Within the last 25 years a remarkable increase of the Arctic near–surface air temperature exceeding the global warming by a factor of two has been observed. This phenomenon is commonly referred to as Arctic Amplification. The warming results in rather dramatic changes of a variety of climate parameters. For example, the Arctic sea ice has declined significantly. This ice retreat has been well identified by satellite measurements. However, coupled regional and global climate models still fail to reproduce it adequately; they tend to systematically underestimate the observed sea ice decline. This model–observation difference implies that the underlying physical processes and feedback mechanisms are not appropriately represented in Arctic climate models. Thus, the predictions of these models are also likely to be inadequate. It is mandatory to identify the origin of this disagreement.

(a) Linear trend of annual–mean near–surface air temperature (1960–2012) in units of Kelvin (K) per century, (b) zonal annual mean temperature anomaly (K) with respect to the 1951–1980 mean. Data are provided by the NASA Goddard Institute for Space Studies Team (GISTEMP Team, 2015).
(a) Linear trend of annual–mean near–surface air temperature (1960–2012) in units of Kelvin (K) per century, (b) zonal annual mean temperature anomaly (K) with respect to the 1951–1980 mean. Data are provided by the NASA Goddard Institute for Space Studies Team (GISTEMP Team, 2015).

The Arctic climate has several unique features, for example, the mostly low solar elevation, regularly occurring polar day and night, high surface albedo, large area and volume of sea ice, frequent abundance of low–level mixed–phase clouds, and an often shallow Atmospheric Boundary Layer (ABL). These characteristics influence the physical and bio–geochemical processes (such as feedback mechanisms of water vapour, clouds, temperature, and lapse–rate), atmospheric composition (trace gases, aerosol particles, clouds and precipitation), as well as meteorological (including energy fluxes) and surface parameters. In addition, meridional atmospheric and oceanic transports and exchanges between ocean, troposphere, and stratosphere largely control the Arctic climate. The oxidizing capacity of the ABL and free troposphere, and algae and phytoplankton production depend on these processes and their changes. Although many individual consequences of changes in the above parameters and processes are known, their combined influence and relative importance for Arctic Amplification are complicated to quantify and difficult to disentangle. As a result, there is no consensus about the mechanisms dominating Arctic Amplification.

Over recent decades, significant progress has been made in two main scientific areas: (i) the capabilities of in–situ measurements and remote sensing techniques to observe key physico–chemical atmospheric constituents and surface parameters at high latitudes have advanced impressively, and (ii) the computational skills and power used to model individual feedback mechanisms on small scales have improved notably. It is, therefore, timely to exploit synergistically these new developments and thereby to enhance our knowledge of the origins of the observed Arctic climate changes. This is required to improve the accuracy of its projections. To achieve this aim we propose to focus and combine the scientific expertise and competency of three German universities and two non–university research institutes in the framework of the Transregional Collaborative Research Centre TR 172. Observations from instrumentation on satellites, aircraft, tethered balloons, research vessels, and a selected set of ground–based sites will be integrated in dedicated campaigns, as well as being combined with long–term measurements. The field studies will be conducted in different seasons and meteorological conditions, covering a suitably wide range of spatial and temporal scales. They will be performed in an international context and in close collaboration with modelling activities. The latter utilize a hierarchy of process, meso–scale, regional, and global models to bridge the spatio–temporal scales from local individual processes to appropriate climate signals. The models will serve to guide the campaigns, to analyse the measurements and sensitivities, to facilitate the attribution of the origins of observed Arctic climate changes, and to test the ability of the models to reproduce observations.

The overarching scientific objective of (AC)³ is to identify, investigate, and evaluate the key processes contributing to Arctic Amplification, improve our understanding of the major feedback mechanisms, and quantify their relative importance for Arctic Amplification. In Phase I the research will focus on atmospheric and surface processes, because the ongoing rapid changes in the Arctic climate imply that mechanisms involve important atmospheric influences. In the Phases II and III the interactions between oceanic and atmospheric components in Arctic Amplification and related global aspects will be addressed in more detail. The combination of observational and modelling studies aims to improve future projections of Arctic climate development.