Master thesis defence by Anne-Katrine Faber – Niels Bohr Institute - University of Copenhagen

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Master thesis defence by Anne-Katrine Faber

Title: Influence of the polar front position on stable water isotopes in Greenland precipitation 

A strong spatial relation exist between the isotopic signal and surface temperatures. Using this relation to investigate past temporal changes in the isotope records it is possible to reconstruct past climate variability from Greenland ice cores. Yet, for present climate a significant year-to-year variability is seen in the d18O-values in ice core records. The knowledge  about the causes for this observed interannual variability is very limited. It is therefore highly desired to refine the understanding of the physical processes that influence the isotopic composition.

This study investigates the connection between the position of the polar front in the North Atlantic and the interannual variability observed in d18O records for Greenland.   Based on re-analysis data from ERA-40, existing computational methods of thermal measures were applied to identify fronts. It was found that these measures could not express the actual position of the polar front. Therefore demands for a suitable representation of the polar front were formulated and an alternative approach of polar front identification were designed.

The polar front  location method is essentially based on the thermal frontal parameter (TFP). Issues of non-continuity and varying intensity of the polar front are considered by the method. The output of the method is a two-dimensional field, representing the spatial distribution of the polar front.

Using the output of the method, estimates for moisture sources temperatures for Greenland precipitation were computed. These source temperatures had values in the range 12-18 degrees celcius.

The estimated source temperatures were used to describe the differences in radiative cooling that the air masses experience during transport towards Greenland. These  temperatures were compared with the interannual variations in d18O. Data from the GCM isoCAM3 were used for comparison with variations in d18O.  Evidence was found that the position of the PF influence d18O in Greenland precipitation, but the effect of this correlation was neglectable, due to the pronounced correlation with local site temperatures.

Supervisor: Peter L. Langen