Tracer studies with Arctic and subArctic coupled ice-ocean models: dispersion of radionuclides and oxygen isotopes

Full text: Natural and man-made soluble isotopes which enter the oceanic environment are advected by the ocean-currents or the ice-drift and are distributed over large areas far from their sources. By this they trace the dominant flow patterns and exchanges processes on timescales from years to decades. By introducing such isotopes as tracers into coupled ice-ocean models used to study the climate system of the Arctic and Subarctic we receive progress in two areas: The intercomparison of model derived tracer distributions with observations offers the opportunity to validate the model experiments. If consistent with observations, the model experiments in turn may serve as an interpretative tool to understand the evolution of the observed distributions. The latter is especially valuable in areas like the Arctic Ocean, where the observations are sparse in time and space. We present examples from two projects introducing the natural isotope δ18O as a tracer for river water in the Arctic and the man-made radionuclide Technetium-99, which has been emitted from west-European nuclear reprocessing facilities in increased amounts in the 1990s. The natural isotope δ18O enters the Arctic Ocean via the rivers carrying runoff from the Siberian and North American catchment areas. Since the signature of δ18O for the rivers is markedly different from oceanic values it can serve as a tracer for the riverine component of freshwater. The investigation of the dynamics of freshwater in form of ice, ice-melt and river water in the Arctic Ocean is closely linked to a better understanding of the variability of the global thermohaline circulation. The latter is apparently influenced by the amount of freshwater released from the Arctic Ocean into the convective areas of the Nordic Seas and the Labrador Sea. The intercomparison of the modelled and observed patterns of δ18O leads to better insight of the state of the large circulation systems which store and advect freshwater in the interior Arctic, namely the Beaufort Gyre and the Transpolar Drift, which have undergone significant changes in the last decade. The man-made radionuclide Technetium-99 is emitted from the west-European reprocessing facilities in Sellafield and La Hague. Since 1994 the release has increased significantly and introduced an intense signal into the current system of the North Sea, the Nordic Seas and the Arctic Ocean. We simulate the dispersion of this radionuclide for the last decades. The results of this experiment feed into a Norwegian research project which aims an improvement of box-models used for assessment studies and monitoring strategies. At the same time an intercomparison of the simulated technetium signal with observations in the Nordic Seas and the central Arctic Ocean allows a better insight into the ventilation processes of those areas with respect to the inflowing water masses. We will present recent advances in the described applications of isotopes in climate studies for the Arctic and subArctic domains and want to demonstrate the large potential of such combined model-observation studies with isotopes for the investigation of the climate system in the north, as well as for applied purposes like the dispersion of dissolved pollutants in the ocean. (author)