PIs: Annette Rinke, Gunnar Spreen

The central aim of the project is (i) the improvement in understanding of feedback mechanisms between the atmosphere and sea ice–ocean in the Arctic and (ii) a quantification of the individual contributions of atmospheric circulation, Atmospheric Boundary Layer (ABL) and cloud processes, and sea ice changes to recently observed Arctic climate change. The differences between the observed sea ice retreat and simulation results from state–of–the–art regional coupled climate models will be assessed. Ensemble simulations with the coupled regional atmosphere–sea ice–ocean model of the Arctic climate system HIRHAM–NAOSIM are compared with new satellite–derived sea ice concentration, thickness and snow depth data. One of the main objectives is to realistically simulate the regional patterns of Arctic sea ice loss, changes in sea ice and snow thickness, and sea ice drift patterns. The aim is to identify and quantify the individual external and internal drivers and feedback mechanisms behind these changes. Combined with a comprehensive model evaluation, a series of model sensitivity studies with respect to key processes (sea ice/snow albedo, vertical mixing in the ABL, mixed–phase clouds) will help to quantify and improve the associated regional feedback processes in the model. Furthermore, we will attribute the Arctic Amplification to regional feedback processes arising from non–linear interactions between the atmosphere and sea ice–ocean and to the large–scale atmospheric circulation patterns and synoptic–scale processes including cyclones.

In order to test the hypothesis, we will address the following central questions:

• What are the mechanisms for the rapid Arctic sea ice loss?

• Which are the involved key regional atmosphere–sea ice feedback mechanisms?

• How can they appropriately be described in climate models?

• D03 provides sea-ice data and regional feedback descriptions
• D03 relies on the data from (AC)³ for process- and climate-oriented evaluation