Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability

Abstract

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...

Sensitivity of the dynamics of Pine Island Glacier, West Antarctica, to climate forcing for the next 50 years

“…The other mode of retreat is a grounding line retreat of several hundreds of kilometers per century. This mode is consistent with control run and m1 scenarios of Favier et al between the basal melting rates that we are applying (based on high-resolution MITgcm simulations), which are significantly lower that the ones used in Favier et al (2014).…”

“…Williams et al (2012) concluded from a model study that high-frequency forcings (decadal to subdecadal) are transmitted by membrane stress and not by driving stress, and rapidly propagate several tens of kilometers inland. Favier et al (2014) used a three-dimensional (3-D) full Stokes (FS) model and parameterization of the oceaninduced melting rate to study the grounding line retreat of Pine Island Glacier. They showed that the grounding line of Pine Island Glacier is likely to have started an irreversible retreat on the downward sloping bed of the main trunk and that its contribution to sea level rise could reach 100 Gt yr −1 in the next 20 yr.…”

Increased West Antarctic and unchanged East Antarctic ice discharge over the last 7 years

“…Increased ice discharge from the Amundsen Sea Embayment and subsequent partial re-stabilization have been attributed to changes in ice shelf buttressing (Jacobs et al, 1996;Macgregor et al, 2012) that resulted from increased ice shelf basal melt rates (Jacobs et al, 2011;Jenkins et al, 1997) and more recently to a decrease in ocean melting resulting from changes in the temperature of intermediate depth waters (Dutrieux et al, 2014). Increased discharge from glaciers feeding into the Getz Ice Shelf is likely in response to rapid thinning of the ice shelf due to changes in ocean circulation and the depth of warmer modified Circumpolar Deep Water .…”

Increased West Antarctic ice discharge and East Antarctic stability over the last seven years

“…a glacier can decelerate but still be discharging ice at a rate that exceeds the surface mass budget minus basal melt. Increased ice discharge from the Amundsen Sea Embayment and subsequent partial re-stabilization have been attributed to changes in ice shelf buttressing (Jacobs et al, 1996;Macgregor et al, 2012) that resulted from increased ice shelf basal melt rates (Jacobs et al, 2011;Jenkins et al, 490 1997) and more recently to a decrease in ocean melting resulting from changes in the temperature of intermediate depth waters (Dutrieux et al, 2014). Increased discharge from glaciers feeding into the Getz Ice Shelf is likely in response to rapid thinning of the ice shelf due to changes in ocean circulation and the depth of warmer modified Circumpolar Deep Water .…”