ALBERT

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.

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.

Description: Global Change and its predicted key impact on the Arctic bring the Laptev Sea to the centre of climate-related polar research. This Shelf Sea is known as being a highly productive area for the formation of new ice throughout the winter season. A main part of the ice production occurs in flaw polynyas which appear recurrently at the edge of the fast ice surrounding the coastal zones during wintertime. This work attempts to provide a method to reliably estimate the ice production in the Laptev Sea polynyas from model result s of the numerical weather prediction model COSMO. Our modeling approach contains the use of COSMO with 15 and 5 km horizontal resolution nested in global GME/ERA-Interim data to calculate ice production in polynyas. To account for realistic polynya representation polynya area is prescribed to the COSMO model by daily AMSR-E satellite data. The potential volume ice production is calculated from atmospheric net radiation fluxes. In contrast to preceding studies our new COSMO-based method takes into account the effect of polynyas on the atmosphere. Over open water, warmer 2m temperatures (COSMO in comparison to NCEP) lead to lower ice production. Over thin ice, surface temperature depends on air temperature and reduced air surface temperature gradients cause lower heat fluxes and less ice production than over open water. As warm-biased NCEP values are balancing the effects of our improvements the comparison of ice production retrieval based on NCEP data with our results show minor total differences only. Both methods are leading to results in same order of magnitude if the polynya is assumed to be covered with 10cm of thin-ice. This supports the thesis that either of them leads to feasible ice production values if thin ice within the polynya is accounted for in the calculation. In case of an open water polynya, however, our study underlines the impact of the atmospheric data on the ice production. Thus we conclude that it is of major importance to choose a validated ice thickness parameterization for the model.

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: 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.

Description: Processes of the exchange of energy and momentum at the sea ice-ocean-atmosphere interface are key processes for the polar climate system. Heat and moisture fluxes are strongly modulated by open water fractions associated with polynyas, having important consequences for the atmosphere, ocean processes, ice formation, brine release, gas exchange and biology. Our paper aims at the study of atmospheric processes forcing and maintaining polynyas in the Laptev Sea of the Siberian Arctic. This region is known as being a highly productive area for the formation of new ice throughout the winter season. We study polynya processes using passive satellite remote sensing data, high-resolution (5km) sea-ice/ocean and atmospheric models, as well as in-situ data obtained during experimental studies in that area. Passive microwave sensor data (SSM/I, AMSR) are used together with atmospheric reanalysis to characterize the long-term spatiotemporal characteristics of polynya events. A special focus lies on the detection of thin ice in polynya areas, which is studied using thermal infrared data (MODIS, AVHRR). Thin ice statistics combined with microwave data allows for estimations of ice production rates for the last decades. The NWP model COSMO is used together with the sea-ice/ocean model FESOM to study polynya dynamics. Model simulations are validated using satellite data and in-situ measurements from two campaigns in the Laptev Sea area.

Description: Simulations for Greenland with focus on the wind regime are presented using the high-resolution non-hydrostatic model COSMO (Consortium for Small scale modeling). The simulations are performed at 15 km, 5.5 km and 1.3 km resolution for the time period of June 2010. The Nares Strait, including the North Water (NOW) polynya, in northwest Greenland was selected as focus of the simulations, since comprehensive measurements of the structure of the boundary layer are available from an aircraft study. The observations on four different days show a shallow stable boundary layer over the polynya and a pronounced low-level jet associated with the flow channeling in the Nares Strait, particularly at Smith Sound. The reproduction of the vertical patterns of wind and temperature by the simulations is realistic at all resolutions and best results are found for 5.5 km and 1.3 km resolution. A vertical displacement of the patterns and an overestimation of the temperature was found. The measured low-level inversion is not simulated well, but overall the vertical structures of the simulation and observation correlate highly. Thus, the model is well suited for simulations in particular for the situation of flow channeling in a topographically complex area. The analysis of the synoptic situations associated with channeled flow through the Nares Strait shows that the wind speed increases with higher pressure difference between the Lincoln Sea and Baffin Bay. Channeling effects lead to a prevailing flow direction towards Baffin Bay. A strong increase of the wind speed occurs at Smith Sound, where the flow also passes over mountains of the Greenland coast. The wind maximum is found downstream of Smith Sound, and typical low-level jets with wind speeds of around 20 m/s occur at a height of 100 m.