Description: The ability of state‐of‐the‐art regional climate models to simulate cyclone activity in the Arctic is assessed based on an ensemble of 13 simulations from 11 models from the Arctic‐CORDEX initiative. Some models employ large‐scale spectral nudging techniques. Cyclone characteristics simulated by the ensemble are compared with the results forced by four reanalyses (ERA‐Interim, National Centers for Environmental Prediction‐Climate Forecast System Reanalysis, National Aeronautics and Space Administration‐Modern‐Era Retrospective analysis for Research and Applications Version 2, and Japan Meteorological Agency‐Japanese 55‐year reanalysis) in winter and summer for 1981–2010 period. In addition, we compare cyclone statistics between ERA‐Interim and the Arctic System Reanalysis reanalyses for 2000–2010. Biases in cyclone frequency, intensity, and size over the Arctic are also quantified. Variations in cyclone frequency across the models are partly attributed to the differences in cyclone frequency over land. The variations across the models are largest for small and shallow cyclones for both seasons. A connection between biases in the zonal wind at 200 hPa and cyclone characteristics is found for both seasons. Most models underestimate zonal wind speed in both seasons, which likely leads to underestimation of cyclone mean depth and deep cyclone frequency in the Arctic. In general, the regional climate models are able to represent the spatial distribution of cyclone characteristics in the Arctic but models that employ large‐scale spectral nudging show a better agreement with ERA‐Interim reanalysis than the rest of the models. Trends also exhibit the benefits of nudging. Models with spectral nudging are able to reproduce the cyclone trends, whereas most of the nonnudged models fail to do so. However, the cyclone characteristics and trends are sensitive to the choice of nudged variables.

Description: In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) started a priority program ‘Antarctic Research’ (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate-relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients and trace metals in the ocean with special emphasis on the Weddell Sea.

Description: A new climate model has been developed that employs a multi-resolution dynamical core for the sea ice-ocean component. In principle, the multi-resolution approach allows one to use enhanced horizontal resolution in dynamically active regions while keeping a coarse-resolution setup otherwise. The coupled model consists of the atmospheric model ECHAM6 and the finite element sea ice-ocean model (FESOM). In this study only moderate refinement of the unstructured ocean grid is applied and the resolution varies from about 25 km in the northern North Atlantic and in the tropics to about 150 km in parts of the open ocean; the results serve as a benchmark upon which future versions that exploit the potential of variable resolution can be built. Details of the formulation of the model are given and its performance in simulating observed aspects of the mean climate is described. Overall, it is found that ECHAM6–FESOM realistically simulates many aspects of the observed climate. More specifically it is found that ECHAM6–FESOM performs at least as well as some of the most sophisticated climate models participating in the fifth phase of the Coupled Model Intercomparison Project. ECHAM6–FESOM shares substantial shortcomings with other climate models when it comes to simulating the North Atlantic circulation.

Description: The European Centre for Medium-Range Weather Forecasts Reanalysis ERA40, National Centers for Environmental Prediction (NCEP) 20th-century reanalysis, and three station observations along an Antarctic traverse from Zhongshan to Dome-A stations are used to assess 2-m temperature simulation skill of a regional climate model. This model (HIRHAM) is from the Alfred Wegener Institute for Polar and Marine Research in Germany. Results show: (1) The simulated multiyear averaged 2-m temperature field pattern is close to that of ERA40 and NCEP; (2) the cold bias relative to ERA40 over all of Antarctic regions is 1.8°C, and that to NCEP reaches 5.1°C; (3) bias of HIRHAM relative to ERA40 has seasonal variation, with a cold bias mainly in the summer, as much as 3.4°C. There is a small inland warm bias in autumn of 0.3°C. Further analysis reveals that the reason for the cold bias of 2-m temperature is that physical conditions of the near-surface boundary layer simulated by HIRHAM are different from observations: (1) During the summer, observations show that near-surface atmospheric stability conditions have both inversions and non-inversions, which is due to the existence of both positive and negative sensible heat fluxes, but HIRHAM almost always simulates a situation of inversion and negative sensible heat flux; (2) during autumn and winter, observed near-surface stability is almost always that of inversions, consistent with HIRHAM simulations. This partially explains the small bias during autumn and winter.

Description: Characteristics of cyclones (frequency, intensity and size) and their changes in the Arctic region in a warmer climate have been analyzed with the use of the HIRHAM regional climate model simulations with SRES-A1B anthropogenic scenario for the twenty first century. The focus was on cyclones for the warm (April–September) and cold (October–March) seasons. The present-day cyclonic characteristics from HIRHAM simulations are in general agreement with those from ERA–40 reanalysis data. Differences noted for the frequency of cyclones are related with different spatial resolution in the model simulations and reanalysis data. Potential future changes in cyclone characteristics at the end of the twenty first century have been analyzed. According to the model simulations, the frequency of cyclones is increasing in warm seasons and decreasing in cold seasons for a warmer climate in the twenty first century, but these changes are statistically insignificant. Noticeable changes were detected for the intensity and size of cyclones for the both seasons. Significant increase was found for the frequency of weak cyclones during cold season. Further, a general increase in the frequency of small cyclones was calculated in cold seasons, while its frequency decreases in warm seasons.