Description: This bibliography unites the individual data collected by different types of autonomous platforms deployed during MOSAiC in 2019/2020.

Description: In September 2019, the research icebreaker Polarstern started the largest multidisciplinary Arctic expedition to date, the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) drift experiment. Being moored to an ice floe for a whole year, thus including the winter season, the declared goal of the expedition is to better understand and quantify relevant processes within the atmosphere–ice–ocean system that impact the sea ice mass and energy budget, ultimately leading to much improved climate models. Satellite observations, atmospheric reanalysis data, and readings from a nearby meteorological station indicate that the interplay of high ice export in late winter and exceptionally high air temperatures resulted in the longest ice-free summer period since reliable instrumental records began. We show, using a Lagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiC floe carrying the Central Observatory (CO) formed in a polynya event north of the New Siberian Islands at the beginning of December 2018. The results further indicate that sea ice in the vicinity of the CO (〈40 km distance) was younger and 36 % thinner than the surrounding ice with potential consequences for ice dynamics and momentum and heat transfer between ocean and atmosphere. Sea ice surveys carried out on various reference floes in autumn 2019 verify this gradient in ice thickness, and sediments discovered in ice cores (so-called dirty sea ice) around the CO confirm contact with shallow waters in an early phase of growth, consistent with the tracking analysis. Since less and less ice from the Siberian shelves survives its first summer (Krumpen et al., 2019), the MOSAiC experiment provides the unique opportunity to study the role of sea ice as a transport medium for gases, macronutrients, iron, organic matter, sediments and pollutants from shelf areas to the central Arctic Ocean and beyond. Compared to data for the past 26 years, the sea ice encountered at the end of September 2019 can already be classified as exceptionally thin, and further predicted changes towards a seasonally ice-free ocean will likely cut off the long-range transport of ice-rafted materials by the Transpolar Drift in the future. A reduced long-range transport of sea ice would have strong implications for the redistribution of biogeochemical matter in the central Arctic Ocean, with consequences for the balance of climate-relevant trace gases, primary production and biodiversity in the Arctic Ocean.

Description: We developed a new version of the Alfred Wegener Institute Climate Model (AWI-CM3), which has higher skills in representing the observed climatology and better computational efficiency than its predecessors. Its ocean component FESOM2 (Finite-volumE Sea ice-Ocean Model) has the multi-resolution functionality typical of unstructured-mesh models while still featuring a scalability and efficiency similar to regular-grid models. The atmospheric component OpenIFS (CY43R3) enables the use of the latest developments in the numerical-weather-prediction community in climate sciences. In this paper we describe the coupling of the model components and evaluate the model performance on a variable-resolution (25-125 km) ocean mesh and a 61 km atmosphere grid, which serves as a reference and starting point for other ongoing research activities with AWI-CM3. This includes the exploration of high and variable resolution and the development of a full Earth system model as well as the creation of a new sea ice prediction system. At this early development stage and with the given coarse to medium resolutions, the model already features above-CMIP6-average skills (where CMIP6 denotes Coupled Model Intercomparison Project phase 6) in representing the climatology and competitive model throughput. Finally we identify remaining biases and suggest further improvements to be made to the model.

Description: A new climate model supporting multi-resolution meshes in the ocean component has been established at the Alfred Wegener Institute (AWI) in Bremerhaven. The atmospheric component is ECHAM6 with T63L47 setting, while the ocean is simulated by the AWI multi-resolution model FESOM, supporting triangular unstructured meshes. Two multi-century simulations with ECHAM6-FESOM, REF and TRO, document the beneficial role of an increased tropical ocean resolution for ENSO simulations. REF features a tropical ocean resolution of about 1°, TRO employs more than 0.25° in a narrow equatorial band, with resolution gradually decreasing to 1° as in REF. Outside the tropical belt (15°N to 15°S), both meshes are identical. REF and TRO simulate a mean climate comparable to some of the best CMIP5 models. In TRO, however, both the cold tongue SST bias and the western Pacific SST standard deviation bias appear to improve along with the Nino-3 index statistics. Also, advanced ENSO diagnostics including the Nino-3.4 seasonal variance, the annual cycle representation, and its interaction with ENSO tend to improve. The robustness of these improvements is analyzed and their physical explanations are explored.

Description: We analyse the ENSO-like variability in the newly established global climate model ECHAM-FESOM. This is the first global coupled model with an ocean module supporting unstructured meshes. The Finite Element Sea Ice - Ocean Model (FESOM) is a dynamical ocean model development at AWI Bremerhaven. In contrast to conventional ocean models, the spatial discretization is based on the Finite Element method. This method allows a variable spatial resolution of the triangular surface mesh with high mesh-stretching factors. FESOM has been used in numerous recent, yet uncoupled, studies. Its validation in the climate context is still ongoing activity. ECHAM is a state-of-the-art spectral atmosphere model developed at the Max-Planck-Institute for Meteorology in Hamburg for climate modelling purposes. We apply the latest generation, version 6, with a T63L47 resolution. FESOM and ECHAM are currently coupled by the OASIS3-MCT coupler and a structured exchange mesh. We analyse two simulation runs that differ in the tropical ocean mesh resolution between 15°N and 15°S. Setup 1 uses a reference mesh with a resolution of about 1° in the tropics. In contrast, Setup 2 has a higher resolution of 1/4° (in a narrow band around the equator) that gradually decreases to 1°. Outside the tropics both meshes are identical. Modelled Nino3.4 indices are compared with observations and the influence of the mesh resolution is discussed.

Description: Icebergs are commonly ignored in current general circulation models despite their connections to ocean stratification, phytoplankton growth and redistribution of freshwater in the Southern Ocean. On the way to fully including icebergs in ocean circulation models, we present FESOM-IB, the high resolution Finite Element Sea Ice - Ocean Model (FESOM) enhanced by an IceBerg drift and decay module developed at AWI Bremerhaven. By solving the momentum equations for iceberg drift, the iceberg trajectory is computed from an evaluation of the FESOM ice/ocean velocity fields and sea surface height at every time step. Icebergs are assumed to be cubical-shaped and treated as Lagrangian point masses having properties such as length, width and height. Simple diagnostic equations for computing the melt rates of icebergs are applied and iceberg dimensions are adjusted accordingly. Therefore the numerical method's stability for the solution of the momentum equations has to be independent from iceberg size. Our numerical procedure proved to be stable across the full range of iceberg classes; small to giant icebergs may be modelled. We present a 3-year simulation of 308 artifical icebergs from 4 different size classes started at 77 circum-Antarctic locations. Melt rates as well as the components of iceberg momentum balance are quantified and the influence of iceberg size on the drift patterns is discussed. In our simulation giant icebergs tend to stay close to the Antarctic coast. They drift westwards in the coastal current and may only leave it at well-defined bifurcation points in the Weddell Sea, the Ross Sea and over the Kerguelen Plateau. In contrast, smaller icebergs show an off-shore drift component early in their lives. Independent of the iceberg size, the dominant iceberg velocity component is changed into eastward as soon as icebergs reach the ACC.

Description: The ice strength parameter P* is a key parameter in dynamic/thermodynamic sea ice models that cannot be measured directly. Stochastically perturbing P* in the Finite Element Sea Ice–Ocean Model (FESOM) of the Alfred Wegener Institute aims at investigating the effect of uncertainty pertaining to this parameterization. Three different approaches using symmetric perturbations have been applied: 1) reassignment of uncorrelated noise fields to perturb P* at every grid point, 2) a Markov chain time correlation, and 3) a Markov chain time correlation with some spatial correlation between nodes. Despite symmetric perturbations, results show an increase of Arctic sea ice volume and a decrease of Arctic sea ice area for all three approaches. In particular, the introduction of spatial correlation leads to a substantial increase in sea ice volume and mean thickness. The strongest response can be seen for multiyear ice north of the Greenland coast. An ensemble of eight perturbed simulations generates a spread in the multiyear ice comparable to the interannual variability of the model. Results cannot be reproduced by a simple constant global modification of P*.