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: Mesoscale eddies in the open ocean are mostly formed by baroclinic instability, in which the available potential energy from the large-scale slope of the isopycnals is converted into the kinetic energy of the flow around the eddy. As a permissible form of motion within a rapidly rotating and stratified fluid eddies driven by baroclinic instability are important for the poleward and vertical transport, not only of physical properties, but also biogeochemical ones. In this paper, we present observations from four cyclonic eddies in the Antarctic Circumpolar Current. We have sorted them by apparent age, based on altimeter data and consideration of the degree of homogenisation of the potential temperature-salinity(θS) relationship, and then looked at the spatial distribution of measures of fine-scale variability in the upper thermocline. The youngest eddy shows isopycnals which are domed upwards and it contains a variety of waters with differing temperature-salinity characteristics. The fine-scale variability is higher in the core of the eddy. The older eddies show a core which is more homogeneous in potential temperature and salinity. The isopycnals are flatter in the centre of the eddy, and in cross-section, they can be M-shaped, so that the steepest gradients are concentrated around the edge. The fine-scale variability is more concentrated around the edges where the density gradients are stronger. We hypothesise that lateral stirring and mixing processes within the eddy homogenise the water so that the temperature-salinity relationship becomes tighter. When the eddy eventually collapses, this modified water can be released back into the flow. Thus, we see how the interplay of mesoscale and small-scale processes are modifying water mass properties and, potentially, regulate biogeochemical processes.

Description: The Weddell Gyre is a fundamental component of the global climate system, in that it supplies heat to underneath the Antarctic ice shelves, and regulates the density of water masses that feed the deepest limb of the global over-turning circulation. Here we utilise Argo float profile and trajectory data spanning the entire Weddell Gyre from 2002 to 2016, in order to determine the large-scale mean horizontal circulation and heat distribution within the upper Weddell Gyre. An elongated, double-cell, cyclonic circulation is revealed, where the eastern cell is stronger than the western cell. The transport of heat within the Warm Deep Water layer, which is the primary heat source to the Weddell Gyre, is demonstrated by diagnosing the heat budget for a 1000 m thick layer encompassing the core of Warm Deep Water. While the heat budget does not close at the resolution of the grid cells, it does close when integrated over large areas, within the range of uncertainty provided by a range of values for horizontal and vertical diffusivity. In the southern limb of the gyre, heat transport convergence due to mean horizontal advection balances with divergence due to horizontal turbulent diffusion (representative of eddy processes). In contrast, within the interior circulation cell of the Weddell Gyre, heat transport divergence due to mean horizontal advection balances with convergence due to horizontal turbulent diffusion. We show that heat is advected into the Weddell Gyre along the southern limb, some of which is diffused northwards into the interior circulation cell, while some is diffused southwards towards the shelf seas. This implies that horizontal turbulent diffusion plays a role in transporting heat towards the ice shelves. Horizontal turbulent diffusion is also a mechanism by which heat can enter the Weddell Gyre across the open northern boundary. This work highlights the importance of understanding the role of eddy processes in redistributing heat throughout the Weddell Gyre, in particular within the vicinity of the ice shelves, where basal ice melt can contribute to rising sea levels.