Masters Thesis Defense by Cecilie Drost Aakjær – Niels Bohr Institutet - Københavns Universitet

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Masters Thesis Defense by Cecilie Drost Aakjær




Dynamics of the Arctic Ocean freshwater storage in the EC-Earth coupled climate model


Abstract:
This study reports on the freshwater storage in the Arctic Ocean. The dynamics of the freshwater storage in the Arctic Ocean are of great interest due to the large climatic  changes in the area. These are estimated from a preindustrial control run of the ocean-atmosphere coupled climate model, EC-Earth. A temporal representation of the freshwater volume (FWV) is found for four different integration areas, the Arctic Ocean (including shelf), 10 · 106 km3, the Arctic Ocean (excluding shelf), 4.4 · 106 km3, the Canadian Basin, (excluding shelf)(CB), 2.8 · 106 km3, and the Eurasian Basin (excluding shelf)(EB), 1.6 · 106 km3. The mean FWV is found as: The Arctic Ocean (including shelf): 59340 km3, the Arctic Ocean (excluding shelf): 38203 km3, the CB: 31599 km3 and the EB: 6716.8 km3. From a spectral and autocorrelation analysis, the variability of the FWV was found highly autocorrelated, with a one year autocorrelation of 0.99 and no significant periodicity. Histograms and scatter plots revealed a non-Gaussian distribution of the FWV and no correlation with the changes in freshwater volume. This, together with the high autocorrelation of the freshwater volume, lead to a new hypothesis of the distribution of FWV:

The state of freshwater (FWV) is a free variable, collected as a simple accumulative process on the basis of random changes.

Preliminary tests on the hypothesis, such as a comparison with a model comparison project by Jahn et al. [2012] and investigations of accumulated sums of random changes, were able to support this hypothesis. To investigate the hypothesis further, the source of these random changes was estimated. Running a high frequency filter on the changes of FWV revealed a significant pattern of the mean sea level pressure and wind stress curl, similar to Arctic Oscillation pattern, consistent with the theory of Ekman convergence and divergence. An over all convergent flow was found, but with variations in magnitude corresponding to periods of accumulation and release of freshwater in the Beaufort Sea. The above resulted in the revised hypothesis that the freshwater storage simply collects/loses freshwater as a cumulated sum of random changes applied by the atmosphere. Variations in FWV were comparable with present day changes, indicating that present day FWV changes are not necessarily due to climate changes, but could be the result of the natural variability of the atmosphere.