Arctic Amplification

AC3 is the short name of the DFG funded Transregio project: Arctic Amplification: Climate relevant atmospheric and surface processes and feedback mechanisms. The project has successfully completed its first project phase and is currently in its second phase. The overarching goals for phase II are to identify, investigate, and evaluate the key processes contributing to Arctic amplification, to improve the understanding of the local and remote feedback mechanisms, and to quantify their relative importance for Arctic amplification. For details please find the AC3 project home page: www.ac3-tr.de 

The AWI sea ice physics section contributes to two sub-projects of AC3 (see boxes below): Project C01 on spatial heterogeneity and temporal evolution of surface properties and Project D04 on meridional heat transports and regional processes.

Sub-project C0: Surface properties

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The spatial heterogeneity and temporal evolution of surface properties of the Arctic Ocean influence the radiative energy transfer through the coupled compartments (atmosphere, sea ice, open ocean) of the Arctic climate system. Radiative effects of interactions between these components are not well studied, however, they may play an important role in the Arctic climate system. For example, temporal changes of radiative energy fluxes during the transition period between the onset of sea ice melting and freeze-up are critical, also because these processes are not well represented in the models and, thus, may cause significant uncertainties in projections of the Arctic climate system. Furthermore, the effects of spatially heterogeneous surface conditions, in particular in case of clouds, are not well investigated, although clouds are an important player in Arctic amplification. Therefore, this project will observe the inter-annual and seasonal changes of solar and thermal-infrared radiative flux densities within and through the compartments of the coupled atmosphere-sea ice-ocean system during different sea ice regimes as a function of spatially heterogeneous surface properties (e.g., albedo, temperature, sea ice, and snow thickness). These characteristics and further, more specific surface features (e.g., sea ice types, melt ponds, leads, loe size distributions) as well as the transfer of radiative energy fluxes through the compartments of the system, will be investigated on different spatial and temporal scales by (i) in–situ observations over the full annual cycle below and above the sea ice during MOSAiC, (ii) aircraft measurements on regional and seasonal scales during MOSAiC and the HALO-(AC)³ campaign and (iii) multi-year satellite observations. Based on these sources, we will quantify the influence of the heterogeneity of the surface properties on (i) radiative flux densities in the atmosphere and ocean compartments, (ii) atmospheric cloud radiative forcing (CRF), and (iii) sea ice-ocean interface interactions. Transfer functions quantifying the transition of solar and thermal infrared radiative flux densities between the system compartments will be derived and parameterised. Furthermore, we will continue to improve surface albedo parameterisations established during phase I (e.g., for HIRHAM-NAOSIM) by including additional factors (e.g., cloud cover, surface temperature, melt pond coverage, snow depth). We will analyse airborne data from the previous ACLOUD, PAMARCMiP, and AFLUX campaigns, and collect new measurements during the planned MOSAiC and HALO-(AC)³ observations. In addition we will use satellite data (MERIS, Sentinel-3) in our analysis. 

Sub-project D04

Atlantic overturning and gyre circulation carry heat from low and mid-latitudes to the Arctic. A part of this heat is released to the sea ice and the atmosphere. At present strong oceanic stratification still inhibits efficient heat release in most parts of the Arctic. Recent observations indicate, however, that in some regions major changes are taking place in the upper layers of the Arctic Ocean. In particular, the cold halocline layer, which separates warm Atlantic water from the upper ocean mixed layer, appears to be affected by Arctic climate change. On the one hand, warming in the shelf regions was found to affect the formation of the cold halocline. On the other hand, warm Atlantic water was suggested to destabilise the cold halocline from below. This project aims to better understand the oceanic processes that shape the response of the Arctic climate system, especially the ocean-atmosphere heat fluxes, to greenhouse gas warming using regional and global modelling in addition to analysing existing results from global climate models. A focus will be the Barents Sea, where strong surface heat fluxes foster low sea level pressure and shallowdepth export of water and sea ice. The export is compensated for by a corresponding inflow of Atlantic water, which closes a positive feedback loop. Ocean stratification and dense water formation are affected and, as a consequence, these processes can contribute to further Arctic warming. 

An initial analysis of model results from the Coupled Model Intercomparison Project Phase 5 (CMIP5) suggests that global climate models simulate an increased breakdown frequency of the cold halocline in a future climate change scenario and that these breakdowns inluence the surface ocean temperature. 

 

Additional work is proposed here to further investigate the potential importance of an increased cold halocline breakdown frequency for Arctic climate change and to evaluate simulations of cold halocline breakdowns in climate models. Earlier experiments with a coupled ocean-sea ice model (North Atlantic Arctic Ocean Sea Ice Model, NAOSIM) show a destabilisation of the halocline due to increased freezing in polynyas of Arctic shelf seas. We are planning to combine global and regional modelling with observations to better quantify and understand the relevant processes.