Alfred Wegener Institute for Polar and Marine Research
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Description and validation of the ice-sheet model Nix v1.0
Get PDFAbstract. We present a physical description of the ice-sheet model Nix v1.0, an open-source project intended for collaborative development. Nix is a two-dimensional (flowline combined with a vertical dimension) thermomechanical model written in C and C++ that simultaneously solves for the momentum balance equations, mass conservation and temperature evolution. Nix's velocity solver includes a hierarchy of Stokes approximations: Blatter–Pattyn, depth-integrated higher order and shallow shelf. The grounding-line position is explicitly solved by a moving coordinate system that avoids further interpolations. The model can be easily forced with any external boundary conditions. Nix has been verified for standard test problems, showing versatility from regular machines (lightweight memory allocation) to high-performance computing (multi-threading capabilities). Resolutions below 0.1 km are attainable even with minimal computational resources: Nix's serial run finalizes within hours on a single CPU. Here we show results for a number of benchmark experiments from the Marine Ice Sheet Intercomparison Project (MISMIP) and assess grounding-line migration with an overdeepened bed geometry. Lastly, we further exploit the thermomechanical coupling by designing a suite of experiments where the forcing is a physical variable, unlike previously idealized forcing scenarios where ice temperatures are implicitly fixed via an ice rate factor. That is, we use atmospheric temperature and oceanic temperature anomalies to assess model hysteresis behaviour with active thermodynamics. Our results show that hysteresis in an overdeepened bed geometry is similar for atmospheric and oceanic forcings. Notably, the classical hysteresis loop is widened for both forcing scenarios (i.e. atmospheric and oceanic) if the ice sheet is thermomechanically active as a result of the internal feedback among ice temperature, stress balance and viscosity. These results show that a temperature-dependent ice viscosity provides inertia and stability to the ice sheet, regardless of the particular external forcing applied. In summary, Nix combines rapid computational capabilities with a Blatter–Pattyn stress balance fully coupled to a thermomechanical solver, not only validating against established benchmarks but also offering a powerful tool for advancing our insight into ice dynamics and grounding-line stability
The Role of Model Biases in the Simulated North Atlantic Jet Stream Response to Global Warming
No full textAbstract Future projections of the European climate suffer from uncertainties in the changes of the North Atlantic jet stream. Most climate models simulate an annual-mean poleward shift of the jet in response to global warming, but the responses have a large spread. Previous studies comparing climate models suggest that the projected meridional shift of the jet is negatively correlated with its simulated present-day mean latitude. Model basic-state biases are a possible cause for the uncertainty, but their effect is difficult to assess because the spread in simulations may be caused by any intermodel differences. In this study, the effect of model biases on future projections was isolated by modifying the basic state of a single atmospheric model with a run-time correction method aiming to adjust the model climatology toward those of three other models and a reanalysis. The effect of model biases was found to be strongly seasonal. In winter, changes in the frequencies of two of the three preferred positions of the jet were found to be sensitive to the model biases, causing considerable uncertainty in the jet shift and contributing to the anticorrelation between the present-day jet latitude and the future shift, but in summer, the impact of biases is small relative to the magnitude of the changes. While the anticorrelation of jet latitude and shift is only partly caused by biases, our results demonstrate there is potential to reduce uncertainty in the projected jet stream changes by improving model basic states. Significance Statement Future projections of climate models are affected by biases, i.e., systematic errors, in simulations of the present-day climate. We investigated the contribution of model biases to the large spread of simulated future changes of the North Atlantic jet stream, a varying current of fast winds strongly affecting the European climate. We found that biases cause large uncertainty in, especially, the changes of the position of the winter jet stream. The results also show that a known dependence of the latitudinal shift of the jet on its simulated current position is not as strong as suggested by previous studies. However, reducing model biases provides opportunities for more reliable climate predictions
Implementation of water tracers in the Met Office Unified Model
Get PDFAbstract. There is an increasing need to understand how water is cycled and transported within the atmosphere, to aid water management. Here, atmospheric water tracers are added to the Met Office Unified Model (UM) to allow tracking of water within the model. This requires the implementation of water tracers in the following parts of the model code: large-scale advection, surface evaporation, boundary layer mixing, large-scale precipitation (microphysics), large-scale clouds, stochastic physics, and convection. A single water tracer is found to track all water in the model to a high degree of accuracy during a 35 year simulation; the differences are typically less than 10−16 kg kg−1 at the end of every time step, prior to a very small adjustment to prevent the build up of numerical error. The increase in computing time for each water tracer is between 3.1 % and 3.8 %, depending on the model resolution. The model development is tested by using the water tracers to find the sources of precipitation in a historical UM simulation. As expected, the majority of precipitation is found to be sourced directly from the ocean, with the recycling of water over land becoming increasingly important downwind across continents. The UM results for the mean evaporative source properties of precipitation are comparable to those of the ECHAM6 atmospheric model, with some interesting local differences over Antarctica, Greenland, and the Indian monsoon region. Finally, global model hydrological fluxes are derived from the water tracers to show the amount of precipitation sourced from the land and ocean separately, illustrating the additional information that can be provided from the new development
Hybrid nonlinear-Kalman ensemble transform filtering for data assimilation in systems with different degrees of nonlinearity
Get PDFControl of simulated ocean ecosystem indicators by biogeochemical observations.
Get PDFTo protect marine ecosystems threatened by climate change and anthropic stressors, it is essential to operationally monitor ocean health indicators. These are metrics synthetizing multiple marine processes relevant to the users of operational services. Here we assess if selected ocean indicators simulated by operational models can be controlled (here meaning constrained effectively) by biogeochemical observations, by using a newly proposed methodological framework. The method consists in firstly screening the sensitivities of the indicators with respect to the initial conditions of the observable variables. These initial conditions are perturbed stochastically in Monte Carlo simulations of one-dimensional configurations of a multi-model ensemble. Then, the models are applied in three-dimensional ensemble assimilation experiments, where the reduction of the ensemble variance corroborates the controllability of the indicators by the observations. The method is applied for ten relevant ecosystem indicators (ranging from inorganic chemicals to plankton production), seven observation types (representing data from satellite and underwater platforms), and an ensemble of five biogeochemical models of different complexity, employed operationally by the European Copernicus Marine Service. We demonstrate that all the indicators are controlled by one or more types of observations. In particular, the indicators of phytoplankton phenology are controlled and improved by the merged observations from the surface ocean colour and chlorophyll profiles from biogeochemical-ARGO floats. Similar observations also control and reduce the uncertainty of the plankton community structure and production. However, the uncertainty of the trophic efficiency and POC increases when assimilating chlorophyll-a data, though observations were not available to assess whether that was due to a worsen model skill. We recommend that the assessment of controllability proposed here becomes a standard practice in designing operational monitoring, reanalysis and forecast systems, to ultimately provide the users of operational services with more precise estimates of ocean ecosystem indicators
Formation of mega-scale glacial lineations far inland beneath the onset of the Northeast Greenland Ice Stream
Get PDFAbstract. Rapidly-flowing ice streams drain the interior of the Greenland Ice Sheet, currently accounting for around half of its annual mass loss. The Northeast Greenland Ice Stream (NEGIS) is one of the largest, recognisable almost 600 km inland, and extends close to the central ice divide. Numerical ice sheet models are unable to accurately reproduce the configuration of the NEGIS, but understanding its bed properties and spatial and temporal evolution is critical to predicting its future contribution to sea-level change. Here, we use swath radar imaging to create a high-resolution Digital Elevation Model of the bed close to where the NEGIS initiates. Surprisingly, this reveals a landscape interpreted to include mega-scale glacial lineations (MSGLs) that are often assumed to be indicative of rapid ice stream flow (100s m yr−1), under present-day flow velocities of only ∼ 60 m yr−1. Given that MSGLs are thought to form under much higher flow velocities, their presence so far inland at an onset zone raises important questions about their formation and preservation under ice streams, as well as past configurations of the NEGIS. Elongate bedrock landforms outside the current shear margins also suggest that the NEGIS was wider than its present configuration at some point in the past
From science to policy: evolving marine biodiversity targets
Get PDFThe Montreal‐Kunming Global Biodiversity Framework (GBF) substantially advances biodiversity protection. We systematically reviewed the scholarly literature published during the UN Decade on Biodiversity (2010–2020) to assess whether GBF targets align with scientific approaches and improve upon the Aichi Targets in recognizing the complexity of marine biodiversity. Our findings showed that the new targets have improved to address the full suite of essential biodiversity variable (EBV) classes, reducing the risk of changes in crucial aspects of biodiversity being overlooked. We observed a high degree of alignment between research and policy in EBVs and a relative increase in the reliance of the GBF on secondary variables such as ecosystem function. While this alignment mirrors that within other global frameworks, we caution against overemphasizing secondary variables at the expense of foundational variables such as community composition. Our analysis demonstrates that global policy targets align well with scientific understanding of marine biodiversity. Future efforts should focus on improving national‐level implementation and refining indicators to foster transformative change in biodiversity conservation
Arctic Ocean virus communities and their seasonality, bipolarity, and prokaryotic associations
Get PDFViruses of microbes play important roles in ocean environments as agents of mortality and genetic transfer, influencing ecology, evolution and biogeochemistry. However, we know little about the diversity, seasonality, and host interactions of viruses in polar waters. Here, we study dsDNA viruses in the Arctic Fram Strait across four years via 47 long-read metagenomes of the cellular size-fraction. Among 5662 vOTUs, 98% and 2% are Caudoviricetes and Megaviricetes, respectively. Viral coverage is, on average, 5-fold higher than cellular coverage, and 8-fold higher in summer. Viral community composition shows annual peaks in similarity and strongly correlates with prokaryotic community composition. Using network analysis, we identify putative virus-host interactions and six ecological modules associated with distinct environmental conditions. The network reveals putative novel cyanophages with time-lagged correlations to their hosts (in late summer) as well as diverse viruses correlated with Flavobacteriaceae, Pelagibacteraceae, and Nitrosopumilaceae. Via global metagenomes, we find that 42% of Fram Strait vOTUs peak in abundance in high latitude regions of both hemispheres, and encode proteins with biochemical signatures of cold adaptation. Our study reveals a rich diversity of polar viruses with pronounced seasonality, providing a foundation for understanding viral regulation and ecosystem impacts in changing polar oceans