Hilmer, Michael (2001) A Model Study of Arctic Sea Ice Variability. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 157 pp. . Berichte aus dem Institut für Meereskunde an der Christian-Albrechts-Universität Kiel, 320 . DOI 10.3289/ifm_ber_320.

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Abstract

The focus of the present study is the investigation of the variability of the Arctic sea ice cover on interannual to multi-decadal time scales. Because there is only a rather small number of direct observations, this investigation is based on simulated data . Using realistic daily fields of near surface wind and air temperature derived from the NCEP/NCAR reanalysis, the Arctic sea ice cover is simulated with a dynamic-thermodynamic sea ice model integrated over the time period 1951 - 1999. Additionally, sensitivity experiments with prescribed interannual variability of either only winds or only air temperatures are performed in order to identify their relative contributions to the simulated variability of the ice pack. A comparison of available observations from satellites, drifting buoys, submarines, and oceanographic moorings reveals a good correspondence of the long-term averaged quantities of the simulated ice concentration, ice drift, and ice thickness, respectively. Moreover, it is shown that the simulation is able to reproduce the observed interannual variations of the sea ice extent. The simulation results reveal pronounced interannual variability of the main quantities of the Arctic sea ice cover as, for instance, its thickness, its areal extent, and the ice volume export into the North Atlantic. These variations are investigated with respect to their main causes in the atmospheric forcing fields and to its possible impacts on the freshwater input into the North Atlantic Ocean. Moreover, from statistical analyses of the model output dominant modes of interannual variability are identified and partly related to the dominant modes of atmospheric variability of the northen high latitudes, that are the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). It is found that the AO leads to cyclonic and anticyclonic ice drift variations which are associated with changes of the ice mass advection between the Beaufort and East Siberian Seas leading to enhanced ice thickness variability in those regions. The NAO is responsible for the out-of-phase variability of the wintertime ice extent between the regions east and west of Greenland. Moreover, since the mid-1970s the NAO exhibits coherence with variations of the wintertime ice volume export through Fram Strait which is missing in the time period before. The temporal change of the link between both processes is explained by an eastward shift of the NAO's centers of interannual variability around the mid-1970s. This eastward shift was associated with increased anomalous meridional wind components near Fram Strait during high and low NAO winters and, hence, with increased coherence between the NAO and sea ice exports through Fram Strait on interannual time scales. On the background of interannual variability the simulation also reveals significant long-term trends of the Arctic ice pack. Because of a rise of surface air temperatures, the total ice volume of the Arctic has linearly decreased by about 16% since the mid 1960s. In agreement with observational estimates, largest thinnings occur in the Eastern Arctic and near the North Pole whereas the ice concentration and thickness north of the Canadian Archipelago and in the Baffin Bay have slightly increased. The deepening of Arctic surface air pressures led to a weakened anticyclonic large-scale flow of the ice pack. Whereas the effects of long- term changes of the wind field generally tend to damp the thermally induced thinning of the ice cover, the dynamic effects during the last 15 years of the simulation led to an acceleration of the decrease of the Arctic ice pack.

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