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Historical and future influences on zonal wind anomalies. Panels show the internally generated, total externally forced, and GHG- and ozone-forced contributions to wind changes in the three regions shown in Fig. 2. Future changes are shown under the strong-forcing RCP8.5 scenario. All panels show annual-mean time series (thin lines). Panels (a), (c), and (e) also show the same time series after a 13-year running mean (thick lines) to highlight the interdecadal evolution of the responses. Panels (b), (d), and (f) also show historical and future trends, plotted solid if the trend is significant at the 95 % confidence level (see text) and dashed otherwise. Ozone is plotted as the ozone response before 2000 and the non-GHG response afterwards. Internally generated anomalies are plotted relative to 1979–2005, as in Fig. 4.
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Ocean-driven ice loss from the West Antarctic Ice Sheet is a significant contributor to sea-level rise. Recent ocean variability in the Amundsen Sea is controlled by near-surface winds. We combine palaeoclimate reconstructions and climate model simulations to understand past and future influences on Amundsen Sea winds from anthropogenic forcing and...
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Antarctic sea ice extent has seen a slight increase over recent decades, yet since 2016, it has undergone a sharp decline, reaching record lows. While the precise impact of anthropogenic forcing remains uncertain, natural fluctuations have been shown to be important for this variability. Our study employs a series of coupled model experiments, reve...
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... The speed variability is indeed superimposed on a longer-term speed-up. This may be driven by a centennial trend of ocean warming on the continental shelf (Naughten et al., 2022b), likely driven by a trend in winds over the continental shelf break (Holland et al., , 2022, or it could be driven by internal ice dynamic feedbacks (such as MISI) (Reed et al., 2024) or ice-ocean feedbacks, such as changes in cavity circulation due to ice shelf thinning and grounding line retreat (Bradley et al., 2022). ...
The ice streams feeding the Dotson and Crosson ice shelves are some of the fastest changing in West Antarctica. We use satellite observations to measure the change in ice speed and flow direction on eight ice streams in the Pope, Smith, and Kohler region of West Antarctica from 2005 to 2022. Seven ice streams have sped up at the grounding line, with the largest increase in ice speed at Smith West Glacier (87 %), whilst Kohler West Glacier has slowed by 10 %. We observe progressive redirection of ice flowlines from Kohler West into the more rapidly thinning and accelerating Kohler East Glacier, resulting in the deceleration of Kohler West Glacier and eastward migration of the ice divide between Dotson and Crosson ice shelves. These observations reveal previously undocumented impacts of spatially varying ice speed and thickness changes on flow direction and ice flux into downstream ice shelves, which may influence ice shelf and ice sheet mass change during the 21st century.
... Sea ice formation is relatively weak on the warm shelves but causes some water mass transformation during the winter months. As sea ice is formed, the upper layers of the ocean are mixed, and lose their heat to the atmosphere and form WW, which can modulate ice shelf melt rates at interannual timescale (Jenkins et al., 2018;Holland et al., 2022). ...
... Model simulations suggest that the Amundsen Sea warmed in response to changes in wind patterns over the 20th century (Gómez-Valdivia et al., 2023;Naughten et al., 2022). Ocean warming would explain why the WAIS failed to recover from a natural climate anomaly, and why increased basal melting continues to the present day (Holland et al., 2022). However, observations of water mass properties on the continental shelf are sparse in both space and time (Schmidtko et al., 2014). ...
... This took place in regions where grounded ice is highly sensitive to ice shelf and grounding line change, thereby leading to accelerated discharge of grounded ice into the ocean 11,12 . These changes have been driven by enhanced incursion of modified Circumpolar Deep Water (mCDW) onto the continental shelf, increasing ocean melting of ice shelves in the Amundsen Sea Sector, through some combination of a gradual anthropogenic warming during the 20th century and historical warm anomalies triggering ongoing change 13,14 . Due to the topography of the bed under the WAIS, and the Amundsen Sea Sector in particular, current changes may reflect an ongoing Marine Ice Sheet Instability and self-sustained, irreversible, retreat [15][16][17][18][19][20] . ...
The retreat of the Antarctic Ice Sheet is conventionally attributed to increased ocean melting of ice shelves, potentially enhanced by internal instability from grounding lines near retrograde bed slopes. Ocean melting is enhanced by increased intrusion of modified Circumpolar Deep Water (mCDW) into ice shelf cavities. Upwelling from the release of subglacial meltwater can enhance mCDW’s melting ability, though its efficacy is not well understood and is not represented in current ice sheet loss projections. Here we quantify this process during an exceptional subglacial lake drainage event under Thwaites Glacier. We found that the buoyant plume from the subglacial discharge temporarily doubled the rate of ocean melting under Thwaites, thinning the ice shelf. These events likely contributed to Thwaites’ rapid thinning and grounding line retreat during that period. However, simulations and observations indicate that a steady subglacial water release would more efficiently enhance basal melt rates at Thwaites, with melt rate increasing like the square root of the subglacial discharge. Thus, it remains unclear whether increased subglacial flooding events provide a stabilizing influence on West Antarctic ice loss by reducing the impact of subglacial water on ocean melting, or a destabilizing influence by triggering rapid changes at the grounding zone.
... A part of the internal climate variability can be characterised as modes such as the El Niño-Southern Oscillation and Interdecadal Pacific Oscillation, which have remote connections with the Amundsen Sea Low (ASL; Holland et al., 2022;Dalaiden et al., 2024). The ASL is a low-pressure system located over the South Pacific sector of the Southern Ocean, which generates decadal wind anomalies that affect the oceanic undercurrent along the continental slope, thereby modulating the amount of warm water flowing towards the ice shelves of the Amundsen Sea Embayment (Silvano et al., 2022). ...
Identifying and quantifying irreducible and reducible uncertainties in the Antarctic Ice Sheet (AIS) response to future climate change is essential for guiding mitigation and adaptation policy decision. However, the impact of the irreducible internal climate variability, resulting from processes intrinsic to the climate system, remains poorly understood and quantified. Here, we characterise both the atmospheric and oceanic internal climate variability in a selection of three Coupled Model Intercomparison Project Phase 6 (CMIP6) models (UKESM1-0-LL, IPSL-CM6A-LR, and MPI-ESM1.2-HR) and estimate their impact on the Antarctic contribution to sea-level change over the 21st century under the SSP2-4.5 scenario. To achieve this, we use a standalone ice-sheet model driven by the ocean through parameterised basal melting and by the atmosphere through emulated surface mass balance estimates. The atmospheric component of internal climate variability in Antarctica has a similar amplitude in the three CMIP6 models. In contrast, the amplitude of the oceanic component strongly depends on the climate model and its representation of convective mixing in the ocean. A low bias in sea-ice production and an overly stratified ocean lead to a lack of deep convective mixing which results in weak ocean variability near the entrance of ice-shelf cavities. Internal climate variability affects the Antarctic contribution to sea-level change until 2100 by 45 % to 93 % depending on the CMIP6 model. This may be a low estimate, as the internal climate variability in the CMIP models is likely underestimated. The effect of atmospheric internal climate variability on the surface mass balance overwhelms the effect of oceanic internal climate variability on the dynamical ice-sheet mass loss by a factor of 2 to 5, except in the Dronning Maud area and the Amundsen, Getz, and Aurora basins, where both contributions may be similar depending on the CMIP model. Based on these results, we recommend that ice-sheet model projections consider (i) several climate models and several members of a single climate model to account for the impact of internal climate variability and (ii) a longer temporal period when correcting historical climate forcing to match present-day observations.
... In the future, under global warming, there is the risk of additional mass loss from the AIS under both sustained warming of the Southern Ocean and the increased tendency to produce upwelling of Circumpolar Deep Water (CDW) caused by stronger westerly winds (Holland et al., 2020(Holland et al., , 2022. Complete disappearance of the WAIS could lead to an estimated sea level rise of 1.91-5.08 ...
The Antarctic Ice Sheet (AIS) has experienced accelerated loss of ice over the last decades and could become the main contributor to sea level rise in the coming centuries. However, the associated uncertainty is very large. The main sources of this uncertainty lie in the future scenarios, the climatic forcing and, most notably, the structural uncertainty due to our lack of understanding of ice–ocean interaction processes, in particular, the representation of subshelf basal melt. In this study, we use a higher-order ice sheet model to investigate the impact of these three sources of uncertainty on the contribution of the AIS to sea level in the coming centuries in the context of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) but extending the projections to 2500. We test the sensitivity of the model to basal melting parameters using several forcings and scenarios simulated in the CMIP5 and CMIP6 ensembles. Results show a strong dependency on the values of the parameter that controls the heat exchange velocity between ice and ocean as well as the forcing and scenario. Higher values of the heat exchange parameter lead to higher sea level rise, with the contribution depending on the forcing–scenario configuration and reaching in some cases more than 3 m in sea level equivalent by the end of 2500. Idealized simulations considering the individual effects of the atmospheric and the oceanic forcing have been performed, demonstrating that the oceanic forcing plays a dominant role over the western sector of the AIS, while the atmospheric forcing is more important for the eastern sector and the interior.
... Holland et al. (2019) analyzed climate model simulations that were constrained to follow the observed variability in tropical Pacific sea-surface temperatures over the 20th century, allowing them to separate the modeled influence of anthropogenic and tropical forcings respectively on winds over the Amundsen Sea. Using the paleoclimate reconstruction of O'Connor et al. (2021) and additional model simulations, Holland et al. (2022) quantified the relative contributions of internal variability and external forcing to wind changes over the 20th century. Building upon this previous work, here we combine evidence from paleoclimate records and historical climate model simulations to examine the relative contribution of multi-decadal tropical Pacific variability and anthropogenic forcing in regulating the large-scale atmospheric circulation in the West Antarctic sector, specifically the ASL, from 1870. ...
... Previous studies indicate that the response of the ASL to external forcing is driven by both stratospheric ozone depletion and greenhouse gas forcing Holland et al., 2022). To isolate the impact of the stratospheric ozone depletion and greenhouse gas forcing, we make use of the ensemble of experiments conducted with CESM1 where single forcings were omitted (Deser, Phillips, et al., 2020;Landrum et al., 2017). ...
Plain Language Summary
Changes in the West Antarctic Ice Sheet mass balance (i.e., the difference between the gain and loss of ice mass) are partly influenced by large‐scale winds, and in particular, a climatological low‐pressure feature located off the West Antarctic coast called the Amundsen Sea Low (ASL). Yet, although the long‐term strengthening of the ASL since the mid‐20th century has been demonstrated to be related to anthropogenic forcing, our understanding of the variability of the ASL on time‐scales of decades is poorly known. In this paper, we therefore investigate the origins of this variability since 1870, and quantify the relative contributions of human‐caused climate changes and natural variability of the climate system. For this purpose, we use several ensembles of model simulations as well as new climate reconstructions that combine paleoclimate records with model simulations using a statistical method. Our results indicate that the multi‐decadal variability of the ASL is strongly driven by tropical variability in the Indo‐Pacific through atmospheric connections between this region and the Amundsen Sea. Our reconstruction, when compared with a large ensemble of model simulations, indicates that since 1950, human‐induced climate forcing has become a dominant driver of long‐term ASL variability, contributing equally to tropical variability.
... In the ASE, increased ice flux through basal melting has not been driven by some simple increasing trend in ambient ocean temperature, but rather intermittent changes in thermocline depth, driven by complex non-local processes, leading to a spatially and temporally heterogeneous response in ice shelf melting (Holland et al., 2022;Jenkins et al., 2018). Increasingly, coupled 30 ice-sheet/ocean models are becoming available that address this shortcoming (e.g. ...
... These scenarios are further divided into 10 ensemble members each, which sample different realizations of internal climate variability within CESM1. Capturing this variability is important, since is likely to have a strong influence on the region and may have contributed considerably to observed trends (Holland et al., 2019(Holland et al., , 2022. Therefore, all of our simulations randomly sample 245 from these different ensemble members, and the ensemble ID (E ID ) is an input to our surrogate model described in Sect.2.4.2 and thus contributes to our uncertainty estimates. ...
The Amundsen Sea region in Antarctica is a critical area for understanding future sea level rise due to its rapidly changing ice dynamics and significant contributions to global ice mass loss. Projections of sea level rise from this region are essential for anticipating the impacts on coastal communities and for developing adaptive strategies in response to climate change. Despite this region being the focus of intensive research over recent years, dynamic ice loss from West Antarctica and in particular the glaciers of the Amundsen Sea represent a major source of uncertainty for global sea level rise projections. In this study, we use ice sheet model simulations to make sea level rise projections to the year 2100 and quantify the associated uncertainty. The model is forced by climate and ocean model simulations for the RCP8.5 and Paris2C scenarios, and is carefully calibrated using measurements from the observational period. We find very similar sea level rise contributions of 19.0 ± 2.2 mm and 18.9 ± 2.7 mm by 2100 for Paris2C and RCP8.5 scenarios, respectively. A subset of these simulations, extended to 2250, show an increase in the rate of sea level rise contribution and clearer differences between the two scenarios emerge as a result of differences in snow accumulation. Our model simulations include both a cliff-height and hydrofracture driven calving processes and yet we find no evidence of the onset of rapid retreat that might be indicative of a tipping point in any simulations within our modelled timeframe.
... 2654 D. T. Bett et al.: Coupled ice-ocean interactions during future retreat of West Antarctic ice streams 2018; Dutrieux et al., 2014). In addition, it has been suggested that there is an average anthropogenic warming trend superimposed on this internal variability (Holland et al., 2022;Naughten et al., 2022). Future anthropogenic warming of the Amundsen Sea is a key mechanism by which human activities may influence SLR from the Antarctic Ice Sheet (Holland et al., 2022;Jourdain et al., 2022;Holland et al., 2019). ...
... 2654 D. T. Bett et al.: Coupled ice-ocean interactions during future retreat of West Antarctic ice streams 2018; Dutrieux et al., 2014). In addition, it has been suggested that there is an average anthropogenic warming trend superimposed on this internal variability (Holland et al., 2022;Naughten et al., 2022). Future anthropogenic warming of the Amundsen Sea is a key mechanism by which human activities may influence SLR from the Antarctic Ice Sheet (Holland et al., 2022;Jourdain et al., 2022;Holland et al., 2019). ...
... In addition, it has been suggested that there is an average anthropogenic warming trend superimposed on this internal variability (Holland et al., 2022;Naughten et al., 2022). Future anthropogenic warming of the Amundsen Sea is a key mechanism by which human activities may influence SLR from the Antarctic Ice Sheet (Holland et al., 2022;Jourdain et al., 2022;Holland et al., 2019). ...
The Amundsen Sea sector has some of the fastest-thinning ice shelves in Antarctica, caused by high, ocean-driven basal melt rates, which can lead to increased ice streamflow, causing increased sea level rise (SLR) contributions. In this study, we present the results of a new synchronously coupled ice-sheet–ocean model of the Amundsen Sea sector. We use the Wavelet-based, Adaptive-grid, Vertically Integrated ice sheet model (WAVI) to solve for ice velocities and the Massachusetts Institute of Technology general circulation model (MITgcm) to solve for ice thickness and three-dimensional ocean properties, allowing for full mass conservation in the coupled ice–ocean system. The coupled model is initialised in the present day and run forward under idealised warm and cold ocean conditions with a fixed ice front. We find that Thwaites Glacier dominates the future SLR from the Amundsen Sea sector, with a SLR that evolves approximately quadratically over time. The future evolution of Thwaites Glacier depends on the lifespan of small pinning points that form during the retreat. The rate of melting around these pinning points provides the link between future ocean conditions and the SLR from this sector and will be difficult to capture without a coupled ice–ocean model. Grounding-line retreat leads to a progressively larger Thwaites Ice Shelf cavity, leading to a positive trend in total melting, resulting from the increased ice basal surface area. Despite these important sensitivities, Thwaites Glacier retreats even in a scenario with zero ocean-driven melting. This demonstrates that a tipping point may have been passed in these simulations and some SLR from this sector is now committed.
... Glacier speed-up has been attributed to an increase in glacier melt in contact with warm, salty ocean waters of circumpolar origin, or circumpolar deep water (CDW) (2). CDW has gained access to the continental shelf, ice cavities, and glaciers over the past 40 y (3) due to an increase in the strength of the westerly winds (4), itself caused by the combined effect of rapid climate warming over the rest of the planet from human-induced greenhouse gas emissions and a cooling of the Antarctic stratosphere from the human-induced depletion of the stratospheric ozone (5,6). ...
Warm water from the Southern Ocean has a dominant impact on the evolution of Antarctic glaciers and in turn on their contribution to sea level rise. Using a continuous time series of daily-repeat satellite synthetic-aperture radar interferometry data from the ICEYE constellation collected in March–June 2023, we document an ice grounding zone, or region of tidally controlled migration of the transition boundary between grounded ice and ice afloat in the ocean, at the main trunk of Thwaites Glacier, West Antarctica, a strong contributor to sea level rise with an ice volume equivalent to a 0.6-m global sea level rise. The ice grounding zone is 6 km wide in the central part of Thwaites with shallow bed slopes, and 2 km wide along its flanks with steep basal slopes. We additionally detect irregular seawater intrusions, 5 to 10 cm in thickness, extending another 6 km upstream, at high tide, in a bed depression located beyond a bedrock ridge that impedes the glacier retreat. Seawater intrusions align well with regions predicted by the GlaDS subglacial water model to host a high-pressure distributed subglacial hydrology system in between lower-pressure subglacial channels. Pressurized seawater intrusions will induce vigorous melt of grounded ice over kilometers, making the glacier more vulnerable to ocean warming, and increasing the projections of ice mass loss. Kilometer-wide, widespread seawater intrusion beneath grounded ice may be the missing link between the rapid, past, and present changes in ice sheet mass and the slower changes replicated by ice sheet models.
... In addition to natural decadal variability, the Amundsen Sea region is impacted by anthropogenic effects, driving trends in the regional winds (Goyal et al., 2021;Holland et al., 2022) which can drive trends in ocean conditions, in particular increasing the on-shelf heat content Spence et al., 2014). Climate projections show that ice-shelf basal melt in the Amundsen Sea is expected to increase over the next century (Jourdain et al., 2022;Naughten et al., 2023). ...
The ice streams flowing into the Amundsen Sea, West Antarctica, are losing mass due to changes in oceanic basal melting of their floating ice shelves. Rapid ice‐shelf melting is sustained by the delivery of warm Circumpolar Deep Water to the ice‐shelf cavities, which is first supplied to the continental shelf by an undercurrent that flows eastward along the shelf break. Temporal variability of this undercurrent controls ice‐shelf basal melt variability. Recent work shows that on decadal timescales the undercurrent variability opposes surface wind variability. Using a regional model, we show that undercurrent variability is induced by sea‐ice freshwater fluxes, particularly those north of the shelf break, which affect the cross‐shelf break density gradient. This sea‐ice variability is linked to tropical Pacific variability impacting atmospheric conditions over the Amundsen Sea. Ice‐shelf melting also feeds back onto the undercurrent by affecting the on‐shelf density, thereby influencing shelf‐break density gradient anomalies.
