of the FW export vary among models. The best model-to-model agreement is found for the interannual and seasonal variability of the solid FW export and the solid FW storage, which also agree well with observations. For the interannual and seasonal variability of the liquid FW export, the agreement among models is better for the Canadian Arctic Archipelago (CAA) than for Fram Strait. The reason for this is that models are more consistent in simulating volume flux anomalies than salinity anomalies and volume-flux anomalies dominate the liquid FW export variability in the CAA but not in Fram Strait. The seasonal cycle of the liquid FW export generally shows a better agreement among models than the interannual variability, and compared to observations the models capture the seasonality of the liquid FW export rather well. In order to improve future simulations of the Arctic FW budget, the simulation of the salinity field needs to be improved, so that model results on the variability of the liquid FW export and storage become more robust.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2014-07-04
    Description: Simulations of ice shelf basal melting in future climate scenarios from the IPCC’s Fourth Assessment Report (AR4) have revealed a large uncertainty and the potential of a rapidly increasing basal mass loss particularly for the large cold-water ice shelves in the Ross and Weddell Seas. The large spread in model results was traced back to uncertainties in the freshwater budget on the continental shelf, which is governed by sea ice formation. Differences in sea ice formation, in turn, follow the regional differences between the atmospheric heat fluxes imprinted by the climate models. A more recent suite of BRIOS and FESOM model experiments was performed with output from two members of the newer generation of climate models enganged in the IPCC’s Fifth Assessment Report (AR5). Comparing simulations forced with output from the AR5/CMIP5 models HadGem2 and MPI-ESM, we find that uncertainties arising from inter-model differences in high latitudes have reduced considerably. Projected heat fluxes and thus sea ice formation over the Southern Ocean continental shelves have converged to an ensemble with a much smaller spread than between the AR4 experiments. For most of the ten larger ice shelves in Antarctica, a gradual (but accelerating) increase of basal melt rates during the 21st century is a robust feature throughout the various realisations. Both with HadGem2 and with MPI-ESM forcing, basal melt rates for Filchner-Ronne Ice Shelf in FESOM increase by a factor of two by the end of the 21st century in the RCP85 scenario. For the smaller, warm-water ice shelves, inter-model differences in ice shelf basal mass loss projections are still slightly larger than differences between the scenarios RCP45 and RCP85; compared to AR4 projections, however, the model-dependent spread has been strongly reduced.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2015-02-24
    Description: An intercomparison of four low-resolution remotely sensed ice-drift products in the Arctic Ocean is presented. The purpose of the study is to examine the uncertainty in space and time of these different drift products. The comparison is based on monthly mean ice drifts from October 2002 to December 2006. The ice drifts were also compared with available buoy data. The result shows that the differences of the drift vectors are not spatially uniform, but are covariant with ice concentration and thickness. In high (low) ice-concentration areas, the differences are small (large), and in thick (thin) ice-thickness areas, the differences are small (large). A comparison with the drift deduced from buoys reveals that the error of the drift speed depends on the magnitude of the drift speed: larger drift speeds have larger errors. Based on the intercomparison of the products and comparison with buoy data, uncertainties of the monthly mean drift are estimated. The estimated uncertainty maps reasonably reflect the difference between the products in relation to ice concentration and the bias from the buoy drift in relation to drift speed. Examinations of distinctive features of Arctic sea ice motion demonstrate that the transpolar drift speed differs among the products by 13% (0.32 cm s−1) on average, and ice drift curl in the Amerasian Basin differs by up to 24% (3.3 × 104 m2 s−1). These uncertainties should be taken into account if these products are used, particularly for model validation and data assimilation within the Arctic.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
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  • 4
    Publication Date: 2015-09-28
    Description: Antarctic ice sheet mass loss and thus part of global sea-level rise is related to enhanced ice stream discharge to the fringing ice shelves. The transfer of ice into the ocean occurs via iceberg calving and ice shelf basal melting. For decades the balance of both terms was assumed to be in favor of the calving, but recent results, based on remote sensing, revealed that basal melting seems to be at least of similar importance. A recent model study indicates that future atmospheric conditions in the southern Weddell Sea may switch the continental shelf, formerly dominated by the formation of cold saline waters, to one influenced by warm open ocean waters with consequences for the basal mass flux and ice shelf/ice sheet dynamics. Here, we continue the simulations showing a warming of the Filchner-Ronne Ice Shelf cavity, applying 20th-century atmospheric and basal mass flux forcing at different future points in time. Our numerical study indicates that once the system reaches the 'warm phase', a positive meltwater feedback stabilizes the shelf circulation such that only a reduction to 20th century basal mass flux can stop warm water from penetrating onto the continental shelf and into the sub-ice cavity. This has implications for the future of the Antarctic Ice Sheet, since a major decrease of basal melting only can be achieved by a significant disintegration of the floating portion of the ice sheet.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed , info:eu-repo/semantics/conferenceObject
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  • 5
    Publication Date: 2015-10-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2016-11-29
    Description: Recent decades have shown substantial changes in the Arctic Ocean, yet observations are still relatively sparse compared to most other parts of the world's oceans. Results from numerical models still differ in the distribution of key variables, such as the pathways of liquid freshwater. From salinity and temperature profiles observed by a variety of platforms since 1992 we are able to show a substantial freshening in the upper Arctic Ocean impacting an increase in stratification between the mixed-layer and the lower halocline. Based on temperature and salinity profiles, we will present an objective analysis of mixed-layer depth, sea surface height and geostrophic velocity during the recent two decades. We are able to derive decadal trends as well as seasonal cycles during the most recend decade. Surface geostrophic velocity points to a flow in the Amerasian Basin toward the Lomonosov Ridge. Although most of the freshwater volume increase occurred on the Amerasian side of the Arctic Ocean, the changes on the Eurasian side have strong implications not only for the stratification but also the vertical exchange of nutrients in the upper ocean.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 7
    Publication Date: 2017-12-15
    Description: Warm water of open ocean origin on the continental shelf of the Amundsen and Bellingshausen Seas causes the highest basal melt rates reported for Antarctic ice shelves with severe consequences for the ice shelf/ice sheet dynamics. Ice shelves fringing the broad continental shelf in the Weddell and Ross Seas melt at rates orders ofmagnitude smaller. However, simulations using coupled ice–ocean models forced with the atmospheric output of the HadCM3 SRES-A1B scenario run (CO2 concentration in the atmosphere reaches 700 ppmv by the year 2100 and stays at that level for an additional 100 years) show that the circulation in the southern Weddell Sea changes during the twenty-first century. Derivatives of Circumpolar Deep Water are directed southward underneath the Filchner–Ronne Ice Shelf, warming the cavity and dramatically increasing basal melting. To find out whether the open ocean will always continue to power the melting, the authors extend their simulations, applying twentieth-century atmospheric forcing, both alone and together with prescribed basal mass flux at the end of (or during) the SRES-A1B scenario run. The results identify a tipping point in the southern Weddell Sea: once warm water flushes the ice shelf cavity a positive meltwater feedback enhances the shelf circulation and the onshore transport of open ocean heat. The process is irreversible with a recurrence to twentieth-century atmospheric forcing and can only be halted through prescribing a return to twentieth-century basal melt rates. This finding might have strong implications for the stability of the Antarctic ice sheet.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
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  • 8
    Publication Date: 2018-03-06
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
  • 10
    Publication Date: 2018-12-07
    Description: An automatic parameter optimization system for a coupled ocean-sea ice model is applied to investigate uniqueness of parameter set obtained from data assimilation. We set up a parameter optimization experiment, in which 15 model parameters are optimized simultaneously using a 23-years optimization window. A series of 11 independent experiments are conducted to examine spread of objective functions, optimized sea ice fields and associated optimal parameters. The result shows sufficiently small spreads of objective functions and ice fields, whereas a significantly large spread of optimized parameters. This indicates the system gives an unique solution regarding the simulated ice fields, whereas multiple solutions regarding the associated model parameters. A correlation analysis shows the optimal parameters are inter-related and covariant. A principal component analysis (PCA) reveals that the first 3 principal components explain 70% of the variation of the optimal parameter sets, indicating a contraction of the model parameter space.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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