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...

Observations beneath the floating section of Pine Island Glacier have revealed the presence of a subglacial ridge which rises up to 300 m above the surrounding bathymetry. This topographic feature probably served as a steady grounding line position until sometime before the 1970s, when an ongoing phase of rapid grounding line retreat was initiated. As a result, a large ocean cavity has formed behind the ridge, strongly controlling the ocean circulation beneath the ice shelf and modulating the ocean water properties that cause ice melting in the vicinity of the grounding line. In order to understand how melt rates have changed during the various phases of cavity formation, we use a high-resolution ocean model to simulate the cavity circulation for a series of synthetic geometries. We show that the height of the ridge and the gap between the ridge and ice shelf strongly control the inflow of warm bottom waters into the cavity, and hence the melt rates. Model results suggest a rapid geometrically controlled increase of meltwater production at the onset of ice thinning, but a weak sensitivity to geometry once the gap between the ridge and ice shelf has passed a threshold value of about 200 m. This provides evidence for a new, coupled, ice-ocean feedback acting to enhance the initial retreat of an ice stream from a bedrock high. The present gap is over 200 m, and our results suggest that observed variability in melt rates is now controlled by other factors, such as the depth of the thermocline.

Abstract. We use repeat-pass SAR data to produce detailed maps of surface motion covering the glaciers draining into the former Larsen B Ice Shelf, Antarctic Peninsula, for different epochs between 1995 and 2013. We combine the velocity maps with estimates of ice thickness to analyze fluctuations of ice discharge. The collapse of the central and northern sections of the ice shelf in 2002 led to a near-immediate acceleration of tributary glaciers as well as of the remnant ice shelf in Scar Inlet. Velocities of most of the glaciers discharging directly into the ocean remain to date well above the velocities of the pre-collapse period. The response of individual glaciers differs and velocities show significant temporal fluctuations, implying major variations in ice discharge as well. Due to reduced velocity and ice thickness the ice discharge of Crane Glacier decreased from 5.02 Gt a −1 in 2007 to 1.72 Gt a −1 in 2013, whereas Hektoria and Green glaciers continue to show large temporal fluctuations in response to successive stages of frontal retreat. The velocity on Scar Inlet ice shelf increased 2-3-fold since 1995, with the largest increase in the first years after the break-up of the main section of Larsen B. Flask and Leppard glaciers, the largest tributaries to Scar Inlet ice shelf, accelerated. In 2013 their discharge was 38 % and 46 % higher than in 1995.