ALBERT

Description: Die Arktis erwärmt sich schneller als der Rest der Welt. Damit verringert sich der Temperaturunterschied zwischen Äquator, gemäßigten Breiten auf der einen und Arktis auf der anderen Seite. Dieser Unterschied treibt jedoch das starke Westwindband der Nordhalbkugel an und bestimmt dessen Bahnen und Stärke mit. Der sogenannte Polar-Front-Jetstream hat große Bedeutung für unser Wetter in Deutschland.

Description: The Arctic has become a hot spot of climate change, but the nonlinear interactions between regional and global scales in the coupled climate system responsible for Arctic amplification are not well understood and insufficiently described in climate models. Here, we compare reanalysis data with model simulations for low and high Arctic sea ice conditions to identify model biases with respect to atmospheric Arctic–mid‐latitude linkages. We show that an appropriate description of Arctic sea ice forcing is able to reproduce the observed winter cooling in mid‐latitudes as result of improved tropospheric‐stratospheric planetary wave propagation triggering a negative phase of the Arctic Oscillation/North Atlantic Oscillation in late winter.

Description: With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic.

Description: Atmospheric circulation transports moist static energy from the mid-latitudes to the Arctic as compensation for radiative imbalances between Arctic and lower latitudes. Variations in sea ice cover, ocean temperature and atmospheric conditions can potentially modulate these moist static energy transports. We compare the transports of atmospheric energy in total as well as its components between an early and a late period of Arctic amplification. In particular, we analyze the dependence of the transport changes on region, height and season in reanalysis data. A barotropic wind field correction is applied before the calculations of the atmospheric energy transport accounting for inconsistencies in mass-fluxes due to the assimilation process. An increased meridional energy transport during summer is mostly compensated by a decrease in late winter. The changes in the dry static and latent components of the energy transport differ between individual months. In the vertical, changes in dry static energy transport extend into the stratosphere. Our hypothesis on the influence of changing background conditions on the atmospheric energy transport and Arctic amplification is tested with model experiments. An ensemble of idealized model simulations with the global icosahedral non-hydrostatic atmosphere model ICON was carried out. The model experiments distinguish between effects of sea ice retreat, ocean warming, and increased concentrations of greenhouse gases. Parts of the observed changes are reproduced in the simulations. To elucidate potential mechanisms of Arctic-midlatitude linkages, we analyze the changing frequency of tropospheric and stratospheric circulation regimes and the coupling between troposphere and stratosphere.