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Abstract:
The Fram Strait is an important interface for the transfer of Arctic climate signals to the North Atlantic Ocean. This thesis investigates the variability of the sea ice export through Fram Strait and its impact on northern hemispheric climate. For this purpose a 500-year control integration of the global coupled atmosphere-ocean-sea ice model ECHAM5/MPI-OM is analysed and sensitivity experiments are performed. The model provides a realistic mean Arctic climate and reproduces the observed variability well. The simulated mean Fram Strait ice export amounts to 0.1 Sverdrup. It is highly variable at interannual to decadal time scales. This variability is mainly determined by variations in the sea level pressure gradient across Fram Strait and thus geostrophic wind stress. The North Atlantic Oscillation has no significant influence on the ice export in this model. In contrast, variations in the stratospheric polar vortex and atmospheric planetary waves turned out to be of considerable importance for the wind stress in Fram Strait.
A combination of anomalous northerly winds in Fram Strait and increased sea ice thickness leads to particularly large ice exports. After such events the ice/fresh water signal propagates southward in the East Greenland Current and reaches the Labrador Sea one to two years later. This negative salinity anomaly decreases or even suppresses oceanic convection in the Labrador Sea. The associated cooler sea surface and increased sea ice cover reduce the ocean heat release to the atmosphere, which has a significant impact on air temperatures and atmospheric circulation. Sea level pressure anomalies respond in large areas of the high northern latitudes to oceanic perturbations in the Labrador Sea.
A decadal climate mode could be identified in the Arctic: it is characterised by wind forced formation of ice thickness anomalies at the Siberian coast, their subsequent propagation across the Arctic towards Fram Strait in 4 to 5 years and enhanced ice export. Negative ice thickness anomalies are generated at the Siberian coast simultaneously with the enhanced export. They reach Fram Strait another 4 to 5 years later. Furthermore, there is a feedback of the ice anomalies on the atmospheric circulation. However, it is rather weak compared to atmospheric variability. The mode is thus strongly damped.
Sensitivity studies show that the isolated effect of a prescribed ice/fresh water anomaly in Fram Strait explains a large part of the climate variability in the Labrador Sea. The ice export through Fram Strait can thus be used for predictability of Labrador Sea climate for the next two years.
