This post is a follow-up to my last post, where I promised to elaborate on possible implications of recently identified changes to "lens of water" or subglacial cavities, upstream of the grounding lines of key marine glaciers in West Antarctica.
The first image shows a figure showing the location of recently active subglacial cavities in Antarctica, where "active" means that sufficient water has either entered or been discharged from the subglacial cavity to be measured by satellite altimetry since the satellite data became good enough (circa mid-1990's). Thus these subglacial cavities naturally/normally discharge water downstream (or to the ocean) and accumulate basal glacial meltwater from upstream. However, limited model findings indicate that with continued global warming that not only will the grounding lines migrate upstream but that these subglacial cavities will become more active, and will grow until these merge with the grounding lines (see the second attached image).
Now I note that with continued global warming the Antarctic ice shelves both cold (like FRIS, RIS and Amery) and warm (like PIIS and Thwaites) will continue to thin (due both to basal ice melting and acceleration of the ice flow velocities) which will markedly increasing their susceptibility to abrupt ice mass loss events both now and in the next few decades due to events that might increase the hydrodynamic, and hydrostatic, pressure within the sub-ice-shelf cavities. Sources of hydrodynamic & hydrostatic pressure that could destabilize the Pine Island Ice Shelf, PIIS (or other marine glaciers with rapidly retreating grounding lines), include:
(a) Large El Nino events, could temporarily raise eustatic sea level by 6 to 8mm (due to increased rainfall over the ocean and concurrent increased drought over land) over a one or two year period, and could also induce the ABSL to direct more wind and ocean currents into the ASE,
(b) Accelerated land water mining due to increasing anthropogenic water demand;
(c) The fingerprint effect associate with ice mass loss from Greenland. Note that several Greenland marine terminating glaciers appear to be primed for rapid grounding line retreat over the next approximately twenty years;
(d) Storm surge & storm tide could increase due to increased storm activity in the Amundsen Sea.
(e) King tide (high astronomical tides) amplitudes can increase with increasing regional sea level;
(f) Local steric sources: The Southern Ocean is freshening rapidly, resulting in regional steric SLR.
(g) Winds (such as that associate with the ABSL) and ocean currents (such as the CDW) re-directed into the ASE, which would increase ocean elevation in the embayment, and stagnation pressure beneath the PIIS, Thwaites Ice Tongue base cavity, etc.;
(h) Tsunamis have been proven to induce cracking in Antarctic ice shelves (see Walker et al. 2013, DOI: 10.1002/2013JF002742), and a large Pacific seismic event could readily direct a large tsunami into the ASE and from there into the PIIS cavity, Thwaites Ice Tongue base cavity, etc.
(i) Hydraulic connections (jokulhlaup or glacial outburst flood) of the sub-ice-shelf cavity to the pressurized basal meltwater subglacial hydrological system underneath the PIG. The second attached image shows red dots where satellites have measured rapid changes in the ice surface elevation, which indicate a rapid movement of pressurized basal meltwater (ie. subglacial drainage events). This image indicates that there is a significant amount of subglacial basal meltwater periodically being released from beneath the PIG into the sub-ice-shelf cavity. Note that the build-up of hydrostatic pressure from say storm surge, or a tsunami, could serve to trigger a jokulhlaup event; so the simultaneous increase of hydrostatic, and increase of hydrodynamic, pressure is not improbable.
(j) Passing high pressure atmospheric systems, could temporarily increase the hydrostatic pressure in the sub-ice shelf cavities.
(k) Continuing eustatic SLR contributions from mountain glaciers.
(l) Local seismic activity could temporarily increase hydraulic pressure within the partially confined sub ice shelf cavities.
(m) Tidal amplification due to funnel effect within a sub-ice-shelf cavity that narrows upstream.
Next the linked Discover Magazine article about the Pine Island Bay glaciers includes a nice video of the use of ESA's SAR satellite radar interferometry use to estimate the retreat of the PIG grounding line (see third attached image and the YouTube link below). I note that 2010-11 was a strong La Nina event, so we will need to wait & see whether the rate of grounding line retreats accelerates during the current possible 2014-15 El Nino event:
http://blogs.discovermagazine.com/imageo/2014/04/24/antarctic-glaciers-flow-faster-iceberg-drifts-toward-sea/Quote related to the following video: "In the visualization, based on data from radar instruments on European Space Agency satellites, the Pine Island Glacier is seen where it empties into Pine Island Bay. Past what’s known as the “grounding line,” where the glacier rests on bedrock, the ice floats and is part of a giant, permanent ice shelf that fringes the coast and tends to hold back the flow of the glaciers.
In the visualization, the ice shelf can be seen flexing up and down from tidal action. And as sea water, which has become warmer at least in part from human-caused global warming, circulates under the ice, it causes the shelf to thin. With less of a buttress to hold things back, the glacier speeds up. This, in turn, causes the grounding line to retreat."
http://www.esa.int/spaceinvideos/Videos/2014/03/Pine_Island_retreatNext, I note that other surging marine glaciers already exhibit "hydro-thermodynamic" feedback as discussed in the following linked reference (and fourth attached image). While in this case the marine terminating glacier was in the NH and had summer icemelt that leaked to the bottom of the glacier to accelerate its motion; nevertheless, in the Antarctic which is currently too cold for significant surface melt water, the basal water in the subglacial cavities can increase due to acceleration of the ice flow, and more significantly (w.r.t. Margaret Davidson's comments to the insurance industry) tidal/barometric fluctuations can pump ocean water in and out of subglacial cavities that are located near the grounding lines. Also, when GMST departures above pre-industrial, reach between 2 to 2.7C (which could occur as soon as 2030 to 2060), then per DeConto & Pollard 2016, surface ice melting in Antarctica could lead to hydrofracturing of such low elevation ice shelves as: PIIS, FRIS and RIS:
Dunse, T., Schellenberger, T., Hagen, J. O., Kääb, A., Schuler, T. V., and Reijmer, C. H.: Glacier-surge mechanisms promoted by a hydro-thermodynamic feedback to summer melt, The Cryosphere, 9, 197-215, doi:10.5194/tc-9-197-2015, 2015.
http://www.the-cryosphere.net/9/197/2015/tc-9-197-2015.htmlAlso see:
Per johnm33 in the Jakobshavn thread, the following link shows the local bed topology near several key marine (terminating) glaciers both in Greenland and Antarctica, indicating the close relationship between bathymetry and tides (hydrostatic pressure), bottom gradients and ice-ocean interactions (including local calving, water (fresh & brackish) lens upstream of the grounding line). As a side note, there is growing evidence that Jakobshavn calves more frequently on high tides, so in a few decades, key WAIS marine glaciers (Thwaites, PIG, etc) may have cliff faces and may contribute to cold spots in the Southern Ocean by major calving events to produce fleets of icebergs that slowly move out from coastal areas to circulate around the Southern Ocean in great armadas (ala Hansen).
http://nholschuh.com/glaciers.html
