Midterm colloquium by Rasmus A. Pedersen – Niels Bohr Institute - University of Copenhagen

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Midterm colloquium by Rasmus A. Pedersen

The effect of a reduced sea ice cover on the vertical structure of Arctic Warming

The focus of this thesis will be Arctic amplification of global warming, and the work will aim to assess the role of the sea ice cover in the temperature amplification. There is an ongoing debate in recent studies on whether the observed Arctic warming is caused primarily by surface processes or by poleward energy transport higher in the atmosphere. To distinguish between these different origins of warming, the vertical structure of the warming is examined. Chung and Räisänen (2011) concludes that climate models tend to overestimate the contribution from the poleward heat transport compared to the surface processes, when model results from CMIP3 is compared to the reanalysis data from ERA Interim. This thesis raises the question of whether this observation is “real” or if it is caused by differences in the surface parameters – especially the sea ice cover. The CMIP3 only covers the period from 1979-99 and thus excludes the 10 years of significant sea ice loss from 2000-10, which are included in ERA Interim. Thus a study is proposed, where a climate model (CAM3) is used to calculate the Arctic warming using two different sea ice patterns – the CMIP3 “model” sea ice cover and the “observed” sea ice cover from ERA Interim, which is expected to have a significantly lower extent. The vertical profiles of the warming in the two scenarios are compared, and the conclusions of Chung and Räisänen are assessed in the view of the new results.

A second study is proposed to simulate the “full” response from the changed sea ice cover – as the study above is run in a fixed SST and sea ice setup. The second study is done with a slab ocean, allowing the upper ocean and sea ice to respond to the introduced changes – i.e. warming from increased greenhouse gas concentration. Additional to the issue described above, Stroeve et.al. (2007) conclude that climate models in general tend to underestimate the reduction of the sea ice cover in a warming environment. Thus this study will implement a stronger response in sea ice change to temperature changes, simulated by changing the albedo of the sea ice. The vertical profiles of the temperature change from these calculations are compared to the results from the fixed SST runs and the results of Chung and Räisänen, and assessed in light of the trends presented Stroeve et.al. (2007)