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  • 1
    Publication Date: 2016-11-29
    Description: Mesoscale model simulations were conducted for the Weddell Sea region for the autumn and winter periods of 2008 using a high-resolution, limited-area, non-hydrostatic atmospheric model. A sea ice–ocean model was run with enhanced horizontal resolution and high-resolution forcing data of the atmospheric model. Daily passive thermal and microwave satellite data was used to derive the polynya area in the Weddell Sea region. The focus of the study is on the formation of polynyas in the coastal region of Coats Land, which is strongly affected by katabatic flows. The polynya areas deduced from two independent remote sensing methods and data sources show good agreement, while the results of the sea ice simulation show some weaknesses. Linkages between the pressure gradient force composed of a katabatic and a synoptic component, offshore wind regimes and polynya area are identified. It is shown that the downslope surface offshore wind component of Coats Land is the main forcing factor for polynya dynamics, which is mainly steered by the offshore pressure gradient force, where the katabatic force is the dominant term. We find that the synoptic pressure gradient is opposed to the katabatic force during major katabatic wind events.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2014-12-11
    Description: The thermohaline circulation of the world ocean is partly driven by deep water formation at high-latitudes. In the Southern Ocean, deep and bottom water formation in the marginal seas is induced by high freezing rates as generally found at coastal polynyas. Atmospheric cooling and brine-release enable the production of very cold and saline water masses. In the southwestern Weddell Sea, wide shelves allow for a strong salinification of the whole water column and the formation of High Salinity Shelf Water (HSSW). The impact of coastal polynyas on ice production and water mass formation in the southwestern Weddell Sea was studied employing the Finite Element Sea ice-Ocean Model (FESOM) of the Alfred Wegener Institute, Bremerhaven. FESOM is a coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. Simulations were conducted on a global unstructured mesh with a strong focus on the southwestern Weddell Sea coastline (up to 3 km resolution). In vertical direction, the grid features 37 z-coordinate depth levels of which 6 are within the uppermost 100 m. The model runs were initialised in 1980 and forced with NCEP reanalysis data (daily resolution). The year 2008 was also simulated with higher-resolution GME and regional COSMO forcing data. For data evaluation and analysis the period 1990-2009 is used. A comparison of AMSR sea ice concentration and model results shows good accordance in spatial and temporal polynya extent. Also, calculated vertical temperature and salinity profiles agree well with CTD measurements. Our simulations feature a 20-year winter mean area of coastal polynyas of 6.7 x 10-3 km-2 (0.4% of the continental shelf area) in the southwestern Weddell Sea which is in good agreement with observations. Winter sea ice production within the coastal polynyas exceeds the ice production of the surrounding ice-covered area by a factor of 7 in the 20-year mean, so that the polynya contribution to total sea ice formation averages at about 3%. This small percentage is due to their even smaller areal percentage and the existence of leads and small polynyas in the so-called ice-covered ocean. The latter give a major contribution to sea ice production, but do not contribute to bottom water formation since they are transient elements that open, move and close dependent on the ice drift, whereas coastal polynyas are spatially fixed and open often for days, which is essential to achieve the salinification necessary for the formation of HSSW. From our simulations we derive a 20-year mean HSSW-formation of 4.2 x 10-5 km-3/season, but only 0.5 Sv thereof are exported over the shelf break, the rest stays on the shelf and is warmed and diluted during summer. The WSBW formation rate for the southwestern Weddell Sea continental shelf in our simulation is about 6.3 x 10-4 km-3/yr (2 Sv), which is on the low side but still reasonable compared to independent estimates. We conclude that in the Weddell Sea the role of coastal polynyas for sea ice production is not as big as is widely assumed, but they are indispensable for the formation of HSSW and thus for bottom water formation.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
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    In:  [Other] In: 24. Internationale Polartagung, 06.09.-10.09.2010, Obergurgl, Austria .
    Publication Date: 2014-12-18
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
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    In:  [Poster] In: SCAR and Open Science Conference "Antarctic Perspectives - Connecting the Arts to Science", 16.07.-19.07.2012, Portland, Oregeon, USA .
    Publication Date: 2014-12-12
    Description: For any climate signal to leave an imprint on the Antarctic Bottom Water (AABW) that fills the World Ocean abyss, it has to pass through the process of bottom water formation in the marginal seas of the Southern Ocean. An indispensable component of AABW is the dense shelf waters created on the continental shelves around Antarctica, particularly in the Ross and Weddell Seas. At coastal polynyas we find strong atmospheric cooling and high freezing rates that lead to a strong salinification of the water column. Here the bulk of High Salinity Shelf Water (HSSW) is formed. The impact of coastal polynyas on ice production and water mass formation in the southwestern Weddell Sea was studied employing the Finite Element Sea ice-Ocean Model (FESOM) of Alfred Wegener Institute, Bremerhaven. FESOM is a coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. Simulations were conducted on a global unstructured mesh with a strong focus on the southwestern Weddell Sea coastline (up to 3 km resolution). The model runs were initialised in 1980 and forced with NCEP reanalysis data (daily resolution). For 2008 also higher-resolution GME data and results from the regional COSMO atmosphere model of University Trier were applied as atmospheric forcing data. The period 1990-2009 is used for data analysis. Our simulations indicate that mean winter sea ice production within the coastal polynyas exceeds the surrounding area’s ice production by a factor of 7, giving a polynya contribution to total sea ice formation of 3 %. This small percentage is due to their even smaller areal percentage (0.4 %), and also the existence of leads and small polynyas in the ‘ice-covered’ ocean. The latter contribute substantially to sea ice production, but not to bottom water formation since they are transient elements that open, move and close dependent on the ice drift, whereas coastal polynyas are fixed in space and often open for days, enabling the salinification necessary for HSSW formation. From our simulations we derive a mean HSSW-formation of 4.2∙10^5 km^3/winter, but only 0.5 Sv thereof are exported over the shelf break, the rest stays on the shelf and is warmed and diluted during the following summer. The WSBW formation rate for the southwestern Weddell Sea continental shelf in our simulation is about 6.3∙10^4 km^3/yr (2 Sv), which is on the low side but still reasonable compared to independent estimates.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
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    Cambridge Univ. Press
    In:  Antarctic Science, 27 (4). pp. 388-402.
    Publication Date: 2015-07-21
    Description: The development of coastal polynyas, areas of enhanced heat flux and sea ice production strongly depend on atmospheric conditions. In Antarctica, measurements are scarce and models are essential for the investigation of polynyas. A robust quantification of polynya exchange processes in simulations relies on a realistic representation of atmospheric conditions in the forcing dataset. The sensitivity of simulated coastal polynyas in the south-western Weddell Sea to the atmospheric forcing is investigated with the Finite-Element Sea ice-Ocean Model (FESOM) using daily NCEP/NCAR reanalysis data (NCEP), 6 hourly Global Model Europe (GME) data and two different hourly datasets from the high-resolution Consortium for Small-Scale Modelling (COSMO) model. Results are compared for April to August in 2007–09. The two coarse-scale datasets often produce the extremes of the data range, while the finer-scale forcings yield results closer to the median. The GME experiment features the strongest winds and, therefore, the greatest polynya activity, especially over the eastern continental shelf. This results in higher volume and export of High Salinity Shelf Water than in the NCEP and COSMO runs. The largest discrepancies between simulations occur for 2008, probably due to differing representations of the ENSO pattern at high southern latitudes. The results suggest that the large-scale wind field is of primary importance for polynya development.
    Type: Article , PeerReviewed
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  • 6
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    In:  [Other] In: EGU General Assembly 2011, 03.04.-08.04.2011, Vienna, Austria .
    Publication Date: 2014-12-11
    Description: Coastal polynyas are frequent in many polar areas even in winter and play an important role for the coupling between the components of the sea ice-ocean-atmosphere system. Locally enhanced surface exchange processes in areas of open water have important consequences for the atmosphere and ocean processes, as well as for ice formation and the associated brine release. The non-hydrostatic, mesoscale model COSMO from the German Weather Service (DWD) has been applied for the Weddell Sea Region and therefore, adjustments of the COSMO model have been made to allow for the distinctive polar conditions. Besides a new parameterization of the roughness length, we have changed the soil/ice parameterization of the Antarctic continent in the model. For a better representation of the ice shelf elevations, we incorporated the ETOPO1 ice surface dataset. Furthermore, we use a thermodynamic sea ice model, which allows for a realistic treatment of the sea ice-atmosphere interactions. Remote sensing data from the passive microwave sounder AMSR-E was used to derive a high resolution, daily sea ice coverage for the model simulations. Results for several case studies show the improvements by the model adaptions. Whereas the model simulations show very good agreement with measurements in topographically homogeneous regions, larger differences occur with measurements in topographically heterogeneous regions. With these model adaptions and a 2 step nesting, we have produced a high-resolution COSMO dataset (15 km and 5 km) for one winter season (6 months), to provide forcing data for the high resolution sea ice-ocean model FESOM. The benefit of high-resolution forcing data for the sea ice-ocean model FESOM is investigated. Due to the higher spatial and temporal resolution, a more realistic simulation of polynya dynamics is expected, compared to the common forcing with global reanalyzes, like NCEP or GME.
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  • 7
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    In:  [Talk] In: 24. Internationale Polartagung, 10.09.2010, Obergurgl, Austria .
    Publication Date: 2014-12-12
    Description: For the formation and modification of water masses in the polar oceans the surface processes of freezing and melting are of great importance. At coastal polynyas, a major fraction of the annual ice production of the high-latitude oceans takes place, since they are usually kept open mechanically, primarily by winds, and the ocean surface is at freezing point. The very thin sea ice cover or the total lack thereof allows for locally enhanced exchange processes between ocean and atmosphere, especially an increase in the heat flux that leads to very high freezing rates and the high brine rejection associated. In polynya areas, very cold and salty water masses are formed and thus the duration and extent of polynya events have a substantial effect on bottom water formation. In the western Weddell Sea, recurring coastal polynyas are formed in front of the Filchner-Ronne Ice Shelf and in the area of the decayed Larsen A/B Ice Shelf. Simulations of oceanic processes linked to polynya occurrence are performed using the Finite-Element Sea-ice Ocean Model FESOM on a global grid with a high resolution (〈 3 km) area along the coasts of the Weddell Sea. FESOM is a fully coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. The model was forced with data from NCEP reanalysis, fields from GME, and from high-resolution COSMO simulations. The high-resolution experiments give very distinct polynya signatures. We clearly see reoccurring areas of strongly reduced or zero ice concentration and high negative fresh water flux (i.e. strong salt input) along the eastern coast of the Antarctic Peninsula and in front of the Ronne Ice Shelf, but also off Brunt Ice Shelf and Riiser-Larsen Isen. Comparisons with coarse-scale model runs show rather blurred evidence of polynya events. In simulations with 1.5° horizontal resolution, merely the recurring polynya located at Ronne Basin and the high freezing rates over Berkner Bank are visible as local maxima of negative fresh water flux, and a very weak reduction of ice thickness can be seen along the eastern coast of the Antarctic Peninsula.At a location at the outer edge of Ronne Basin, the differences between simulations with different resolution become obvious when looking at vertical profiles of temperature and salinity. It is an area of recurring polynyas which in the high-resolution case results in high salt input and formation of High Salinity Shelf Water (HSSW) entailing strong convection and an almost homogeneously mixed subsurface water column, whereas the coarse grid can not resolve the polynya and features a stratified ocean with a cold surface layer above Modified Warm Deep Water (MWDW). Also in the high-resolution simulation, plumes of cold water can be seen forming at the southwestern corner of the continental shelf and flowing down the slope. They subsequently mix with the warmer water masses around while drifting north. Signatures of similar cold water plumes have been observed during the ISPOL campaign in summer 2004/2005.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  [Other] In: EGU General Assembly 2011, 03.04.-08.04.2011, Vienna, Austria .
    Publication Date: 2014-12-11
    Description: Coastal polynyas are areas in the ice-covered ocean from which the sea-ice cover has been mechanically removed, primarily by winds. They are areas of enhanced exchange processes between ocean and atmosphere. The increased heat flux allows for exceptionally high freezing rates, which lead to locally increased brine-rejection. In the southwestern Weddell Sea, wide continental shelves and a weak exchange with the open ocean provide conditions that allow for substantial salinity enrichment, forming the cold and saline High Salinity Shelf Water (HSSW), which is the densest water mass in the region. HSSW is one of the ingredients of Weddell Sea Bottom Water (WSBW) and is thus essential for the formation of Antarctic Bottom Water, which covers large parts of the World Ocean’s abyss. Thus, production rates of HSSW and WSBW are of crucial importance in the ocean’s global thermohaline circulation. To study the influence of coastal polynyas on ice production and water mass formation in the southwestern Weddell Sea, we performed simulations using the Finite Element Sea ice-Ocean Model (FESOM) of the Alfred Wegener Institute, Bremerhaven. FESOM is a coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea-ice model. Simulations were conducted on a global unstructured mesh, focussing on the southwestern Weddell Sea coastline with up to 3 km resolution. In vertical direction, the grid features 37 z-coordinate depth levels of which 6 are within the uppermost 100 m. The model runs were initialised in 1980 and forced with NCEP daily reanalysis data. In addition, a hindcast for the year 2008 was computed with GME 6-hourly data forcing. For the winter period 2008, the (hourly) output from the high-resolution regional atmosphere model COSMO of the University Trier was nested into the GME fields, covering the area of the western Weddell Sea. For data evaluation and analysis the period 1990-2009 is used. A comparison of model results to AMSR sea ice concentration shows good agreement in spatial and temporal polynya extent. Also, simulated vertical temperature and salinity profiles agree well with CTD measurements. The total area of coastal polynyas is very small compared to the area of the Weddell Sea continental shelf. Winter sea ice production within the coastal polynyas, however, exceeds the ice production of the surrounding ice-covered area by a factor of 8 in the 20-year mean, so that the polynya contribution to total sea ice formation is always larger than their areal fraction. When looking at ice production, it should be kept in mind that also in the so-called ice-covered ocean, leads and small polynyas exist with an areal fraction of typically 5 %, which integrates to a total area that is much larger than the total area of coastal polynyas - but consists of small and transient elements. Thus this "fractal polynya" in the offshore Weddell Sea yields a major contribution to sea ice production, but does not contribute to bottom water formation, whereas coastal polynyas are spatially coherent for days or even weeks, which is essential to achieve the necessary salinity enrichment. Only in coastal polynyas and directly adjoining areas does surface salinity exceed 34.65, which is the defining minimum salinity for HSSW. From our simulations we derive a formation rate of 4.2 x 10-5 km-3/yr (13 Sv) of HSSW as a 20-year mean, with peak formation rates of 3 x 10-5 km-3 /month (116 Sv) in the autumn months. The WSBW formation rate in our model was found to be 6.3 x 10-4 km-3/yr (2 Sv) which is on the low side although not unrealistic when compared to observation-based estimates.
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  • 9
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    In:  [Other] In: EGU General Assembly 2010, 02.05.-07.05.2010, Vienna, Austria .
    Publication Date: 2014-12-12
    Description: Coastal polynyas play a prominent role in the formation and modification of water masses in the polar oceans. A coastal polynya is usually kept open mechanically, primarily by winds, and the ocean surface is at freezing point. Thus a major fraction of the annual ice production of the high-latitude oceans occurs in polynyas and hence the duration and extent of their appearance has a substantial effect on bottom water formation. In the western Weddell Sea, recurring coastal polynyas are formed in front of the Filchner-Ronne Ice Shelf and in the area of the decayed Larsen A/B Ice Shelf. Simulations to study polynya formation and their impact on ice production and bottom water formation in the western Weddell Sea were performed with the Finite Element Sea ice-Ocean Model (FESOM) of Alfred-Wegener-Institute (AWI). FESOM is a fully coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. The simulations were conducted on a global grid with a resolution varying between roughly 300 km in tropical latitudes and 〈5 km along the coast of the southwestern Weddell Sea. In vertical direction, the grid uses terrain-following coordinates. The model results give insight into the mechanisms governing the formation of transient and persistent polynyas and their influence on ice production and deep water formation. Water mass formation and ice export rates are quantified and compared to observation-based estimates.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
    facet.materialart.
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    In:  [Other] In: EGU General Assembly 2013, 07.04.-12.04.2013, Vienna, Austria .
    Publication Date: 2014-12-11
    Description: Dense shelf water is an essential ingredient to the formation of Antarctic Bottom Water (AABW). It is formed on the continental shelves surrounding Antarctica, when freezing rates are sufficiently high to push ocean salinity to values of 34.65 and higher. Coastal polynyas, where the ice is driven away from the coastline, maintain the highest freezing rates in Antarctic winter. Since theWeddell Sea is considered the most productive source region of AABW, we investigate the dense water formation on the continental shelves of the southwestern Weddell Sea, with a focus on the role of coastal polynyas, using the Finite Element Sea ice-Ocean Model (FESOM), a primitive-equation, hydrostatic ocean model coupled with a dynamic-thermodynamic sea ice model. The horizontal resolution of the global, unstructured mesh is up to 3 km at the southwestern Weddell Sea coastline; in vertical direction the mesh features 37 depth levels (resolution increases toward the surface). The model was initialized on 01/01/1980 with data from the Polar Hydrographic Climatology and forced with NCEP/NCAR Reanalysis data. The 20-year period 1990-2009 is used for analysis. Our results indicate that in an average year, the polynya freezing rates of 9 cm d--1 (corresponding to a salt input of 2.5 kg m--2d--1) cause a seasonal variation in salinity of 0.3 psu under the Ronne polynya and result in the production of 5.10-4 km-3 dense shelf water, which leaves the continental shelf (outlined by the 700 m isobath in this study) at a long-term mean volume flux of 5.2 Sv. Some of this water contributes to the formation of Weddell Sea Deep/BottomWater, but a large fraction is diluted by mixing with ambient water and leaves the Weddell Sea at intermediate levels.
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