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Free keywords:
cyclones , statistic , sea ice concentration
Abstract:
Arctic sea-ice is a barrier between ocean and atmosphere and as such, plays an important role in the climate system. In winter, a closed ice cover reduces the sensible and latent heat fluxes between ocean and atmosphere to a great extent. In summer, the sea ice reflects the most part of the incoming shortwave radiation. Strong winds, as they occure in cyclones, lead to sea-ice drift and influence the fraction of ocean surface which is covered by sea ice. This study investigates the impact of cyclones on sea ice, with a focus an the sea-ice concentration in the central Arctic. The impact of cyclones is analyzed on the basis of observations of the field campaigns DAMOCLES 2007 and DAMOCLES 2008, on the basis of satellite measurements (AMSR-E ice concentrations) and furthermore on the basis of simulations with a coupled sea-ice-ocean model. For the simulations the dynamic-thermodynamic model NAOSIM (North Atlantic Arctic Ocean Sea Ice Model) is forced with 6-hourly ECMWF-analyses (European Centre for Medium- Range Weather Forecasts). The comparison of simulated ice drift and concentration with observations made clear that the choosen model configuration is appropriate for the performed studies. Sensitivity studies were performed with a wind field that represents a cyclone passing through the Arctic. The experiments show, that the ice concentration is reduced in general under the influence of a cyclone. The reduction is the stronger, (1) the slower the cyclone, (2) the higher the pressure difference between core and surrounding, (3) the smaller the deviation angle between 10 m-wind and geostrophic wind, and (4) the higher the initial ice concentration is. Between reduction of ice concentration and initial ice thickness no correlation has been found. The comparison of simulated ice drift with data of drift buoys reveals, that the model overestimates the drift speed in general whereas extreme events like storms are unterestimated. A systematic deviation in simulated and observed drift direction is found. Furthermore it is shown, in which region the agreement between simulated and messuared ice drift is high and in which regions it is low. In Winter, the model simulates realistic ice concentrations and realistic ice extent. In summer, the ice concentration is too low and the ice extent is too large. A new approach has been made in this study: for a statistical analyses of the impact of a large number of real cyclones on simulated sea ice, 6-hourly positions of cyclones on the basis of the ECMWF sea level pressure field are used. It is investigated how the ice drift, ice concentration and the windfactor are changing at the cyclone’s positions. These investigations include the intensity of the cyclones, the initial ice conditions, the seasonal variability and regional distribution. In summer there is an important climatologic impact of cyclone due to the reduced albedo of a reduced ice cover. Thus, the absorption of solar radiation is increased until the next freezing period. In summer, an increase of cyclone activity accelerates the reduction of the arctic ice concentration.
