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Simulation of coastal polynyas and associated ocean processes in the western Weddell Sea

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Haid, V. , Timmermann, R. , Heinemann, G. and Ebner, L. (2010): Simulation of coastal polynyas and associated ocean processes in the western Weddell Sea , Internationale Polartagung, Obergurgl.-10. September 2010. .
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Abstract:

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.

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