ALBERT

Description: What are the benefits of limiting the global warming to 1.5 degree with respect to pre-industrial conditions for the vulnerable region of West Antarctica which might be prone to positive feedback mechanisms between ocean circulation, melting of shelf ice and instabilities of the ice sheet? There are indications that West Antarctic ice sheet instabilities have occurred in the Last Interglacial around 125.000 years ago. At that time the polar surface temperature was about 2K warmer than today. The question under which circumstances a tipping point may be reached and if this may happen again is therefore highly relevant, especially since a disintegration of the West Antarctic ice sheet could cause a global sea level rise between 3 and 5 m. Here we address this question with variable resolution, global coupled ice sheet - shelf ice - ocean - atmosphere multi-century simulations. With our innovative ocean modelling approach in the Finite Element Sea-ice Ocean Model FESOM it is possible to refine the ocean resolution to up to 3 km in the Amundsen Sea and 10 km around the whole Antarctica while keeping it relatively coarse in the order of a couple of hundred km in dynamically not very active regions such as the subtropical regions. This means that we can simulate the feedback between ocean and ice in the relevant regions highly resolved given that the ice sheet model runs at a resolution of 5 to 10 km. Three different emission scenarios are applied up to 2100, two of them limiting the global mean temperature increase to 1.5 ◦ C and 2 ◦ C respectively and one of them assuming business-as-usual conditions (IPCC SRES RCP8.5 scenario). The simulations are extended to 2400 with the greenhouse gas and aerosol concentrations kept constant at 2100 levels, respectively, to be able to simulate the long-term implications of different global warming levels.

Description: While summer sea ice reduced dramatically/significantly, and the atmospheric warming is amplified over the Arctic, changes in the ocean are less obvious due to its higher inertia. The understanding of the ongoing changes at polar latitudes and its linkages to mid-latitude climate has become a top subject among climate research community. The ocean circulation response to an idealized decline in Arctic sea ice is investigated in a set of novel fully-coupled climate model (AWI-CM) experiments. The atmosphere and thermodynamics is resolved by ECHAM6.3 in a resolution of ca. 180Km, whereas FESOM resolves the ocean and dynamical aspects of the sea ice with resolution ranging from 25 to 150 km. A 250-year reference simulation (REF) is initialized with CORE II and WOA01 data and forced by 1990 greenhouse gases and aerosol concentrations. We conduct a comparative study in which three distinct thermodynamical perturbations are applied on the sea ice to induce a gradual sea ice reduction over 150-year period simulations. Our sensitivity experiments consist of three different approaches to induce an Arctic sea ice reduction: I) the albedo is modified by the increase of snow aging factor; II) reducing the lead closing parameter which resembles a loss of sea ice thickness rather than sea ice area; III) imposing an anomalous heat flux on the sea ice by adding 0.5 W/m2 of long wave radiation. To check the robustness of our results we undertake a second realization of each sensitivity experiment simply by initializing the experiments 30 years later. It is shown that ocean responses establish comparably in all sensitivity experiments. Dynamical adjustments of ocean fluxes and currents are not confined to the polar latitudes. The North Atlantic high-latitude indicates a southward shift of the North Atlantic Current pathway. Although the atmosphere seems to play a secondary role in responding and forcing dynamical changes in the Arctic Ocean, we believe that a negative annular-mode like trend explains the weakening of the westerly winds along the poleward flank of the jet stream, which in turn alters the upper ocean circulation.

Description: 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.

Description: The influence of the Arctic atmosphere on Northern Hemisphere mid-latitude tropospheric weather and climate is explored by comparing the skill of two sets of 14-day weather forecast experiments with the ECMWF model with and without relaxation of the Arctic atmosphere towards ERA-Interim reanalysis data during the course of the integration. Two pathways are identified along which the Arctic influences mid-latitude weather, one pronounced one over Asia and Eastern Europe and a secondary one over North America. In general, linkages are found to be strongest (weakest) during boreal winter (summer) when the amplitude of stationary planetary waves over the Northern Hemisphere is strongest (weakest). No discernable Arctic impact is found over the North Atlantic and North Pacific region, which is consistent with predominantly southwesterly flow. An analysis of the flow-dependence of the linkages shows that anomalous northerly flow conditions increase the Arctic influence on mid-latitude weather over the continents. Specifically, an anomalous northerly flow from Kara Sea towards Western Asia leads to cold surface temperature anomalies not only over Western Asia but also over Eastern and Central Europe. Finally, the results of this study are discussed in the light of potential mid-latitude benefits of improved Arctic prediction capabilities.