Alfred Wegener Institute for Polar and Marine Research
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Description and validation of the ice-sheet model Nix v1.0
Full text linkAbstract. 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
Full text linkAbstract. 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
Full text linkControl of simulated ocean ecosystem indicators by biogeochemical observations.
Full text linkTo 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
Full text linkAbstract. 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
The Expedition PS141 of the Research Vessel POLARSTERN to the Davis Sea and Mawson Sea in 2024
Full text linkAdvances in Permafrost Representation: Biophysical Processes in Earth System Models and the Role of Offline Models
Full text linkABSTRACTPermafrost is undergoing rapid changes due to climate warming, potentially exposing a vast reservoir of carbon to be released to the atmosphere, causing a positive feedback cycle. Despite the importance of this feedback, its specifics remain poorly constrained, because representing permafrost dynamics still poses a significant challenge for Earth System Models (ESMs). This review assesses the current state of permafrost representation in land surface models (LSMs) used in ESMs and offline permafrost models, highlighting both the progress made and the remaining gaps.We identify several key physical processes crucial for permafrost dynamics, including soil thermal regimes, freeze–thaw cycles, and soil hydrology, which are underrepresented in many models. While some LSMs have advanced significantly in incorporating these processes, others lack fundamental elements such as latent heat of freeze–thaw, deep soil columns, and Arctic vegetation dynamics. Offline permafrost models provide valuable insights, offering detailed process testing and aiding the prioritization of improvements in coupled LSMs.Our analysis reveals that while significant progress has been made in incorporating permafrost‐related processes into coupled LSMs, many small‐scale processes crucial for permafrost dynamics remain underrepresented. This is particularly important for capturing the complex interactions between physical and biogeochemical processes required to model permafrost carbon dynamics. We recommend leveraging advancements from offline permafrost models and progressively integrating them into LSMs, while recognizing the computational and technical challenges that may arise in coupled simulations. We highlight the importance of enhancing the representation of physical processes, including through improvements in model resolution and complexity, as this is a fundamental precursor to accurately incorporate biogeochemical processes and capture the permafrost carbon feedback.</jats:p
Theatre of enforcement at sea: The global fight against ‘illegal fishing’ and the criminalisation of fisher peoples and exploitation of fish workers
Full text linkIllegal, unreported, and unregulated (IUU) fishing has been internationally branded as a major threat to oceans. Frequently depicted as having profound societal impacts and operational synergies with other forms of criminal activities, which justify the need for a so-called global fight against IUU fishing to protect the marine commons and secure marine spaces. Whereas industrial fishing is the prime culprit, policy reforms are being promoted to regulate and formalise small-scale/artisanal fishing practices. This raises questions on how enforcement and formalisation processes are translated into practice and shaped by economic interests within and beyond the ocean. In this intervention, we focus on the governance of IUU fishing in Colombia and anchor our critique into two acts — the act of criminalisation and the act of impunity — to uncover a theatre of enforcement at sea. We argue that the punitive approach to IUU fishing criminalises fisher peoples, while domestic, foreign, and transnational capitalist actors continue to operate, depleting oceans and exploiting fish workers’ labour with very limited control. We conclude by asserting that the fight against IUU fishing is in part a fight against precarious fish workers and fisher peoples, rather than against ‘ocean grabbers’, reflecting biased criminalisation processes with differentiated impacts at the intersections of class, gender, and race