Atmosphere-ice forcing in the transpolar drift stream: results from the DAMOCLES ice-buoy campaigns 2007-2009

During the EU research project Developing Arctic Modelling and Observing Capabilities for Long-term Environmental Studies (DAMOCLES), 18 ice buoys were deployed in the region of the Arctic transpolar drift (TPD). Sixteen of them formed a quadratic grid with 400 km side length. The measurements lasted from 2007 to 2009. The properties of the TPD and the impact of synoptic weather systems on the ice drift are analysed. Within the TPD, the speed increases by a factor of almost three from the North Pole to the Fram Strait region. The hourly buoy position fixes would show that the speed is underestimated by 10-20% if positions were taken at only 1-3 day intervals as it is usually done for satellite drift estimates. The geostrophic wind factor U-i/U-g (i.e. the ratio of ice speed U-i and geostrophic wind speed U-g), in the TPD amounts to 0.012 on average, but with regional and seasonal differences. The constant U-i/U-g relation breaks down for U-g < 5 m s(-1). The impact of synoptic weather systems is studied applying a composite method. Cyclones (anticyclones) cause cyclonic (anticyclonic) vorticity and divergence (convergence) of the ice drift. The amplitudes are twice as large for cyclones as for anticyclones. The divergence caused by cyclones corresponds to a 0.1-0.5% per 6 h open water area increase based on the composite averages, but reached almost 4% within one day during a strong August 2007 storm. This storm also caused a long-lasting (over several weeks) rise of U-i and U-i/U-g and changed the ice conditions in a way which allowed large amplitudes of inertial ice motion. The consequences of an increasing Arctic storm activity for the ice cover are discussed