Beschreibung: The Filchner-Ronne Ice Shelf (FRIS), the largest (by volume) floating extension of the Antarctic Ice Sheet (AIS), fringes the southern Weddell Sea known to be the dominant source of the globally relevant Antarctic Bottom Water. As a link between ocean and ice sheet, this ice shelf plays an important role for the stability of the AIS and the preconditioning of water masses participating in the global thermohaline circulation. The dominant process serving this pivotal role of FRIS is the exchange of heat, salt and tracers at the base of the ice shelf. While the southern Weddell Sea has been considered as largely invulnerable to climate warming, recent projections point to a potential tipping of the ocean state from cold to warm by the end of this century. The lack of detailed knowledge about the ocean underneath FRIS and the possibility of dramatic changes in the near future brought together scientists from the UK, Norway, and Germany. In the framework of the Filchner Ice Shelf Project, they intensively investigate the southern Weddell Sea continental shelf, including the FRIS cavity, by means of ship-based observations, moorings in front of and beneath the ice shelf, sub-ice shelf water sampling, and numerical modeling. This presentation reviews the achievements of the Alfred Wegener Institute over the past 6 years focused on observation, modeling, and comprehensive understanding of on-shore flow, dense water formation, sub-ice shelf circulation, meltwater production, and Ice Shelf Water spreading on the southern Weddell Sea continental shelf. All together has an impact on the ice shelf mass balance and thus on the discharge of inland ice with consequences for global sea level rise.

Beschreibung: The Filchner-Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near-freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank, and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW-sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer-based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two-year travel time, and the other located in the central Filchner Trough following a ∼six year-long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW-derived ISW (Ronne-mode) or more locally derived Berkner-HSSW (Berkner-mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW-cores along the trough's western flank and onto Berkner Bank, which suggests that Ronne-ISW preconditions Berkner-HSSW production. The negligible density difference between Berkner- and Ronne-mode waters indicates that each contributes cold dense shelf waters to protect FRIS against inflowing mWDW.

Beschreibung: We have conducted a series of atmosphere-only and coupled model experiments on time scales from weather to climate and with different methods to address the question how the large scale circulation of the Northern mid-latitudes is affected by the shrinking Arctic sea ice as well as by the overlying atmosphere. A major pathway has been found from the Barents Sea / Kara Sea area to Eastern Europe and Northern Asia and a secondary one from the Canadian Arctic into North America. In contrast, the atmosphere above ocean areas is less affected by the Arctic. A recurring response feature to declined Arctic sea ice is the slowdown and southward shift of the jet stream with less cyclone activity north of it leading to around 0.5 K colder conditions over some limited regions of North America and North Siberia in winter consistent with a negative Arctic Oscillation index. This happens despite the tendency of less intense cold advection due to the warmer Arctic in cases of anomalous northerly flow. It should be noted that for robust responses large ensemble simulations are needed due to low signal-to-noise ratio. In this respect it has been proven helpful to perform simulations in a Numerical Weather Prediction setting as the short simulation time enables us to easily run ensembles of several hundreds of realizations. Furthermore, in such a setting the initial response to a suddenly changed Arctic sea ice cover can be studied giving us hints how anomalies in the atmosphere develop. Coupled model simulations hint at no discernable influence of shrinking Arctic sea ice on the ocean on time scales of a year while on decadal to centennial time scales the ocean starts to react with possible feedbacks to the atmosphere. Due to less and thinner sea ice cover the momentum flux into the ocean increases which spins up the Beaufort Gyre. This response propagates towards the North Atlantic as an increased outflow through the Fram Strait, which drives increased volume transport into the Barents Sea, thus fostering the Atlantification of the basin. The response is not confined to the interior of the Arctic and our results suggest that it may reach as far south as the North Atlantic Current as a combined response to the dynamical ocean adjustment triggered within the Arctic and, secondarily, to the atmospheric weakening of the westerly winds.

Beschreibung: We have conducted a series of idealized atmosphere-only and coupled model experiments on time scales from weather to climate and with different methods to address the question how the large scale circulation of the Northern mid-latitudes is affected by the shrinking Arctic sea ice. A recurring response feature to declined Arctic sea ice is the slowdown and southward shift of the jet stream with less cyclone activity north of it leading to around 0.5 K colder conditions over some limited regions of North America and North Siberia in winter. This happens despite the tendency of less intense cold advection due to the warmer Arctic in cases of anomalous northerly flow. It should be noted that for robust responses large ensemble simulations are needed due to low signal-to-noise ratio. In this respect it has been proven helpful to perform simulations in a Numerical Weather Prediction setting as the short simulation time enables us to easily run ensembles of several hundreds of realizations. Furthermore, in such a setting the initial response to a suddenly changed Arctic sea ice cover can be studied giving us hints how anomalies in the atmosphere develop. Coupled simulations hint at no discernable influence of shrinking Arctic sea ice on the ocean on time scales of a year while on decadal to centennial time scales the ocean starts to react with possible feedbacks to the atmosphere.

Beschreibung: Arctic sea ice has undergone substantial decline in the last 3 decades and climate model studies show that Arctic could be ice-free in the late summer even by the middle of this century. Sea ice plays an important role in the climate system, therefore, it is important to investigate the sole influence of the Arctic sea ice loss on the atmosphere and ocean circulations. In the present study, results of the experiments carried out with the coupled global model ECHAM6-FESOM were analyzed. In an ensemble of 100 members, Arctic sea ice thickness was reduced by 80% on 1st of June in the sensitivity experiments, and changes through the following year were compared to the reference experiments. The reduction in the Arctic sea ice thickness led to an ice-free Arctic for the next 5 months in the sensitivity experiments and, as a consequence, it led to a strong increase in the surface temperature over the Arctic region in the following autumn and winter, making the response of other parameters most pronounced in those seasons. Strong baroclinic circulation anomalies were found over the Arctic, while barotropic response was found over north-eastern and southern parts of Europe. Precipitation increased over central Arctic area and also Mediterranean area in the winter, which resembles the synoptic activity shift in the same season from the northern Atlantic towards southern Europe. Changes in wind-driven ocean circulation were found in the Mediterranean Sea.

Beschreibung: Arctic sea ice decline is expected to continue throughout the 21st century as a result of increased greenhouse gas concentrations. Here we investigate the impact of a strong Arctic sea ice decline on the atmospheric circulation and low pressure systems in the Northern Hemisphere through numerical experimentation with a coupled climate model. More specifically, a large ensemble of 1-year long integrations, initialized on 1 June with Arctic sea ice thickness artificially reduced by 80%, is compared to corresponding, unperturbed control experiments. The sensitivity experiment shows an ice-free Arctic from July to October; during autumn the largest near-surface temperature increase of about 15 K is found in the central Arctic, which goes along with a reduced meridional temperature gradient, a decreased jet stream, and a southward shifted Northern Hemisphere storm track; and the near-surface temperature response in winter and spring reduces substantially due to relatively fast sea ice growth during the freezing season. Changes in the maximum Eady growth rate are generally below 5% and hardly significant, with reduced vertical wind shear and reduced vertical stability counteracting each other. The reduced vertical wind shear manifests itself in a decrease of synoptic activity by up to 10% and shallower cyclones while the reduced vertical stability along with stronger diabatic heating due to more available moisture may be responsible for the stronger deepening rates and thus faster cyclone development once a cyclone started to form. Furthermore, precipitation minus evaporation decreases over the Arctic because the increase in evaporation outweighs that for precipitation with implications for the ocean stratification and hence ocean circulation.