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.

Description: The Laptev Sea area of the Siberian Arctic is known as a region of high polynya activities throughout the winter season. We analyze the impact of open-water and thin-ice covered polynyas on heat and moisture fluxes and the atmospheric boundary layer (ABL) using downscaled NWP simulations. ERA-Interim reanalysis data are used as forcing for dynamically downscaled COSMO runs with 15 and 5 km horizontal resolution. Sea ice information is taken from AMSR-E data and the period of investigation is 2002-2011. Our results clearly prove that polynyas moisten and heat the air downwind the polynya (up to several hundred kilometers) and additionally increase cloudiness. The analysis of surface energy balance components shows the sensible heat flux H0 as the largest contributor to ice production. Mean monthly H0 over polynyas is about 150 W/m2 for Dec.-Feb. 2002-2011. This is about three times higher than the energy loss by net radiation. Small polynyas have the largest heat loss (and ice production) per surface unit. In comparison with most previous studies our results suggest that most preceding studies overestimated the polynya ice production of the Laptev Sea.

Description: The interaction between polynyas and the atmospheric boundary layer is examined in the Laptev Sea using the regional, non-hydrostatic Consortium for Small-scale Modelling (COSMO) atmosphere model. A thermodynamic sea-ice model is used to consider the response of sea-ice surface temperature to idealized atmospheric forcing. The idealized regimes represent atmospheric conditions that are typical for the Laptev Sea region. Cold wintertime conditions are investigated with sea-ice ocean temperature differences of up to 40 K. The Laptev Sea flaw polynyas strongly modify the atmospheric boundary layer. Convectively mixed layers reach heights of up to 1200 m above the polynyas with temperature anomalies of more than 5 K. Horizontal transport of heat expands to areas more than 500 km downstream of the polynyas. Strong wind regimes lead to a more shallow mixed layer with strong near-surface modifications, while weaker wind regimes show a deeper, well-mixed convective boundary layer. Shallow mesoscale circulations occur in the vicinity of ice-free and thin-ice covered polynyas. They are forced by large turbulent and radiative heat fluxes from the surface of up to 789 W m-2, strong low-level thermally induced convergence and cold air flow from the orographic structure of the Taimyr Peninsula in the western Laptev Sea region. Based on the surface energy balance we derive potential sea-ice production rates between 8 and 25 cm d-1. These production rates are mainly determined by whether the polynyas are ice-free or covered by thin ice and by the wind strength.

Description: The interaction between polynyas and the atmospheric boundary layer is examined in the Laptev Sea using the regional, non-hydrostatic Consortium for Small-scale Modelling (COSMO) atmosphere model. A thermodynamic sea-ice model is used to consider the response of sea-ice surface temperature to idealized atmospheric forcing. The idealized regimes represent atmospheric conditions that are typical for the Laptev Sea region. Cold wintertime conditions are investigated with sea-ice ocean temperature differences of up to 40 K. The Laptev Sea flaw polynyas strongly modify the atmospheric boundary layer. Convectively mixed layers reach heights of up to 1200 m above the polynyas with temperature anomalies of more than 5 K. Horizontal transport of heat expands to areas more than 500 km downstream of the polynyas. Strong wind regimes lead to a more shallow mixed layer with strong near-surface modifications, while weaker wind regimes show a deeper, well-mixed convective boundary layer. Shallow mesoscale circulations occur in the vicinity of ice-free and thin-ice covered polynyas. They are forced by large turbulent and radiative heat fluxes from the surface of up to 789 W m-2, strong low-level thermally induced convergence and cold air flow from the orographic structure of the Taimyr Peninsula in the western Laptev Sea region. Based on the surface energy balance we derive potential sea-ice production rates between 8 and 25 cm d-1. These production rates are mainly determined by whether the polynyas are ice-free or covered by thin ice and by the wind strength.

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.