Description: The surface of the world’s oceans has been warming since the beginning of industrialization. In addition to this, multidecadal sea surface temperature (SST) variations of internal origin exist. Evidence suggests that the North Atlantic Ocean exhibits the strongest multidecadal SST variations and that these variations are connected to the overturning circulation. This work investigates the extent to which these internal multidecadal variations have contributed to enhancing or diminishing the trend induced by the external radiative forcing, globally and in the North Atlantic. A model study is carried out wherein the analyses of a long control simulation with constant radiative forcing at preindustrial level and of an ensemble of simulations with historical forcing from 1850 until 2005 are combined. First, it is noted that global SST trends calculated from the different historical simulations are similar, while there is a large disagreement between the North Atlantic SST trends. Then the control simulation is analyzed, where a relationship between SST anomalies and anomalies in the Atlantic meridional overturning circulation (AMOC) for multidecadal and longer time scales is identified. This relationship enables the extraction of the AMOC-related SST variability from each individual member of the ensemble of historical simulations and then the calculation of the SST trends with the AMOC-related variability excluded. For the global SST trends this causes only a little difference while SST trends with AMOC-related variability excluded for the North Atlantic show closer agreement than with the AMOC-related variability included. From this it is concluded that AMOC variability has contributed significantly to North Atlantic SST trends since the mid nineteenth century.

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.

Description: The influence of two simple descriptions for the sea ice distribution on boundary layer values is investigated by comparing model results from the regional climate model REMO with measured data in the Fram Strait in April 1999. One method for determining the sea ice distribution in REMO is to diagnose the sea ice cover from the prescribed surface temperature and allow each grid cell to be either completely free of ice or completely covered by ice (REMO-original). The other one is to employ a partial sea ice concentration in each REMO grid cell with the input data derived from satellite data (REMO-partial). Surface fluxes are average values of the ice and water partial fluxes. There is a clearly better agreement between measured and simulated surface and boundary layer temperatures and humidities when using REMO-partial compared to REMO-original. The closed ice cover in REMO-original leads to downward sensible heat fluxes over ice, whereas the ice cover with leads and polynyas in REMO-partial leads to smaller downward or even upward sensible heat fluxes. The introduction of the partial sea ice concentration smoothes unrealistically sharp gradients between ice-covered and ice-free regions. which can influence cloud cover and precipitation. An additional result of the study is that the simulation of the albedo could be improved in allowing a larger range of sea ice albedos and introducing a water albedo dependent on sun zenith angle.

Description: 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.

Description: The Arctic plays a major role in the global circulation, and its water and energy budget is not as well explored as that in other regions of the world. The aim of this study is to calculate the climatological mean water and energy fluxes depending on the season and on the North Atlantic Oscillation (NAO) through the lower, lateral, and upper boundaries of the Arctic atmosphere north of 70°N. The relevant fluxes are derived from results of the regional climate model (REMO 5.1), which is applied to the Arctic region for the time period 1979–2000. Model forcing data are a combination of 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15) data and analysis data. The annual and seasonal total water and energy fluxes derived from REMO 5.1 results are very similar to the fluxes calculated from observational and reanalysis data, although there are some differences in the components. The agreement between simulated and observed total fluxes shows that these fluxes are reliable. Even if differences between high and low NAO situations occur in our simulation consistent with previous studies, these differences are mostly smaller than the large uncertainties due to a small sample size of the NAO high and low composites.