Antarctic sea ice is experiencing a weak overall increase in area that is the residual of opposing regional trends. This study considers their seasonal pattern. In addition to traditional ice ...concentration and total ice area, temporal derivatives of these quantities are investigated (“intensification” and “expansion,” respectively). This is crucial to the attribution of trends, since changes in forcing directly affect ice areal change (rather than ice area). Diverse regional trends all contribute significantly to the overall increase. Trends in the Weddell and Amundsen‐Bellingshausen regions compensate in magnitude and seasonality. The largest concentration trends, in autumn, are actually caused by intensification trends during spring. Autumn intensification trends directly oppose autumn concentration trends in most places, seemingly as a result of ice and ocean feedbacks. Springtime trends are reconcilable with wind trends, but further study of changes during the spring melting season is required to unravel the Antarctic sea ice increase.
Key Points
A study of seasonality of trends in Antarctic sea ice concentration and growth
Largest concentration trends in autumn are caused by spring ice loss changes
All sectors cause the increase; Weddell‐Amundsen‐Bellingshausen compensate
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.
Plain Language Summary
The glaciers that flow toward the Amundsen Sea, West Antarctica, are losing ice faster than most others about the continent. Once these glaciers reach the coast, they extend out onto the ocean surface, forming ice shelves. The rapid loss of ice is caused by changes in melting by relatively warm ocean waters beneath the floating ice shelves. In the Amundsen Sea, a deep ocean current is responsible for delivering warm water from the deep ocean to the ice shelves. We present model results that show that this deep current varies on decadal timescales as a consequence of systematic sea‐ice melt and formation patterns. A faster current drives more rapid ice shelf melting which, via a feedback process, further accelerates the current. Climate variability originating in the tropical Pacific Ocean is responsible for the variability in the sea‐ice, and is therefore also responsible for the effects on melting of the ice shelves.
Key Points
In the Amundsen Sea decadal variability of an undercurrent flowing along the shelf break drives decadal variability in ice‐shelf basal melt
Sea‐ice freshwater fluxes and positive feedbacks from ice‐shelf basal melt drive the undercurrent variability
Tropical Pacific teleconnections induce atmospheric anomalies over the Amundsen Sea which drive the sea‐ice freshwater flux variability
A three-dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean-warming scenarios. The model predicts that the total ice shelf basal ...melt increases quadratically as the ocean offshore of the ice front warms. This occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf, both of which are found to increase linearly with ocean warming. The behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations, and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature. This study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results, particularly the claim that a single melt rate sensitivity to warming is universally valid. The hypothesized warming does not necessarily require a heat input to the ocean, as warmer waters (or larger volumes of “warm” water) may reach ice shelves purely through a shift in ocean circulation. Since ice shelves link the Antarctic Ice Sheet to the climate of the Southern Ocean, this finding of an above-linear rise in ice shelf mass loss as the ocean steadily warms is of significant importance to understanding ice sheet evolution and sea level rise.
Coupled ice sheet–ocean models capable of simulating moving grounding lines are just becoming available. Such models have a broad range of potential applications in studying the dynamics of marine ...ice sheets and tidewater glaciers, from process studies to future projections of ice mass loss and sea level rise. The Marine Ice Sheet–Ocean Model Intercomparison Project (MISOMIP) is a community effort aimed at designing and coordinating a series of model intercomparison projects (MIPs) for model evaluation in idealized setups, model verification based on observations, and future projections for key regions of the West Antarctic Ice Sheet (WAIS). Here we describe computational experiments constituting three interrelated MIPs for marine ice sheet models and regional ocean circulation models incorporating ice shelf cavities. These consist of ice sheet experiments under the Marine Ice Sheet MIP third phase (MISMIP+), ocean experiments under the Ice Shelf-Ocean MIP second phase (ISOMIP+) and coupled ice sheet–ocean experiments under the MISOMIP first phase (MISOMIP1). All three MIPs use a shared domain with idealized bedrock topography and forcing, allowing the coupled simulations (MISOMIP1) to be compared directly to the individual component simulations (MISMIP+ and ISOMIP+). The experiments, which have qualitative similarities to Pine Island Glacier Ice Shelf and the adjacent region of the Amundsen Sea, are designed to explore the effects of changes in ocean conditions, specifically the temperature at depth, on basal melting and ice dynamics. In future work, differences between model results will form the basis for the evaluation of the participating models.
In recent decades, Antarctic sea ice has expanded slightly while Arctic sea ice has contracted dramatically. The anthropogenic contribution to these changes cannot be fully assessed unless climate ...models are able to reproduce them. Process-based evaluation is needed to provide a clear view of the capabilities and limitations of such models. In this study, ice concentration and drift derived from AMSR-E data during 2003–10 are combined to derive a climatology of the ice concentration budget at both poles. This enables an observational decomposition of the seasonal dynamic and thermodynamic changes in ice cover. In both hemispheres, the results show spring ice loss dominated by ice melting. In other seasons ice divergence maintains freezing in the inner pack while advection causes melting at the ice edge, as ice is transported beyond the region where it is thermodynamically sustainable. Mechanical redistribution provides an important sink of ice concentration in the central Arctic and around the Antarctic coastline. This insight builds upon existing understanding of the sea ice cycle gained from ice and climate models, and the datasets may provide a valuable tool in validating such models in the future.
Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea-level rise. Increased oceanic melting of its ice shelf is thought to have triggered those ...changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice-shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.
A potentially irreversible threshold in Antarctic ice shelf melting would be crossed if the ocean cavity beneath the large Filchner-Ronne Ice Shelf were to become flooded with warm water from the ...deep ocean. Previous studies have identified this possibility, but there is great uncertainty as to how easily it could occur. Here, we show, using a coupled ice sheet-ocean model forced by climate change scenarios, that any increase in ice shelf melting is likely to be preceded by an extended period of reduced melting. Climate change weakens the circulation beneath the ice shelf, leading to colder water and reduced melting. Warm water begins to intrude into the cavity when global mean surface temperatures rise by approximately 7 °C above pre-industrial, which is unlikely to occur this century. However, this result should not be considered evidence that the region is unconditionally stable. Unless global temperatures plateau, increased melting will eventually prevail.
Accelerating ice loss from Thwaites Glacier is contributing approximately 5% of global sea‐level rise, and could add tens of centimeters to sea level over the coming centuries. We use an ocean model ...to calculate sub‐ice melting for a succession of Digital Elevation Models of the main trunk of Thwaites Glacier from 2011 to 2022. The ice evolution during this period induces a strong geometrical feedback onto melting. Ice thinning and retreat provides a larger melting area, thicker and better‐connected sub‐ice water column, and steeper ice base. This leads to stronger sub‐ice ocean currents, increasing melting by over 30% without any change in forcing from wider ocean conditions. This geometrical feedback over just 12 years is comparable to melting changes arising from plausible century‐scale changes in ocean conditions and subglacial meltwater inflow. These findings imply that ocean‐driven ice loss from Thwaites Glacier may only be weakly influenced by anthropogenic emissions mitigation.
Plain Language Summary
The West Antarctic Ice Sheet is losing ice, making a substantial contribution to global sea‐level rise. This ice loss is known to be triggered by changes in ocean melting of the floating parts of the ice sheet. Computer predictions show that this ice loss could make a large contribution to global sea‐level over the coming centuries, but the future trajectory is very uncertain. In this study we simulated the ocean melting of Thwaites Glacier during 2011–2022, a period when the glacier rapidly thinned and retreated. We show that the geometrical evolution of the glacier during this period led to a substantial increase in ocean melting, caused by the exposure of more ice base to warm ocean waters, and changing ocean currents beneath the ice. This change in melting is similar to what might be expected from 100 years of ocean warming under anthropogenic climate change. These results imply that the future melting of such glaciers is strongly controlled by the geometrical evolution of the ice through internal ice and ocean feedbacks, and will therefore only weakly be influenced by reductions in the emissions of greenhouse gases.
Key Points
Model simulations are used to investigate oceanic melting of the main trunk of Thwaites Glacier during its rapid retreat between 2011 and 2022
The evolution of the ice geometry leads to an increase in melting by more than 30% without any change in ocean forcing
This strong feedback means the future ocean melting of Thwaites Glacier may only be weakly influenced by changes in anthropogenic forcing
In the vicinity of the Antarctic Peninsula observations show diminishing sea ice and a rapid warming of atmosphere and ocean. These changes have led to the collapse of ice shelves and retreat, ...acceleration, and thinning of inland ice. However, ocean observations in the center of the nearby Bellingshausen Sea are spatially and temporally coarse. In this study, ocean and sea ice models forced by atmospheric reanalyses are used to investigate this data gap by simulating processes in the Bellingshausen Sea for the years 1979–2007. The model suggests flow features in the region and predicts the basal melting of local ice shelves. Modeled ocean conditions are found to be less variable than in the nearby Amundsen Sea, which is situated closer to foci of annual and interannual atmospheric variability. Melt rates beneath George VI Ice Shelf are investigated in detail, concluding that the ice shelf may have been melting out of balance (and therefore thinning) for decades. The melt rate contains significant interannual variability that the model links to variation in sea ice conditions offshore of the southern end of the ice shelf. This stands in contrast to the Amundsen Sea, where models suggest that ice shelf melting is controlled by variable transport of Circumpolar Deep Water onto the continental shelf.
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