Antarctic Marginal Ice Zone Research Articles

Estimation of Antarctic sea ice thickness through observation of wave attenuation

The Close-Packing model – a physically based model for wave attenuation in sea ice – is used to infer sea ice thickness from wave observations collected in the Antarctic marginal ice zone during the PIPERS experiment. The model, calibrated for Arctic conditions, predicts ice thickness in good agreement with independent satellite measurements. The calibrated Close -Packing model, which is expressed in a simple monomial form, appears to have broad validity and, therefore, can be a suitable option for operational purposes.

Analysis of the Antarctic Marginal Ice Zone Based on Unsupervised Classification of Standalone Sea Ice Model Data

AbstractThe Antarctic marginal ice zone, the regularly wave‐affected outer band of the sea ice covered Southern Ocean, typically contains an unconsolidated ice cover comprised of smaller, thinner floes than the inner ice pack. Thus, it is a highly dynamic region and susceptible to rapid expansion and contraction, making it a focal area for understanding and predicting the response of Antarctic sea ice to a changing climate. This novel study uses unsupervised statistical clustering of sea ice data simulated by a global sea ice model (standalone CICE6 combined with a wave propagation module and prescribed ocean) to address the outstanding challenge of separating the marginal ice zone from the inner ice pack in sea ice data sets. The method identifies a marginal ice zone with the desired characteristics and floe size is shown to be the key variable in the classification. Simulated marginal ice zone widths are similar to those derived from satellite observations of wave penetration distances, but contrast with those using the standard 15%–80% areal sea ice concentration proxy, particularly during austral winter. The simulated marginal ice zone is found to undergo a seasonal transition due to new ice formation in winter, increased drift in spring, and increased rates of wave‐induced breakup and melting in summer. The understanding gained from the study motivates incorporation of wave and floe‐scale processes in sea ice models, and the methods are available for application to outputs from high‐resolution and coupled sea ice–ocean–wave models for more detailed studies of the marginal ice zone (in both hemispheres).

A contrast in sea ice drift and deformation between winter and spring of 2019 in the Antarctic marginal ice zone

Abstract. Two ensembles of buoys, deployed in the marginal ice zone (MIZ) of the north-eastern Weddell Sea region of the Southern Ocean, are analysed to characterise the dynamics driving sea ice drift and deformation during the winter-growth and the spring-retreat seasons of 2019. The results show that although the two buoy arrays were deployed within the same region of ice-covered ocean, their trajectory patterns were vastly different. This indicates a varied response of sea ice in each season to the local winds and currents. Analyses of the winter data showed that the Antarctic Circumpolar Current modulated the drift near the sea ice edge. This led to a highly energetic and mobile ice cover, characterised by free-drift conditions. The resulting drift and deformation were primarily driven by large-scale atmospheric forcing, with negligible contributions due to the wind-forced inertial response. For this highly advective coupled ice–ocean system, ice drift and deformation linearly depended on atmospheric forcing. We also highlight the limits of commercial floating ice velocity profilers in this regime since they may bias the estimates of sea ice drift and the ice type detection. On the other hand, the spring drift was governed by the inertial response as increased air temperatures caused the ice cover to melt and break up, promoting a counterintuitively less wind-driven ice–ocean system that was more dominated by inertial oscillations. In fact, the deformation spectra indicate a strong decoupling to large-scale atmospheric forcing. Further analyses, extended to include the deformation datasets from different regions around Antarctica, indicate that, for similar spatial scales, the magnitude of deformation varies between seasons, regions, and the proximity to the sea ice edge and the coastline. This implies the need to develop rheology descriptions that are aware of the ice types in the different regions and seasons to better represent sea ice dynamics in the MIZ.

Small-scale computational fluid dynamics modelling of the wave induced ice floe-grease ice interaction in the Antarctic marginal ice zone

Computational sea ice models have been developed to simulate sea ice formation, melt and motion characteristics on a regional scale. However, the highly dynamic sea ice behaviour in the Antarctic marginal ice zone (MIZ), the area where sea ice and ocean waves interact, still eludes reliable prediction. This is due to the complex sea ice composition consisting of relatively small and mobile ice floes governed by collision and fracture mechanisms. To improve the accuracy of sea ice models, the realistic sea ice distribution needs to be accounted for at a high resolution so that key aspects defining the interplay of ice motion and wave propagation can be suitably distinguished. In this work a computational fluid dynamics model based on the small-scale continuum approach has been developed to simulate the dynamics of a heterogeneous sea ice cover. The model makes use of a realistic sea ice layout and studies the mechanical behaviour of sea ice as affected by wave forcing and the specific material properties of ice floes and grease ice. Taking advantage of the continuum approach paired with a heterogeneous sea ice cover, the floe-grease ice interaction is elucidated by discussing detailed temporal and spatial distributions of the mechanical response of sea ice. Results show that the interplay between waves and the ice floe collision behaviour is directly controlled by sea ice inertia, where the frequency and severity of ice floe collisions increase with the wave period. Furthermore, the interaction of grease ice with ice floes through form drag at the interface leads to high localised grease ice strain rate gradients and low viscosity values due its shear thinning characteristics.

Rafting of Growing Antarctic Sea Ice Enhances In‐Ice Biogeochemical Activity in Winter

AbstractThe study of Antarctic first‐year sea‐ice biogeochemistry has been limited by samples mostly being collected in pack ice during summer, with few winter data available. Measurements from the Antarctic marginal ice zone (AMIZ) have proven even more difficult to obtain. The AMIZ is a broad, circumpolar feature of the Southern Ocean found at different latitudes during the year where light and nutrients are high enough to sustain primary production and influence ecosystem functioning. We present the first biogeochemical data set from growing ice collected in the Atlantic AMIZ during winter 2019, including measurements from young pancake ice (YI) and consolidated first‐year ice (FYI). Measurements of sea‐ice temperature, salinity, crystal structure, δ18O, chlorophyll, and nutrient concentrations were used to investigate the winter sea‐ice habitat and decipher the conditions under which the ice formed and grew. Model simulations support the hypothesis that nutrient accumulation in advancing sea ice cannot be explained by passive seawater entrainment and thermodynamics alone. Our data confirm that winter sea ice is biogeochemically active and accumulates remineralized nutrients. We further propose that mechanical thickening enhances the reservoir of nutrients during the ice growth season. The biogeochemical transition from YI to FYI does not appear to be a linear progression of thickness with habitat space reduction as sea ice consolidates. Instead, FYI bulk biogeochemistry results from multiple cycles of rafting of YI, which conserves the biogeochemical properties of YI in the FYI, ultimately increasing the overall nutrient and chlorophyll content.

Direct Observations of Wave‐Sea Ice Interactions in the Antarctic Marginal Ice Zone

AbstractWave energy propagating into the Antarctic marginal ice zone effects the quality and extent of the sea ice, and wave propagation is therefore an important factor for understanding and predicting changes in sea ice cover. Wave‐sea ice interactions are notoriously hard to model and in situ observations of wave activity in the Antarctic marginal ice zone are scarce, due to the extreme conditions of the region. Here, we provide new in situ data from two drifting Surface Wave Instrument Float with Tracking (SWIFT) buoys deployed in the Weddell Sea in the austral winter and spring of 2019. The buoy location ranges from open water to more than 200 km into the sea ice. We estimate the attenuation of swell with wave periods 8–18 s, and find an attenuation coefficient α = 4 · 10−6 to 7 · 10−5 m−1 in spring, and approximately five‐fold larger in winter. The attenuation coefficients show a power law frequency dependence, with power coefficient close to literature. The in situ data also shows a change in wave direction, where wave direction tends to be more perpendicular to the ice edge in sea ice compared to open water. A possible explanation for this might be a change in the dispersion relation caused by sea ice. These observations can help shed further light on the influence of sea ice on waves propagating into marginal ice zones, aiding development of coupled wave‐sea ice models.

High‐Resolution Thermal Imaging in the Antarctic Marginal Ice Zone: Skin Temperature Heterogeneity and Effects on Heat Fluxes

AbstractInsufficient in situ observations from the Antarctic marginal ice zone (MIZ) limit our understanding and description of relevant mechanical and thermodynamic processes that regulate the seasonal sea ice cycle. Here we present high‐resolution thermal images of the ocean surface and complementary measurements of atmospheric variables that were acquired underway during one austral winter and one austral spring expedition in the Atlantic and Indian sectors of the Southern Ocean. Skin temperature data and ice cover images were used to estimate the partitioning of the heterogeneous surface and calculate the heat fluxes to compare with ERA5 reanalyses. The winter MIZ was composed of different but relatively regularly distributed sea ice types with sharp thermal gradients. The surface‐weighted skin temperature compared well with the reanalyses due to a compensation of errors between the sea ice fraction and the ice floe temperature. These uncertainties determine the dominant source of inaccuracy for heat fluxes as computed from observed variables. In spring, the sea ice type distribution was more irregular, with alternation of sea ice cover and large open water fractions even 400 km from the ice edge. The skin temperature distribution was more homogeneous and did not produce substantial uncertainties in heat fluxes. The discrepancies relative to reanalysis data are however larger than in winter and are attributed to biases in the atmospheric variables, with the downward solar radiation being the most critical.

Sea‐Ice Impacts Inter‐Annual Variability of Phytoplankton Bloom Characteristics and Carbon Export in the Weddell Sea

AbstractThe Antarctic Marginal Ice Zone (MIZ) accounts for 15% of the Southern Ocean's primary production (PP), but limited data has hindered understanding of its variability and connection to carbon export. Using a combination of gliders, biogeochemical Argo floats and satellite observations in the northeast Weddell Sea, we show that years with more sea‐ice formation over winter are followed by more intense phytoplankton blooms (∼15% greater daily PP) and export to 100 m (∼50% higher daily carbon export) the following summer. However, the carbon export beyond the deepest winter mixed layer did not vary in proportion to PP, suggesting different drivers of carbon export at depth compared to surface waters. Furthermore, across the entire MIZ, the response of blooms to sea‐ice volume was spatially variable, indicating the need to consider spatial heterogeneity in the response of the biological carbon pump to future sea‐ice changes.

High-resolution thermal imaging in the Antarctic marginal ice zone: Skin temperature heterogeneity and effects on heat fluxes

High-resolution thermal imaging in the Antarctic marginal ice zone: Skin temperature heterogeneity and effects on heat fluxes

Physical and morphological properties of first-year Antarctic sea ice in the spring marginal ice zone of the Atlantic-Indian sector

AbstractThis study presents the first dataset of physical and textural properties of sea ice collected in the South Atlantic and Indian Ocean sector of the Antarctic marginal ice zone (MIZ). Observations of sea ice from this region in the austral spring 2019, including sea-ice core temperature, salinity, crystal size, texture, oxygen isotopes and stratigraphy, were used in conjunction with a Lagrangian back-tracking algorithm and atmospheric reanalyses. This method relates the reconstructed synoptic conditions to sea-ice growth along the transect. A significant difference was found between the stratigraphy of consolidated pack ice samples collected at the same latitude and spanning over 550 km eastwards. The eastward group was found to have more disturbances in their stratigraphy which is attributed to the highly variable atmospheric and sea-ice conditions together with varying wave penetration through the sea-ice pack, notably during the passage of an intense polar cyclone, while the westward group showed no signs of disturbance or deformation. These results indicate that consolidated Antarctic sea-ice floes of similar thickness and from the same latitude in the MIZ have distinct stratigraphic properties, which will influence their physical and biogeochemical features.

An indicator of sea ice variability for the Antarctic marginal ice zone

Abstract. Remote-sensing records over the last 40 years have revealed large year-to-year global and regional variability in Antarctic sea ice extent. Sea ice area and extent are useful climatic indicators of large-scale variability, but they do not allow the quantification of regions of distinct variability in sea ice concentration (SIC). This is particularly relevant in the marginal ice zone (MIZ), which is a transitional region between the open ocean and pack ice, where the exchanges between ocean, sea ice and atmosphere are more intense. The MIZ is circumpolar and broader in the Antarctic than in the Arctic. Its extent is inferred from satellite-derived SIC using the 15 %–80 % range, assumed to be indicative of open drift or partly closed sea ice conditions typical of the ice edge. This proxy has been proven effective in the Arctic, but it is deemed less reliable in the Southern Ocean, where sea ice type is unrelated to the concentration value, since wave penetration and free-drift conditions have been reported with 100 % cover. The aim of this paper is to propose an alternative indicator for detecting MIZ conditions in Antarctic sea ice, which can be used to quantify variability at the climatological scale on the ice-covered Southern Ocean over the seasons, as well as to derive maps of probability of encountering a certain degree of variability in the expected monthly SIC value. The proposed indicator is based on statistical properties of the SIC; it has been tested on the available climate data records to derive maps of the MIZ distribution over the year and compared with the threshold-based MIZ definition. The results present a revised view of the circumpolar MIZ variability and seasonal cycle, with a rapid increase in the extent and saturation in winter, as opposed to the steady increase from summer to spring reported in the literature. It also reconciles the discordant MIZ extent estimates using the SIC threshold from different algorithms. This indicator complements the use of the MIZ extent and fraction, allowing the derivation of the climatological probability of exceeding a certain threshold of SIC variability, which can be used for planning observational networks and navigation routes, as well as for detecting changes in the variability when using climatological baselines for different periods.

Physical and mechanical properties of winter first-year ice in the Antarctic marginal ice zone along the Good Hope Line

Abstract. As part of the 2019 Southern oCean seAsonal Experiment (SCALE) Winter Cruise of the South African icebreaker SA Agulhas II, first-year ice was sampled at the advancing outer edge of the Antarctic marginal ice zone along a 150 km Good Hope Line transect. Ice cores were extracted from four solitary pancake ice floes of 1.83–2.95 m diameter and 0.37–0.45 m thickness as well as a 12×4 m pancake ice floe of 0.31–0.76 m thickness that was part of a larger consolidated pack ice domain. The ice cores were subsequently analysed for temperature, salinity, texture, anisotropic elastic properties and compressive strength. All ice cores from both solitary pancake ice floes and consolidated pack ice exhibited predominantly granular textures. The vertical distributions of salinity, brine volume and mechanical properties were significantly different for the two ice types. High salinity values of 12.6±4.9 PSU were found at the topmost layer of the solitary pancake ice floes but not for the consolidated pack ice. The uniaxial compressive strengths for pancake ice and consolidated pack ice were determined as 2.3±0.5 and 4.1±0.9 MPa, respectively. Young's and shear moduli in the longitudinal core direction of solitary pancake ice were obtained as 3.7±2.0 and 1.3±0.7 GPa, respectively, and of consolidated pack ice as 6.4±1.6 and 2.3±0.6 GPa, respectively. Comparing Young's and shear moduli measured in longitudinal and transverse core directions, a clear directional dependency was found, in particular for the consolidated pack ice.

Altimetric observation of wave attenuation through the Antarctic marginal ice zone using ICESat-2

Abstract. The Antarctic marginal ice zone (MIZ) is a highly dynamic region where sea ice interacts with ocean surface waves generated in ice-free areas of the Southern Ocean. Improved large-scale (satellite-based) estimates of MIZ extent and variability are crucial for understanding atmosphere–ice–ocean interactions and biological processes and detection of change therein. Legacy methods for defining the MIZ are typically based on sea ice concentration thresholds and do not directly relate to the fundamental physical processes driving MIZ variability. To address this, new techniques have been developed to measure the spatial extent of significant wave height attenuation in sea ice from variations in Ice, Cloud and land Elevation Satellite-2 (ICESat-2) surface heights. The poleward wave penetration limit (boundary) is defined as the location where significant wave height attenuation equals the estimated error in significant wave height. Extensive automated and manual acceptance/rejection criteria are employed to ensure confidence in along-track wave penetration width estimates due to significant cloud contamination of ICESat-2 data or where wave attenuation is not observed. Analysis of 304 ICESat-2 tracks retrieved from four months of 2019 (February, May, September and December) reveals that sea-ice-concentration-derived MIZ width estimates are far narrower (by a factor of ∼ 7 on average) than those from the new technique presented here. These results suggest that indirect methods of MIZ estimation based on sea ice concentration are insufficient for representing physical processes that define the MIZ. Improved large-scale measurements of wave attenuation in the MIZ will play an important role in increasing our understanding of this complex sea ice zone.

Atmospheric drivers of a winter-to-spring Lagrangian sea-ice drift in the Eastern Antarctic marginal ice zone

AbstractSea-ice drift in the Antarctic marginal ice zone (MIZ) is discussed using data from a 4-month-long drift of a buoy deployed on a pancake ice floe during the winter sea-ice expansion. We demonstrate increased meandering and drift speeds, and changes in the dynamical regimes of the absolute dispersion during cyclone activity, together with high correlations between drift velocities and wind from atmospheric reanalyses. This indicates a dominant physical control of wind forcing on ice drift and the persistence of free-drift conditions. These conditions occurred despite the buoy remaining largely in >80% ice concentrations and at distances >200 km from the estimated ice edge. The drift is additionally characterised by a strong inertial signature at 13.47 h, which appears initiated by passing cyclones. A wavelet analysis of the buoy's velocity confirms that the momentum transfer from winds at the multi-day frequencies is due to atmospheric forcing, while the initiation of inertial oscillations of sea ice has been identified as the secondary effect. Propagating storm-generated waves may initiate inertial oscillations by increasing the mobility of floes and enhance the drag of the inertial current. This analysis indicates that the Antarctic MIZ in the Indian Ocean sector remains much wider and mobile, during austral winter-to-spring, than defined by sea-ice concentration.

The Antarctic Marginal Ice Zone and Pack Ice Area in CMEMS GREP Ensemble Reanalysis Product

Global ocean reanalyses provide consistent and comprehensive records of ocean and sea ice variables and are therefore of pivotal significance for climate studies, particularly in data-sparse regions such as Antarctica. Here, for the first time, we present the temporal and spatial variability of sea ice area in the ensemble of global ocean reanalyses produced by the Copernicus Marine Environment Monitoring Service (CMEMS) for the period 1993–2019. The reanalysis ensemble robustly reproduces observed interannual and seasonal variability, linear trend, as well as record highs and lows. While no consensus has been reached yet on the physical source of Antarctic-wide ice changes, our study also emphasizes the importance of understanding the different responses of ice classes, marginal ice zone (MIZ) and pack ice, to climate changes. Modifications of the distribution of MIZ and pack ice have implications for the level of air/sea exchanges and for the marine ecosystem. Analysis of the spatial and temporal variability of ice classes can provide further insights on long-term trends and help to improve predictions of future changes in Antarctic sea ice. We assess the ability of the reanalysis ensemble to adequately capture variability in space and time of the MIZ and pack ice area, and conclude that it can provide consistent estimates of recent changes in the Antarctic sea ice area. Our results show that the Antarctic sea ice area agrees well with satellite estimates, and the hemispheric and regional sea ice area variability are properly reproduced on seasonal and interannual time scales. Although the ensemble reanalysis product tends to slightly overestimate MIZ in summer, results show that it properly represents the variability of MIZ minima and maxima as well as its interannual variability during the growing and melting seasons. Our results confirm that Global Reanalysis Ensemble Product is able to reproduce the observed substantial regional variability, in regions covered by marginal ice.

Concept-driven extraction of the Antarctic marginal sea ice zone from remote sensing image time series

Sea ice plays a significant role in global climate change. Marginal ice zone (MIZ) is defined as the transition zone between open ocean and pack ice where intensive air–ice–ocean–wave​ interactions between the ocean and the atmosphere occur. This definition of MIZ is rather vague, which affects its mapping. Previous data-driven methods extracted MIZ from single source data and conveniently used a single classification threshold. In this study, we propose to apply a concept-driven top down extraction method. We use a fuzzy inference system (FIS) to integrate multiple ice properties based upon the MIZ definition, and use membership functions to quantify the uncertainty of the threshold for several ice characteristics. This concept-driven method is applied to mapping the Antarctic MIZ between 2010 and 2021. Results show that a FIS successfully combines the ice concentration and ice thickness, while it allowed us to map the MIZ as objects with vague boundaries, thus well-presenting its indeterminate nature in space.

Numerical study on effect of brash ice on water exit dynamics of ventilated cavitation cylinder

This study numerically examines the evolution of cavitation, trajectory, and impact load during the process of water exit of a submerged and ventilated cavitation cylinder in a brash ice zone by coupling the computational fluid dynamics (CFD) method with the discrete element method (DEM). A two-way coupling scheme is used to study the influence on the fluid due to the brash ice. The ice conditions off Wilkes Land in the Antarctic marginal ice zone in late winter were used to formulate a DEM particle model without failure limitation. The brash ice zone was established according to random positions of the ice generated by self-programming. The CFD-DEM coupling method was verified through comparisons with both experimental results and empirical formulae in the literature. The results show that the cavity quickly broke while crossing the water surface, and a prominent formation of spikes was observed in brash ice conditions. The displacement and deflection angle of the cylinder had different features in the two directions perpendicular to the axis. Furthermore, strong alternating loads were observed when the water surface was crossed in comparison with those in ice-free conditions.

Experimental study on the effects of brash ice on the water-exit dynamics of an underwater vehicle

To investigate the impact of brash ice on the dynamics of the water exit of an axisymmetric vehicle, an experimental study was conducted on the water exit of an axisymmetric vehicle in brash ice using a water tank, launching system, control system, data acquisition system, and high-speed camera system. The brash ice zone was modeled using actual observational data from the Antarctic marginal ice zone off Wilkes Land in late winter. Brash ice was created from polypropylene material, and the influence of brash ice on the dynamics of the water-exit of the vehicle is analyzed. The results show that the collapse of bubbles during water-exit of a vehicle in the presence of brash ice creates a state of eruption, which splashes upward from the shoulder to the center of the vehicle, and the underwater cavities undergo shrinkage and shedding. The effects of brash ice on the magnitude of the force and moment on the vehicle during the process are diverse and operate in different directions. The overall deflection angle is greater than that under ice-free conditions. As water-exit speed increases, the splash time becomes earlier, and the extent of the splash becomes more dispersive. In addition, pressure falls as the vehicle approaches the water surface, and the extent and magnitude of pressure fluctuation during the whole water-exit process becomes more severe.

Frazil Ice in the Antarctic Marginal Ice Zone

Frazil ice, consisting of loose disc-shaped ice crystals, is the first ice that forms in the annual cycle in the marginal ice zone (MIZ) of the Antarctic. A sufficient number of frazil ice crystals form the surface “grease ice” layer, playing a fundamental role in the freezing processes in the MIZ. As soon as the ocean waves are sufficiently damped by a frazil ice cover, a closed ice cover can form. In this article, we investigate the rheological properties of frazil ice, which has a crucial influence on the growth of sea ice in the MIZ. An in situ test setup for measuring temperature and rheological properties was developed. Frazil ice shows shear thinning flow behavior. The presented measurements enable real-data-founded modelling of the annual ice cycle in the MIZ.

Stirring of Sea‐Ice Meltwater Enhances Submesoscale Fronts in the Southern Ocean

AbstractIn the sea‐ice‐impacted Southern Ocean, the spring sea‐ice melt and its impact on physical processes set the rate of surface water mass modification. These modified waters will eventually subduct near the polar front and enter the global overturning circulation. Submesoscale processes modulate the stratification of the mixed layer (ML) and ML properties. Sparse observations in polar regions mean that the role of submesoscale motions in the exchange of properties across the base of the ML is not well understood. The goal of this study is to determine the interplay between sea‐ice melt, surface boundary layer forcing, and submesoscale flows in setting properties of the surface ML in the Antarctic marginal ice zone. High‐resolution observations suggest that fine‐scale lateral fronts arise from either/both mesoscale and submesoscale stirring of sea‐ice meltwater anomalies. The strong salinity‐driven stratification at the base of the ML confines these fronts to the upper ocean, limiting submesoscale vertical fluxes across the ML base. This strong stratification prevents the local subduction of modified waters by submesoscale flows, suggesting that the subduction site that links to the global overturning circulation does not correspond with the location of sea‐ice melt. However, surface‐enhanced fronts increase the potential for Ekman‐driven cross‐frontal flow to modulate the stability of the ML and ML properties. The parameterization of submesoscale processes in coupled‐climate models, particularly those contributing to the Ekman buoyancy flux, may improve the representation of ML heat and freshwater transport in the ice‐impacted Southern Ocean during summer.