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...

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.

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.

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.