Masters Thesis Defence by Rasmus Anker Pedersen – Niels Bohr Institutet - Københavns Universitet

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Masters Thesis Defence by Rasmus Anker Pedersen

Arctic Warming. The Effect of a Reduced Sea Ice Cover on the Vertical Structure of Warming.

The Arctic sea ice cover has declined rapidly in recent years. The sea ice loss is primarily
ascribed to anthropogenic global warming, which has been observed to be stronger in the
Arctic than anywhere else. This increased warming is termed Arctic amplification, and
is arising from the combined effect of several climatic feedback processes. Investigation
of the mechanisms behind the amplification has resulted in a scientific debate on the
relative importance of remote and local sources of warming – in practice often meaning
the relative warming contributions from changed atmospheric heat transport and
surface-based, sea ice related processes. The vertical profile of warming has been widely
used as an indicator of the relative contributions, as the warming signals from local and
remote sources are expected to be seen near the surface and aloft respectively. Such an
analysis of the vertical structure of Arctic warming, performed by Chung and R¨ais¨anen
[2011], has been used to argue that climate models tend to over-estimate the warming
from atmospheric heat transport, as the warming aloft in the models exceeds what is
seen in reanalysis data. This finding has been contested in this thesis, which contributes
to the Arctic amplification debate with an assessment of the effect of a reduced Arctic
sea ice cover on the vertical profile of warming.

This analysis is based on simulations with the atmospheric general circulation model
CAM3, which was used to simulate the atmospheric response to a reduced Arctic sea ice
cover. Two different approaches has been used to induce the diminished sea ice cover:
The first experiment uses prescribed, fixed sea ice conditions from the ERA Interim
reanalysis, while the second incorporates an active upper ocean and sea ice cover, and
induce the sea ice reduction through an albedo change.

The results show that, the sea ice reduction causes substantial surface-based warming,
which exceeds the warming aloft. This indicates that, the basis for the conclusion
by Chung and Räisänen [2011] is invalid, while the results presented here still indicate
the need for further investigation of the simulated atmospheric heat transport in general
circulation models. The incorporation of an active upper ocean and sea ice cover includes
additional feedbacks in the simulation, which improves the estimated vertical warming
profile in the model compared to the reanalysis data. The improvement is found to be
linked to changes in the atmospheric circulation, and in line with results from similar
studies it seems that the sea ice cover somehow is linked to the large-scale atmospheric
circulation. Consequently, changes in the sea ice cover have impacts for the climate both
within and beyond the Arctic. The details of the coupling remain unclear, while the
results here suggest that the crucial factors lie in the feedback processes involving the
sea ice and the upper ocean.