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Over recent decades the primary climatic forcing on the ice shelves in the Amundsen Sea Embayment (ASE) has been the influx of warm water into sub‐ice‐shelf ocean cavities. By contrast, the contribution of a globally warming atmosphere has been negligible but may play a more significant role in the future. Warm, moist air intrusions from the ocean onto the ice sheet surface play an essential role in the surface energy budget and constitute the primary driver for surface melting in West Antarctica. This study employs observations from automatic weather stations deployed on the Pine Island Glacier and numerical model outputs from the Antarctic Mesoscale Prediction System to investigate warm air intrusion (WAI) events and examine their impacts on surface glacier melt in the ASE. A deep low‐pressure center over the Amundsen/Ross Seas and a blocking ridge over the southeast Pacific created favorable conditions for developing an atmospheric river that directs warm and moist air toward the ASE. An analysis of meteorological observations through 2013–2014 shows that 2013 received two times higher frequency WAI events than 2014. 2013 year coincides with more substantial pressure gradients in the western Ross Sea and the eastern Amundsen Sea. The February and March 2013 WAI events induce about 3 days of surface melting over the Pine Island Glacier and the Thurston Island area. A continued increase in the large‐scale advection of warm air from the midlatitudes toward the ASE could lead to an increased surface melting frequency with implications for ice shelves in the study area. Key Points: Warm air intrusion events over the Amundsen Sea Embayment are shown to be caused by the large‐scale moisture advection associated with an atmospheric river2013 experienced two times as higher frequency warm air intrusion events than 2014 due in part to stronger pressure gradients in the study areaWarm air intrusion events cause about 3 days of surface melting over the Pine Island Glacier [ABSTRACT FROM AUTHOR]

Dotson Ice Shelf (DIS) in West Antarctica is undergoing rapid basal melting driven by intrusions of warm, saline Circumpolar Deep Water (CDW) onto the continental shelf. Meltwater from DIS is thought to influence biology in the adjacent Amundsen Sea Polynya (ASP), which exhibits the highest net primary productivity (NPP) per unit area of any coastal polynya in the Southern Ocean. However, the relative importance of iron and light in colimiting the spring phytoplankton bloom in the ASP remains poorly understood. In this modeling study we first investigate the mechanisms by which ice shelves impact NPP, then map spatio-temporal patterns in iron-light colimitation, and finally examine the environmental drivers of iron and light supply. We find that ice shelf melting leads to greater upper ocean iron concentrations, both directly due to release of iron from sediments entrained at the glacier bed, and indirectly via a buoyancy-driven overturning circulation which pulls iron from CDW to the surface. Both of these mechanisms increase NPP compared to experiments where ice shelf melt is suppressed. We then show that the phytoplankton self-shading feedback delays the bloom and reduces peak NPP by 80% compared to experiments where light penetration is independent of chlorophyll. Compared to light limitation, iron limitation due to phytoplankton uptake is more important a) later in the season, b) higher in the water column, and c) further from the ice shelf. Finally, sensitivity experiments show that variability in CDW intrusion influences NPP by controlling the horizontal spreading of iron-rich meltwater. Plain Language Summary The seas around Antarctica are covered for much of the year with sea ice. When gaps (known as polynyas) develop, the exposed ocean surface can exchange carbon dioxide with the atmosphere. These polynyas often host large seasonal blooms of floating algae--phytoplankton--which take up carbon as they photosynthesize. The most intense blooms tend to form close to the floating ice shelves which form the margins of the Antarctic Ice Sheet. In this study we construct a computer model of a polynya off West Antarctica. We show that the ice shelf helps to supply phytoplankton with the iron that they need to grow, allowing the polynya to take up more carbon. On the other hand, the early growth of the bloom results in a progressive "greening" of the polynya, which prevents sufficient light from reaching deeper phytoplankton. This slows the development of the bloom and reduces overall carbon uptake of the polynya. Similarly, the uptake of iron by growing phytoplankton reduces the availability of iron at later times in the summer growing season. Finally, we show that the phytoplankton bloom is smaller in some years with higher melt rates, due to a trapping of meltwater close to the coastline. [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]

The Amundsen Sea Polynya (ASP) is distinguished by having the highest net primary production per unit area in the coastal Antarctic. Recent studies have related this high productivity to the presence of fast‐melting ice shelves, but the mechanisms involved are not well understood. In this study we describe the first numerical model of the ASP to represent explicitly the ocean‐ice interactions, nitrogen and iron cycles, and the coastal circulation at high resolution. The study focuses on the seasonal cycle of iron and carbon, and the results are broadly consistent with field observations collected during the summer of 2010–2011. The simulated biogeochemical cycle is strongly controlled by light availability(dictated by sea ice, phytoplankton self‐shading, and variable sunlight). The micronutrient iron exhibits strong seasonality, where scavenging by biogenic particles and remineralization play large compensating roles. Lateral fluxes of iron are also important to the iron budget, and our results confirm the key role played by inputs of dissolved iron from the buoyancy‐driven circulation of melting ice shelf cavities (the "meltwater pump"). The model suggests that westward flowing coastal circulation plays two important roles: it provides additional iron to the ASP and it collects particulate organic matter generated by the bloom and transports it to the west of the ASP. As a result, maps of vertical particulate organic matter fluxes show highest fluxes in shelf regions located west of the productive central ASP. Overall, these model results improve our mechanistic understanding of the ASP bloom, while suggesting testable hypotheses for future field efforts. Plain Language Summary: Winds in the coastal Antarctic regions tend to create openings in the sea ice cover called "coastal polynyas." Such polynyas are found in multiple locations around the Antarctic continent including in the Amundsen Sea. This particular embayment features the fastest‐melting ice shelves of the continent, but it is also distinguished by a large production of algae during the austral summer season. Recent studies have reported a statistical link between these two features but the mechanisms involved are not well understood. We use advanced computer simulations to gain insight into these mechanisms and use measurements collected in the Amundsen Sea to evaluate how realistic the simulations are. The results emphasize the two roles of the coastal circulation: it provides a necessary micronutrient (iron) to the algae and it also transports the algae away from where it grows during the summer. The computer simulations improve our understanding of why the Amundsen Sea Polynya is so productive, and they suggest new hypotheses that can be tested in future field efforts. Key Points: This is the first study that uses a 3‐D ice shelf‐sea ice‐ocean coupled model to represent the phytoplankton bloom of the Amundsen Sea PolynyaOceanic circulation plays two roles: provides dissolved iron to the polynya (meltwater pump) and also transports organic matter westwardModel results improve our mechanistic understanding of the ASP bloom while suggesting testable hypotheses for future field efforts [ABSTRACT FROM AUTHOR]

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]

Basal melting of Antarctic ice shelves is expected to increase during the twenty-first century as the ocean warms, which will have consequences for ice sheet stability and global sea level rise. Here we present future projections of Antarctic ice shelf melting using the Finite Element Sea Ice/Ice-Shelf Ocean Model (FESOM) forced with atmospheric output from models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). CMIP5 models are chosen based on their agreement with historical atmospheric reanalyses over the Southern Ocean; the best-performing models are ACCESS 1.0 and the CMIP5 multimodel mean. Their output is bias-corrected for the representative concentration pathway (RCP) 4.5 and 8.5 scenarios. During the twenty-first-century simulations, total ice shelf basal mass loss increases by between 41% and 129%. Every sector of Antarctica shows increased basal melting in every scenario, with the largest increases occurring in the Amundsen Sea. The main mechanism driving this melting is an increase in warm Circumpolar Deep Water on the Antarctic continental shelf. A reduction in wintertime sea ice formation simulated during the twenty-first century stratifies the water column, allowing a warm bottom layer to develop and intrude into ice shelf cavities. This effect may be overestimated in the Amundsen Sea because of a cold bias in the present-day simulation. Other consequences of weakened sea ice formation include freshening of High Salinity Shelf Water and warming of Antarctic Bottom Water. Furthermore, freshening around the Antarctic coast in our simulations causes the Antarctic Circumpolar Current to weaken and the Antarctic Coastal Current to strengthen. [ABSTRACT FROM AUTHOR]

The Amundsen Sea is a key region of Antarctica where ocean, atmosphere, sea ice, and ice sheet interact. For much of Antarctica, the relatively warm water of the open Southern Ocean (a few degrees above freezing) does not reach the Antarctic continental shelf in large volumes under current climate conditions. However, in the Amundsen Sea, warm water penetrates onto the continental shelf and provides heat that can melt the underside of the area's floating ice shelves, thinning them. Here, we discuss how the ocean's role in melting has come under increased scrutiny, present 2014 observations from the Amundsen Sea, and discuss their implications, highlighting aspects where understanding is still incomplete. [ABSTRACT FROM AUTHOR]

The grounded ice in the Totten and Dalton glaciers is an essential component of the buttressing for the marine-based Aurora basin, and hence their stability is important to the future rate of mass loss from East Antarctica. Totten and Vanderford glaciers are joined by a deep east-west running subglacial trench between the continental ice sheet and Law Dome, while a shallower trench links the Totten and Dalton glaciers. All three glaciers flow into the ocean close to the Antarctic circle and experience ocean-driven ice shelf melt rates comparable with the Amundsen Sea Embayment. We investigate this combination of trenches and ice shelves with the BISICLES adaptive mesh ice-sheet model and ocean-forcing melt rates derived from two global climate models. We find that ice shelf ablation at a rate comparable with the present day is sufficient to cause widespread grounding line retreat in an east-west direction across Totten and Dalton glaciers, with projected future warming causing faster retreat. Meanwhile, southward retreat is limited by the shallower ocean facing slopes between the coast and the bulk of the Aurora sub-glacial trench. However the two climate models produce completely different future ice shelf basal melt rates in this region: HadCM3 drives increasing sub-ice shelf melting to ~2150, while ECHAM5 shows little or no increase in sub-ice shelf melting under the two greenhouse gas forcing scenarios. [ABSTRACT FROM AUTHOR]

The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using 18 years of continuous satellite radar altimeter observations, we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 cubic kilometers per year for 1994-2003 to rapid loss of 310 ± 74 cubic kilometers per year for 2003-2012. West Antarctic losses increased by ~70% in the past decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades. [ABSTRACT FROM AUTHOR]

The article discusses the documentation of the spatial distribution of long-term large-scale trends in temperature, salinity, and core depth over the Antarctic continental shelf and slope. Topics include the role of ice shelf basal melt in Antarctic ice sheet mass loss, circulation of Circumpolar Deep Water (CDW), hydrographic properties of the Antarctic Continental Shelf Bottom Water (ASBW) temperature, and factors contributing to warming at the seabed in the Bellingshausen and Amundsen seas.

The ice shelves in the Amundsen Sea are thinning rapidly, and the main reason for their decline appears to be warm ocean currents circulating below the ice shelves and melting these from below. Ocean currents transport warm dense water onto the shelf, channeled by bathymetric troughs leading to the deep inner basins. A hydrographic mooring equipped with an upward-looking ADCP has been placed in one of these troughs on the central Amundsen shelf. The two years (2010/11) of mooring data are here used to characterize the inflow of warm deep water to the deep shelf basins. During both years, the warm layer thickness and temperature peaked in austral fall. The along-trough velocity is dominated by strong fluctuations that do not vary in the vertical. These fluctuations are correlated with the local wind, with eastward wind over the shelf and shelf break giving flow toward the ice shelves. In addition, there is a persistent flow of dense lower Circumpolar Deep Water (CDW) toward the ice shelves in the bottom layer. This bottom-intensified flow appears to be driven by buoyancy forces rather than the shelfbreak wind. The years of 2010 and 2011 were characterized by a comparatively stationary Amundsen Sea low, and hence there were no strong eastward winds during winter that could drive an upwelling of warm water along the shelf break. Regardless of this, there was a persistent flow of lower CDW in the bottom layer during the two years. The average heat transport toward the ice shelves in the trough was estimated from the mooring data to be 0.95 TW. [ABSTRACT FROM AUTHOR]

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]

The thinning and acceleration of the West Antarctic Ice Sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf. A hydrographic section including lowered acoustic Doppler current profiler measurements showing such an inflow in the channel leading to the Getz and Dotson Ice Shelves is presented here. The flow rate was 0.3–0.4 Sv (1 Sv ≡ 106 m3 s−1), and the subsurface heat loss was estimated to be 1.2–1.6 TW. Assuming that the inflow persists throughout the year, it corresponds to an ice melt of 110–130 km3 yr−1, which exceeds recent estimates of the net ice glacier ice volume loss in the Amundsen Sea. The results also show a 100–150-m-thick intermediate water mass consisting of Circumpolar Deep Water that has been modified (cooled and freshened) by subsurface melting of ice shelves and/or icebergs. This water mass has not previously been reported in the region, possibly because of the paucity of historical data. [ABSTRACT FROM AUTHOR]

The article discusses findings of autonomous underwater vehicle exploration of the ocean cavity beneath an Antarctic ice shelf focusing on the Pine Island Glacier (PIG) in Amundsen Sea, Antarctica. It relates the rise of the global mean sea level due to the mass loss from the Antarctic Ice Sheet. It describes the growth in the submarine melting of PIG based on observations aboard the research vessel, Nathaniel B. Palmer.