Orals: Wed, 30 Apr | Room L2
In winter 2015, Antarctic sea ice underwent a drastic change, transitioning from a period of record high sea ice coverage to a period of record low sea ice coverage. While both an intensified atmospheric circulation and warmer ocean temperatures have been invoked as possible causes for this sea ice regime shift, a detailed process understanding is still missing. Using ~110,000 hydrographic profiles from the seasonal ice zone of the Southern Ocean and atmospheric reanalysis, we reconcile how storm-driven mixing interacted with subsurface warming to change the sea ice state. We observe a gradual thinning of Antarctic Winter Water that acts as a barrier between the warmer deep water and the surface over the period 2005 to 2015 (~2 m per year). This thinning is likely induced by an increased near-surface density stratification in this period, hampering Winter Water formation. As a result, the reservoir of warmer deep water moved closer to the surface and the sea ice. In winter 2015, anomalously strong winds enhanced mixing across the thin Winter Water layer, which broke down stratification over the upper ocean and enhanced connectivity between the ocean mixed layer and deeper interior. Consequently, this reduced stratification allows warmer deep waters to melt sea ice. Our findings thus show that an oceanic preconditioning was a prerequisite for the potential sea ice regime shift that was ultimately triggered by strong storm-driven mixing in 2015.
How to cite: Spira, T., du Plessis, M., Haumann, A., Giddy, I., Silvano, A., Narayanan, A., and Swart, S.: Thinning of Antarctic Winter Water preconditions recent storm-triggered Antarctic sea ice decline, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-426, https://doi.org/10.5194/egusphere-egu25-426, 2025.
The formation of dense water in the Southern Ocean plays a key role in the global overturning circulation of the ocean, and thus affects the distributions of heat, carbon, oxygen and nutrients across the World Ocean. However, the simulation of dense water properties by climate models remains problematic. These models often generate dense water in incorrect locations and for wrong reasons, primarily through deep convection in the center of the Weddell and Ross Seas. We hypothesize that this inability to simulate the formation and fate of dense water stems partly from the erroneous or absent representation of coastal polynyas and their key drivers, particularly landfast ice. A recent study presented a restoring method that accurately represents Antarctic landfast ice and demonstrated its essential role in shaping coastal polynyas and enhancing sea ice production in the NEMO4-SI3 model. Here, building on this study, we investigate the impact of this landfast ice representation on water mass properties simulated by the model over the Antarctic continental shelf. We perform two simulations: one with the landfast ice scheme activated and one with this scheme turned off. A comparison of the simulation results confirms the expected densification of water masses within polynyas when landfast ice is represented. However, the results also reveal unexpected regions of fresher water beneath landfast ice, which influence the polynya dynamics downstream. On a circumpolar scale, incorporating landfast ice enhances the model's agreement with observations, particularly in terms of bottom salinity, temperature and mixed layer depth. Notably, the mixed layer depth undergoes significant changes, which in turn affect the Southern Ocean's coastal dynamics and lead to enhanced ice shelf melting. Overall, representing landfast ice improves the simulation of dense water formation and shelf ocean dynamics, thereby advancing our understanding of key physical processes in these critical regions.
How to cite: Pirlet, N., Fichefet, T., Vancoppenolle, M., and de Lavergne, C.: Impact of a representation of Antarctic landfast ice on the shelf water properties simulated by NEMO4-SI³, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8172, https://doi.org/10.5194/egusphere-egu25-8172, 2025.
Antarctic Bottom Water (AABW) is formed in select locations around the Antarctic margin, filling the bottom 40% of the world’s ocean and circulating heat, carbon and nutrients throughout all basins. Recent observations suggest that almost half of AABW is formed in the western Weddell Sea and that since 1992, its formation has decreased by around 40%. A combination of anthropogenic warming, through the addition of freshwater from melting glaciers and ice shelves, and natural climate variability, is thought to have led to this drastic decrease. The Interdecadal Pacific Oscillation (IPO), known to exhibit teleconnections to the Weddell Sea, has been proposed as responsible for some of this decrease. However, it remains unclear to what extent recent shifts in the IPO have influenced AABW formation because limited observations include the impact of all natural and anthropogenic climate drivers and not just the IPO. Here we use the 1/10° global ocean-sea ice model ACCESS-OM2 to simulate the isolated impact of the IPO on Weddell Sea dense shelf and bottom water formation. We find indications that southward wind anomalies associated with a negative IPO phase push sea ice towards the coastline, prevent polynyas from opening and have reduce dense shelf and bottom water formation. In the Ross Sea we see the opposite, with more dense shelf water formation during negative IPO phases compared to positive phases, especially for the highest density ranges. This indicates that during IPO phase shifts, Weddell Sea AABW changes might be compensated by changes in the Ross Sea. These findings have implications for interpreting decadal-scale variability in dense shelf and AABW production and its impacts on the global ocean circulation under a rapidly warming climate.
How to cite: Huguenin, M., Ryan, S., Ummenhofer, C., and England, M.: Linking the recent decrease in Weddell Sea dense shelf water formation to shifts in the Interdecadal Pacific Oscillation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-462, https://doi.org/10.5194/egusphere-egu25-462, 2025.
The Southern Ocean plays a crucial role in global climate regulation through its influence on the oceanic and atmospheric circulation. Due to the very cold temperatures in the polar Southern Ocean, salinity exerts a fundamental control on vertical mixing, water mass renewal, and the global overturning circulation. Thus, understanding the causes and consequences of salinity changes in the Southern Ocean is not only essential for understanding changes in regional climate dynamics but also implications the changes have on the global climate system. We here investigate spatial and temporal changes in the salinity of Antarctic Bottom Water (AABW) from historic hydrographic data to understand the coupled system of freshwater inputs and salinity changes in the Southern Ocean. Using a novel database of seawater isotopes and noble gases as tracers, combined with in-situ salinity measurements, we identify freshwater sources in AABW within the Weddell Sea, and their changes over time. We find that AABW has freshened and become enriched in δ18O from 1995 to 2016, suggesting a coupled relationship between reduced sea ice export from the Weddell Sea continental shelf, increased sea ice melting, and reduced basal melting of the adjacent Filchner-Ronne Ice Shelf. The observed decline in meteoric water contributions, including basal melt, further supports this inference, suggesting that declines in sea ice export from the continental shelf significantly impact AABW formation and export.
How to cite: Robertson, E., Haumann, A., and Meredith, M.: Identifying Changes in Ice-Ocean-Atmosphere Fluxes in Antarctic Bottom Water, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1904, https://doi.org/10.5194/egusphere-egu25-1904, 2025.
The ice shelves in Amundsen Sea are experiencing a rapid melting phase, largely due to the intrusion of warm Circumpolar Deep Water (CDW) from off the continental shelf. At intrusion sites on the continental slope, the CDW, along with an eastward undercurrent, breaks the Taylor–Proudman theory, causing southward cross-isopycnal intrusion. To explore the mechanisms of the intrusion, we developed a coupled ocean-sea ice-ice shelf numerical model for the Ross Sea-Amundsen Sea system, reconstructing the circulations and simulating the CDW intrusion. The vorticity budget along the continental shelf break of Amundsen Sea is examined using the depth-averaged vorticity budget equation based on the model’s outputs. Results show that the advection of planetary vorticity (APV) and the joint effect of baroclinicity and relief (JEBAR) dominate the vorticity balance at the CDW intrusion sites on the shelf break. The intensity and vertical structure of the eastward undercurrent upstream significantly affect the density structure in the downstream intrusion area, promoting the JEBAR effect. The velocity of the eastward undercurrent is linked to the local wind field. We find that stronger eastward undercurrent speeds are associated with stronger westerly winds and weaker wind stress curl. Westerly winds can drive undercurrents via modifying the meridional sea surface altitude gradient, while wind stress curl reduces the undercurrent by weakening the strength of the continental slope front, which represents the wind field's own internal constraints on the undercurrent. Stronger negative wind stress curl in the Amundsen Sea could drive a stronger geostrophic component of Sverdrup transport under the Ekman layer, which may compress local isopycnals to alter the undercurrent on a seasonal timescale.
How to cite: Li, Z., Wang, C., and Zhou, M.: Mechanisms of wind-field regulation of eastward undercurrents in the Amundsen Sea, West Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4674, https://doi.org/10.5194/egusphere-egu25-4674, 2025.
Transport of warm Circumpolar Deep Water (CDW) into ice shelf cavities is known to be a primary source of heat driving Antarctic Ice Sheet mass loss. This CDW originates in the open ocean, and thus, basal melt rate variability is often linked to wind-driven fluctuations in cross-shelf CDW transport. While cross-shelf heat fluxes are certainly an important part of the story, less focus has been placed on the offshore processes that bring CDW from the open ocean to the shelf break. Here, we use in situ data from profiling floats in combination with Lagrangian particle release experiments in an ocean model to investigate the pathways by which CDW moves toward the continental slope, which is a necessary precursor to the cross-shelf exchange that has been studied in more depth. Observations and models suggest that CDW transport exhibits considerable spatial heterogeneity in the form of concentrated pathways linked to bathymetric features, both on- and off- shore of the continental slope. This suggests that pathways to the shelf break are characterized by distinct timescales and varying degrees of water mass transformation across different sectors. In addition, temporal variability on mesoscale, seasonal, and interannual timescales is present. This is potentially important context through which to understand regional and long-term variations in continental shelf heat content and ice shelf basal melt, which in turn, has implications for future sea level rise.
How to cite: Prend, C., Girton, J., MacGilchrist, G., and Thompson, A.: Remote pathways of ocean heat transport toward the Antarctic Ice Sheet, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7499, https://doi.org/10.5194/egusphere-egu25-7499, 2025.
The cavity underneath the second largest ice shelf in Antarctica, the Filchner-Ronne Ice Shelf, could flip under strong climate warming from its current 'cold' state into a 'warm' state (Hellmer et al., 2012). Numerical models show that this regime shift occurs relatively abrupt, within a decade, with sub-shelf melt rates increasing 21-fold (Naughten et al., 2021). The increase in melting will reduce the ice shelfs buttressing capacity, thereby driving grounded ice loss, and a contribution to sea-level rise. Moreover, changes in sub-shelf melting, and the cavity geometry, in turn, can influence the ocean circulation, creating feedbacks that only emerge when considering the ice and ocean systems together. It is unclear, how these feedbacks influence the regime shift and subsequent evolution of the system, as well as a potential reversibility of the cavity. Here we run regional, numerical simulations of the coupled ice sheet and ocean system to investigate the role of ice-ocean feedbacks on the ocean regime shift, its reversibility, and the impact on ice sheet dynamics. We find that while sub-shelf melt rates increase only half as much as in the coupled system due to the geometric changes, the feedbacks do not influence a reversibility of the regime shift that we find in our simulations. Importantly, the reversal occurs more gradual than the 'cold' to 'warm' flip, and meanwhile the ice sheet continues losing ice and retreating. Our results imply that melt rate projections are ideally conducted in a coupled system, however, the regime shift and reversal of Filchner-Ronne cavity appears to be controlled by local atmospheric conditions, and is not qualitatively influenced by ice-ocean feedbacks.
How to cite: Reese, R., De Rydt, J., and Naughten, K.: Regime shift of Filchner-Ronne Ice Shelf cavity remains reversible , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17058, https://doi.org/10.5194/egusphere-egu25-17058, 2025.
Freshwater hosing experiments are a widely used tool for understanding the impacts of Antarctic ice shelf basal melting on the Southern Ocean and global climate. Most existing coupled climate models lack the necessary physics to explicitly simulate basal melting. Therefore, freshwater anomalies have to be imposed as a proxy. Previous studies have employed diverse freshwater scenarios and application methods. In this study, we explore variations in the application and representation of meltwater anomalies around Antarctica. We compare simulations where freshwater anomalies are introduced at the ocean surface over different spatial extents and at specified depths along the continental slope for a more realistic representation of plume dynamics resulting from basal melting. Additionally, we investigate ocean heat fluxes variability when accounting for the latent heat of fusion required to melt the ice. It is possible to observe similarities and differences in ocean responses depending on the methodology used to impose the freshwater anomaly. Surface application scenarios tend to exhibit more diffuse impacts on ocean stratification and circulation, while depth-specific applications lead to localized but more intense changes in water mass transformation. Accounting for latent heat can introduce further complexity, altering the thermal structure and influencing buoyancy-driven dynamics. By comparing these approaches, we want to highlight the sensitivity of simulated ocean dynamics to the spatial and physical representation of meltwater inputs. Accurately parameterizing ice-ocean interactions in models is necessary to improve the reliability of projections regarding Antarctic contributions to sea level rise and global climate variability.
How to cite: Zapponini, M., Sidorenko, D., Scholz, P., Semmler, T., Streffing, J., and Jung, T.: The role of Antarctic basal meltwater , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4705, https://doi.org/10.5194/egusphere-egu25-4705, 2025.
Observations beneath ice-covered oceans and within ice-shelf cavities are central to understanding the ocean-ice interactions that influence ice-shelf stability, contribute to global sea-level change, and shape large-scale ocean circulation patterns. However, direct observations in these regions, particularly those capturing wintertime conditions, remain scarce due to the logistical challenges posed by persistent ice cover. Emerging autonomous technologies and new applications now offer opportunities to address these observational gaps.
Since 2020, we have deployed 20 unconventionally programmed Argo floats in key regions of the Ross Sea, including the Terra Nova Bay and Ross Ice Shelf polynyas, the critically under-sampled Eastern Gate, and along the Ross Ice Shelf front. These floats provide year-round thermohaline and biochemical measurements, which, among other capabilities, allow for the quantification of water mass transformations, sub-ice dynamics, and key processes such as the production of High Salinity Shelf Water, a precursor to Antarctic Bottom Water. This represents a significant advancement, as previous studies have largely relied on summer ship-based or satellite-derived observations, which fail to capture the full seasonal cycle.
Futhermore, with measurements from three Argo floats operating for several months beneath the Ross Ice Shelf, we directly observed and quantified processes that had previously only been hypothesized. These include the intrusion of seasonally warmed Antarctic Surface Water, identified as a primary driver of frontal and basal melting, along with its associated effects on ocean heat content and basal melt rates, as well as the outflow of Ice Shelf Water, the coldest ocean water in the world.
Building on the insights gained over the past 4 years, we argue that broadening the deployment of grounded-mode Argo floats across Antarctica can provide a unique understanding of ocean-ice interactions. By enabling continuous, autonomous measurements — even in winter and under ice — this approach can improve our capacity to quantify key processes, such as the lateral and vertical extent of shelf water production and the mechanisms driving basal melt. Our results demonstrate that Argo floats offer direct evidence of how heat absorbed at the surface is transported into ice-shelf cavities, contributing to basal melting and reshaping our understanding of water mass formation processes in coastal polynyas. Expanding the float network would enhance our ability to detect interannual variability, characterize longer-term trends, and reduce uncertainties in ice-shelf and sea-level projections, ultimately supporting more accurate climate model predictions for these critical polar environments.
How to cite: Falco, P., Krauzig, N., Castagno, P., Garzia, A., Martellucci, R., Cotroneo, Y., Flocco, D., Menna, M., Pirro, A., Mauri, E., Memmola, F., Solidoro, C., Pacciaroni, M., Notarstefano, G., Budillon, G., and Zambianchi, E.: Winter thermohaline evolution along andbelow the Ross Ice Shelf, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2291, https://doi.org/10.5194/egusphere-egu25-2291, 2025.
The melting of Antarctic ice shelves is driven by heat fluxes from the underlying ocean to the ice. The relationship between basal heat fluxes and ocean conditions is an active topic of research, as current state-of-the-art parameterizations perform relatively poorly in all but the fully-turbulent well-mixed regimea. Indeed, discrepancies between observed and predicted melt rates under certain ice-shelves have been detectedb. Here, we perform laboratory experiments of tabular ice cuboids melting in salty water. We aim to improve our understanding of the basal melting of ice shelves in the diffusive-convective regime, for which there is currently no parameterizationc. To this end, we investigate the melting rate and underlying fluid dynamics over a broad range of water salinity and temperature, without any external forcing. Our work uniquely complements field observations, which are difficult and sparse, and simulations, which most often approximate the dynamics for computational expediency.
We use a meter-scale tank, which we fill with saltwater and place a freshwater tabular ice cuboid to melt on top. A bottom heating plate is used to maintain the bottom saltwater temperature at a prescribed value and the setup is placed in a cold room. The depth-dependent seawater temperature, salinity, currents, and the melt rate are explored for different bottom water temperatures and initial salinity values. We use a moving temperature and salinity sensor, PIV data and shadowgraphy images of the retreating ice-water front to provide a relatively comprehensive data set from which we derive a mapping between the average melting rate and the flow statistics (kinetic energy density, dissipation rate, temperature gradient) of interest to polar oceanography.
We find that temperature and salinity vertical gradients in the system can create a layered system, depending on the conditions. In particular, we observe the formation of a freshwater layer insulating the ice plate, and slowing the melting, at relatively low temperature. When the bottom temperature is relatively large, the two-layer organisation disappears as convection becomes vigorous enough to penetrate and mix the freshwater layer with the ambient.
References :
a - Malyarenko, A., Wells, A. J., Langhorne, P. J., Robinson, N. J., Williams, M. J., \& Nicholls, K. W. (2020). A synthesis of thermodynamic ablation at ice–ocean interfaces from theory, observations and models. Ocean Modelling, 154, 101692.
b - Kimura, S., Nicholls, K. W., \& Venables, E. (2015). Estimation of ice shelf melt rate in the presence of a thermohaline staircase. Journal of Physical Oceanography, 45(1), 133-148.
c - Rosevear, M. G., Gayen, B., \& Galton-Fenzi, B. K. (2022). Regimes and transitions in the basal melting of Antarctic ice shelves. Journal of Physical Oceanography, 52(10), 2589-2608.
How to cite: Collin, B., Couston, L.-A., Joubaud, S., and Volk, R.: Ice melting in saltwater: laboratory experiments in the diffusive-convective regime, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6361, https://doi.org/10.5194/egusphere-egu25-6361, 2025.
Posters on site: Wed, 30 Apr, 16:15–18:00 | Hall X5
The Antarctic sea ice extent has displayed two large drops over the past 65 years, a small one at the end of the 1970s and a more substantial one after 2016. The atmospheric forcing provided a dominant contribution to those drops. Wind changes strongly control the spatial pattern of sea ice reduction, especially in the Pacific sector of the Southern Ocean and in the western Weddell Sea. The relationship between the winds and sea ice seems less direct in the east Antarctic sector (i.e., mainly in the eastern Weddell Sea and in the Indian sectors), where oceanic processes are expected to play a larger role. This contribution of oceanic processes in the sea ice reduction in the east Antarctic sector is estimated here using simulations performed with the ocean-sea-ice model NEMO, substantiated by observations. The simulations cover the period 1958-2023, driven by both the ERA5 reanalysis and a forcing derived from a recent reconstruction that displays more homogenous time series than ERA5 over the whole period. Observations are used at first to evaluate the model behaviour over the past decades when the data network is denser. The simulations, then, allow the analysis to be extended over the past 65 years, to estimate the changes in oceanic heat transport, heat content and oceanic heat flux towards the sea ice. Several simulations with NEMO are compared, using different initial conditions and parameters influencing mixing to estimate how they influence the sea ice extent variations and thus to disentangle the role of different oceanic processes in the observed changes.
How to cite: Goosse, H., Francis, F., Mezzina, B., Richaud, B., and Dalaiden, Q.: Contribution of ocean processes to the drops in Antarctic sea ice extent at the end of the 1970s and after 2016, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2487, https://doi.org/10.5194/egusphere-egu25-2487, 2025.
The Southern Ocean is the source of the Antarctic Bottom Water (AABW), a dense water mass that occupies 60 % of the global ocean. AABW is formed in different places around Antarctica as a mixture of dense shelf waters (DSW) and Circumpolar Deep Water (CDW).
CDW occupies the deep Southern Ocean at depths between 200-1500 m, and is considerably warmer (temperature higher than 0°C) than the water masses found at similar depths over the continental shelf. CDW is carried eastward around the continent by the Antarctic Circumpolar Current (ACC) but close to the shelf break it flows westward within the Antarctic Slope Current (ASC).
The ASC is a quasi-circumpolar feature that starts in the Bellingshausen Sea and vanishes next to the Antarctic Peninsula in the Weddell Sea. The ASC is connected to the flooding of the continental shelf with CDW modified by the interaction with local surface waters (modified CDW – mCDW) and the presence of mCDW over the continental shelf can affect the sea ice formation and represents a risk for the ice shelves fringing the Antarctic continent.
The presence of warmer mCDW and freshwater resulting from the ice shelf melt reduce the sea ice production rates, a crucial part of DSW formation. When the ocean freezes salt (brine) is rejected into the water increasing local density which creates an instability and can trigger convection and produce DSW, when less sea ice is formed the deep convection potential is reduced; the DSW formed this way is termed HSSW.
HSSW can flow offshore and slide down the continental slope mixing with ambient waters along its path until it reaches the equilibrium depth as AABW. HSSW can also flow underneath the ice shelf cavity and reach the grounding line where the freezing temperature is lower than at the surface due to the pressure effect. There, it causes melting and the mixture with glacial melt water generates the supercooled (colder than surface freezing temperature) and slightly fresher Ice Shelf Water (ISW). ISW can mix with ambient waters forming a dense water type that sinks when it reaches the continental slope producing another type of AABW.
When the DSW leaves the continental shelf they are carried westward by the ASC together with CDW and lighter waters influenced by the ice shelf melt water. Part of the waters on the continental shelf are advected in the same direction by the Antarctic Coastal Current (ACoC), a westward flow observed in various sites around Antarctica. The waters transported by the ACoC and ASC can have an impact on the neighbouring basins.
Starting from a Finite volumE Sea Ice-Ocean Model (FESOM2) that is able to reproduce the above-mentioned key characteristics around the Antarctic continent we prepared 3 experiments to investigate the effects of the ice shelves upstream from 3 key DSW formation sites: the Filchner-Ronne Ice Shelf, Amery Ice Shelf and Ross Ice Shelf. We will present the experiment design and preliminary results.
How to cite: van Caspel, M., Timmermann, R., and Janout, M.: How upstream ice shelves affect Dense Water formation: Insights from FESOM2 Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18340, https://doi.org/10.5194/egusphere-egu25-18340, 2025.
Importance of the Weddell Sea for the global ocean circulation is irrefutable as it is a hotspot of dense water formation, a precursor of the Antarctic Bottom Water (AABW), which dominates the abyssal ocean ventilation. Furthermore, the region hosts the largest by volume floating ice shelf, Filchner-Ronne Ice Shelf (FRIS), which is vulnerable to episodic Warm Deep Water (WDW) inflows that induce basal melting at the ice shelf front and within its cavities. The resulting meltwater enters the system as freshwater forcing and alters local density gradients and water mass distribution, ultimately contributing to global sea level rise and potentially disrupting the global overturning circulation. Hence, knowledge about the seasonality and magnitude of the WDW inflows carries significance both locally and globally.
In 2017 and 2018, the anomalously warm inflows were observed yet there is still no agreement on the mechanism for their trigger. The on-shore propagation of WDW inherently depends on several factors, such as e.g circulation at the continental margins, the seasonal cycle of sea ice, the thermocline depth, position and magnitude of the Antarctic Slope Front (ASF). The intricate interplay between these factors yields a fairly complex system, which is difficult to unravel with the scarce available observations. Albeit the Weddell sea remains fairly undersampled up to this day, significant efforts are being undertaken to supplement data gaps with numerical modeling.
The aim of my research is to shed light on seasonal and interannual variability of the coastal circulation upstream of the FRIS and its sensitivity to external forcings, using existing in-situ observations, reanalyses and the high-resolution eddy-permitting Finite-Element/Volume Sea Ice-Ocean Model (FESOM), in order to investigate the impact of wind and buoyancy forcing. Preliminary results indicate that the maximum thermocline depth exhibits a distinct seasonal cycle which is not consistent with sea ice formation, as previously speculated. Apparently, it reaches its deepest position in the Dronning Maud Land region in austral summer, then downstream at Kapp Norvegia in austral autumn and further downstream next to the FRIS – in austral winter, which rather implies generation and along-coast propagation of a large-scale baroclinic signal and further accentuates the importance of along-coast connectivity. Furthermore, the character of thermocline also needs to be considered. For instance, in 2017-2018, when the anomalous warm inflows were observed at the FRIS, thermocline thickness was increased rather than its depth, which raises the question of necessary and sufficient conditions for said inflows to occur.
How to cite: Volkova, V., Janout, M., and Kanzow, T.: Upstream governors of the Warm Deep Water inflows towards the Filchner-Ronne Ice Shelf, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5829, https://doi.org/10.5194/egusphere-egu25-5829, 2025.
Coupled with the Antarctic Slope Front (ASF), the Antarctic Slope Current (ASC) encircles Antarctica and has variable structures. Two types of the ASC/ASF have been identified in the Ross Sea. Yet, the spatial characteristics of the ASC/ASF have not been depicted in detail, and the transition zone between the two different types is still unclear. Using an eddy-permitting coupled regional ocean-sea ice-ice shelf model, we aim to investigate the spatial characteristics and energy sources of the ASC/ASF in the Ross Sea. Based on the simulated results, three distinct structures of the ASC/ASF have been identified in three regimes from east to west: (i) in Regime I, the ASC is characterized by a westward flow in the upper layer and an eastward countercurrent in the lower layer; (ii) in Regime II, the undercurrent of the ASC reverses to the west and features a bottom intensification; (iii) in Regime III, the ASC in the upper layer is eastward, and the westward undercurrent still occupies the lower layer. By analyzing the momentum budget of the ASC, we quantified the respective contributions of barotropic and baroclinic pressure terms in determining the structure of the ASC/ASF. Furthermore, by analyzing the Mean Kinetic Energy budget of the ASC, we found that the Mean Available Potential Energy plays an important role in converting energy to the Mean Kinetic Energy, indicating that the maintenance of the ASC is closely associated with the available potential energy released from the ASF.
How to cite: Liu, Y., Liu, C., Wang, Z., Yan, L., Han, X., Liu, K., Haigh, M., Xia, Y., Zeng, J., Li, X., and Liang, X.: Spatial Characteristics and Dynamic Mechanisms of the Antarctic Slope Current in the Ross Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4882, https://doi.org/10.5194/egusphere-egu25-4882, 2025.
Subsurface warming has been identified as a likely causal factor for the sustained low Southern Ocean sea ice extent in recent years. Subsurface-to-surface heat transport is impacted by the water mass structure of the water column and the depth of vertical mixing, which can in turn be altered by sea ice processes. These interactions create potential feedback effects that remain insufficiently explored in the context of the recent low Southern Ocean sea ice extent. In this study, we investigate interaction mechanisms and feedbacks between Southern Ocean sea ice and the underlying water column through a simple one-column box model, focusing on water mass structure and properties. The model represents the surface mixed layer, subsurface Winter Water, and the Upper Circumpolar Deep Water as distinct ocean boxes. A fourth box represents sea ice when present, interacting with the mixed layer through heat and salt exchange. The evolving mixed layer depth is calculated using a mixed layer model, with subsurface Winter Water formed when the mixed layer shoals and re-entrained when the mixed layer deepens. In this contribution, we present the box model framework and discuss preliminary insights, as well as challenges encountered during the model development process.
How to cite: Schramm, M. and Haumann, F. A.: Southern Ocean Sea Ice-Ocean Interactions in a Simple Box Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11729, https://doi.org/10.5194/egusphere-egu25-11729, 2025.
Despite the increase in global mean temperature and massive sea ice loss in the Arctic, the Antarctic sea ice extent has not changed significantly throughout reliable satellite records starting in 1979. Long-term trends rather show an increase in the Antarctic sea ice area, resulting in record high anomalies in 2014 and 2015. However, after the moderate expansion in the sea ice extent, a sharp decline occurred in 2016 and has remained low since then. The record Antarctic sea ice loss in recent years may be a sign the region has entered a new regime of low sea ice coverage in a warming world. Meanwhile, Antarctic Bottom Water (AABW), driving the lower limb of the global meridional overturning circulation and ventilating the abyssal ocean interior has warmed and freshened in recent decades, leading to a decrease in AABW formation. Ross Sea shelf water which is responsible for 20–40% of the total AABW production, has experienced the largest freshening. However, repeat hydrographic data have shown that since the mid-2010s the salinity of Ross Sea shelf water has sharply rebounded from the multidecadal freshening trend. Here, it is interesting that the abrupt transition from a high to low state of Antarctic sea ice since the mid-2010s coincides with the onset of the salinity rebound of dense shelf water on the Antarctic continental shelf.
As the planet warms global sea ice has continued to get a lot of attention due to the substantial implications for planetary albedo, ice sheet and ice shelf stability, atmosphere-ocean interactions, cryosphere ecosystems, biogeochemical cycle, and the Southern Ocean freshwater cycle. Particularly, sea ice’s growth and melting play an important role in water mass transformations. Here, we investigate how the rapid decline in the Antarctic sea ice in recent years has contributed to the rebound of shelf water salinity in the Ross Sea, using satellite observations of sea ice, as well as oceanic and atmospheric reanalysis data. Our result shows that despite the rapid decrease in the Antarctic sea ice in recent years, the sea ice formation rate in the Ross Sea continental shelf has increased. During the salinification period since the mid-2010s, local anomalous winds and surface heat flux associated with the remote and large-scale forcing that drive the recent change in the Antarctic sea ice, induced the reduced sea ice cover and larger polynya area on the Ross Sea continental shelf, increasing sea ice formation rate. Furthermore, data-based sea ice budget analysis indicates that due to the anomalous wind forcing, the sea ice has moved to the outer shelf through dynamic processes such as advection and divergence, creating a sustained favorable environment for sea ice formation and brine rejection.
How to cite: Kim, T., Choo, S.-H., Moon, J.-H., Jin, E. K., Kim, D., and Cha, H.: Reversal of freshening trend of Ross Sea shelf water links to an abrupt transition from high to low state of Antarctic sea ice since the mid-2010s, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14844, https://doi.org/10.5194/egusphere-egu25-14844, 2025.
Sea ice extent around the Antarctic exhibits a high level of variability on interannual and longer timescales, characterized by a positive trend since the satellite era and interruptions due to e.g., the emergence of the Maud Rise Polynya in 2016. Given the relatively short period of observational data and the high level of natural variability it has remained challenging to unequivocally identify the anthropogenic fingerprint in Antarctic sea ice. Moreover, to properly study the Antarctic sea-ice and its response to future warming, it is necessary to capture important dynamics, such as polynyas, the Antarctic slope current and coastal leads. Many models within the CMIP6 model portfolio do not even have the spatial resolution to adequately resolve these features. This implies that their Antarctic projections may not be as trustworthy and robust as those for the Arctic Ocean.
In this study, we employ the high-resolution OpenIFS-FESOM (AWI-CM3) coupled general circulation (nominally 31 km atmosphere and 4-25 km ocean resolutions) to investigate the Antarctic sea ice response to greenhouse warming, following a SSP5-8.5 greenhouse gas emission scenario. Our simulation exhibits a sudden decline of Antarctic sea ice in the Weddell Sea (WS) which can be explained by a combination of physical processes that involve continued strengthening of westerlies, increasing of horizontal density and pressure gradients, intensifying of atmosphere-ocean momentum transfer due to sea ice decline, a spin-up of the cyclonic gyre and westward current and corresponding vertical and horizontal supply of heat into the Weddell Sea. The resulting decrease of sea ice further leads to heat accumulation in austral summer due to the absorption of short-wave radiation, which can further weaken winter sea ice extent and intensify the momentum transfer and associated heat transport into the Weddell Sea.
Our study highlights the relevance of positive atmosphere-sea ice-ocean feedback in triggering the abrupt decline in Antarctic sea ice.
How to cite: Kim, D.-W., Zapponini, M., Sharma, S., Jung, T., Kim, M.-H., Koldunov, N., Puthiyaveettil, N., Sidorenko, D., Streffing, J., Timmermann, A., Semmler, T., and Park, W.: Positive low cloud feedback accelerates abrupt Southern Ocean sea-ice decline in high-resolution global climate model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14845, https://doi.org/10.5194/egusphere-egu25-14845, 2025.
Icebergs, formed by the calving of the Antarctic ice sheet, are among the most emblematic natural features of the polar regions. Their presence in the Southern Ocean, an essential carbon and heat sink, raises important questions about their role in the future evolution of the ocean and climate. As the mass balance of the Antarctic ice sheet continues to decline, it is crucial to better understand how iceberg-sea ice interactions influence ocean dynamics and sea ice variability in the Southern Ocean.
In this study, we use the NEMO version 4.2.2 ocean model coupled to the ICB module (Iceberg) to investigate these interactions. Our approach has two main objectives: (1) We assess the impact of icebergs on the ocean and sea ice by running two 30-year regional hindcast simulations of the Southern Ocean, one with the ICB module enabled and the other with the module disabled. This allows us to isolate the influence of iceberg dynamics on sea ice concentration and thickness, along with ocean physical properties such as surface temperature and salinity. And (2) by implementing a sea ice locking process in the ICB module we aim to better represent the mechanical interactions between icebergs and sea ice, particularly when icebergs become 'trapped' by thick, highly concentrated sea ice. Sea ice locking has a major impact on the distribution of freshwater fluxes in the ocean, by influencing the trajectory of icebergs.
This study is a first step towards improving our understanding of the coupled iceberg-sea ice-ocean system and its implications for the future evolution of the Southern Ocean in a changing climate.
How to cite: Lemaire, E., Massonnet, F., Fichefet, T., Pirlet, N., Mathiot, P., Marini Marson, J., and Olivé Abelló, A.: Investigating Iceberg–Sea Ice Interactions in the Southern Ocean Using NEMO-ICB, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6705, https://doi.org/10.5194/egusphere-egu25-6705, 2025.
Despite the ongoing decline in Arctic sea ice extent over the past 30 years, an increase had been observed in Antarctic sea ice until 2016. That year, however, the sea ice melt season started unusually early, leading to a decrease in sea ice extent. Since then, this decline has continued, culminating in a record low sea ice extent in 2023. This reduction in sea ice is primarily linked to changes in atmospheric patterns, along with the impact of rising ocean surface temperatures. Additionally, it has been identified that the decrease in Antarctic sea ice extent significantly affects the atmosphere by increasing the surface heat loss from the Southern Ocean. Simultaneously, a continuous thinning of the sea ice has been observed. This decline in ice thickness is expected to lower the drag coefficient between the sea ice and the ocean, which would, in turn, enhance the influence of wind on the flow of sea ice. Notably, increased sea ice flow during the melting season could accelerate the melting process and increase seasonal sea ice variability. We analyzed the changes in the sea ice-ocean drag coefficient using satellite-observed sea ice concentration and thickness, along with reanalysis wind data over the past 20 years. Although regional differences exist, the overall trend indicates a clear decline in the sea ice-ocean drag coefficient.
How to cite: Kim, T.-W., Yang, H., Kim, Y., and Park, J.: Changes in the Sea Ice-Ocean Drag Coefficient Due to the Decrease in Antarctic Sea Ice, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7568, https://doi.org/10.5194/egusphere-egu25-7568, 2025.
To quantify the ocean-driven Antarctic ice mass loss and the subsequent sea level rise, the geophysical modeling community is pushing towards frameworks that fully couple increasingly complex models of atmosphere, ocean, sea ice and ice sheets & shelves. We will present results from an 8km ocean – ice shelf – ice sheet - bathymetry coupled model of Antarctica, based on the frameworks of the Regional Ocean Modelling System and the Parallel Ice Sheet Model.
Our projections for two different global warming trajectories (rcp2.6 & 8.5) suggest that warming of the Antarctic shelf seas diverge between scenarios from the 2050s onward. Our preliminary analysis focuses on the two largest ice shelves, the Ross and the Ronne-Filchner, both currently hosting cold ocean cavities.
Under the rcp8.5 trajectory, episodic warm water hosing over the eastern shelf in the Weddell Sea becomes a permanent feature at mid century, leading to a 1.5 degree increase of water temperature within a decade over the central and the eastern shelf. Basal melt rates of the entire Filchner-Ice-Shelf and the southern Ronne-Ice-Shelf exceed 2 m/yr, which in some areas is a magnitude larger than current rates.
In contrast the Ross-Ice-Shelf appears to remain stable under both climate trajectories. In the rcp8.5 scenario the shelf sea over the north-south stretching banks warms moderately by 0.25-0.5 degrees but this increased heat has no access to the cavity according to our model results.
We will present insight to the mechanisms that drive the sudden warming in the Weddell Sea and construct a hypothesis of why the Ross Ice Shelf appears more protected from Southern Ocean heat.
How to cite: Jendersie, S., Alevropoulos-Borrill, A., Lowry, D., and Golledge, N.: Indications of the Ronne-Filchner-Ice-Shelf becoming host to a warm cavity within the second half of the 21st century under high emission scenario, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20716, https://doi.org/10.5194/egusphere-egu25-20716, 2025.
The increasing release of meltwater from Antarctica represents one of the most profound yet uncertain consequences of global climate change. The lack of interactive ice sheets in state-of-the-art climate models, including those participating in the Coupled Model Intercomparison Project (CMIP6), combined with the inadequate representation of key processes driving ice shelf basal melting, prevents the direct calculation of ice-ocean feedbacks and leaves a high uncertainty on the magnitude and impacts of meltwater discharge. Previous studies that explored meltwater impacts produced partially contradictory findings, largely relied on experiments with single models, had inconsistent experimental designs, and imposed varying freshwater forcing rates. To address these shortcomings, this study employs results from the new "Southern Ocean Freshwater Input from Antarctica” (SOFIA) initiative to assess the effect of meltwater-induced ocean warming on basal melting and potential future Antarctic mass loss. We evaluate the ocean response to meltwater across a suite of 10 CMIP6 models and compare it to future scenarios simulations without additional meltwater (SSP5-8.5), assessing model bias and both meltwater- and global warming-induced anomalies in the Southern Ocean. Applying these anomalies to a regional basal melting parameterization, constrained by a new observational hydrographic climatology, our findings reveal that meltwater feedbacks amplify warming on the continental shelf and enhance ice loss in many sectors around Antarctica. However, in the West Antarctic regions where the greatest ice mass loss was observed in recent years, most models show either cooling or reduced warming on the shelf, hence indicating a negative feedback due to the meltwater input. Consistent with previous studies, we confirm that regional disparities are driven by advection and acceleration of the Antarctic Slope Current. Our results suggest that mass loss from East Antarctica will become increasingly important under future global warming. The meltwater-induced feedback causes an additional 750 Gt/year of ice loss in the multi-model median response to our perturbation experiments. For comparison, observations estimate current anomalous ice shelf loss at approximately 1,000 Gt/year, while SSP5-8.5 simulations, which account for global warming without additional Antarctic meltwater, project an anomalous 3,400 Gt/year of ice loss by the end of the century.
How to cite: Muilwijk, M., Hattermann, T., and Beadling, R. and the SOFIA team: Uncertainty in Future Southern Ocean Warming and Antarctic Ice Shelf Melting Due to Meltwater-Driven Climate Feedbacks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11288, https://doi.org/10.5194/egusphere-egu25-11288, 2025.
Ice shelves in the Amundsen Sea, West Antarctica, are being melted rapidly from below by warm ocean waters, causing sea-level rise. Amundsen Sea oceanography and ice-shelf melting are both subject to long-term (centennial) trends and natural decadal variability. We study the atmospheric drivers of the decadal variability using perturbation experiments in which the mechanical (winds) and thermodynamic atmospheric forcings are applied individually in an ice-ocean model of the Amundsen Sea. We find that the decadal variability is predominantly driven by mechanical forcing of the winds, through impacts on the melting and formation of sea ice. This variability in the sea ice drives variability in the Amundsen Sea undercurrent and the heat fluxes towards the ice shelves, which in turn leads to decadal variability in the melting of the ice shelves. While winds are the primary driver of this variability, it is also found that a significant part of the variability is due to nonlinear effects, and cannot be explained by the individual impacts of either winds or thermodynamics. Our results also highlight how the processes that drive variability differ depending on the timescale (e.g., annual, decadal, centennial) of interest.
How to cite: Haigh, M. and Holland, P.: Decadal variability of ice-shelf melting in the Amundsen Sea driven by winds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2551, https://doi.org/10.5194/egusphere-egu25-2551, 2025.
Subglacial discharge beneath ice shelves is a source of freshwater, and therefore buoyancy, at the grounding line. Being released at depth, it accelerates an ascending plume along the ice-shelf base, enhancing entrainment of ambient waters, and increasing melt rates. By now it is understood that subglacial discharge is a key control on melt rate variability at the majority of Greenland's glaciers. However, its importance in present-day and future Antarctic melt rates is less clear. To address this point, we use the Energy Exascale Earth System Model (E3SM) and investigate the effects of subglacial discharge addition in both idealized setups and realistic, global, sea-ice ocean coupled configurations. For realistic Antarctic configurations, we use the subglacial hydrology model from the MALI ice-sheet model run at 4-20 km resolution to calculate steady state subglacial discharge across the grounding line under historical ice-sheet conditions. This subglacial meltwater discharge is implemented as a grounding line freshwater flux in MPAS-Ocean, the ocean component of E3SM.
Results from idealized, rotating ice-shelf configurations show a stronger melt rate dependence on discharge than in previously studied non-rotating Greenland-like fjord scenarios. We also find that the melt-rate response is strongly sensitive to the location of the discharge along the grounding line; the efficiency of subglacial discharge, in terms of total melt-rate increase, grows with distance from the area where meltwater accumulates due to rotational effects. The analysis of subglacial discharge effects in realistic, global configurations focuses on ice-shelf melt rates, cavity circulation, continental shelf properties, and sea-ice conditions around Antarctica. Results from realistic, global configurations indicate that, although some regions are more affected than others, overall the present-day levels of subglacial discharge result only in relatively minor changes in ice-shelf melt rates and continental shelf properties. Significant oceanic changes would require at least an order of magnitude stronger subglacial discharge than present-day estimates.
How to cite: Vaňková, I., Asay-Davis, X., Branecky Begeman, C., Comeau, D., Hager, A., Hoffman, M., Price, S., and Wolfe, J.: Subglacial discharge effects on ice-shelf basal melting in Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14740, https://doi.org/10.5194/egusphere-egu25-14740, 2025.
Hydrographic surveys in the Dotson Trough in the Amundsen Sea in January-February 2022 using a fleet of ocean gliders reveal deep (~400 m) isolated lenses of cold, dense water. The water contained within these lenses is colder, saltier, deeper and denser than the typical regional Winter Water (temperature-minimum) layer that appears above the lenses at about 200 m in summer. The lenses occur in the stratified layer influenced by the warm, salty and dense modified Circumpolar Deep Water below, that provides heat to the underside of the vulnerable ice shelves in this region. We do not have evidence of the lenses travelling beneath the ice shelves, but they are at about the right depth to do so. If they did, they would insulate the base of the ice shelf from the warmer water below, helping to prevent basal melting that is prominent in this region.
The lenses are colder (close to the local freezing point of seawater) than the surrounding waters at the same depths and densities, and fresher than the surrounding water at the same densities. Their dissolved oxygen concentration is similar to that of Winter Water and their pH is lower than Winter Water, but both properties are increased compared with surrounding water at the same depth. Thus, they provide a mechanism to sequester carbon and oxygen deeper than typical Winter Water formation can achieve.
We explore possible formation mechanisms for the lenses and the water mass they contain, using wintertime profiles of temperature and salinity obtained from tags on seals. One possibility is that local chimneys of deep convection succeed in penetrating sporadically to 400 m, and are subsequently capped by other water masses. We do not find convincing evidence to support this. Our favoured hypothesis is that the shallower regions (less than 500 m water depth) surrounding the Dotson Trough (e.g. Bear and Martin Peninsulas) host enhanced surface heat loss and subsequently intense brine rejection during sea ice formation, leading to very cold, dense water in winter. The seal tag profiles do indeed show wintertime water masses in these shallower regions of the same temperature, salinity and density as the lenses. We speculate that this water spills off into the deeper water sporadically (perhaps akin to the formation mechanism for Meddies). They would then populate the layer at which they are neutrally buoyant, beneath the typical Winter Water and invading the uppermost layers of modified Circumpolar Deep Water.
How to cite: Heywood, K. J., Pickup, D., Bakker, D., Glassup, F., and Webber, B.: Cold deep lenses in the Dotson Trough, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12192, https://doi.org/10.5194/egusphere-egu25-12192, 2025.
