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The Amundsen Sea of West Antarctica is one of the regions of particularly interest to the oceanography community due to accelerated basal melting of its ice shelves. Quantitative estimates of the composition of water masses and their sources are essential for understanding the heat and salt budgets and the stability of ice shelves in this region. In this study, the water mass distributions on the Amundsen Sea continental shelf are estimated based on FOAM-GLOSEA5v13, a global oceanographic reanalysis product, using a modified statistical based optimal water mass analysis method. The reanalysis product effectively captures the signal of primary water masses, including the Winter Water (WW) and the Circumpolar Deep Water (CDW) below the Antarctic Summer Surface Water (SSW). The horizontal and vertical distributions and temporal evolution of three water mass endmembers (WW, CDW and DSW) and their mixing ratios are calculated, based on which an intrusion location detection method was established. Using the reanalysis product and intrusion detection method, all potential CDW intrusion sites along the shelf break are identified. Based on the reanalysis product, an intrusion site over the shelf break, away from major deep troughs was captured and confirmed by numerical particle tracking experiments. [ABSTRACT FROM AUTHOR]

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 meltSea‐ice freshwater fluxes and positive feedbacks from ice‐shelf basal melt drive the undercurrent variabilityTropical Pacific teleconnections induce atmospheric anomalies over the Amundsen Sea which drive the sea‐ice freshwater flux variability [ABSTRACT FROM AUTHOR]

The inflow of warm and salty Circumpolar Deep Water affects the melting of the ice shelf on the Amundsen Sea, a significant contributor to global sea level rise. Multi‐year mooring data (2014–2016 and 2018–2020) from the front of the Dotson Ice Shelf show the modified Circumpolar Deep Water layer was thicker during 2018–2020 than during 2014–2016. During 2014–2016, Ocean surface stress curl influenced the barotropic process and strengthened southward velocity, while during 2018–2020, it caused lift and downwelling of thermocline depth, increasing the impact of the baroclinic process in ocean circulation. The heat transport to the ice shelf during 2018–2020 (57.42 MW m−1) was half as much as it was during 2014–2016 (111.06 MW m−1) due to a weaker lower layer current. The difference in ocean circulation between two periods, caused by a difference in warm layer thickness, ultimately impacts the heat transport entering the ice shelf cavity. Plain Language Summary: Warm and salty water from the deep ocean flows into the ice shelf cavities in the West Antarctic, causing the ice to melt and contribute to global sea level rise. We measured the current and water properties in front of the Dotson Ice Shelf during 2014–2016 and 2018–2020 and found that the warm layer was thicker during the latter period. Unlike during 2014–2016 when ocean surface stress curl created a spatial imbalance in sea level and affected the southward current due to pressure gradients, during 2018–2020, ocean surface stress curl changed the density by causing upwelling and downwelling. This density change influenced the velocity variation toward the ice shelf. Although the mCDW layer was ticker, the heat influx to the ice shelf during 2018–2020 was half as much as it was during 2014–2016. The difference in ocean current during these two periods, due to differences in the warm layer thickness, ultimately affects how much heat is transported into the ice shelf cavity. Key Points: The modified circumpolar deep water layer was thicker in 2018–2020 than that during 2014–2016The baroclinic effect plays a more important role in the variability of the current entering the ice shelf during 2018–2020Differences in the seasonal cycle of the ocean surface stress curl can affect ocean circulation by changing ocean conditions [ABSTRACT FROM AUTHOR]

Mass loss from the Antarctic Ice Sheet is dominated by basal melting–induced warm ocean water. Ice-sheet mass loss and thinning of buttressing ice shelves occur primarily in the Amundsen and Bellingshausen Seas. Here, we show that in a global ocean simulation using the 0.25° Nucleus for European Modeling of Ocean (NEMO) model driven by the JRA55 reanalysis from 1982 to 2017, the Amundsen sector of the Antarctic continental shelf acts as a gateway, regulating the on-shelf access of warm Circumpolar Deep Water (CDW) from the deep ocean and its westward transfer to other sectors up to ca. 90° E, particularly the Ross Sea. As a result, anomalies in Antarctic-shelf-averaged temperature mainly originate in the Amundsen sector. These changes are primarily governed by shifts in the Amundsen Sea Low associated with tropical climate variability, modulating the on-shelf transport of CDW via wind-driven perturbations to ocean currents. The ensuing temperature anomalies progress westward from the Amundsen Sea via three distinct routes: a slow, convoluted westward pathway on the shelf via the Antarctic Coastal Current; a faster westward pathway along the shelf break via the Antarctic Slope Current and then onto the shelf along topographic troughs; and a third, eastward route toward the Bellingshausen sector, whereby temperature anomalies are transported into a region of local wind-generated changes farther north. These results emphasize the importance of the Amundsen sector for climate variability over the Antarctic shelves. [ABSTRACT FROM AUTHOR]

Abstract 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.

The collapse of ice shelves is a process that can severely increase the rise of global sea-levels through the reduction of the buttressing effect of ice shelves and the consequent acceleration of the ice flow of ice sheets. In recent years, the West Antarctic ice shelves in the Amundsen Sea, whose buttressing effect is essential for a great part of the West Antarctic ice sheet, have been experiencing the most rapid melting and thinning in the world. The melting of the West Antarctic ice shelves is caused primarily by heat transported by Circumpolar Deep Water (CDW). For this reason, it is important to investigate ice-ocean interactions that could influence the melting of ice shelves and evaluate the stability of West Antarctic ice shelves. A lot of researchers have been actively investigating the West Antarctic ice shelves in the Amundsen Sea. High-impact journals have recognized the importance of and published studies on ice-ocean interactions occurring near and under the ice shelves as well as the connections among ice shelves. However, in situ observations are limited due to extreme weather and sea-ice conditions near the ice shelves; therefore, many scientific questions remain unanswered. This study introduces the characteristics of the Amundsen Sea and investigate the past and latest research issues in this region. This study also gives suggestions regarding important scientific questions and directions for future research that should help early-career scientists take the lead in future research on the melting dynamics of the West Antarctic ice shelves in the Amundsen Sea. [ABSTRACT FROM AUTHOR]

Melt rates of West Antarctic ice shelves in the Amundsen Sea track large decadal variations in the volume of warm water at their outlets. This variability is generally attributed to wind‐driven variations in warm water transport toward ice shelves. Inspired by conceptual representations of the global overturning circulation, we introduce a simple model for the evolution of the thermocline, which caps the warm water layer at the ice‐shelf front. This model demonstrates that interannual variations in coastal polynya buoyancy forcing can generate large decadal‐scale thermocline depth variations, even when the supply of warm water from the shelf‐break is fixed. The modeled variability involves transitions between bistable high and low melt regimes, enabled by feedbacks between basal melt rates and ice front stratification strength. Our simple model captures observed variations in near‐coast thermocline depth and stratification strength, and poses an alternative mechanism for warm water volume changes to wind‐driven theories. Plain Language Summary: Ice loss from the West Antarctic Ice Sheet contributes significantly to current and projected rates of global sea‐level rise. The ice sheet is primarily losing mass via glaciers that flow from the Antarctic continent into the Amundsen Sea, where floating ice shelves are exposed to much warmer ocean waters than elsewhere around Antarctica. In this work we present a simplified mathematical model for the volume of warm water at Amundsen Sea ice shelf fronts that reproduces observed patterns of warm water variability. The modeled variability relies on interactions between ice shelf melt and coastal polynyas, regions where enhanced wintertime sea‐ice production can trigger mixing that diverts heat carried by warm waters away from the ice shelf and into the atmosphere. Higher melt rates inhibit polynya convection, allowing more warm water into the ice shelf cavity and reinforcing a high melt state, whilst lower melt rates facilitate polynya convection, diverting heat away from the ice shelf and reinforcing a low melt state. Interannual variations in polynya sea‐ice production trigger shifts between these reinforcing states. Our results promote the importance of coastal processes in explaining observed variations in Amundsen Sea ice shelf melt, which have previously been attributed to remote wind patterns. Key Points: Rates of ocean‐driven Amundsen Sea ice shelf melt respond to variations in warm water transport to the coast and modification at the coastA simple Amundsen Sea continental shelf overturning model, based on water mass transformation, reveals bistable high and low melt regimesFeedbacks between glacial melt and polynya convection are central to the bistability and produce variability consistent with observations [ABSTRACT FROM AUTHOR]

Ocean warming around Antarctica has the potential to trigger marine ice‐sheet instabilities. It has been suggested that abrupt and irreversible cold‐to‐warm ocean tipping points may exist, with possible domino effect from ocean to ice‐sheet tipping points. A 1/4° ocean model configuration of the Amundsen Sea sector is used to investigate the existence of ocean tipping points, their drivers, and their potential impact on ice‐shelf basal melting. We apply idealized atmospheric perturbations of either heat, freshwater, or momentum fluxes, and we characterize the key physical processes at play in warm‐to‐cold and cold‐to‐warm climate transitions. Relatively weak perturbations of any of these fluxes are able to switch the Amundsen Sea to an intermittent or permanent cold state, that is, with ocean temperatures close to the surface freezing point and very low ice‐shelf melt rate. The transitions are reversible, that is, canceling the atmospheric perturbation brings the ocean system back to its unperturbed state within a few decades. All the transitions are primarily driven by changes in surface buoyancy fluxes resulting from the freshwater flux perturbation or from modified net sea‐ice production due to either heat flux or sea‐ice advection anomalies. These changes affect the vertical ocean stratification over the continental shelf and thereby the eastward undercurrent at the shelf break, which both impact ice‐shelf melting. As sea‐ice induced deep convection is already quite limited in present‐day conditions, surface buoyancy gain in a warmer climate has relatively little effect on deep ocean properties compared to colder climate conditions. Plain Language Summary: The West Antarctic Ice Sheet is under the threat of a partial collapse, which would induce rapid global sea level rise. This threat is partly related to the thinning of floating ice shelves, and the consequent retreat of the grounding line, which is a self‐sustained ice dynamics process. It is triggered by increased basal melting of the ice shelves, which results from enhanced flow of relatively warm waters onto the continental shelf. It has been suggested that self‐sustained ocean processes may lead to abrupt changes in the flow of warm water into ice‐shelf cavities, which could facilitate the tipping to a marine ice‐sheet instability. Here, we analyze whether such abrupt ocean changes can occur under cold‐to‐warm or warm‐to‐cold transitions in the Amundsen Sea, West Antarctica. We use a regional ocean model with a set of idealized local atmospheric perturbations to characterize the thresholds and reversibility of ocean abrupt changes.We find that the currently warm Amundsen Sea could switch intermittently or permanently to a cold state for relatively weak atmospheric perturbations and could be slightly warmer in the future. All transitions are reversible. The main mechanism involved on decadal scale is related to a change in the surface buoyancy fluxes. Key Points: The currently warm ice‐shelf cavities of the Amundsen sector could become or have been cold for slightly colder climatic conditionsThe transitions are reversible: canceling the atmospheric perturbation brings the ocean back to its unperturbed state within a few decadesAll the transitions are primarily driven, at multi‐decadal scale, by changes in surface buoyancy fluxes over the continental shelf [ABSTRACT FROM AUTHOR]

Warm ocean waters drive rapid ice‐shelf melting in the Amundsen Sea. The ocean heat transport toward the ice shelves is associated with the Amundsen Undercurrent, a near‐bottom current that flows eastward along the shelf break and transports warm waters onto the continental shelf via troughs. Here we use a regional ice‐ocean model to show that, on decadal time scales, the undercurrent's variability is baroclinic (depth‐dependent). Decadal ocean surface cooling in the tropical Pacific results in cyclonic wind anomalies over the Amundsen Sea. These wind anomalies drive a westward perturbation of the shelf‐break surface flow and an eastward anomaly (strengthening) of the undercurrent, leading to increased ice‐shelf melting. This contrasts with shorter time scales, for which surface current and undercurrent covary, a barotropic (depth‐independent) behavior previously assumed to apply at all time scales. This suggests that interior ocean processes mediate the decadal ice‐shelf response in the Amundsen Sea to climate forcing. Plain Language Summary: The West Antarctic Ice Sheet is losing mass, causing sea level rise. Most of this loss occurs in the Amundsen Sea Embayment, due to melting of coastal glaciers by warm ocean waters. These warm waters are transported toward the glaciers by the Amundsen Undercurrent, a near‐seafloor eastward‐flowing current located at the boundary between the deep ocean and the shallower seas around Antarctica. Changes in the undercurrent thus regulate the amount of heat available to melt the glaciers. Here, we use a model to assess the undercurrent's variability on time scales of decades, as decadal ocean forcing drives periods of enhanced ice‐sheet retreat. Contrary to previous work, our model shows that wind fluctuations, associated with surface temperature changes in the tropical Pacific, lead to changes in the interior ocean density field on decadal time scales. Decadal anomalous cyclonic atmospheric circulation over the Amundsen Sea, associated with cooling in the tropical Pacific, accelerates the near‐surface ocean flow westward, but also accelerates the eastward‐flowing undercurrent and enhances glacial melting. Our work suggests that previous assumptions about the decadal oceanic response of the Amundsen Sea to wind variability might need to be reconsidered, with implications for melting of West Antarctic glaciers. Key Points: Our modeled decadal strengthening of the Amundsen Undercurrent corresponds with increase in ice‐shelf basal meltingUndercurrent variability on decadal time scales is predominantly baroclinic (i.e., anticorrelated with the surface flow variability)At decadal timescales, this model shows enhanced ice‐shelf melting under westward wind anomalies, a reverse relationship to previous studies [ABSTRACT FROM AUTHOR]

Antarctic Ice Sheet projections show the highest sensitivity to increased basal melting in the Amundsen Sea. However, little is known about the processes that control future increase in melt rates. We build an ensemble of three ocean–sea‐ice–ice‐shelf simulations for both the recent decades and the late 21st century, constrained by regional atmosphere simulations and the multi‐model mean climate change of the fifth Climate Model Intercomparison Project under the RCP8.5 scenario. The ice‐shelf melt rates are typically multiplied by 1.4–2.2 from present day to future, for a total basal mass loss increased by 346 Gt yr−1 on average. This is equally explained by advection of warmer water from remote locations and regional changes in Ekman downwelling and in the ice‐shelf melt‐induced circulation, while increased iceberg melt plays no significant role. Our simulations suggest that high‐end melt projections previously used to constrain recent sea level projections may have been significantly overestimated. Plain Language Summary: Future sea level rise highly depends on how fast the ocean will melt the floating ice shelves in Antarctica, which modulates the ice flow from the ice sheet into the ocean. This is particularly true for the Amundsen Sea sector where the ice flow into the ocean is very sensitive to ocean‐induced melting. Here, we use a numerical model that represents the evolution of the Amundsen Sea, including under the floating ice shelves. Under a high‐end greenhouse‐gases concentration pathway, our simulations indicate that melt rates beneath the ice shelves may increase by 40%–120%. This is explained by both warmer seawater coming from distant regions and changes in the local wind stress. Our simulations suggest that high‐end melt projections previously used to constrain recent sea level projections may have been overestimated. Key Points: We present 1/12° ocean‐sea‐ice‐ice‐shelf projections at the end of the 21st century under the RCP8.5 scenarioIce shelf melt rates in the Amundsen Sea are typically multiplied by 1.4–2.2 from 1989–2009 to 2080–2100Advection of warmer water from remote locations and reduced Ekman downwelling are the main drivers of changes in ice shelf melt rates [ABSTRACT FROM AUTHOR]

Extensive oceanographic data sets were combined with microbiological parameters to elucidate the tight coupling between glacial meltwater and heterotrophic bacterial production (BP) on the Getz Ice Shelf (GtzIS) in the Amundsen Sea. BP in the eastern GtzIS (EG; 85.8 pM Leu. h−1), where basal glacier meltwater upwells, was significantly higher than BP measured in the western GtzIS (WG; 50.6 pM Leu. h−1) and the Amundsen Sea Polynya (ASP; 27.8 pM Leu. h−1). BP in the EG accounted for 49% of primary production, which was greater than that of the WG (10%) and ASP (9.2%). Enhanced BP in the eastern GtzIS was not coupled with phytoplankton biomass, but correlated significantly with the freshwater fraction containing meltwater‐derived dissolved organic carbon (MW‐DOC). These results suggest that warming‐induced glacier melting weakens carbon sequestration efficiency in Antarctic coastal waters by stimulating heterotrophic metabolism that converts MW‐DOC to CO2. Plain Language Summary: The Getz Ice Shelf (GtzIS) in the Amundsen Sea, the third‐largest ice shelf (34,018 km2) on the West Antarctic Ice Sheet, is regarded as a hot spot for producing large amount of glacier meltwater (144.9 ± 14 Gt yr−1). In general, the amount of dissolved organic carbon (DOC) from terrestrial sources is negligible in Antarctic waters, making DOC supplied by phytoplankton a major carbon source for heterotrophic bacteria growth. Although Antarctica's glacial ice shelves are presumably a large reservoir of organic carbon (ca. 5.5 Pg C), little is known about the effect of glacial melting on the supply of DOC available to heterotrophic bacteria. On the eastern GtzIS, where glacial meltwater upwells to the surface water column, heterotrophic bacteria production has no significant positive relationship with phytoplankton biomass, but is positively correlated with freshwater fraction. This suggests that DOC in glacial meltwater, rather than phytoplankton, provides DOC to support enhanced bacteria production. Our results imply that global warming–related increases in glacial meltwater may stimulate heterotrophic bacterial metabolism that respires DOC to CO2, thereby reducing carbon sequestration efficiency in Antarctic coastal waters. Key Points: Unlike the sea‐ice melting system, enhanced bacterial production (BP) in the eastern Getz Ice Shelf (GtzIS) was uncoupled with phytoplankton biomassDissolved organic carbon in glacier meltwater likely stimulates BP on the eastern GtzISWarming‐induced glacial melting may weaken carbon sequestration in the Antarctic Ocean by stimulating heterotrophic bacterial metabolism [ABSTRACT FROM AUTHOR]

The Amundsen Sea polynya is one of the richest areas of primary productivity in Antarctica. The Antarctic toothfish (Dissostichus mawsoni , TOA) is a commercially valuable circum-Antarctic fish that undertakes ontogenetic movements on the continental shelf. However, the movement and distribution of this species remain unclear in the polynya, especially in the data-poor Amundsen Sea polynya. Along with hydrological and fishery survey data in the Amundsen Sea polynya, the otolith chemistry of TOA from three areas along the ice shelf of the Dotson–Getz Trough was analyzed to explore the movement pattern of this species. The elemental composition deposited during early life in the otolith core did not show significant differences, indicating that TOA in the Amundsen Sea polynya probably experienced a similar hydrological environment at the early life stage or might originate from the same spawning ground. However, the elemental composition deposited at the otolith edge showed that SrCa−1 and LiCa−1 differ in otolith chemistry between areas along the Amundsen Sea coast. In conjunction with additional data from the fish sampled in these areas, this study reveals that TOA show an ontogenetic movement from westward along the ice shelf in the Amundsen Sea polynya. A deeper area (with a depth of over 1000 m) in the west box provides a fish pool for TOA, which needs to be conserved in the fishery management of toothfish populations. • Ontogenetic movement of Dissostichus mawsoni in the polynya is investigated for the first time. • D. mawsoni moves ontogenetically from east to west along ice shelf in the Amundsen Sea polynya. • Mature D. mawsoni prefers to staying in the deep waters > 1000 m. [ABSTRACT FROM AUTHOR]

Rapid ice loss is occurring in the Amundsen Sea sector of the West Antarctic Ice Sheet. This ice loss is assumed to be a long‐term response to oceanographic forcing, but ocean conditions in the Amundsen Sea are unknown prior to 1994. Here we present a modeling study of Amundsen Sea conditions from 1920 to 2013, using an ensemble of ice‐ocean simulations forced by climate model experiments. We find that during the early twentieth century, the Amundsen Sea likely experienced more sustained cool periods than at present. Warm periods become more dominant over the simulations (mean trend 0.33°C/century) causing an increase in ice shelf melting. The warming is likely driven by an eastward wind trend over the continental shelf break that is partly anthropogenically forced. Our simulations suggest that the Amundsen Sea responded to historical greenhouse gas forcing, and that future changes in emissions are also likely to affect the region. Plain Language Summary: Ice loss from the West Antarctic Ice Sheet is one of the fastest growing contributions to sea level rise. The pattern of melting suggests that the ocean has been warming in the Amundsen Sea, but we don't know what the ocean was like in this region before observations started in 1994. In this study, we model the ocean conditions over the last century, using a set of 20 ice‐ocean simulations fed with the output of larger climate models. Our simulations suggest that the Amundsen Sea used to be cooler than it is today. Over the last century, ocean temperatures have increased, but this is hard to detect in observations because the trend is obscured by frequent swings between cold and warm conditions. The main cause of the warming in our simulations is changes in the winds near the edge of the Amundsen Sea that pump more warm water from the deep ocean onto the shallower continental shelf. These wind trends are partly caused by greenhouse gas emissions from the burning of fossil fuels. Whether the Amundsen Sea continues to warm, or whether temperatures stabilize, will likely hinge on the success of future climate policies to reduce emissions. Key Points: An ensemble of ice‐ocean simulations is used to study conditions in the Amundsen Sea over the twentieth centuryThe simulations show a long‐term ocean warming trend on the continental shelf, which is not yet clear in the shorter observational recordThe trends are partly driven by anthropogenic forcing, suggesting the ocean and ice shelves may also be sensitive to future climate policy [ABSTRACT FROM AUTHOR]

Widespread ice shelf thinning has been recorded in the Amundsen Sea in recent decades, driven by basal melting and intrusions of relatively warm Circumpolar Deep Water (CDW) onto the continental shelf. The Dotson Ice Shelf (DIS) is located to the south of the Amundsen Sea polynya, and has a high basal melting rate because modified CDW (mCDW) fills the Dotson‐Getz Trough (DGT) and reaches the base of the ice shelf. Here, hydrographic data in the DGT obtained during seven oceanographic surveys from 2007 to 2018 were used to study the interannual variation in mCDW volume and properties and their causes. Although mCDW volume showed relatively weak interannual variations at the continental shelf break, these variations intensified southward and reached a maximum in front of the DIS. There, the mCDW volume was ∼8,000 km3 in 2007, rapidly decreased to 4,700 km3 in 2014 before rebounding to 7,300 km3 in 2018. We find that such interannual variability is coherent with local Ekman pumping integrated along the DGT modulated by the presence of sea ice, and complementing earlier theories involving shelf break winds only. The interannual variability in strength of the dominant south‐southeast coastal wind modulates the amplitude of Ekman upwelling along the eastern boundary of the Amundsen Sea polynya during the austral summers of the surveyed years, apparently leading to change in the volume of mCDW along the DGT. We note a strong correlation between the wind variability and the longitudinal location of the Amundsen Sea Low. Plain Language Summary: Recent widespread thinning of floating ice shelves around the Amundsen Sea coast has been shown to result from flows of relatively warm water into ocean cavities below the ice shelves. Dotson Ice Shelf is located in the center of the Amundsen Sea, and has seen high rates of ice loss from ocean‐driven melting as warm water spreads southward from the Southern Ocean along the Dotson‐Getz Trough. We analyzed long‐term hydrographic observations from seven oceanographic surveys along the trough between 2007 and 2018 to understand the variation of warm water properties. We show that the warm water volume varies significantly from year to year, and that this variability is largely driven by local winds which are, in turn, connected to the broader scale atmospheric circulation. These results provide insights into how variability in atmospheric circulation due to climate change will affect the water masses on the Antarctic coast and how rapidly ice shelves will melt. Key Points: Interannual variations in the thickness and volume of modified Circumpolar Deep Water intensify southward toward Dotson Ice ShelfModified Circumpolar Deep Water volume in the Dotson‐Getz Trough was 8,000 km3 in 2007, 4,700 km3 in 2014, and 7,300 km3 in 2018Dominant south‐southeast wind over the Dotson‐Getz Trough intensified Ekman upwelling at the eastern boundary of the Amundsen Sea polynya [ABSTRACT FROM AUTHOR]

The flow of warm water toward the western Getz Ice Shelf along the Siple Trough, West Antarctica, is intermittently disrupted during short events of Winter Water deepening. Here we show, using mooring records, that these 5–10 days‐long events reduced the heat transport toward the ice shelf cavity by 25% in the winter of 2016. The events coincide with strong easterly winds and polynya opening in the region, but the Winter Water deepening is controlled by non‐local coastal Ekman downwelling rather than polynya‐related surface fluxes. The thermocline depth anomalies are forced by Ekman downwelling at the northern coast of Siple Island and propagate to the ice front as a coastal trapped wave. During the events, the flow at depth does no longer continue along isobaths into the ice shelf cavity but aligns with the ice front. Plain Language Summary: Ocean‐driven melting of the floating ice shelves that fringe the West Antarctica Ice Sheet has increased during the last decades. The transport of ocean heat toward the base of the ice shelves displays a large variability on daily to decadal timescales; still, the processes behind this variability are not well known. The western Getz Ice Shelf in West Antarctica experienced a persistent inflow of warm ocean water during 2016–2017, but the inflow was disrupted by intermittent events of reduced heat transport. The causes for these events are studied here, and it appears that atmospheric processes in the form of surface winds and locally reduced sea ice cover are important. Strong easterly winds push the sea ice off‐coast and depress the cold surface waters toward the ocean floor. This deepening of the cold layer happens about 100 km away from the mooring site, but influences the heat transport under the ice shelf remotely through waves traveling along the coast. Simultaneously, the ocean currents are deflected at the ice front, which causes a substantial decrease of the heat transport toward the base of the ice shelves during these events. Key Points: Observations at the western Getz Ice Shelf show eight intermittent events of Winter Water deepening below 350 m depth during winter 2016The events are associated with strong easterly winds and caused by non‐local Ekman downwellingThe ocean heat transport into the Getz Ice Shelf cavity is reduced by 25% in the winter of 2016 due to the events [ABSTRACT FROM AUTHOR]

The Amundsen Sea, located in West Antarctica, is experiencing rapid melting due to the intrusion of Circumpolar Deep Water, which is causing ice sheet thinning and basal melting. The resulting changes can affect the biogeochemical cycle of dissolved organic matter (DOM) by supplying iron from sea ice and/or glacier, thereby influencing primary production and ocean circulation. Therefore, it is crucial to understand the dynamics of the DOM in this region. In this study, our primary focus was to examine the optical properties of DOM in the oceanic regions adjacent to the West Getz Ice Shelf (WGIS) and the Dotson Ice Shelf (DIS). Significant differences in DOM optical properties, including the chromophoric DOM (CDOM) absorption coefficient at 350 nm (a 350), spectral slope coefficient (S 275 – 295), and specific UV absorbance at 254 nm (SUVA 254), were observed between the WGIS and DIS regions (t -test, p < 0.05). Notably, the WGIS regions exhibited high a 350 values. Additionally, the S 275 – 295 and SUVA 254 values, which serve as indices of molecular weight, indicated that the DOM pool in the WGIS regions was dominated by high molecular weight compounds with a substantial proportion of aromatic compounds. In contrast, the low values of a 350 and SUVA 254 along with the high S 275 – 295 values in the DIS region suggested the dominance of low molecular weight CDOM associated with compounds of lower aromaticity. Furthermore, significant negative correlations were found between biomass of Phaeocystis antarctica (P. antarctica) and phosphate (PO 4) in the WGIS regions (r2 = 0.82, p < 0.01 for WGIS 1 and r2 = 0.73, p < 0.01 for WGIS 2). However, no significant relationship was observed in the DIS region. These findings suggest that the high value and molecular weight of a 350 , extending from the surface layer to the deep layer, in the WGIS regions were associated with autochthonous sources, primarily driven by the colony-forming bloom of P. antarctica. These findings demonstrate that the quantity and quality of DOM in the Amundsen Sea are strongly affected by bloom conditions. The results emphasize that a combination of physical and biological processes interacts in complex ways to determine the characteristics of DOM in the Amundsen Sea. • Contrasting DOM optical properties were observed between the WGIS and DIS regions. • Optical indices revealed the dominance of high molecular weight DOM in the WGIS. • Low molecular weight DOM dominated the DOM pool in the DIS. • Distinct physical and biological processes impacted DOM in the WGIS and DIS. [ABSTRACT FROM AUTHOR]

Pine Island Ice Shelf, in the Amundsen Sea, is losing mass due to increased heat transport by warm ocean water penetrating beneath the ice shelf and causing basal melt. Tracing this warm deep water and the resulting glacial meltwater can identify changes in melt rate and the regions most affected by the increased input of this freshwater. Here, optimum multiparameter analysis is used to deduce glacial meltwater fractions from independent water mass characteristics (standard hydrographic observations, (NG), and oxygen isotopes), collected during a ship‐based campaign in the eastern Amundsen Sea in February–March 2014. (NG) (neon, argon, krypton, and xenon) and oxygen isotopes are used to trace the glacial melt and meteoric water found in seawater, and we demonstrate how their signatures can be used to rectify the hydrographic trace of glacial meltwater, which provides a much higher‐resolution picture. The presence of glacial meltwater is shown to mask the Winter Water properties, resulting in differences between the water mass analyses of up to 4‐g/kg glacial meltwater content. This discrepancy can be accounted for by redefining the "pure" Winter Water endpoint in the hydrographic glacial meltwater calculation. The corrected glacial meltwater content values show a persistent signature between 150 and 400 m of the water column across all of the sample locations (up to 535 km from Pine Island Ice Shelf), with increased concentration toward the west along the coastline. It also shows, for the first time, the signature of glacial meltwater flowing off‐shelf in the eastern channel. Plain Language Summary: Pine Island Ice Shelf in the Amundsen Sea, Antarctica, is melting due to warm ocean waters. The glacial meltwater that is produced is less salty and carries essential food for biological organisms, so where the glacial meltwater goes once it leaves the front of the ice shelf is important to understand: Less salt in the ocean at the surface makes it easier to form sea ice, and increased productivity from biological organisms can help draw carbon down into the ocean from the atmosphere. We use (NG) to identify where this glacial meltwater is, as the signature that the meltwater leaves in the gases is unique like a fingerprint. We use the noble gas meltwater signature to improve our identification of glacial meltwater using temperature, salinity, and dissolved oxygen (hydrographic observations), which are easier and cheaper to collect so cover a larger area. Using the improved signature from hydrographic observations, we identify the presence of glacial meltwater between 150‐ and 400‐m depths everywhere across the continental shelf. We also show, for the first time, glacial meltwater from the ice shelf flowing off‐shelf in the easternmost channel. These results are important as they show where glacial meltwater is affecting the ocean column most. Key Points: Oxygen isotope and noble gas data identify freshwater in the upper ocean across the Amundsen SeaHydrographic calculations of meltwater are improved using (NG) as ground truthWe identify a new meltwater export pathway in the eastern channel in the Amundsen Sea [ABSTRACT FROM AUTHOR]

Mass loss from the Antarctic Ice Sheet is driven by changes at the marine margins. In the Amundsen Sea, thinning of the ice shelves has allowed the outlet glaciers to accelerate and thin, resulting in inland migration of their grounding lines. The ultimate driver is often assumed to be ocean warming, but the recent record of ocean temperature is dominated by decadal variability rather than a trend. The distribution of water masses on the Amundsen Sea continental shelf is particularly sensitive to atmospheric forcing, while the regional atmospheric circulation is highly variable, at least in part because of the impact of tropical variability. Changes in atmospheric circulation force changes in ice shelf melting, which drive step-wise movement of the grounding line between localized high points on the bed. When the grounding line is located on a high point, outlet glacier flow is sensitive to atmosphere-ocean variability, but once retreat or advance to the next high point has been triggered, ocean circulation and melt rate changes associated with the evolution in geometry of the sub-ice-shelf cavity dominate, and the sensitivity to atmospheric forcing is greatly reduced.

Highlights • Tides enhance simulated ice-shelf melting by 1–39% depending on the ice shelf. • Enhanced melting is mostly due to strengthened turbulence near the ice/ocean interface. • Tidal processes occurring seaward are less important than those occurring underneath ice shelves. • A methodology to prescribe tidal effects on ice shelf melt is proposed. Abstract The representation of tides in regional ocean simulations of the Amundsen Sea enhances ice-shelf melting, with weakest effects for Pine Island and Thwaites (< + 10 %) and strongest effects for Dotson, Cosgrove and Abbot (> + 30 %). Tides increase vertical mixing throughout the water column along the continental shelf break. Diurnal tides induce topographically trapped vorticity waves along the continental shelf break, likely underpinning the tidal rectification (residual circulation) simulated in the Dotson–Getz Trough. However, the primary effect by which tides affect ice-shelf melting is the increase of ice/ocean exchanges, rather than the modification of water masses on the continental shelf. Tide-induced velocities strengthen turbulent heat fluxes at the ice/ocean interface, thereby increasing melt rates. Approximately a third of this effect is counterbalanced by the resulting release of cold melt water that reduces melt downstream along the meltwater flow. The relatively weak tide-induced melting underneath Pine Island and Thwaites could be partly related to their particularly thick water column, which limits the presence of quarter wavelength tidal resonance. No sensitivity to the position of Pine Island and Thwaites with respect to the M 2 critical latitude is found. We refine and evaluate existing methodologies to prescribe the effect of tides on ice-shelf melt rates in ocean models that do not explicitely include tidal forcing. The best results are obtained by prescribing spatially-dependent tidal top-boundary-layer velocities in the melt equations. These velocities can be approximated as a linear function of existing barotropic tidal solutions. A correction factor needs to be applied to account for the additional melt-induced circulation associated with tides and to reproduce the relative importance of dynamical and thermodynamical processes. [ABSTRACT FROM AUTHOR]

Mid-trophic level at high-latitude coastal water in the Southern Ocean reside unique geographical condition with sea ice, coastal polynya, and ice shelf. To investigate the regional differences in their vertical distribution during summer, we examined acoustic backscatter data from scientific echo sounder, collected in the three representative regions in the Amundsen Sea: pack ice zone, coastal polynya zone, and ice shelf zone. The weighted mean depths (WMDs) representing zooplankton were calculated with the high resolution acoustic backscatter (1-m depth) to identify the vertical variability of the sound-scattering layer (SSL). WMDs were mainly distributed between 50 and 130 m exhibiting clear regional differences. The WMDs were detected in the shallow depth ranged between 48 and 84 m within the pack ice and coastal polynya, whereas they were observed at deeper depths around near ice shelf ranged between 117 and 126 m. WMDs varied with changing the stratification of water column structure representing strong linear relationship with the mixed layer depth ( r  = 0.69). This finding implies that understanding the essential forcing of zooplankton behavior will improve our ability to assess the coastal ecosystem in the Southern Ocean facing dramatic change. [ABSTRACT FROM AUTHOR]

Ferrero Bay, located in eastern Pine Island Bay (PIB) of the Amundsen Sea Embayment, is one of the largest and southernmost fjords yet studied in Antarctica. High-resolution multibeam swath bathymetric data, chirp sonar sub-bottom profiles, and three Kasten cores were collected in Ferrero Bay during the IB Oden Southern Ocean 2009-2010 cruise (OSO0910). Core KC-15 from the inner bay yielded two carbonate ages providing a minimum age for ice sheet recession from this sector of PIB by ∼11 cal. kyr BP. In total, seven additional acid insoluble organic (AIO) fraction radiocarbon ages provide a linear age model with an R2 of 0.99. Variations in magnetic susceptibility, grain size, total organic carbon (TOC) and nitrogen, diatom abundance, and foraminiferal assemblage and abundance are used to interpret glacial history and paleoceanographic conditions. Grounding line retreat was characterized by advection of planktic foraminifera beneath an ice shelf that may have extended across the middle continental shelf. Following initial deglaciation, the Cosgrove Ice Shelf covered Ferrero Bay and productivity was virtually absent during the mid-Holocene, while benthic foraminifera indicate periodic incursion of warm Circumpolar Deep Water. The ice shelf persisted until 2.3 cal. kyr BP, when TOC and diatom abundance increased as the bay opened and coastal areas deglaciated. Abundant diatoms demonstrate open marine conditions and seasonal sea ice during the recent open water phase, while high benthic foraminiferal abundance indicates active benthos. The retreat of the Cosgrove Ice Shelf was out of phase with Antarctic Peninsula ice shelves and ice-core proxy temperatures, implying that it did not respond to Holocene climate events but rather to the influence of Circumpolar Deep Water and possibly to internal glacial dynamics. [ABSTRACT FROM AUTHOR]

Abstract: The Amundsen Sea, West Antarctica, is home to the most productive polynyas of the Southern Ocean, where summer primary production can reach 3gCm−2 d−1. The remoteness of this region has meant that systematic studies of biogeochemistry in the Amundsen Sea polynyas have been limited, despite their importance to overall Antarctic shelf productivity and proximity to the fastest thinning glaciers on the continent. Particulate iron inputs to the productive shelf waters of the Amundsen Sea may be important to the overall bioavailability of Fe in this region of natural Fe fertilization. Here we discuss findings from the US–Swedish 2007–08 expedition aboard the I/B Oden, during which 12 stations were sampled for particulate trace metal analyses at depths of 8–800m in the eastern and central polynyas as well as in sea ice covered waters, both on the Amundsen continental shelf and in deep waters north of the shelf break. Suspended particulate samples were collected in two size fractions, 0.45–5μm and >5μm. Particulate Fe concentrations ranged from as low as 10pmolL−1 in open Antarctic Circumpolar Current (ACC) waters off the continental shelf to >100,000pmolL−1 near the Crosson Ice Shelf, and were dominated by particles>5μm at all stations. The relative concentrations of total particulate Fe, Al, Mn and P show the near-ubiquitous influence of crustal particles in the water column at stations on the Amundsen continental shelf. However, many samples had Fe/Al and Mn/Al ratios substantially in excess of mean crustal ratios, especially in the small size fraction (0.45–5μm), suggesting that more labile Fe oxyhydroxides and authigenic MnO2 phases, resulting from sediment resuspension, are also present at relatively high concentrations. In contrast, Fe/P ratios indicate that Fe associated with biogenic particles rarely accounts for more than 20% of total particulate Fe, even in offshore stations. A detailed examination of particulate elemental composition and spatial distribution in the context of water mass temperature and salinity gradients suggests that particle delivery processes associated with melting ice shelves and sediment resuspension dominate the particulate Fe sources to the Amundsen Sea water column. [Copyright &y& Elsevier]

Abstract: Two coastal polynyas in the Amundsen Sea (Amundsen Polynya and Pine Island Polynya) were studied as part of the DynaLiFe project because of their unusually high rates of biological production and close association with melting ice shelves. In this component of the study, satellite imagery of ocean color (using Reprocessing R2009.1) and passive microwave sea ice distributions were used to characterize interannual changes in polynya dynamics and phytoplankton bloom development between 1997 and 2010. Interannual differences in sea ice cover were large in both polynyas, but especially so in the smaller Pine Island Polynya, and were related to synoptic scale wind patterns rather than climate state (ENSO or SAM). Open water season lasted 10 days longer in the Amundsen Polynya (132 days) than in the Pine Island Polynya (122 days). The phytoplankton bloom in the Amundsen Polynya lasted approximately two weeks longer and was 30% more productive per unit area than the bloom in the Pine Island Polynya. Because Pine Island Polynya was lower in both mean open water area and annual net primary production (NPP) per unit area, total annual NPP averaged over the 13-year time series was only 60% of that in the Amundsen Polynya (1.96 and 3.26TgCyr−1, respectively). Given the high interannual variability and the short time series duration, we did not observe a secular trend in primary production or any relationship with climate indices such as the Southern Annular Mode. However, with increased iron addition from melting ice shelves into the surface waters of the polynyas, we expect that primary production could be enhanced in the future. [Copyright &y& Elsevier]