Alfred Wegener Institute for Polar and Marine Research
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Evaluation of Novel Airborne Gravity Levelling Methods
Full text linkAbstract
In dynamic gravimetry, i.e. airborne and shipborne gravimetry, levelling methods are used to refine gravity disturbance results based on neighbouring trajectories. In the traditional crossover adjustment, line biases are estimated using gravity disturbance residuals at trajectory line crossings as input to a least-squares adjustment. In an alternative method, the results along the complete trajectory are used to estimate the gravity disturbance field in the survey area and line biases in a one-step least-squares adjustment applying spherical radial basis functions. This makes the bias estimation more robust since the observations are not restricted to a small number of residuals at crossings strongly affected by random errors. Adjustment becomes applicable to a wider range of campaigns including irregular trajectories without many crossings. Within the scope of this work, existing methods that estimate line biases are extended to bias estimation based on trajectory segments with inter-bias interpolation. The extended method can be particularly useful for irregular trajectories without a sufficient number of line crossings. The introduced levelling methods are evaluated at the example of three airborne campaigns: a fixed wing survey at Germany with a very dense grid, a fixed wing survey in East Antarctica with varying line separation, and a helicopter survey on Svalbard with highly irregular trajectories. It is shown that the levelling method based on spherical radial basis functions improves the precision in all evaluated campaigns, even when a traditional crossover levelling is not possible.</jats:p
Simulated density reorganization on the Weddell Sea continental shelf sensitive to atmospheric forcing
Full text linkAbstract. The strong Antarctic Slope Front in the southern Weddell Sea limits the present-day transport of modified Warm Deep Water (mWDW) onto the continental shelf and is associated with a characteristic V-shape in the density structure across the continental slope. The mechanisms controlling today's V-shape are well studied, but its future development is not yet well constrained. In this study, we run ocean model simulations for a 21st century Shared Socioeconomic Pathways (SSP) 3-7.0 emission scenario. The forcing is retrieved from atmospheric model output from simulations with a global climate model and from a higher-resolved regional atmospheric model respectively. We find that the resolution of the atmospheric model component influences the simulated future transport of mWDW onto the continental shelf into the Filchner Trough in the southern Weddell Sea through differences in the evolution of the depth and symmetry of the V-shape over the 21st century. In both simulations, reduced sea-ice formation and weakened Ekman downwelling reduce the depth of the V-shape and increase the sensitivity of its position above the slope to seasonal variations in sea-ice production and in the wind field. Using forcing data from an atmosphere model with higher resolution leads to an acceleration of the density redistribution on the continental shelf compared to the simulations forced with coarse-resolution data. This indicates that the SSP3-7.0 climate scenario may have a greater potential for a regime shift from a cold to a warm Filchner Trough through a cross-slope current before the end of the 21st century than suggested by other ocean simulations for the same scenario but with lower atmospheric resolution. As cross-slope currents disturb the continuity of the V-shape, we define a spatial grade of connectivity to quantify the lateral integrity of the V-shape along the continental slope. We find that the integrity of the V-shape reduces with a delay of 3 months after a strong cross-slope current of mWDW enters Filchner Trough. Atmospheric downscaling increases the potential for a regime shift, dominated by warmer summer air temperatures. The Antarctic Slope Front is temporarily disturbed by cross-slope currents but the primary reason for the regime shift is the cross-slope density gradient
Towards multi-fidelity models of coupled multi-X processes in sea ice within the Antarctic marginal ice zone
Full text linkThe Antarctic Marginal Ice Zone (MIZ) is a dynamic and complex region where sea ice interacts with oceanic, atmospheric, and ecological processes. Accurately modeling the processes related to sea ice requires a multi-fidelity approach that can capture the coupled multi-X (scale, phase, field, ...) physical and biogeochemical processes. Traditional high-fidelity models, while precise, are computationally expensive, limiting their application for large-scale and long-term simulations. On the other hand, low-fidelity models are computationally efficient but often fail to accurately represent the intricate interactions within the MIZ. Apart from this, making time dependent observations in the MIZ is in itself a challenge. This paper presents a framework for developing multi-fidelity models, starting from high-fidelity simulations that act as input data for low-fidelity surrogate models to improve both accuracy and computational efficiency in modeling the coupled processes in the Antarctic MIZ. The approach leverages dimensionality reduction techniques to create surrogate models that bridge the gap between highly detailed small scale process models to low resolution large scale global models . By combining data-driven models with physics-based simulations, the framework captures the complex dynamics of sea ice, including thermodynamic processes, phase transition, and primary production. Initial results demonstrate the potential of this approach to provide accurate predictions with reduced computational costs. The framework is validated against available literature data from the Antarctic MIZ. This work represents a critical step towards more robust and scalable models for understanding and predicting the behavior of sea ice in polar regions, which is crucial for global climate models and ecological studies
Disentangling future effects of climate change and forest disturbance on vegetation composition and land surface properties of the boreal forest
Full text linkAbstract. Forest disturbances can cause shifts in boreal vegetation cover from predominantly evergreen to deciduous trees or non-forest dominance. This, in turn, impacts land surface properties and, potentially, regional climate. Accurately considering such shifts in future projections of vegetation dynamics under climate change is crucial but hindered (e.g., uncertainties in future disturbance regimes). In this study, we investigate how sensitive future projections of boreal forest dynamics are to additional changes in disturbance regimes. We use the dynamic vegetation model LPJ-GUESS to investigate and disentangle the impacts of climate change and intensifying disturbance regimes in future projections of boreal vegetation cover as well as changes in land surface properties such as albedo and evapotranspiration. Our simulations find that (1) warming alone drives shifts towards more densely forested landscapes, (2) more intense disturbances reduce tree cover in favor of shrubs and grasses, and (3) the interaction between climate and disturbances leads to an expansion of deciduous trees. Our results additionally indicate that warming decreases albedo and increases evapotranspiration, while more intense disturbances have the opposite effect, potentially offsetting climate impacts. Warming and disturbances are thus comparably important agents of change in boreal forests. Our findings highlight future disturbance regimes as a key source of model uncertainty and underscore the necessity of accounting for disturbances-induced effects on vegetation composition and land surface–atmosphere feedback
The MOSES Sternfahrt Expeditions of the Research Vessels LITTORINA, LUDWIG PRANDTL and MYA II to the Elbe River, Elbe Estuary and German Bight in 2024
Full text linkFramStrait linkages of phytoplankton community composition to ocean physics and sea ice properties
Full text linkSequence of abrupt transitions in Antarctic drainage basins before and during the Mid-Pleistocene Transition
Full text linkUnraveling the drivers of climate variability during the Mid-Pleistocene Transition (MPT) remains a central challenge in paleoclimate research. This interval marked a shift from 41-kyr to 100-kyr glacial cycles associated with larger ice sheets. While previous studies emphasize interactions between climate and Northern Hemisphere ice sheets, Antarctica’s role remains unclear. We use the Parallel Ice Sheet Model to simulate Antarctic Ice Sheet evolution over the last 3 million years, applying a climate index approach. Our simulations show that between 1.9 and 0.8 Ma, several Antarctic drainage basins underwent structural re-organization at different times, including the formation of a stable marine-based West Antarctic Ice Sheet (WAIS). We analyze the drivers of these thresholds and their associated state transitions. Our findings reveal tri-stability in the Thwaites basin and suggest that WAIS thresholds and their complex interactions amplified ~100-kyr climate variability before and during the MPT, providing new insights into long-term climate dynamics
Surveying marine biodiversity using eDNA metabarcoding of seawater and sediment in a high Arctic fjord during the polar night (Kongsfjorden, Svalbard)
Full text linkThe marine ecosystem of Kongsfjorden (KF), Svalbard, is directly affected by ongoing climate change in the Arctic. Shifts in species composition and distributions are already underway as a result of the transition from Arctic to Atlantic conditions in the fjord. The polar night is a period of challenging conditions and is historically understudied, and thus our understanding of the biodiversity of major eukaryotic groups is particularly limited at this time of year. Here we aimed to provide a comprehensive snapshot of eukaryotic biodiversity present in KF during the polar night using environmental DNA (eDNA) metabarcoding of the mitochondrial cytochrome c oxidase subunit 1 gene (COI). Additionally, we sought to establish a baseline dataset for gelatinous zooplankton (GZP) during this period by combining eDNA with net sampling. Lastly, we tested for the impact of PCR-inhibition on the recovery of eDNA from turbid fjord waters. We successfully generated species lists for a large component of the pelagic community known to inhabit KF, benthic and hyperbenthic species typical to the area, as well as a number of new detections. We recovered taxa from major functional groups in the fjord, including macroalgae, phytoplankton, zooplankton, large vertebrates and benthic invertebrates. Finally, we recovered a richer polar night GZP community than previously detected with morphology-based methods, including other seasons. This study demonstrates the versatility of eDNA metabarcoding for marine biodiversity surveys in challenging environmental conditions and provides a baseline for future polar night eDNA-based biodiversity monitoring in KF
Morphological, molecular and toxinological characterization of potentially toxigenic microalgal strains from the western Black Sea
Full text linkHarmful algal blooms (HAB) of various toxic microalgae negatively affect the Black Sea but there is limited data on the sources of phycotoxins in this area. During the PHYCOB cruise in September 2021, two strains of one diatom and thirty strains of ten dinoflagellate species of potentially toxic plankton were characterized in terms of morphology, phylogeny, toxin profiles, and toxin cell quotas. Twenty strains of potentially yessotoxins (YTXs) producing dinoflagellate species were investigated. All six strains of Protoceratium reticulatum contained yessotoxin (YTX), with cell quotas between 1.9 and 5.4 pg cell-1. Additionally, several YTX variants were detected in minor amounts in nine of twelve strains of Lingulaulax polyedra, whereas no YTXs were detected in two strains of other gonyaulacoids (Gonyaulax sp. and Sourniaea diacantha). All strains of Alexandrium spp. (four strains of A. tamutum, one strain each of A. andersonii, A. ostenfeldii, and A. pseudogonyaulax) were analyzed for all toxins known to be produced by the genus. None of the strains contained detectable levels of paralytic shellfish toxins (PSTs) or gymnodimines. A. pseudogonyaulax, produced goniodomine A (GDA) and GDA-seco acid at cellular levels of 14.0 and 0.33 pg cell-1, respectively. Moreover, four previously unreported spirolide analogues (SPX) were detected in the A. ostenfeldii strain, with cell quotas between 1.0 and 1.6 pg cell-1. All strains of potentially ichthyotoxic species (Karlodinium sp., Polykrikos hartmanii) did not show extracellular lytic activity. In conclusion, these findings improve our understanding of the potential sources, diversity and dynamics of phycotoxins in the Black Sea
Landfast ice in the Kara Sea stabilizes the Arctic halocline and may slow down Atlantification of the Eurasian Basin
Full text linkObservations show an Atlantification of the Eurasian Basin of the Arctic Ocean, with deeper penetration, shoaling, and ventilation of Atlantic waters in the eastern Arctic and an associated weakening of the cold halocline layer. These processes have a profound impact on the sea ice cover above and potentially on the transition of the Arctic to a seasonal ice cover. Here we show, using a coupled ice-ocean model, that a proper simulation of the landfast ice cover in the relatively small but deeper peripheral Kara Sea has a disproportionately large influence on the halocline stability in the Eurasian Basin and beyond. Specifically, landfast ice in the Kara Sea reduces ice growth and therefore salt rejection into the surface ocean. This negative salinity anomaly is advected eastward with a coastal current along the continental shelf in the Makarov Basin and then out of the Arctic through Fram Strait by the Transpolar Drift Stream on timescales of less than ten years. Global Climate Models, however, do not yet include landfast ice parameterizations. Therefore, they are missing this key process affecting the halocline stability, Atlantification of the Makarov Basin, and potentially the timing of a seasonally ice-free Arctic