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Pine Island, Thwaites, Smith, and Kohler glaciers in the Amundsen Sea Embayment (ASE) sector of West Antarctica experience rapid mass loss and grounding line retreat due to enhanced ocean thermal forcing from Circumpolar Deep Water (CDW) reaching the grounding lines. We use simulated Lagrangian particles advected with a looping 1 year output from the Southern Ocean high‐resolution model to backtrack the transport and cooling of CDW to these glaciers. For the simulated year 2005–2006, we find that the median time needed to reach the grounding lines from the edge of the ASE is 3 years. In addition, the Antarctic Coastal Current contributes an equal number of particles as off‐shelf sources to the grounding lines of Pine Island and Thwaites. For CDW coming from off‐shelf, results from SOhi indicate that 25%–66% of the cooling occurs within ice shelf cavities. Plain Language Summary: The glaciers in the Amundsen Sea Embayment (ASE) sector of West Antarctica are contributing rapidly to sea level rise in response to enhanced melt by warm, salty Circumpolar Deep Water (CDW). We use an ocean numerical model to trace the sources, pathways, and cooling of warm waters to reach glacier grounding lines. We find that it takes multiple years for off‐shelf perturbations to reach the glaciers and that the colder Antarctic Coastal Current contributes the same amount of water to the grounding lines of Thwaites and Pine Island glaciers as off‐shelf CDW sources. The model also reveals that half of the cooling of CDW occurs under the ice shelves, which is an effect not accounted for in most models used for projecting sea level rise. Key Points: From a 2005–2006 simulation, it takes about 3 years for water to reach the grounding lines from the edge of the Amundsen Sea EmbaymentThe simulation shows the Antarctic Coastal Current contributing equal amounts of water to Pine Island and Thwaites as off‐shelf sourcesSOhi results indicate that 1/4 to 2/3 of the cooling between the shelf break and the grounding lines occurs within the cavities [ABSTRACT FROM AUTHOR]

We investigate the atmospheric drivers of extreme precipitation over the Amundsen Sea Embayment (ASE) of West Antarctica (WA) using daily output from the RACMO2 model and reanalysis data (1979–2016). Overall, 93.7% of days with extreme precipitation at the two coastal stations of ASE are associated with the four dominant Empirical Orthogonal Function (EOF) modes of geopotential height anomalies (at 850 hPa) over WA. The second EOF mode, associated with a coupled pattern consisting of an Amundsen Sea Low and a blocking high to the east, is the main driver of extreme precipitation over ASE, linked to 44.75% of extreme precipitation days. This is followed by EOF‐3 (associated with El Niño Southern Oscillation/PSA‐1), EOF‐4 (likely associated with more frequent "atmospheric river" events), and EOF‐1 (i.e., Southern Annular mode) with a contribution of 22.16%, 21.1%, and 12%, respectively. Extreme precipitation linked to EOF‐2 and EOF‐4 is more intense (by ∼2 mm/day) than the rest. Plain Language Summary: Snowfall is a key component of the mass balance or "stability" of the West Antarctica (WA) ice sheet. Around 35% of the total precipitation over the Amundsen Sea Embayment of WA comes from "extreme" snowfall events. We analyze the output from a regional climate model and global reanalysis data to study the seasonal distribution of these extreme events. We identify the atmospheric patterns responsible for these events and quantify their relative importance. The low pressure system over the Amundsen Sea was found to be the dominant driver of these events. Remote forcing due to periodic warming of the tropical Pacific Ocean can also lead to extreme snowfall over the region. Moreover, atmospheric patterns conducive to frequent "atmospheric rivers" events over WA can cause intense snowfall events in the region. Key Points: The main drivers of extreme snowfall events over the Amundsen Sea Embayment are identified, and their relative contribution is quantifiedCoupled pattern consisting of Amundsen Sea low and a blocking high to the east is the dominant driver of extreme snowfall during 1979–2016El Nin∼ $\tilde{\mathrm{n}}$o Southern Oscillation and "atmospheric rivers" are also suspected to be linked to several extreme snowfall events over the region [ABSTRACT FROM AUTHOR]

Major ice loss in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is hypothesized to have triggered ice sheet collapses during past warm periods such as those in the Pliocene. International Ocean Discovery Program (IODP) Expedition 379 recovered continuous late Miocene to Holocene sediments from a sediment drift on the continental rise, allowing assessment of sedimentation processes in response to climate cycles and trends since the late Miocene. Via seismic correlation to the shelf, we interpret massive prograding sequences that extended the outer shelf by 80 km during the Pliocene through frequent advances of grounded ice. Buried grounding zone wedges indicate prolonged periods of ice‐sheet retreat, or even collapse, during an extended mid‐Pliocene warm period from ∼4.2–3.2 Ma inferred from Expedition 379 records. These results indicate that the WAIS was highly dynamic during the Pliocene and major retreat events may have occurred along the Amundsen Sea margin. Plain Language Summary: Collapses of the West Antarctic Ice Sheet (WAIS) during past warm times are suggested to begin in the Amundsen Sea sector. During a drilling expedition of the International Ocean Discovery Program (IODP), deep‐sea sediments were retrieved from the Amundsen Sea. These sediment cores contain records of colder and warmer periods in the Pliocene (5.3–2.6 million years ago) which relate to the behavior of the WAIS. By analyzing seismic images that allow correlation of sediment layers from the drill sites to the continental shelf, we show that the shelf grew oceanward by 80 km due to the erosion of sediments below the WAIS and their deposition at the shelf break as the result of frequent Pliocene advances of grounded ice across the shelf. The shelf sediment layers contain grounding zone wedges predominantly formed during ice sheet retreat. The preservation of these grounding zone wedges testifies that they were not eroded by subsequent ice advances, with their burial requiring periods of prolonged WAIS retreat during warm intervals. This interpretation is consistent with our IODP drill core observations of a prominent decrease in terrigenous sedimentation between 4.2 and 3.2 million years ago, which indicates a highly dynamic WAIS during the Pliocene. Key Points: Seismic stratigraphic continental rise‐to‐shelf link from International Ocean Discovery Program (IODP) Expedition 379 drill sites on the Amundsen Sea riseMajor prograding shelf growth in the Amundsen Sea Embayment by frequent grounded ice advances in the early PlioceneExtended period of ice sheet retreat in mid‐Pliocene (4.2–3.2 Ma) from buried grounding zone wedges on shelf and core records from rise [ABSTRACT FROM AUTHOR]

Satellite data and a 35-year hindcast of the Amundsen Sea Embayment summer climate using the Weather Research and Forecasting model are used to understand how regional and large-scale atmospheric variability affects thinning of ice shelves in this sector of West Antarctica by melting from above and below (linked to intrusions of warm water caused by anomalous westerlies over the continental shelf edge). El Niño episodes are associated with an increase in surface melt but do not have a statistically significant impact on westerly winds over the continental shelf edge. The location of the Amundsen Sea Low and the polarity of the Southern Annular Mode (SAM) have negligible impact on surface melting, although a positive SAM and eastward shift of the Amundsen Sea Low cause anomalous westerlies over the continental shelf edge. The projected future increase in El Niño episodes and positive SAM could therefore increase the risk of disintegration of West Antarctic ice shelves. [ABSTRACT FROM AUTHOR]

The Antarctic ice sheet is the largest block of ice on Earth, a tiny change of its ice sheet will have a significant impact on sea level change, so it plays an important role in global climate change. The Gravity Recovery and Climate Experiment (GRACE) mission, launched in 2002, provides an alternative method to monitor the Antarctic ice mass variation. The latest Release Level 05 (RL05) version of GRACE time-variable gravity (TVG) data, derived from GRACE observations with improved quality and time-span over 10 years, were released by three GRACE data centers (CSR, JPL and GFZ) in April 2012, which gives us a chance to re-estimate the ice mass change over Antarctic more accurately. In this paper, we examine ice mass changes in regional scale, including Antarctic Peninsula (AP, West Antarctica), Amundsen Sea Embayment (ASE, West Antarctica), Lambert-Amery System (LAS, East Antarctica) and 27 drainage basins based on three data sets. The AP mass change rates are −12. 03±0. 74 Gt/a (CSR, 2004–2012), −13. 92±2. 33 Gt/a (JPL, 2004 −2012), −12. 28±0. 76 Gt/a (GFZ, 2005-2012), with an acceleration of −1.50±0. 25 Gt/a2, −1.54± 0. 26 Gt/a2, −0. 46±0. 28 Gt/a2 respectively, the ASE mass change rates are −89. 22±1. 93 Gt/a, −86. 28± 2. 20 Gt/a, −83. 67±1. 76 Gt/a with an acceleration of −10. 03±0. 65 Gt/a2, −8. 74±0. 74 Gt/a2 and -5. 69 ±0. 68 Gt/a2, and the LAS mass change rates are −4. 31±1. 95 Gt/a, −7. 29±2. 84 Gt/a, 1. 20±1. 35 Gt/a with an acceleration of −0. 18±0. 62 Gt/a2, 3. 55±0. 95 Gt/a2 and 0. 97±0. 49 Gt/a2. The mass change rates derived from the three RL05 data are very close to each other both in AP and ASE with the uncertainties much smaller than the change rates, and mass losses are significantly accelerated since 2007 in AP and 2006 in ASE, respectively. However, the mass change rates are significantly different in LAS, negative rate from CSR and JPL data, but positive rate from GFZ data, the uncertainties are even larger than the correspondent change rates. With regard to the 27 drainage basins, seven basins (basin 3–9) located in the east Antaxctica show positive mass change rates, and the rest twenty basins are chamcterizecl by negative mass change rates during the time span of the three RL05 data.

Ice streams in the Pine Island-Thwaites region of West Antarctica currently dominate contributions to sea level rise from the Antarctic ice sheet. Predictions of future ice-mass loss from this area rely on physical models that are validated with geological constraints on past extent, thickness and timing of ice cover. However, terrestrial records of ice sheet history from the region remain sparse, resulting in significant model uncertainties. We report glacial-geological evidence for the duration and timing of the last glaciation of Hunt Bluff, in the central Amundsen Sea Embayment. A multi-nuclide approach was used, measuring cosmogenic 10 Be and in situ 14 C in bedrock surfaces and a perched erratic cobble. Bedrock 10 Be ages (118–144 ka) reflect multiple periods of exposure and ice-cover, not continuous exposure since the last interglacial as had previously been hypothesized. In situ 14 C dating suggests that the last glaciation of Hunt Bluff did not start until 21.1 ± 5.8 ka – probably during the Last Glacial Maximum – and finished by 9.6 ± 0.9 ka, at the same time as ice sheet retreat from the continental shelf was complete. Thickening of ice at Hunt Bluff most likely post-dated the maximum extent of grounded ice on the outer continental shelf. Flow re-organisation provides a possible explanation for this, with the date for onset of ice-cover at Hunt Bluff providing a minimum age for the timing of convergence of the Dotson and Getz tributaries to form a single palaeo-ice stream. This is the first time that timing of onset of ice cover has been constrained in the Amundsen Sea Embayment. [ABSTRACT FROM AUTHOR]

Focused research on the Pine Island and Thwaites glaciers, which drain the West Antarctic Ice Shelf (WAIS) into the Amundsen Sea Embayment (ASE), revealed strong signs of instability in recent decades that result from variety of reasons, such as inflow of warmer ocean currents and reverse bedrock topography, and has been established as the Marine Ice Sheet Instability hypothesis. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e., basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations. We present an approach to investigate ranges of realistic heat fluxes in the ASE by different methods, discuss direct observations, and 3-D numerical models that incorporate boundary conditions derived from various geophysical studies, including our new Depth to the Bottom of the Magnetic Source (DBMS) estimates. Our in situ temperature measurements at 26 sites in the ASE more than triples the number of direct GHF observations in West Antarctica. We demonstrate by our numerical 3-D models that GHF spatially varies from 68 up to 110 mW m−2. [ABSTRACT FROM AUTHOR]

The Amundsen Sea Embayment of West Antarctica hosts one of the most rapidly changing sectors of the West Antarctic Ice Sheet. With the fastest-flowing ice streams in Antarctica, the region around Pine Island Bay is characterized by rapid ice-sheet thinning and grounding-line retreat. Published surface-exposure data are limited to a few isolated nunataks making it difficult to assess the long-term deglacial history of the area. To address this, we correlate existing records of lateral ice-stream retreat from marine sediment cores with onshore glacial thinning in two key areas of eastern Marie Byrd Land: the Kohler Range and Pine Island Bay. Our 10Be surface-exposure ages are the first from the isolated Kohler Range and show that the nunataks there became ice-free between 8.6 and 12.6 ka. This implies a minimum long-term average thinning rate of 3.3 ± 0.3 cm/yr, which is one order of magnitude lower than recent rates based on satellite data. We also present pre- to early Holocene 10Be surface-exposure ages from two islands located approximately 80 km downstream of the Pine Island Glacier ice-shelf front to constrain the lateral deglacial history in the Pine Island Bay area. This study provides insight into the significance of local ice sheet variations and suggests that the post-LGM history in the Amundsen Sea sector was characterized by glacial thinning as well as lateral retreat in pre- to early Holocene times. [ABSTRACT FROM AUTHOR]

• Deglacial history of Pope Glacier reconstructed using surface exposure dating. • 560 m of early Holocene thinning at an average rate of 0.13 ± 0.09/0.04 m yr−1. • Thinning coincides with enhanced upwelling of warm water onto continental shelf. • Reduction in ice shelf buttressing may have triggered widespread glacier thinning. The Amundsen Sea sector of the Antarctic ice sheet presently dominates the contribution from Antarctica to sea level rise. Several large ice streams that currently drain the sector have experienced rapid flow acceleration, grounding line retreat and thinning during the past few decades. However, little is known of their longer-term – millennial-scale – retreat history, despite the reliance of several ice sheet and glacial-isostatic adjustment models on such data for improving sea level prediction from this critical region. This study investigates the timing and extent of surface lowering of one of those ice streams, Pope Glacier, since the Last Glacial Maximum (LGM), using glacial-geological evidence for former ice cover. We present a new deglacial chronology for the glacier, derived from surface exposure dating of glacially-deposited cobbles and ice-scoured bedrock from Mount Murphy and its surrounding peaks. Cosmogenic 10Be exposure ages from 44 erratic cobbles and 5 bedrock samples, and in situ 14C exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5.5-16 ka. Although 10Be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples, none of the ages pre-date the LGM. From these results we infer that the surface of Pope Glacier lowered by 560 m during the early- to mid-Holocene (9-6 ka), at an average rate of 0.13 ± 0.09/0.04 m yr−1. The lowering coincided with a period of enhanced upwelling of warm Circumpolar Deep Water onto the continental shelf in the region. A reduction in buttressing − facilitated by such upwelling − by an ice shelf that is thought to have spanned the embayment until 10.6 cal kyr BP could have triggered simultaneous early Holocene thinning of Pope Glacier and glaciers elsewhere in the Amundsen Sea Embayment. [ABSTRACT FROM AUTHOR]

Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β , of 1.5-1.7 with 80-100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts., Key Points: Abbot Ice Shelf is underlain by E-W rift basins created at ∼90 Ma Amundsen shelf shaped by subsidence, sedimentation, and passage of the ice sheet Bellingshausen plate boundary is located near the base of continental slope and rise.