Dispersion von Meereis unter Nutzung der Lagrangeschen Betrachtungsweise: Eine Studie dynamischer Systeme
Zusammenfassung der Projektergebnisse
Relative separation of sea ice was studied in the Fram Strait for the years 1999 - 2009 using observations of sea ice buoys position. The relative dispersion was characterized by the use of probability distribution functions (PDFs) of the separation; mean squared separation versus time and FSLEs. We focussed on the following questions: • What dispersion regimes, such as ballistic or Richardson, are present in sea ice? • Is there a transition between different regimes at different scales? • How does the dispersion changes from the sea ice environment to the oceanic or atmospheric environment? • How do these regimes impact the export of sea ice from a region such as the Fram Strait, that it one of the main gateways for sea ice into the North Atlantic Ocean? The region under consideration is dynamically dominated by the presence of the East Greenland Current, which is associated to very strong horizontal shear which dominates the dispersion. As a consequence of this we did split the separation in the along and across mean flow direction. The split was done by using the angle of the mean flow from the horizontal by considering the trajectory of the center of mass of the buoys for each year separately. Finally, the buoys were clustered by different initial separation. Results show a scale dependent superdiffusive dispersion of sea ice. In details, the buoys show a diffusive behavior, until ~ 10 km scale. At larger scales, i.e. in the range 10 - 200 km, the buoys show ballistic dispersion, which is associated to the horizontal shear of the East Greenland Current. Interestingly, for larger scales, i.e. in the range 200 - 700 km, the dispersion is superdiffusive, which might be associated to the acceleration of one of the buoys of the pairs, as they reach the sea ice margin. If the relative dispersion is split in the long and across mean flow components it is possible to observe the following. The along mean flow component of the relative separation between buoys shows a spatial interval between 600 m - 2 km in which the separation is exponential and the dynamics correspond then to chaotic advection. At scales between 2 - 100 km the dispersion shows a diffusive behavior, followed by a ballistic regime in the range 100 - 200 km. For larger scales the dispersion shows signs of superdiffusive behavior. The across mean flow component of the relative separations has instead a different behavior than the along component. For scale less than ~ 100 km, the FSLEs do not show any inertial range. In the range 100 - 700 km the FSLEs show a slope between -1 and -2/3. The slope of -2/3 corresponds to self-similar Richardson dispersion and is expected for two dimensional turbulence. The study here reported shows that sea ice follows a complex scale dependent dispersion. Results could be of use for estimates of export of sea ice from the Fram Strait as well as for the development of parameterizations for turbulent sea ice transport.
