Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea level rise. Increased oceanic melting of its ice shelf is thought to have triggered those changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.One Sentence Summary: Ocean melting of the Pine Island Glacier ice shelf was halved in two years as an underlying seabed ridge makes it highly sensitive to climatic forcing. Main Text:Austral summer observations in the Amundsen Sea, West Antarctica, show that lightlymodified, warm (0.5-1.2°C) and saline (>34.6) Circumpolar Deep Water (CDW), 2-4°C above the in-situ freezing point, pervades a network of glacially scoured seabed troughs (1, Fig. 1A).The CDW reaches nearby Antarctic glaciers and delivers heat to the base of their 200-1000 mthick ice shelves (2-4). It is overlain by a 200-300 m-thick layer of cold (-1.5°C) and fresh (salinity<34.4) Winter Water (WW, Fig. 2A) that is seasonally replenished by interaction with the atmosphere and sea ice.Pine Island Glacier (PIG), a major outlet glacier feeding one such ice shelf, has shown apparently continuous thinning (5, 6) and intermittent acceleration (7-9) from 1973 to 2009.During this period, its ice shelf has also thinned (6,(10)(11)(12), and the reduction in buttressing driven by oceanic melting is believed to be responsible for the changes inland. Earlier analysis indicated that a higher CDW volume and temperature in Pine Island Bay (PIB) in January 2009caused an increase in ice-shelf melting and in the associated meltwater-driven circulation, relative to 1994 (2). The lack of sub-annual variability in CDW temperature during one yearlong measurement in PIB (1) and the long-term correlation between the oceanic melting and the mass loss required to sustain thinning of the ice shelf gave the impression that the ice-ocean system had shown progressive change over the last two decades. This is consistent with a positive geometrical feedback, with oceanic melt enlarging the cavity under the ice shelf, allowing stronger circulation and further melting.However, such ice-ocean systems are likely to be more complex. The glacier's rapid change over the last few decades was probably triggered by its ungrounding from a the top of a seabed ridge transverse to the ice flow at some time before the 1970s (4). Subsequent migration of the glacier's grounding line (13) down the seabed slope upstream from the ridge crest was probably an inevitable respon...
In 1994, ocean measurements near Antarctica's Pine Island Glacier showed that the ice shelf buttressing the glacier was melting rapidly 1 . This melting was attributed to the presence of relatively warm, deep water on the Amundsen Sea continental shelf. Heat, salt and ice budgets along with ocean modelling provided steady-state calving and melting rates 2,3 . Subsequent satellite observations and modelling have indicated large system imbalances, including ice-shelf thinning and more intense melting, glacier acceleration and drainage basin drawdown 4-10 . Here we combine our earlier data with measurements taken in 2009 to show that the temperature and volume of deep water in Pine Island Bay have increased. Ocean transport and tracer calculations near the ice shelf reveal a rise in meltwater production by about 50% since 1994. The faster melting seems to result mainly from stronger sub-ice-shelf circulation, as thinning ice has increased the gap above an underlying submarine bank on which the glacier was formerly grounded 11 . We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 • C above freezing can now more readily access the grounding zone.In the eastern Amundsen Sea (Fig. 1), Circumpolar Deep Water (CDW) intrudes onto the continental shelf at depths >300 m, salinities >34.6 and temperatures >3.5 • C above the freezing point (T − T f , Fig. 2a). A thermocline (halocline) defined by strong vertical temperature (salinity) gradients lies between the relatively warm, salty CDW and colder, fresher surface waters. Although water in and above the resulting pycnocline (density gradient) is cooled and modified by sea surface processes, the CDW warmth and large areas of fast ice prevent formation of the cold, high-salinity shelf water that characterizes most other sectors of the Antarctic continental shelf. The new measurements show the pycnocline deepening southward into Pine Island Bay (PIB), then rising within a persistent cyclonic gyre also delineated by shipboard Acoustic Doppler Current Profiling (ADCP). Outer continental shelf sills may preferentially channel CDW across the shelf break 12,13 , but a persistent temperature maximum extending southward from the upper CDW in Fig. 2a suggests inflows well above the sea floor. Temperature and salinity decrease by ∼0.5 • C and 0.05 along the 27.75 isopycnal surface, with most of that change occurring over the outer shelf. Autosub mapped seawater properties beneath the Pine Island Glacier ice shelf (PIG), finding the same isopycnal directly above the crest of a 300 m-high, 700 m-deep submarine ridge transverse to the ice flow direction 11 . The properties of sea water with access to the PIG grounding zone differ little from those in the upper CDW >400 km away.Combining the
Thinning ice in West Antarctica
Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or ongoing, possibly unstable response to a past climate anomaly. Lengthening satellite records appear incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000-2016 near Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of ~4 between cool and warm extremes via a non-linear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice shelf thinning and retreat of the grounding line, which readvanced during a post-2011 cool phase. Those observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The non-linear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high.
Recent ice loss from the West Antarctic Ice Sheet has been caused by ocean melting of ice 9 shelves in the Amundsen Sea. Eastward wind anomalies at the shelf break enhance the 10 import of warm Circumpolar Deep Water onto the Amundsen Sea continental shelf, which 11 creates transient melting anomalies with an approximately decadal period. No 12 anthropogenic influence on this process has been established. Here, we combine 13 observations and climate model simulations to suggest that increased greenhouse-gas 14 forcing caused shelf-break winds to transition from mean easterlies in the 1920s to the 15 near-zero mean zonal winds of the present day. Strong internal climate variability, primarily 16 linked to the tropical Pacific, is superimposed upon this forced trend. We infer that the 17 Amundsen Sea experienced decadal ocean variability throughout the 20 th century, with 18 warm anomalies gradually becoming more prevalent, offering a credible explanation for 19 the ongoing ice loss. Existing climate model projections show that strong future 20 greenhouse-gas forcing creates persistent mean westerly shelf-break winds by 2100, 21 suggesting a further enhancement of warm ocean anomalies. These wind changes are 22 weaker under a scenario in which greenhouse gases are stabilised.23
[1] A dense grid of ice-penetrating radar sections acquired over Pine Island Glacier, West Antarctica has revealed a network of sinuous subglacial channels, typically 500 m to 3 km wide, and up to 200 m high, in the ice-shelf base. These subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf. Above the apex of most channels, the radar shows isolated reflections from within the ice shelf.Comparison of the radar data with acoustic data obtained using an autonomous submersible, confirms that these echoes arise from open basal crevasses 50-100 m wide aligned with the subglacial channels and penetrating up to 1/3 of the ice thickness. Analogous sets of surface crevasses appear on the ridges between the basal channels. We suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition. Finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and, if the channels were formed sufficiently rapidly, would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets. We conclude that ice-shelf basal melting plays a role in determining patterns of surface and basal crevassing. Increased delivery of warm ocean water into the sub-ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf, perhaps, as a prelude to eventual collapse.
A 1/12° ocean model configuration of the Amundsen Sea sector is developed to better understand the circulation induced by ice‐shelf melt and the impacts on the surrounding ocean and sea ice. Eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf/ocean interface are performed. The total melt rate simulated in each cavity is function of the thermal Stanton number, and for a given thermal Stanton number, melt is slightly higher for lower values of the drag coefficient. Sub‐ice‐shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity. The related volume flux into a cavity is 100–500 times stronger than the melt volume flux itself. Ice‐shelf melt also induces a coastal barotropic current that contributes 45 ± 12% of the total simulated coastal transport. Due to the presence of warm circumpolar deep waters, the melt‐induced inflow typically brings 4–20 times more heat into the cavities than the latent heat required for melt. For currently observed melt rates, approximately 6–31% of the heat that enters a cavity with melting potential is actually used to melt ice shelves. For increasing sub‐ice‐shelf melt rates, the transport in the cavity becomes stronger, and more heat is pumped from the deep layers to the upper part of the cavity then advected toward the ocean surface in front of the ice shelf. Therefore, more ice‐shelf melt induces less sea‐ice volume near the ice sheet margins.
Pine Island Glacier (PIG) terminates in a rapidly melting ice shelf, and ocean circulation and temperature are implicated in the retreat and growing contribution to sea level rise of PIG and nearby glaciers. However, the variability of the ocean forcing of PIG has been poorly constrained due to a lack of multi-year observations. Here we show, using a unique record close to the Pine Island Ice Shelf (PIIS), that there is considerable oceanic variability at seasonal and interannual timescales, including a pronounced cold period from October 2011 to May 2013. This variability can be largely explained by two processes: cumulative ocean surface heat fluxes and sea ice formation close to PIIS; and interannual reversals in ocean currents and associated heat transport within Pine Island Bay, driven by a combination of local and remote forcing. Local atmospheric forcing therefore plays an important role in driving oceanic variability close to PIIS.
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