Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf

Abstract

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

Sustained retreat of the Pine Island Glacier

“…Although additional grounding line retreat may result from creep thinning associated with altered ice stream dynamics [Thomas et al, 2011], our observations of accelerated thinning are consistent with measurements of changing oceanographic conditions at the glacier terminus over the period 1994-2009 [Jacobs et al, 2011]. These oceanographic data reveal strengthening circulation in an enlarged sub-ice shelf cavity, leading to faster ice melting.…”

“…The wide range of current sea level projections (2-14 cm by the year 2100) is bounded, at the lower end, by the results of a coupled ice-ocean interaction model of the glacier response to modest ocean forcing [Joughin et al, 2010] and, at the upper end, by the results of a hypothetical assessment of ice mass losses under a heuristic scaling of the present-day imbalance [Pfeffer et al, 2008]. Our observations of accelerated thinning and ongoing rapid hinge-line retreat, coupled with the observed strengthening of ocean melting within the sub-ice shelf cavity [Jacobs et al, 2011], suggest that the lower limit may be found on a conservative forcing scenario. An accurate projection of the glacier response to climate forcing will need to capture details of how the ocean circulates within the sub-ice shelf cavity and how it interacts with the ice shelf base.…”

Complex network of channels beneath an Antarctic ice shelf

“…More recent studies found basal crevasses and channels of various sizes oriented both perpendicular and parallel to the main ice-flow direction at Larsen C, Amery and Getz, Pine Island Glacier, and Fimbul, as well as in Greenland [Luckman et al, 2012;McGrath et al, 2012;Vaughan et al, 2012;Mankoff et al, 2012;Humbert and Steinhage, 2011;Nicholls et al, 2006;Rignot and Steffen, 2008]. The majority of the basal channels reported to date are from ice shelves exposed to ocean waters with a high thermal potential for melting the overlying ice shelf, such as at Pine Island Glacier [Jacobs et al, 2011;Dutrieux et al, 2013]. Based on these reports and recent observations, a number of mechanisms have been proposed by which basal channels can be formed.…”

The interdisciplinary marine system of the Amundsen Sea, Southern Ocean: Recent advances and the need for sustained observations

“…The observed losses on the ice sheet in the Amundsen are due primarily to thinning of the floating ice shelves (Paolo et al, 2015) caused by basal melting driven by warm ocean water that floods the continental shelf and accesses the ice shelf cavities via submarine channels in the seafloor (Jacobs et al, 2011;Pritchard et al, 2012). Warm dense inflows have been observed on the Amundsen Sea shelf Ha et al, 2014;Jacobs et al, 2011;Wåhlin et al, 2010;Walker et al, 2007) and several processes are hypothesized to regulate the inflow of warm deep water. Putative drivers transporting warm water onto the shelf include eastward undercurrents (Chavanne et al, 2010;Walker et al, 2013), bottom Ekman transports (Wåhlin et al, 2012), eddies and wind Thoma et al, 2008;Wåhlin et al, 2013).…”

“…The nearby Getz Ice Shelf is the largest contributor to the overall volume loss of Antarctic ice shelves, with an average loss of -54 ± 5 Gt yr -1 (Paolo et al, 2015). Changes are believed to be due to increasing access of heat from the deep waters of the ACC to the underside of the ice shelves, via the penetration of these waters onto and across the shelf (Jacobs et al, 2011;Schmidtko et al, 2014), although this has not yet been shown in synoptic time series of basal melt and ocean temperature. This has significant consequences for global sea-level rise, a fact underlined by recent assertions that the retreat of this part of the ice sheet is irreversible (Rignot et al, 2014).…”