Hazard Analysis for the FRIS/RIS in the 2012 to 2060 Timeframe

[AbruptSLR]

Due to other time commitments I previously omitted opening a thread focused on a hazard analysis for the Filchner Ronne, and the Ross, Ice Shelfs, (FRIS/RIS); and I lumped this topic together with other topics in the "Collapse Main Period" thread.  Unlike the Bellingshausen/Amundsen Sea marine ice sheets where a significant portion of the ice mass loss contributes directly to SLR; most of the FRIS/RIS ice mass loss will not contribute to SLR, and furthermore, I postulate that the acceleration of ice mass loss from the marine ice sheets in the Weddell and Ross Sea areas (which do contribute to SLR) will not happen until the FRIS/RIS rapidly degrade sometime after 2060.

However, it should be noted that the conventional thinking is that the cold FRIS/RIS ice shelves will largely remain intact until well after 2300; and that most researchers will likely dismiss my hazard assessment of the risk of the FRIS/RIS collapsing soon after 2060 as being without merit, possibly due to their belief in the stationary nature of both: (a) the water adjoining the FRIS/RIS  in the Continential Zone, within the Continental Water Boundary, CWB, shown in the first re-posted image; and (b) the protective circulation pattern beneath a cold ice shelf that helps to keep warm CDW out from beneath as cold ice shelf, as indicated in the second accompanying figure for the FRIS (at the Filchner Trough) per Hellmer et al 2012.

In the series of follow-up posts I hope to contrast my different scenarios for FRIS and RIS from 2012 to 2060 in order to clarify why both of them may collapse shortly after 2060 by following two different paths.

[AbruptSLR]

First, I will start by looking at the current FRIS condition by adding to the other observation that I made in the "Collapse Main Period" thread that a warm CDW inflow is probably already passing along the Filchner Trough to the grounding lines of both the Filchner and the Ronne Ice shelves(for which evidence includes: (i) that at the grounding line of FRIS sub-ice-shelf melt rates as high as 7m/yr have been currently (2012) observed; (ii) the maked decrease in AABW production observed since the 1970's and (iii) an observed tongue of warm CDW descending from the north towards the FRIS).  These additional observations include:

(1) The Weddell Polynya previously contributed to AABW production; however according to Gordon et al 2006 (see attached pdf): "Shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the Weddell Sea. During the years 1974–1976, a 250 by 103 km2 area within the seasonal sea ice cover was virtually free of winter sea ice. This feature, the Weddell Polynya, was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer. Though smaller, less persistent polynyas associated with topographically induced upwelling at Maud Rise frequently form in the area, there has not been a reoccurrence of the Weddell Polynya since 1976. Archived observations of the surface layer salinity within the Weddell gyre suggest that the Weddell Polynya may have been induced by a prolonged period of negative Southern Annular Mode (SAM). During negative SAM the Weddell Sea experiences colder and drier atmospheric conditions, making for a saltier surface layer with reduced pycnocline stability. This condition enables Maud Rise upwelling to trigger sustained deep-reaching convection associated with the polynya. Since the late 1970s SAM has been close to neutral or in a positive state, resulting in warmer, wetter conditions over the Weddell Sea, forestalling repeat of the Weddell Polynya. A contributing factor to the Weddell Polynya initiation may have been a La Niña condition, which is associated with increased winter sea ice formation in the polynya area. If the surface layer is made sufficiently salty due to a prolonged negative SAM period, perhaps aided by La Niña, then Maud Rise upwelling meets with positive feedback, triggering convection, and a winter persistent Weddell Polynya."  Furthermore, I believe that both the loss of the Larsen A & B ice shelves, and the local CDW upwelling near the Antarctic Peninsula, have also contributed (together with the neutral to positive SAM conditions) to warmer and wetter conditions in the Weddell Sea; all of which I believe have contributed to the decrease in AABW production near FRIS.
(2) As stated in the "Forcing" thread, I believe that the extended period of ENSO neutral or La Nina conditions for the past 12 to 13 years have delivered more ocea heat content in the 700 to 2000m range from the South Pacific Current to the Antarctic Circumpolar Current, ACC; which has both caused the volume and temperature of CDW to increase.  It appears that this increase in CDW has expanded the Southern Boudary (SB in the attached figure) of the ACC to the south; to the extent that, in the case of Weddell Gyre, a warm CDW tongue has entered the Weddell Gyre east of the Greenwich Meridion (in the attached figure see how close the SB previously came to the eastern side of the Weddell Gyre current;  also see in small type the location of the Weddell Polynya), where it loops back to the west and is guided directly to the entrance of the Filchner Trough.  Furthermore, I believe that the warmer Weddell Sea conditions, and reduced AABW production, discussed in item (1) have contributed to a disruption of the prior protective under-the-ice-shelf circulation patterns that use to keep any warm CDW out of the Filchner Trough; thus letting progressively more warm CDW under the FRIS, since about 2000.
(3) I believe that the basal ice shelf melting from the warm CDW has further freshened the surface waters thus resulting in still less AABW being produced and more sea ice cover in the Weddell Sea (which insulates the upwelling CDW from cooling to the atmosphere).  This creates a positive feedback loop further disrupting the traditional/protective sub-ice-shelf circulation patterns that then allow more volumes of progressively warmer CDW under the FRIS (note that the CDW not only warming because of the insulation from the sea ice, but also as the is less mixing of the AABW with the ACC as the AABW goes over the edge of the Continential Shelf down to the Continental Slope).

[AbruptSLR]

Primarily, due to the extreme rate of warming in the Antarctic Peninsula (six time the global mean rate of surface temperature rise); I believe that both the Larsen C, and the George VI, Ice Shelves will collapse within the next 5 to 10 years (possibly sooner); which I believe will further warm the Weddell Sea surface temperatures (due both to more exposure of warm CDW to the atmosphere where these two ice shelves use to be, and due to the decreased albedo in these areas); which I believe will result in a further reduction in AABW in the Weddell Sea area; which will allow more (larger volume) intrusion of a warm CDW into the Filchner Trough all the way to the western edge of the Ronne Ice Shelf grounding line (see attached figure from Hellmer et al 2012).  As the present average rate of ice melt from the Ronne Ice Shelf grounding line is about 2m/year (with a peak of 7m/yr), I believe that it is reasonable with the postulated loss of the Larsen C, and George VI, Ice Shelves by about 2020 and the continued projected increase in warming and volume of the CDW entering the Filchner Trough (FT in the figure) that is reasonable to assume an average sub-ice-shelf at the grounding line of FRIS of approximately 10m/year until 2060.  In this location the FRIS is over 530 m thick, so a thickness loss of (2060-2012) x 10m/yr = 480 m loss would mean that the FRIS may be close to collapse after 2060 when Hellmer et al 2012 posulate that changes in the sea ice in the Weddell Sea will drive still more warm CDW below the FRIS (see the "Collapse Main Period" thread).

[AbruptSLR]

The Ross Sea area and the RIS have number of factors that will delay (as compared to the timing for the FRIS previously discussed) the transition of this cold ice shelf to a warm ice shelf, including: (1) the is no adjoining area like the Antarctic Peninsula to accelerate surface warming; (2) there is no sea floor trough leading beneath the RIS comparable to the Filchner Trough; (3) the Antarctic Slope Front & Current in this area is more effective at disrupting/dispersing/diluting and warm CDW coming up the continental slope onto the continental shelf; thus it is more difficult to form a warm CDW tongue of water feeding into the Ross Gyre current; (4) the Ross Gyre current does not lead as directly beneath the RIS as does the Weddell Gyre current for the FRIS; (5) the normal protective/cold sub-ice-shelf circulation pattern beneath RIS have not yet been regularly disrupted (however, there have been reports of periodic disruptions to these circulation patterns for periods of weeks); and (6) the atmospheric low that the Ross Gyre rotates around is stronger and more prevalent than that for the Weddell Gyre. 

Nevertheless, RIS also has its own characteristics that lead me to postulate that it will degrade significantly (or collapse) shortly after 2060, including:
(1) The RIS is already thinner than the FRIS, thus it will take less time for it to thin sufficient for a melt pond mechanism to affect it when the surface temperatures in the WAIS are postulated to raise rapidly after 2060 (due to the projected reduction in antarctic sea ice, and the warming effect from the upwelling warm CDW;
(2) Although the current average sub-ice-shelf ice melting rate for RIS is only about 0.1m/year, most of this subiceshelf melting is located within a few km of the RIS ice face; which resulting in the currently measured rate of face retreat due to ice calving of ove 1 km/yr; which should result in the face retreating over 50km by 2060.
(3) The Getz Ice Shelf degradation is currently the largest single source of fresh melt water into the Southern Ocean (and does not contribute to SLR as the shelf is floating); and as this meltwater flows westward, it helps to dilute any warm CDW that manager to get onto the continental shelf.  As the meltwater contribution from the Getz Ice Shelf will decline before 2060, thus more warm CDW will get onto the continental shelf more frequently and with a warmer temperature; which will lead to an increase frequency and a higher temperature than is currently the case.

[AbruptSLR]

I estimate that by 2050 that the calving face of RIS should have retreated at least 40km (using the current rate of retreat), and I postulate that this retreat of the ice shelf face will cause the Ross Gyre current (see the attached figure) to follow the retreating face until the gyre current is directed at the shallow trough leading to the base of the Byrd Glacier (see the second figure).  Also, by 2050 I believe that that the AABW production will be sufficiently reduced (by both changing atmospheric patterns and the strengthening of the CDW volume & temperature) so that the cold AABW entrained in the Ross Gyre will not be able to adequately dilute the CDW entained in the Ross Gyre (assuming also that the ice melt from the Getz Ice Shelf has slowed sufficiently to reduce the dilution of the CDW crossing the continental shelf with meltwater).  Therefore, I postulate that a warm tongue of CDW will make its way to the grounding line for the Byrd Glacier, which will help to trigger a positive feedback cycle (including a "saline pump" advection action working with the Byrd Glacier) that will alternate the circulation patterns in the Ross Sea Embayment so as to direct the warm tongue from the Byrd Glacier southward along the grounding lines of the Siple Coast ice streams (note that there is a pre-existing shallow trough leading from the Byrd Glacier to the Siple Coast; and also note that I postulate a similiar "horizontal" advective mechansim between PIG and Thwaites Glacier [see the "Surge" thread], which I believe will be repeated here (with the resulting cold ice melt water from the Siple Coast ice streams existing out through another shallow trough along the east side of the Ross Sea Embayment).  As the RIS is already thinner than the FRIS, and the CDW will be warmer by then, I postulate that a large portion of the RIS will be subject to a melt-pond collapse mechanism between 2060 & 2070; which, will reduce the buttressing of the RIS on the Siple Coast ice streams that the thinning ice streams will induce the grounding line treat pattern shown in the "Collapse" thread.

[AbruptSLR]

Regarding the RIS respond in my proposed hazard scenario, I would like to note that:
(1) Even by 2050 I postulate that the calving face of the RIS would have retreated sufficiently so that the tidally induced flexing of the RIS will affect the grounding line stresses sufficiently for the Siple Coast ice stream ice flows to accelerate even before the melt pond mechanism is postulated to occur, thus inducing the "early" grounding line retreat indicated in the "Collapse" thread.

(2) Note that the Byrd Glacier has: (a) the deepest bed topology in the Antarctic thus indicating that the advective process I cite in the immediately prior post may have contributed to bottom scour in the past; and (b) the Byrd Glacier has a very well developed basal meltwater network, that has induced surges of ice flow from the Byrd Glacier in the past; and I postulate that the average ice velocity for the Byrd Glacier will accelerate rapidly after 2050 (which will promote more advection).

(3) RIS already contains many crevasses, and I postulate that the number of these crevasses will increase after 2050 due to the change in sub-ice-shelf circulation patterns that I postulate.

[AbruptSLR]

The following link leads to an article and a video about a mini submarine that was sent down a bore hole into Lake Whillans which is a subglacial lake that sits more than 2,000 feet below the West Antarctic Ice Sheet,on the edge of the RIS.  When the grounding line for the Whillians ice stream retreats to this lake (which I postulate will happen before 2050, see images in the "Collapse" thread), this will accelerate thinning and grounding line retreat for this ice stream.  This type of mini submarine seems ideal for exploring some of the larger subglacial lakes in the Thwaites Drainage basin, in order to better understand the risks that such lakes pose for rapid ice mass loss from such areas:


http://www.businessinsider.com/mini-submarine-in-antarcticas-lake-whillans-2013-3

[AbruptSLR]

To make a brief summary on this topic, I would like to say that I believe:

(1) A "horizontal" advective cell (driven by a salinity gradient of the fresh sub-ice-shelf meltwater and the inflowing CDW through the Filchner Trough) has already been established beneath the FRIS and is (or will be) reinforced by: (a) an increasing volume and temperature of the CDW within a warm tongue of current feeding from the ACC through the Weddell Gyre current; (b) increasing volumes of sub-ice-shelf ice meltwater; (c) decreasing production of AABW; and after 2060, (d) a rapid decline in Antarctic sea ice area, causing both: (i) wind shear to blow more warm CDW into the Filchner Trough (FT); and (ii) an associated increase in surface temperatures.  The establishment of this "horizontal" advective cell (pumping water from the northeastern edge of the FRIS through the FT to the northwestern edge of the FRIS) was facilitated early by interactions between the Antarctic Peninsula & SAM (temporally/periodically disrupting the "protective" sub-ice-shelf circulation pattern) and an intrusion of a warm CDW current from the north.
(2)  A weaker "horizontal" advective cell will be established for RIS after 2050 when the ice shelf face has retreated sufficiently (through calving) in order to direct a warm CDW current (allowed to form when the Getz iceshelf melting slows) towards the Byrd Glacier which will drive as side branch of the warm CDW towards the grounding line of the RIS along the Siple Coast, and then out throught the northeast edge of RIS as a cold current.
(3) In both cases between 2060 and 2070 melt pond mechanisms (once the atmosphere warms due sea ice area declines) may lead to the rapid collapse of most of the ice shelves.

[Pmt111500]

Yes,  the currents are the main concern here for me too. Sorry I haven't read all of the above. I might have missed if you mentioned one atmospheric effect that might have something to contribute here also. That is the southward movement of storm tracks observed in the southern ocean. If this progresses much, we might see storms crossing the Transantarctic mountains and producing a foehn on Filchner-Ronne giving some extra punch to the possible formation of meltponds there. Would this be a plausible scenario and would that have an effect on the main scenario you've described? Pretty much an amateur here, even had to check which outlet was Ronne and which was Filchner... Thanks anyway for spelling out these wild scenarios for the record.

[AbruptSLR]

PMT,

While I have noted that the Southern Ocean storms are becoming more frequent and more intense; I have not noted that they are tracking further to the south, so thanks for the insightful input. 

Regarding your question on melt ponds, calculations have shown that for their current thickness even if melt ponds did occur, they could not induce a collapse mechanism for either the FRIS or the RIS.  However, given the rates of basal melting that I have postulated, I believe that both shelves could be subjected to a melt pond collapse mechanism sometime between 2060 and 2070, and the more southernly storm tracks that you mention could definitely add to the likelihood of such a collapse.

I think that it is important to think about these possible hazard scenarios now, as during the 1980 very few people thought that the organizations such as the World Bank would be publically stating that by 2100 the global mean temperature will likely be between 4 and 6 C, and if the permafrost degrades rapidly such estimates may be non-conservative from a public safety point of view.

[AbruptSLR]

The attached two images are from a 2002 National Geographic map of Antarctica which shows the frontal edge of the FRIS and RIS, respectively, in the 1997 location with dotted lines showing how much the frontal edge of these two ice shelves had retreated by the 2000 to 2001 timeframe.  Furthermore, note the mirror symmetry of the Recovery Glacier feeding into the FRIS where the frontal edge was not retreating and of the Byrd Glacier feeding into the RIS also where the frontal edge was not retreating.  This pattern of retreat supports my postulated/projected (horizontal) advection pattern which could possibly lead to the collapse of these two key ice shelves by the 2060-2070 timeframe.

[AbruptSLR]

The attached image from a 2002 National Geographic map of the Antarctic shows both the sea ice movement and the typical wind flow patterns (circa 2000 to 2001).  The sea ice movement indicates how both the Weddell and the Ross Sea areas manufacture and export sea ice; while the wind pattern shows how some snowfall could be blown into the ocean.

[AbruptSLR]

The attached image from a 2002 National Geographic map of the Antarctic shows both the sea ice movement and the typical wind flow patterns.  The sea ice movement indicates how both the Weddell and the Ross Sea areas manufacture and export sea ice; while the wind pattern shows how some snowfall could be blown into the ocean.

[AbruptSLR]

The attached image from a 2002 National Geographic map of the Antarctic shows how the frontal zone of the RIS (Ross Ice Shelf) in the indicated area is subject to accelerated calving due to local upwelling of warm deep water.

[AbruptSLR]

The first image is a map of FRIS basal accumulation rate (in m a-1) from Joughin and Padman [2003]. Basal melt is  <0. The red line is MOA grounding line GL and ice front (epoch 2003/04). Black line is  =0. The second image is of the maximum tidal current speed (Umax: m s-1) from CATS2008a barotropic tide model(http://www.esr.org/polar_tide_models/Model_CATS2008a.html). The black contour is Umax=0.5 m s-1.  The mean current speeds, time-averaged over a complete spring/neap tidal cycle of ~15 days, are ~0.5Umax.

[AbruptSLR]

I thought that I would post the accompanying two images related to the stability of the FRIS and RIS in the 2012 to 2060 timeframe.

The first image is a little out of date, showing the long-term annual changes in Antarctic surface temperatures from 1981 to 2007; but it clearly illustrates how the most significant temperature changes have occurred at the frontal edge of ice sheets (including FRIS and RIS), due not only to ice shelf calving but also due to increased upwelling in these locations.

The second images illustrates how long period gravity waves (significantly coming from the Pacific Ocean) can induce significant flexural stresses in the ice shelves over 10 km from the frontal edge.  As storm activity in the Pacific Ocean increases with climate change, particularly during strong El Nino events, such gravity wave induced flexural stresses can be expected to promote accelerating calving from the rapidly thinning frontal edge Antarctic ice shelves (particularly for the RIS but also for the ice shelves in the ASE and on the Bellingshausen Sea area).

[AbruptSLR]

The following summary statement focused on FRIS (as part of the Ice2sea study) does not consider such positive feedback factors (that could accelerate the indicated timeline) as: (a) increasing atmospheric methane concentrations over Antarctica; and (b) the timing of (and the end of) the current El Nino hiatus period, that has currently driven more Ocean Heat Content, OHC, into the CDW than previously modelled and which may soon cause accelerated upwelling of this warmer CDW:

"Geophysical Research, Abstracts Vol. 15, EGU2013-10861, 2013.

Southern Ocean warming and increased ice shelf basal melting in the 21st and 22nd centuries based on coupled ice-ocean finite-element modelling
By Ralph Timmermann and Hartmut Hellmer (Ralph.Timmermann@awi.de)

In the framework of the EU project Ice2sea we utilize a global finite element sea ice - ice shelf - ocean model (FESOM), focused on the Antarctic marginal seas, to assess projections of ice shelf basal melting in a warmer climate. Ice shelf - ocean interaction is described using a three-equation system with a diagnostic computation of temperature and salinity at the ice-ocean interface. A tetrahedral mesh with a minimum horizontal resolution of 4 minutes and hybrid vertical coordinates is used. Ice shelf draft, cavity geometry, and global ocean bathymetry have been derived from the RTopo-1 data set.  The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM coupled climate model. Data from their 20th-century simulations are used to evaluate the modeled present-day ocean state. Sea-ice coverage is largely realistic in both simulations. Modeled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM.

Projections for future ice shelf basal melting are computed using atmospheric output for IPCC scenarios E1 and A1B. Trends in sea ice coverage depend on the scenario chosen but are largely consistent between the two forcing models. In contrast to this, variations of ocean heat content and ice shelf basal melting are only moderate in simulations forced with ECHAM5/MPI-OM data, while a substantial shift towards a warmer regime is found in experiments forced with HadCM3 output. A strong sensitivity to salinity distribution at the continental shelf break is found for the Weddell Sea, where in the HadCM3-A1B experiment warm water starts to pulse onto the southern continental shelf during the 21st century. As these pulses reach deep into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases by a factor of three to six compared to the present value of about 100 Gt/yr. By the middle of the 22nd century, FRIS becomes the largest contributor to total ice shelf basal mass loss in this simulation."

[AbruptSLR]

The first attached image from Rignot 2013 shows the 2009 rates of basal melt rate beneath the FRIS and the George VI Ice Shelf, GVIIS; together with the associated amount of ice mass loss via calving (ie in the circle the cross hatched area is calving while the solid black area is basal melting).  This image indicates that in 2009 the maximum basal ice melt beneath the FRIS was about 4 m/yr; while as noted at:

http://forum.arctic-sea-ice.net/index.php/topic,117.0.html

Reply #1:
"… at the grounding line of FRIS sub-ice-shelf melt rates as high as 7m/yr have been currently (2012) observed."

This represents a 75% increase in basal ice melting rate in just 3 years.

The second attached image comes from:

Southern Ocean warming and increased ice shelf basal melting in the 21st and 22nd centuries based on coupled ice-ocean finite-element modelling
by: R. Timmermann and H.H. Hellmer; Ocean Dynamics; Final version submitted on 26. June 2013.

The caption for the second attached image is:

"Basal melt rates (m/yr) for Filchner-Ronne, Larsen C, and George VI Ice Shelves during the 22nd century in a simulation forced with HadCM3-A1B data (ten-year average 2140-2149). Note the non-linear color scale."

This second attached image indicates that per their model the maximum basal ice melting for FRIS should be about 18 m/yr, by about 2145.  However, I believe that this rate of basal ice melting for FRIS will be achieved no later than 2060 for reasons that were not included in the Timmermann and Hellmer, T-H, 2013 analysis, including: (a) the T-H analysis did not consider the high rate of ice calving, or the influence of tide water exchange, for FRIS,; (b) the T-H analysis does not include the large about of Ocean Heat Content, OHC, introduced into the ocean below -2000 m during the current El Nino hiatus period; (c) the T-H analysis does not consider the amount of warm CDW introduced recently into the Weddell Gyre that is now available to enter the Filchner Trough when the seasonal conditions are appropriate.

In summary the Ice2sea program uses the T-H 2013 results (see the immediately preceeding post) to indicate that they do not need to worry about the FRIS collapsing until well into the 22nd Century; but I believe that the FRIS could begin to collapse between 2060 and 2070 when considering the current physical trends affecting this ice sheet.

[AbruptSLR]

In my reply # 3 of this thread, I speculate that when the Getz Ice Shelf collapses, the associated regional changes in seawater temperatures and current flows could contribute to an acceleration of the degradation of the Ross Ice Shelf, RIS.  In this regard, I present that attached image of computer generation ocean water temperatures and current speeds along the Amundsen Sea coastline including the area near the Getz Ice Shelf, indicating that the warm CDW is advecting underneath the Getz Ice Shelf, resulting the relatively high basal meltwater rates beneath the Getz Ice Shelf as discussed in the following three sources from the Internet:

The following reference and abstract, regarding the Getz Ice Shelf, was taken from:

http://onlinelibrary.wiley.com/doi/10.1002/jgrc.20298/abstract

Getz ice shelf melting response to changes in ocean forcing;
S. Jacobs, C. Giulivi, P. Dutrieux, E. Rignot, F. Nitsche, J. Mouginot; 2013; Journal of Geophysical Research: Oceans; DOI: 10.1002/jgrc.20298

"Abstract
The large and complex Getz Ice Shelf extends along nearly half of the West Antarctic coastline in the Amundsen Sea, and is exposed to a more variable ocean environment than most other Pacific sector ice shelves. Ocean temperature, salinity and dissolved oxygen profiles acquired near its sub-ice cavity openings are used here to estimate seawater transports and meltwater fractions. More complete coverage during 2000 and 2007 brackets most of the variability observed from 1994 to 2011, and yearlong records near one ice front support the use of summer profiles to determine annual basal melt rates. We find area average rates of 1.1 and 4.1 m/yr, higher in 2007 when a larger volume of warmer deep water occupied the adjacent continental shelf, and the ocean circulation was stronger. Results are consistent with changes in thermocline depths relative to ice shelf draft and mass transports onto the adjacent continental shelf. We also calculate steady state and actual melting of 2.5 and 4.6 m/yr in 2007-2008 from satellite measurements of ice flux, modeled accumulation, and thinning from 2003-2008. This implies a positive mass balance in 2000, but negative in 2007, when the Getz was producing more meltwater than any of the larger, slower-melting or smaller, faster-melting ice shelves."

The following excerpt regarding the Getz Ice Shelf was taken from a 2011 article at:

http://climatestate.com/2013/05/31/the-fragile-fringe-of-west-antarctica/

"Getz is like a fringe, a lacey ruffle, spanning 500 miles of coastline, with dome-like ice islands poking through it, and a textured surface a bit like well-used sandpaper. The plane flew lower now, seeming to skim the surface, but in fact we were a comfortable 5000 feet above the snow. We crossed just above the coastline and then turned and flew out over the floating ice downstream of the coast, with one hard right to explore a large glacier and then back.
Getz has recently followed the pattern of the much larger Pine Island Glacier, beginning to rapidly lose elevation and mass. The current theory behind this mass loss is that a change in wind patterns related to climate change has led to a warm deep layer of ocean water periodically sloshing onto the continental shelf. For the Pine Island area, this process seems to have begun as far back as the 1980s, but for Getz the changes began just in the past ten years. The warmer water has always been out there offshore, around 2000 feet below the ocean surface, but by the gentle persisting wind changes spanning decades, it has been coaxed up to 600 or 700 feet at the edge of the Antarctic continental shelf, spilling over it. From there, it tends to hug the sea bottom and reach in to the deep underside of the glaciers at the coast, melting them at the point where they emerge from the main ice sheet and begin to float. What we saw in the radar systems on our flight was that the Getz is quite thick (up to 1500 feet) and its deeper ice would sit squarely in this new warmer water. This has led to rapid thinning and acceleration.
With the main mission behind us, the plane climbed back to altitude and headed northward. Ocean and ice passed beneath, then a small iceberg, and then a rugged ice cliff and mountain ridge. In perhaps ten seconds, it was over, and we were back to crossing our thousands of miles of southern ocean. I checked with the teams – every instrument worked perfectly for the whole flight."

The following excerpt regarding  the Getz Ice Shelf  was taken from:

http://www.nature.com/news/oceans-melt-antarctica-s-ice-from-below-1.13200

“This was quite a big gap in our understanding of how the ice sheets interact with their surroundings, and what it shows is that the oceans play a bigger role than we’d previously thought,” says Hamish Pritchard, a glaciologist at the British Antarctic Survey in Cambridge who led the Nature study.”.
Rignot’s analysis suggests that roughly half of the meltwater comes from ten small ice shelves along the southeastern part of the Antarctic Peninsula and West Antarctica, such as the Getz Ice Shelf; the analysis also identifies significant melting at six ice shelves in East Antarctica. The three largest ice shelves, which account for two-thirds of the ice shelf area around Antarctica, are responsible for just 15% of the total basal melting.
Despite being relatively small, Antarctica's Getz Ice Shelf produces more meltwater than ice shelves around the continent that are ten times its size.
Nature; doi:10.1038/nature.2013.13200

[AbruptSLR]

As currently some "modified CDW" is accelerating basal ice melt beneath the FRIS, and this basal melt rate is projected to increase with time; a very extensive monitoring program is planned for the FRIS, as indicated by the attached image and by the information at the following link:

https://www.comnap.aq/Publications/Comnap%20Publications/SOOS_Wahlin_Presentation_17MB.pdf

The extent of this monitoring plan indicates the seriously of the potential collapse of the FRIS this century.

[AbruptSLR]

The following two weblinks provide updates on the progress of the Roosevelt Island  Climate Evolution (RICE) project; which when completed will provide a lot of insight on the paleo-stability of the Ross Ice Shelf.  Also, the attached pdf provides a glimpse of how the ice cores from this project are being processed:

http://www.victoria.ac.nz/antarctic/research/research-prog/rice/

http://climatechange.umaine.edu/roosevelt_island_climate_evolution_rice1

[AbruptSLR]

The following linked reference about the Beardmore Glacier discusses the importance of considering tidal influences on the ice velocities of marine terminating glaciers with significant ice shelves (in this case the RIS).  One can expect that as calving accelerates for RIS the influence of tides on the ice velocities of the glaciers that it buttresses will also accelerate.  The link provides a free pdf:

http://www.the-cryosphere-discuss.net/7/1761/2013/tcd-7-1761-2013.html

Marsh, O. J., Rack, W., Floricioiu, D., Golledge, N. R., and Lawson, W.: Tidally-induced velocity variations of the Beardmore Glacier, Antarctica, and their representation in satellite measurements of ice velocity, The Cryosphere Discuss., 7, 1761-1785, doi:10.5194/tcd-7-1761-2013, 2013

"Abstract. Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, in both linear and non-linear patterns. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, shown here to be present in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of >50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This decreases rapidly to <5% only 15 km upstream of the grounding line. Daily fluctuations are smoothed to <1% in the 11 day repeat pass TerraSAR-X imagery but fortnightly variations over this period are still visible and show that satellite-velocity measurements can be affected by tides over longer periods. The measured tidal displacement observed in radar look direction over floating ice also allows a~new method of grounding line identification to be demonstrated, using differential speckle tracking where phase coherence is too poor for SAR interferometry."

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The following linked reference (see the following abstract, selected text extract and linke to a free pdf) discusses how recently observed slowing of RIS ice discharge velocities (see the first attached image) due to near grounding line shoaling of the seabed should result in of the ice shelf outboard of the shoaling areas thinning, its calving front retreating, leading to a projected increase in ice-stream discharge. Indeed, a cycle of ice-stream deceleration followed by acceleration may be an inevitable consequence of shoaling seabed near the grounding lines of Ice Streams A–C (see the second attached image), with each cycle modulated by the effects of sea-level rise, isostatic uplift of the seabed, and deposition of sediment from the ice streams as they become afloat:

http://www.igsoc.org/journal/59/217/j12J122.pdf

Continued slowing of the Ross Ice Shelf and thickening of West Antarctic ice streams; by: R. THOMAS, B. SCHEUCHL, E. FREDERICK, R. HARPOLD, C. MARTIN, & E. RIGNOT; Journal of Glaciology, Vol. 59, No. 217, 2013 doi: 10.3189/2013JoG12J122


"ABSTRACT. As part of the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS), ice velocities were measured on the Ross Ice Shelf (RIS) during 1973–78. Comparisons of these with velocity estimates at the same locations derived from RADARSAT synthetic aperture radar (SAR) measurements in 1997 and 2009 show velocity reduction in the southeast quadrant of the ice shelf by almost 200ma–1, with deceleration rates increasing with time. Large areas of ice shelf in this region are lightly grounded, forming an ‘ice plain’ that increases local buttressing of the ice streams. ICESat measurements show this ice plain to be thickening. The observed decrease in ice-shelf velocities implies a total reduction in the mass of ice flowing into the RIS from the West Antarctic ice sheet (WAIS) by _23 Gt a–1, shifting the mass balance of the WAIS drainage basin from strongly negative in the 1970s to strongly positive in 2009. The resulting decrease in ice advection should lead to ice-shelf thinning further seaward of the ice plain. This thinning would reduce the lateral drag and back-stress of the shelf ice, further contributing to thinning through an increase in spreading rate. ICESat measurements show recent thinning of most of the freely floating ice shelf."

Selected extract:
"If Ice Streams A and B continue to slow at recent rates, it is quite likely that, as the ice shelf thins, its calving front will retreat, further reducing its buttressing, which would favour increased creep thinning of the ice shelf and possible flotation of the ice plains. This, in turn, would reduce buttressing forces on the ice streams, leading to an increase in ice-stream discharge. Indeed, a cycle of ice-stream deceleration followed by acceleration may be an inevitable consequence of shoaling seabed near the grounding lines of Ice Streams A–C, with each cycle modulated by the effects of sea-level rise, isostatic uplift of the seabed, and deposition of sediment from the ice streams as they become afloat (Thomas and others, 1988). Moreover, this cycle may not be in phase for each ice stream. Ice Stream C ceased to be active in the mid-1800s, whereas slowdown of A and B is quite recent."

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The following two abstracts are taken from the proceedings of the following IGSOC sponsored symposia, and they are relevant to my immediately preceding post, regarding the factors and timing associated with ice drainage basins feeding into the RIS:

International Symposium on Changes in Glaciers and Ice Sheets: observations, modelling and environmental interactions; 28 July–2 August; Beijing, China; Contact: Secretary General, International Glaciological Society


http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm



Tidal pacing, skipped slips and the influence of stick–slip motion on the slowdown of the Whillans Ice Stream, Antarctica
J. Paul WINBERRY, Sridhar ANANDAKRISHNAN, Doug WIENS, Richard B. ALLEY
Corresponding author: J. Paul Winberry
Corresponding author e-mail: winberry@geology.cwu.edu
"Whillans Ice Stream (WIS) is a major route for ice transiting from the interior of the West Antarctic ice sheet (WAIS) into the Ross Sea. It has been observed that WIS has been slowing, contributing to a positive mass balance in the Ross Sea sector of the WAIS. Examination of velocity time series for WIS reveal that the deceleration is not occurring at a steady rate, but varies at the sub-decadal timescale. Superimposed on this decadal-scale trend of deceleration is motion driven by a tidally modulated stick–slip cycle at the daily timescale. These sub-daily oscillations are characterized by extended periods (6–24 hours) of minimal motion followed by brief periods (30 min) of rapid motion when the ice stream lurches forward by ~0.5 m. Comparison of new results collected during 2010–2011 with earlier measurements show that the deceleration has continued and the timing of slip events has become less regular. The reduced regularity of slip events has resulted in a less efficient release of stored elastic strain during slip events, pointing toward non-linear feedbacks at the daily scale that influence the decadal timescale behavior of the ice stream."


Margin lakes and ice-stream grounding-line migration
Mason J. FRIED, Christina HULBE, Mark FAHNESTOCK
Corresponding author: Christina Hulbe
Corresponding author e-mail: christina.hulbe@otago.ac.nz
"The lateral ‘corners’ where Kamb and Whillans Ice Streams (KIS and WIS) discharge into the Ross Ice Shelf share common geometries and ice mechanical settings. At both corners of the now-stagnant KIS outlet, shear margins of apparently different ages confine regions with a relatively flat, smooth surface expression. These features are called ‘the Duckfoot’ on the northern right-lateral side and ‘the Goosefoot’ on the other. The right-lateral outlet of the currently active WIS looks much the same, though with a less-developed inboard margin. It has been suggested, on evidence found in ice internal layers, that the flat ice terrains on KIS were afloat in the recent past, at a time when the ice-stream grounding line was upstream of its present location. The overdeepening in the bed just upstream of the KIS grounding line supports this view of the past geometry. Here, we consider the history of these features and their role in ice-stream variability by comparison of the relict and modern features and via numerical modeling of ice-shelf grounding and ungrounding in response to variations in ice flow. We propose two scenarios for lake (flat ice terrain) development at the outlets of ice streams. In the first, development of a lake in the hydraulic potential low along a shear margin forces a margin jump as shearing develops along the inboard shore of the margin lake. In the second, a remnant lake is formed by grounding-line advance around a relative low in the bed, creating adjacent margins along the lake shores. Discerning which of these scenarios is appropriate at the KIS outlet has implications for understanding the history of the ice-stream grounding line. While the second process is consistent with a paleo-grounding line upstream of the present location, if the margin lake model is instead appropriate, then the accepted interpretation of grounding-line history is incorrect. "

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The following abstracts come from the linked sources and are relevant to the Weddell Sea Sector:

www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htm
Contact: Secretary General, International Glaciological Society


67A016
The basal environment of Evans Ice Stream, West Antarctica, from radio-echo sounding
David ASHMORE, Robert G. BINGHAM, Richard C.A. HINDMARSH
Corresponding author: David Ashmore
Corresponding author e-mail: d.ashmore@abdn.ac.uk
Airborne ice-penetrating radar (radio-echo sounding) is the most efficient method for investigating subglacial environments across polar ice sheets. Theoretically, analyses of the shape and amplitude of the basal reflector can yield physical information on subglacial conditions. Most notably, owing to the high relative permittivity of liquid water a high-amplitude reflection indicates a temperate (unfrozen) bed, the diagnosis of which is pertinent for understanding controls on ice dynamics and, in particular, tributary and fast-flow phenomena. During the past decade a substantial quantity of new airborne radar data has been collected over the Antarctic ice sheet. While these data have been compiled into large maps of subglacial topography, their exploitation with respect to characterizing the basal boundary of the ice sheet remains difficult. Perhaps the greatest difficulty is posed by characterizing how the ice itself attenuates the radar signal. In this study we consider this problem using a 150 MHz centre-frequency airborne radar survey of Evans Ice Stream, a major West Antarctic ice stream, collected by the British Antarctic Survey in 2006/07. Using temperature output from a 3-D finite-difference ice-sheet model we derive a spatially varying parameterization of englacial attenuation. We find a clear association with fast-flow regions and a bimodal frequency distribution of returned power, separated by 10–15 dB, consistent with the reflectivity of the subglacial interface being dominated by the presence of subglacial water. In order to develop these results we present a comparison of these data with several geographical properties. We discuss the glaciological and geophysical implications of these observations. This study demonstrates the potential for the exploitation of existing radar datasets using relatively straightforward techniques.


67A020
Meltwater drainage pathways beneath Institute and Möller Ice Streams, Antarctica
Hugh CORR, Neil ROSS, Martin SIEGERT, Anne LE BROCQ, Fausto FERRACCIOLI
Corresponding author: Hugh Corr
Corresponding author e-mail: hfjc@bas.ac.uk
Subglacial hydrology directly influences the dynamic behaviour of the overlying ice sheet. Meltwater influences the strength of the subglacial sediment and the amount of friction exerted by the bedrock on the ice. Therefore, understanding the distribution and volume of meltwater beneath the ice is essential in resolving ice-flow dynamics. The direct link between ice flow and global sea level means that characterizing and comprehending Antarctic subglacial hydrological networks are of critical importance for predicting how present-day ice sheets may respond to a changing environment. In 2010/11 a high-resolution aerogeophysical survey was acquired over the catchments of Institute and Möller Ice Streams (IIS and MIS), West Antarctica. The data from the airborne ice-sounding radar have provided an unprecedented opportunity to understand the interactions between rugged subglacial topography, the subglacial hydrological system, and the flow regime, structure and physical properties of this sector of West Antarctica. From the revealed bed topography we map the hydraulic potential, which is a function of the elevation potential and the ice overburden pressure. Simulated meltwater pathways that are derived from routing algorithms show distinct regions where the subglacial relief controls the flow rather that the more usual case where the surface relief dominates. However, the two ice streams exhibit the same funnelling characteristic such that the majority of the meltwater that crosses the grounding line is steered into single channels. Despite the unobstructed flow of water to the ocean we find discrete areas where there is evidence of basal accretion. Examination of the basal power-reflection coefficient confirms these interpretations.


67A024
Isochrones and snow accumulation patterns using FM-CW radar data on West Antarctica
Guisella GACITÚA, Andrés RIVERA, José Andrés URIBE, Rodrigo ZAMORA
Corresponding author: Guisella Gacitúa
Corresponding author e-mail: ggacitua@cecs.cl
The West Antarctic ice sheet (WAIS) has been considered potentially unstable because its bedrock is well below sea level and its total disintegration could contribute up to 4.3 m to global sea-level rise. Several recent surveys have studied the area with remotely sensed imagery, airborne platforms and ground surveys, this kind of expedition being very difficult due to the remoteness, difficult logistical approach and several meteorological constraints. However, since 2008 the private company Antarctic Logistics & Expeditions (ALE) has been operating at Union Glacier (79°46′ S, 83°24′ W), providing logistical and technical support to scientific activities from this new gateway for the exploration of inner Antarctica. This support allowed CECs to carry out several expeditions from this base camp, including ice thickness, crevasse detection, ice dynamic, mass-balance and snow accumulation surveys. The last oversnow traverse (2010) started from Union Glacier towards the west covering ~80 km throughout four glaciers before reaching the Antarctic plateau. In this traverse, CECs used a synchronized snow accumulation/ice depth radar system. The snow accumulation was registered by FM-CW radar with an output frequency of 50–400 MHz, which was able to collect nearly 80 m of snow depth data. The processed data showed clear isochrones produced by differential annual snow accumulated. Also, a well-defined ice–snow interface was detected in steep valleys where crevasses were precluding some of the surveys. The obtained data were evaluated, confirming the effectiveness and accuracy of the system when measuring snow layer characteristics, thickness, slopes, inflections and continuity. Additionally, we analysed the patterns of accumulation at the joint between two of the main ice masses in order to characterize the ice dynamic effect on the surface snow layer patterns. In this contribution, the FM-CW system will be described and the surveys performed and the results obtained in the region will be shown.

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The following abstracts come from the linked sources and are relevant to the Ross Sea Sector:

www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htm
Contact: Secretary General, International Glaciological Society


67A058
Flow dynamics of Byrd Glacier, East Antarctica
C.J. VAN DER VEEN, L.A. STEARNS, S.P. GOGINENI
Corresponding author: C.J. Van der Veen
Corresponding author e-mail: cjvdv@ku.edu
Byrd Glacier is a fast-moving outlet glacier transecting the Transantarctic Mountains, funneling an estimated 20.6? Gt a–1 of ice originating on the East Antarctic plateau into the Ross Ice Shelf through a fjord that is ~100 km long and ~20 km wide. The glacier has been the subject of glaciological investigations since the early 1960s, including a comprehensive assessment of balance of forces on the lower trunk by Whillans and others (1989) using surface elevations and ice velocities derived from repeat photogrammetry in the late 1970s. That study, as well as subsequent more recent studies, was limited by lack of detailed information on the bed topography under the glacier. In 2011–2012 the Center for Remote Sensing of Ice Sheets (University of Kansas) conducted extensive airborne radar sounding for mapping the bed under Byrd Glacier, thereby allowing re-evaluation of results from earlier studies and, in particular, to investigate the relation between ‘sticky spots’ and basal relief. The present study aims to investigate flow dynamics and essentially represents an update of the study of Whillans and others (1989). Force-balance calculations reveal large variations in the along-flow component of driving stress that are muted by gradients in longitudinal stress such that basal drag is less variable spatially. The most pronounced sticky spot is located and the downstream end of a basal overdeepening, while smaller regions of high basal drag are co-located with a bed ridge transverse to the flow direction. On the large scale, gradients in longitudinal stress play a small role in balancing the driving stress, and flow resistance is partitioned between basal and lateral drags. Confirming earlier results, there is a significant component of driving stress in the across-flow direction resulting in non-zero basal drag in the direction perpendicular to ice flow. This is an unrealistic result and we propose that there are spatial variations in ice strength similar to those found on other glaciers.
67A059
Crevasse patterns in the catchment of Byrd Glacier, East Antarctica
Logan C. BYERS, Leigh A. STEARNS, C.J. VAN DER VEEN
Corresponding author: Logan C. Byers
Corresponding author e-mail: loganbyers@ku.edu
Complex patterns of surface crevasses are observed in extensive fields dispersed throughout the catchment of Byrd Glacier, Antarctica, using WorldView multispectral visible-band satellite imagery at 0.6 m horizontal resolution. Thorough mapping and orientation analysis reveals that some fields are composed of numerous intersecting and interacting crevasse sets, which experience differential deformation across their domains. A comparison of observed crevasse field occurrence to surface elevation demonstrates that fields are located primarily in regions where the ice surface dips highly downstream and uncrevassed areas tend to occur at positive or less negative downstream slopes. The spatial extent of some of these regions without observable crevasses is equivalent to shallow surface basins that maintain a zone of surficial snow deposition. Analysis of RADARSAT imagery with a horizontal resolution of 25 m and a penetration depth of ~8 m demonstrates that crevasses persist at depth as they are advected through shallow surface basins. As ice advects out of the basin, crevasses continue to be covered but may display some sagging of newly deposited snow. The continued presence of crevasses from high in the catchment past the grounding line may indicate that infilling with snow provides a resistance to closure but has little affect on opening. Infilling would maintain crevasses as structural weaknesses within the ice as advection and differential velocities subject the ice to varied stress states. Surface crevasses have previously been used as indicators and predictors for numerous dynamical properties of glaciers. The findings of this study therefore have implications for automated feature tracking in both visible and long wavelength imagery, the sensing and determination of complex glacier flow paths and controls on glacier flow, and accumulation rates and histories in ice-sheet conditions. This work also challenges the assumption that glacier ice is a structurally homogeneous material and demonstrates a connection exists between observable surface structures and basal conditions that dictate ice motion.

67A062
Insight into the existence and implications of ‘surface waves’ on Byrd Glacier
Sarah CHILD, Leigh STEARNS, C.J. VAN DER VEEN
Corresponding author: Leigh Stearns
Corresponding author e-mail: stearns@ku.edu
Byrd Glacier has one of the largest catchment basins in Antarctica and drains ~20.5 km3 of ice into the Ross Ice Shelf annually. Despite various studies since the late 1970s focusing on flow dynamics of Byrd Glacier, there is still little consensus about its ice–bed coupling and ice-flow relation to bed topography. In this study, we will utilize new bed and surface topography data, in conjunction with high-resolution velocity maps, to model the importance of bed topography and its impact on glacier flow. Our results yield insight into the dynamical flow regime and stability of Byrd Glacier in response to different external forcings. Reusch and Hughes (2003) hypothesized that as Byrd Glacier transitions from sheet flow to stream flow, the ice surface undergoes changes in surface slope (‘surface waves’) that appear to be unrelated to bed topography. The implication is that these surface waves reflect variations in the coupling between ice and the bed and that they may move as individual ice columns and migrate through the glacier. According to this hypothesis, surface waves represent regions of high longitudinal tensile stresses on the ice surface and the dominant resistance for the flow of Byrd Glacier is due to these longitudinal stress gradients. This theory contrasts several studies that conclude the driving stress of Byrd Glacier is primarily resisted by isolated regions of high basal drag (‘sticky spots’). This research investigates the surface wave theory, which has not been tested previously due to the lack of high-resolution bed topography data. From November 2011 to January 2012, the Center for Remote Sensing of Ice Sheets (University of Kansas) collected bed topography and ice thickness data over ~55 000 km2 of Byrd Glacier and its catchment. This dataset, in combination with satellite-derived velocity data, will be used to explore the origin and evolution of surface waves and their relationship to bed topography and longitudinal stress patterns. A correlation of the surface slopes (waves) and the bed topography slope will be performed to determine if wave location is independent of bed relief. Wave migration requires ice–bed decoupling, which could be an indication of Byrd Glacier having a thawed bed in contrast to a frozen one.



67A004
Morphology of basal crevasses at the grounding zone of Whillans Ice Stream, West Antarctica
Robert JACOBEL, Knut CHRISTIANSON, Adam WOOD, Rebecca GOBEL
Corresponding author: Robert Jacobel
Corresponding author e-mail: jacobel@stolaf.edu
The transition from limited- or no-slip conditions at the base of grounded ice to free-slip conditions beneath floating ice occurs across the few-kilometers-wide grounding zone of ice sheets. This transition is either an elastic flexural transition from bedrock to hydrostatically supported elevations (often tidally influenced), or a transition from thicker to thinner ice over a flat bed, or some combination of these processes. In either case, ice must flow across a changing stress field, often resulting in brittle deformation, which is manifested as basal crevassing at the ice-sheet base and tidal strand cracking on the ice-sheet surface. Thus the position and morphology of basal crevasses reveal important information about the stress state across this transition. We acquired gridded ground-based radar surveys at two locations of the Whillans Ice Stream grounding zone, one over a subglacial peninsula where the transition to floatation is abrupt and the second over a subglacial embayment where several dynamic subglacial lakes drain to the ocean, likely resulting in episodic high sediment and water flux across the grounding line. Our surveys indicate a complex pattern of basal crevasses: some are related to basal topography, but others more likely are associated with ice flexure across the basal channel carrying water and sediment to the ocean. Owing to the high reflectivity of sea water and the relatively shallow ice thickness, we image off-nadir crevasses where the radar energy is first reflected from the ice–water interface and then from the crevasse, forming a double image together with the direct reflection. For a basal crevasse with shallow dip, this geometry effectively enables imaging the crevasse from both upper and lower faces simultaneously, producing curious results similar to a clothing-store mirror. Similarly, we see returns from some crevasses that are subsequently reflected to the receiver from the ice–water interface, producing a crevasse signature with a reversed phase echo due to the second reflection. In several cases, these crevasse echoes mimic the geometry of a sub-ice ‘wedge’ dipping into the sediment, while in reality the radar never penetrates below the basal interface. Our results indicate that basal crevasses offer a rich but unexploited dataset for diagnosing stress state and salient processes, such as subglacial stress change over drainage channels, across grounding zones, and that special care is needed when interpreting subglacial returns in radar data."


67A008
Radar/seismic imaging of a subglacial estuary at the grounding zone of Whillans Ice Stream, West Antarctica
Knut CHRISTIANSON, Huw J. HORGAN, Richard B. ALLEY, Robert W. JACOBEL, Sridhar ANANDAKRISHNAN
Corresponding author: Knut Christianson
Corresponding author e-mail: christik@stolaf.edu
The most common view of subglacial water flow across ice-sheet grounding zones is akin to a subaerial waterfall, where water enters the ocean due to the steep gradient in subglacial hydropotential with essentially no marine influence inland. However, our radio-echo sounding and active-source seismic surveys image a grounding zone more analogous to an estuary or tidal lagoon at the downstream end of the hydrologic system that links the active subglacial lakes beneath Whillans Ice Stream to the ocean beneath the Ross Ice Shelf. Kinematic GPS and radar data indicate a hydropotential trough upstream of grounding that continues until the ice goes afloat. Immediately upstream of floatation, irregular basal ringing that persists well below the basal interface is consistent with reflections from on- and off-nadir water-saturated sediments, or, more simply, a till delta. Seismic data also indicate prograding sedimentation as the ice goes afloat and show that the hydropotential trough is linked to the ocean by a large subglacial channel, which has an apparent width of 1 km and maximum depth of 7 m. Pressure differences along the trough axis are within a range that can be overcome by tidally induced processes, making interaction of subglacial and ocean water likely. A shallow water column in the embayment (never thicker than 12 m) and low radar basal reflectivity also imply a well-mixed tidal estuary, with complex interaction of subglacial and ocean water and sediment. Our results highlight the need for joint radar/seismic surveys to properly assess the nature and spatial extent of basal conditions in grounding zones, and the need to consider complex interactions of subglacial and ocean water, sediment and tidal processes across a few-kilometer-wide grounding zone in ice-sheet models.

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While I did not (& I will not) have time to properly comment about the many abstracts that I posted from both the Beijing and Kansas IGSOC Symposia about the ice streams and ice shelves in the Weddell and Ross Sea Sectors; I would like to say that nothing that I posted has changes my opinions about the risks of the potential collapses of these sectors by the end of this century.

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The linked reference indicates that the researchers have discovered large ice channels beneath the Filchner-Ronne Ice Shelf, FRIS, in West Antarctica, that is 250 metres high and hundreds of kilometres long. The channels are largely created due to the channalized outflow of basal meltwater from beneath the grounding line and are likely to influence the stability of the ice shelf:

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1977.html

Evidence from ice shelves for channelized meltwater flow beneath the Antarctic Ice Sheetby: Anne M. Le Brocq, Neil Ross, Jennifer A. Griggs, Robert G. Bingham, Hugh F. J. Corr, Fausto Ferraccioli, Adrian Jenkins, Tom A. Jordan, Antony J. Payne, David M. Rippin & Martin J. Siegert; Nature Geoscience; (2013); doi:10.1038/ngeo1977

Abstract
"Meltwater generated beneath the Antarctic Ice Sheet exerts a strong influence on the speed of ice flow, in particular for major ice streams. The subglacial meltwater also influences ocean circulation beneath ice shelves, initiating meltwater plumes that entrain warmer ocean water and cause high rates of melting. However, despite its importance, the nature of the hydrological system beneath the grounded ice sheet remains poorly characterized. Here we present evidence, from satellite and airborne remote sensing, for large channels beneath the floating Filchner–Ronne Ice Shelf in West Antarctica, which we propose provide a means for investigating the hydrological system beneath the grounded ice sheet. We observe features on the surface of the ice shelf from satellite imagery and, using radar measurements, show that they correspond with channels beneath the ice shelf. We also show that the sub-ice-shelf channels are aligned with locations where the outflow of subglacial meltwater has been predicted. This agreement indicates that the sub-ice-shelf channels are formed by meltwater plumes, initiated by subglacial water exiting the upstream grounded ice sheet in a focused (channelized) manner. The existence of a channelized hydrological system has implications for the behaviour and dynamics of ice sheets and ice shelves near the grounding lines of ice streams in Antarctica."

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The linked reference (with a free access pdf) indicates that both the Institute and Möller Ice Streams are highly sensitive to changes in basal melting either near to their respective grounding lines:

A. P. Wright, A. M. Le Brocq, S. L. Cornford, M. J. Siegert, R. G. Bingham, H.F. J. Corr, F. Ferraccioli, T. A. Jordan, D. M. Rippin, and N. Ross, (2013), "Sensitivity of the Weddell Sea sector ice streams to sub-shelf melting and surface accumulation";  The Cryosphere Discuss., 7, 5475–5508, 2013; www.the-cryosphere-discuss.net/7/5475/2013/; doi:10.5194/tcd-7-5475-2013

http://www.the-cryosphere-discuss.net/7/5475/2013/tcd-7-5475-2013.pdf

Abstract:
"A recent ocean modelling study indicates that possible changes in circulation may bring warm deep ocean water into direct contact with the grounding lines of the Filchner- Ronne ice streams, suggesting the potential for future ice losses from this sector equivalent to ~0.3m of sea-level rise. Significant advancements have been made in our knowledge of both the basal topography and ice velocity in the Weddell Sea sector, thus enabling an assessment to be made of the relative sensitivities of the diverse collection of ice streams feeding the Filchner-Ronne Ice Shelf. Here we use the BISICLES ice sheet model, which employs adaptive-mesh refinement to resolve grounding line dynamics, to carry out such an assessment. The impact of perturbations to the surface and sub-shelf mass balance forcing fields from our 2000 yr “reference” model run indicate that both the Institute and Möller Ice Streams are highly sensitive to changes in basal melting either near to their respective grounding lines, or in the region of the ice rises within the Filchner-Ronne Ice Shelf. These same perturbations have little impact, however, on Rutford, Carlson or Foundation ice streams, while Evans Ice Stream is found to enter a phase of unstable retreat only after melt at its grounding line has increased by an order-of-magnitude from likely present-day values."

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The linked pdf/paper discusses physical model research of the dynamics of water flow into and out of the cavities beneath ice shelves in Antarctica:

http://efdl.cims.nyu.edu/publications/refereed/jfm_iceshelf_12.pdf

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The accompanying image and summary research statement provide an idea of the ice shelf fracturing and flow pattern of the Filchner Ronne Ice Shelf from 2003 to 2004 field data.  Such data is critical to estimating the possible future calving behavior of this ice shelf:

Hulbe, C. L., and C. M LeDoux. 2011. MOA-derived Structural Feature Map of the Ronne Ice Shelf. [indicate subset used]. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5PR7SXR.
 
Summary statement:
"This data set provides a structural feature map of the Ronne Ice Shelf in Antarctica (also known as the Filchner-Ronne Ice Shelf). The map was developed as part of a project to study fracture propagation in the Ronne Ice Shelf, with special focus on the Evans Ice Stream. Features were digitized from the MODIS Mosaic of Antartica (MOA), a composite of individual Moderate Resolution Imaging Spectradiometer (MODIS) images taken between 20 November 2003 and 29 February 2004, with an effective resolution of 125 m. The data set includes estimates of the shelf boundary, including ice stream grounding zones, outlets of glaciers feeding the shelf, extents of islands and ice rises, and the location of the shelf front, and features observed within the shelf, including suture zones between ice streams, streaklines, fractures (crevasses and rifts), and fold-like features. Individual features can be extracted as a group of points and grouping is used to facilitate identification and plotting."

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The linked reference better quantifies the ice mass loss from both calving and basal melting for the major Antarctic ice shelves
The first attached image shows: Calving fluxes (green) and basal mass loss (−BMB; red). Pie chart shows numbers for surveyed ice shelves only. Errors, 1 s.d. b, Ratio between calving flux (green) and BMB (red), in per cent of total flux.
The second attached figure shows a significant correlation (R2 = 0.84 (coefficient of determination); P = 3.13 × 10−5; F-test) between surface lowering rates8 and our mean basal mass-loss rates (−SBMB) for thinning ice shelves.

M. A. Depoorter, J. L. Bamber, J. A. Griggs, J. T. M. Lenaerts, S. R. M. Ligtenberg, M. R. van den Broeke & G. Moholdt, (2013),"Calving fluxes and basal melt rates of Antarctic ice shelves," Nature, doi:10.1038/nature12567

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12567.html?WT.ec_id=NATURE-20130919

Abstract: "Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 gigatonnes per year. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of −1,454 ± 174 gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing."

[sidd]

grounding line fluxes in the supplementary  Table 1:  Amery, Totten, Getz and George VI all exhibiting grounding line flux of 80Gton/yr or larger. Basal melt dominates in all those except Amery. from fig 1 in the ,ain paper.

of course PIG, THW, and the two large ice shelves are the heavies there.

the numbers seem odd for Abbott.

Good paper. They may have found a symptom of vulnerability to CDW incursion through troughs in Northeast, Vanderford, Moscow University, Totten, Sulzberger, Land and Cosgrove ice shelves.

sidd

[AbruptSLR]

The referenced paper (with a link to a free access pdf), adds concerns of firn air depletion to the list of factors to consider with regard to the potential collapse of Antarctic ice shelves.  The caption for the attached figure from the article reads: "Conceptual illustration of firn air depletion and its consequences for ice-shelf hydrology and stability. (a) An ice shelf covered by a firn layer containing sufficient air. The inset shows meltwater being stored in the pore space of the firn. (b) An ice shelf with a depleted firn layer. Due to the absence of pore space, meltwater forms ponds that drain into fractures. Alternatively, water is routed to the fractures efficiently as shown in the leftmost fractures. Courtesy: Journal of Glaciology and Kuipers et al":

"Firn air depletion as a precursor of Antarctic ice-shelf Collapse" Peter Kuipers Munneke, Stefan R.M. Ligtenberg, Michiel van den Broeke, David G. Vaughan, Journal of Glaciology, 60 (220), (2014). DOI: 10.3189/2014JoG13J183.

http://www.igsoc.org/journal/60/220/t13J183.pdf

"ABSTRACT. Since the 1970s, the sudden, rapid collapse of _20% of ice shelves on the Antarctic
Peninsula has led to large-scale thinning and acceleration of its tributary glaciers. The leading hypothesis for the collapse of most of these ice shelves is the process of hydrofracturing, whereby a water-filled crevasse is opened by the hydrostatic pressure acting at the crevasse tip. This process has been linked to observed atmospheric warming through the increased supply of meltwater. Importantly, the low-density firn layer near the ice-shelf surface, providing a porous medium in which meltwater can percolate and refreeze, has to be filled in with refrozen meltwater first, before hydrofracturing can occur at all. Here we build upon this notion of firn air depletion as a precursor of ice-shelf collapse, by using a firn model to show that pore space was depleted in the firn layer on former ice shelves, which enabled their collapse due to hydrofracturing. Two climate scenario runs with the same model indicate that during the 21st century most Antarctic Peninsula ice shelves, and some minor ice shelves elsewhere, are more likely to become susceptible to collapse following firn air depletion. If warming continues into the 22nd century, similar depletion will become widespread on ice shelves around East Antarctica. Our model further suggests that a projected increase in snowfall will protect the Ross and Filchner–Ronne Ice shelves from hydrofracturing in the coming two centuries."

[AbruptSLR]

If nothing else, the linked article indicates that the austral summer sea ice in the Ross Sea will decrease significantly in the coming decades:

Walker O. Smith Jr., Michael S. Dinniman, Eileen E. Hofmann, and John M. Klinck, (2014), "The effects of changing winds and temperatures on the oceanography of the Ross Sea in the 21st century", Geophysical Research Letters, DOI: 10.1002/2014GL059311

http://onlinelibrary.wiley.com/doi/10.1002/2014GL059311/abstract

Abstract: "The Ross Sea is critically important in regulating Antarctic sea ice and is biologically productive, which makes changes in the region's physical environment of global concern. We examined the effects of projected changes in atmospheric temperatures and winds on aspects of the ocean circulation likely important to primary production using a high-resolution sea ice–ocean–ice shelf model of the Ross Sea. The modeled summer sea ice concentrations decreased by 56% by 2050 and 78% by 2100. The duration of shallow mixed layers over the continental shelf increased by 8.5 and 19.2 days in 2050 and 2100, and mean summer mixed layer depths decreased by 12 and 44%. These results suggest that the annual phytoplankton production in the future will increase and become more diatomaceous. Other components of the Ross Sea food web will likely be severely disrupted, creating significant but unpredictable impacts on the ocean's most pristine ecosystem."


See also:

http://blueandgreentomorrow.com/2014/02/28/antarcticas-ross-sea-could-be-ice-free-by-2100-threatening-oceans-most-pristine-ecosystem/

[AbruptSLR]

The following linked reference discusses the influence of ocean variability on ice basal melting near the ice front of the Ross Ice Shelf:

Arzeno, I. B., R. C. Beardsley, R. Limeburner, B. Owens, L. Padman, S. R. Springer, C. L. Stewart, and M. J. M. Williams (2014), Ocean variability contributing to basal melt rate near the ice front of Ross Ice Shelf, Antarctica, J. Geophys. Res. Oceans, 119, 4214–4233, doi:10.1002/2014JC009792.

http://onlinelibrary.wiley.com/doi/10.1002/2014JC009792/abstract

Abstract:  "Basal melting of ice shelves is an important, but poorly understood, cause of Antarctic ice sheet mass loss and freshwater production. We use data from two moorings deployed through Ross Ice Shelf, ∼6 and ∼16 km south of the ice front east of Ross Island, and numerical models to show how the basal melting rate near the ice front depends on sub-ice-shelf ocean variability. The moorings measured water velocity, conductivity, and temperature for ∼2 months starting in late November 2010. About half of the current velocity variance was due to tides, predominantly diurnal components, with the remainder due to subtidal oscillations with periods of a few days. Subtidal variability was dominated by barotropic currents that were large until mid-December and significantly reduced afterward. Subtidal currents were correlated between moorings but uncorrelated with local winds, suggesting the presence of waves or eddies that may be associated with the abrupt change in water column thickness and strong hydrographic gradients at the ice front. Estimated melt rate was ∼1.2 ± 0.5 m a−1 at each site during the deployment period, consistent with measured trends in ice surface elevation from GPS time series. The models predicted similar annual-averaged melt rates with a strong annual cycle related to seasonal provision of warm water to the ice base. These results show that accurately modeling the high spatial and temporal ocean variability close to the ice-shelf front is critical to predicting time-dependent and mean values of meltwater production and ice-shelf thinning."

[AbruptSLR]

While both wili and bligh8 have posted about the following reference in the "Forcing" thread, I have decided to include a post here as this field research shows the importance of mesoscale eddy fields on transporting warmer water beneath the Filchner Ice Shelf, thus contributing to basal ice melt near the calving front in the area indicated in the attached image.

Andrew F. Thompson, Karen J. Heywood, Sunke Schmidtko & Andrew L. Stewart, (2014), "Eddy transport as a key component of the Antarctic overturning circulation", Nature Geoscience, doi:10.1038/ngeo2289

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2289.html

Abstract: "The exchange of water masses across the Antarctic continental shelf break regulates the export of dense shelf waters to depth as well as the transport of warm, mid-depth waters towards ice shelves and glacial grounding lines. The penetration of the warmer mid-depth waters past the shelf break has been implicated in the pronounced loss of ice shelf mass over much of west Antarctica. In high-resolution, regional circulation models, the Antarctic shelf break hosts an energetic mesoscale eddy field, but observations that capture this mesoscale variability have been limited. Here we show, using hydrographic data collected from ocean gliders, that eddy-induced transport is a primary contributor to mass and property fluxes across the slope. Measurements along ten cross-shelf hydrographic sections show a complex velocity structure and a stratification consistent with an onshore eddy mass flux. We show that the eddy transport and the surface wind-driven transport make comparable contributions to the total overturning circulation. Eddy-induced transport is concentrated in the warm, intermediate layers away from frictional boundaries. We conclude that understanding mesoscale dynamics will be critical for constraining circumpolar heat fluxes and future rates of retreat of Antarctic ice shelves."

[AbruptSLR]

The following link discusses an effort by Scripps to establish and monitor a seismic network on the Ross Ice Shelf, which includes the following statement:

"Starting in November, Scripps Institution of Oceanography, UC San Diego, researchers and colleagues will embark on an ambitious and arduous mission funded by National Science Foundation Polar Programs to install a seismic array on Antarctica’s Ross Ice Shelf."

https://scripps.ucsd.edu/news/seismic-network-will-measure-effects-ocean-waves-antarctic-ice-shelves

[AbruptSLR]

The linked reference discusses recent investigation of the FRIS:

Robert G. Bingham, David M. Rippin, Nanna B. Karlsson, Hugh F. J. Corr, Fausto Ferraccioli, Tom A. Jordan, Anne M. Le Brocq, Kathryn C. Rose, Neil Ross, Martin J. Siegert  (2015), "Ice-flow structure and ice dynamic changes in the Weddell Sea sector of West Antarctica from radar-imaged internal layering", Journal of Geophysical Research, DOI: 10.1002/2014JF003291


http://onlinelibrary.wiley.com/doi/10.1002/2014JF003291/full

Abstract: "Recent studies have aroused concerns over the potential for ice draining the Weddell Sea sector of West Antarctica to figure more prominently in sea level contributions should buttressing from the Filchner-Ronne Ice Shelf diminish. To improve understanding of how ice stream dynamics there evolved through the Holocene, we interrogate radio echo sounding (RES) data from across the catchments of Institute and Möller Ice Streams (IIS and MIS), focusing especially on the use of internal layering to investigate ice-flow change. As an important component of this work, we investigate the influence that the orientation of the RES acquisition track with respect to ice flow exerts on internal layering and find that this influence is minimal unless a RES flight track parallels ice flow. We also investigate potential changes to internal layering characteristics with depth to search for important temporal transitions in ice-flow regime. Our findings suggest that ice in northern IIS, draining the Ellsworth Subglacial Highlands, has retained its present ice-flow configuration throughout the Holocene. This contrasts with less topographically constrained ice in southern IIS and much of MIS, whose internal layering evinces spatial changes to the configuration of ice flow over the past ~10,000 years. Our findings confirm Siegert et al.'s (2013) inference that fast flow was diverted from Bungenstock Ice Rise during the Late Holocene and suggest that this may have represented just one component of wider regional changes to ice flow occurring across the IIS and MIS catchments as the West Antarctic Ice Sheet has thinned since the Last Glacial Maximum."

[AbruptSLR]

The linked reference indicates that the Filchner- Ronne Ice Shelf, FRIS, is relatively stable and is not in danger of collapsing due reasonably abrupt ocean warming.  Nevertheless, the model does not consider the possible collapse of the Byrd Subglacial Basin ice forming an ocean passage way from the Amundsen Sea Embayment to the Weddell Sea; and it uses a conservative (ESLD) model:

M. Mengel, J. Feldmann & A. Levermann (2015), 'Linear sea-level response to abrupt ocean warming of major West Antarctic ice basin", Nature Climate Change, doi:10.1038/nclimate2808


http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2808.html


Abstract: "Antarctica’s contribution to global sea-level rise has recently been increasing. Whether its ice discharge will become unstable and decouple from anthropogenic forcing or increase linearly with the warming of the surrounding ocean is of fundamental importance. Under unabated greenhouse-gas emissions, ocean models indicate an abrupt intrusion of warm circumpolar deep water into the cavity below West Antarctica’s Filchner–Ronne ice shelf within the next two centuries. The ice basin’s retrograde bed slope would allow for an unstable ice-sheet retreat, but the buttressing of the large ice shelf and the narrow glacier troughs tend to inhibit such instability. It is unclear whether future ice loss will be dominated by ice instability or anthropogenic forcing. Here we show in regional and continental-scale ice-sheet simulations, which are capable of resolving unstable grounding-line retreat, that the sea-level response of the Filchner–Ronne ice basin is not dominated by ice instability and follows the strength of the forcing quasi-linearly. We find that the ice loss reduces after each pulse of projected warm water intrusion. The long-term sea-level contribution is approximately proportional to the total shelf-ice melt. Although the local instabilities might dominate the ice loss for weak oceanic warming, we find that the upper limit of ice discharge from the region is determined by the forcing and not by the marine ice-sheet instability."

[AbruptSLR]

The linked research on local basal melting rates of the Ross Ice Shelf, RIS, indicates that channelized drainage water from adjoining marine glaciers can form grooves at a measured rate of over 2500% faster than the overall background basal melt rate.  Some decades in the future the presences of such basal grooves could reduce the stability of the ice shelf w.r.t. major iceberg calving, assuming continued global warming:

Oliver J. Marsh, Helen A. Fricker, Matthew R. Siegfried, Knut Christianson, Keith W. Nicholls, Hugh F. J. Corr & Ginny Catania (2015), "High basal melting forming a channel at the grounding line of Ross Ice Shelf, Antarctica", Geophysical Research Letters, DOI: 10.1002/2015GL066612

http://onlinelibrary.wiley.com/doi/10.1002/2015GL066612/full

Abstract: "Antarctica's ice shelves are thinning at an increasing rate, affecting their buttressing ability. Channels in the ice shelf base unevenly distribute melting, and their evolution provides insight into changing subglacial and oceanic conditions. Here we used phase-sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable Ross Ice Shelf. Melt rates of 22.2 ± 0.2 m a−1 (>2500% the overall background rate) were observed 1.7 km seaward of Mercer/Whillans Ice Stream grounding line, close to where subglacial water discharge is expected. Laser altimetry shows a corresponding, steadily deepening surface channel. Two relict channels to the north suggest recent subglacial drainage reorganization beneath Whillans Ice Stream approximately coincident with the shutdown of Kamb Ice Stream. This rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability."


https://scripps.ucsd.edu/news/study-finds-high-melt-rates-antarcticas-most-stable-ice-shelf

Extract: "A new Scripps Institution of Oceanography at UC San Diego-led study measured a melt rate that is 25 times higher than expected on one part of the Ross Ice Shelf. The study suggests that high, localized melt rates such as this one on Antarctica’s largest and most stable ice shelf are normal and keep Antarctica’s ice sheets in balance.
The Ross Ice Shelf, a floating body of land ice the size of France jutting out from the Antarctic mainland, continuously melts and grows in response to changes to both the ice sheet feeding it and the warmer Southern Ocean waters beneath it.

[AbruptSLR]

With a hat-tip to sidd, I provide the linked open access reference that indicates that no later than 2070 we can expect the Filchner-Ronne Ice Shelf to be subjected to an marked increase of warm under-base water flow from off-shelf sources that will accelerate basal melting and calving.

Hartmut H. Hellmer, Frank Kauker, Ralph Timmermann, and Tore Hattermann (2017), "The Fate of the Southern Weddell Sea Continental Shelf in a Warming Climate:, Journal of Climate, doi: 10.1175/JCLI-D-16-0420.1

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0420.1

Abstract: "Warm water of open ocean origin on the continental shelf of the Amundsen and Bellingshausen Seas causes the highest basal melt rates reported for Antarctic ice shelves with severe consequences for the ice shelf/ice sheet dynamics. Ice shelves fringing the broad continental shelf in the Weddell and Ross Seas melt at rates orders of magnitude smaller. However, simulations using coupled ice–ocean models forced with the atmospheric output of the HadCM3 SRES-A1B scenario run (CO2 concentration in the atmosphere reaches 700 ppmv by the year 2100 and stays at that level for an additional 100 years) show that the circulation in the southern Weddell Sea changes during the twenty-first century. Derivatives of Circumpolar Deep Water are directed southward underneath the Filchner–Ronne Ice Shelf, warming the cavity and dramatically increasing basal melting. To find out whether the open ocean will always continue to power the melting, the authors extend their simulations, applying twentieth-century atmospheric forcing, both alone and together with prescribed basal mass flux at the end of (or during) the SRES-A1B scenario run. The results identify a tipping point in the southern Weddell Sea: once warm water flushes the ice shelf cavity a positive meltwater feedback enhances the shelf circulation and the onshore transport of open ocean heat. The process is irreversible with a recurrence to twentieth-century atmospheric forcing and can only be halted through prescribing a return to twentieth-century basal melt rates. This finding might have strong implications for the stability of the Antarctic ice sheet."

[solartim27]

Now that Larsen broke, we can pay attention to these rifts in Ross.  Anyone think it's worthy of it's own thread?  There is a small extension of the rift off the page to the right side.  Click to animate

S1B_EW_GRDM_1SDH_20170712T103517_2B2D_S_1.final.jpg

S1B_EW_GRDM_1SSH_20170210T114000_A87C_S_1.final.jpg

[AbruptSLR]

Anyone think it's worthy of it's own thread?

solartim27,

I think that this type of calving event is rather typical for the RIS, so you would need to document a change in frequency of calving, say over 10 to 20-years, for it to mean very much.  So you can decide how many years you plan to regularly cover, to decide whether a new thread is warranted or not.

Best,
ASLR

[Gray-Wolf]

Suffice to say I had a natter with R Grumbine back in the noughties about the crevasse from Roosevelt toward the U.S. base at McMurdo and they were installing seismographs into the length of the fissure and we've not heard anything dire since?

The arrival of warmer bottom waters ( 2012?) might impact by eating back the grounding line?

[AbruptSLR]

The linked open access reference indicate that while the FRIS is currently essentially in a steady state condition, this could change in the future, and that predictive models need to consider changes in tidally influence advective pathways beneath the FRIS due to global warming:

Mueller, R. D., Hattermann, T., Howard, S. L., and Padman, L.: Tidal influences on a future evolution of the Filchner-Ronne Ice Shelf cavity in the Weddell Sea, Antarctica, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-110, in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-110/

Abstract: "Recent modeling studies of ocean circulation in the southern Weddell Sea, Antarctica, project an increase over this century of ocean heat into the cavity beneath Filchner-Ronne Ice Shelf (FRIS). This increase in ocean heat would lead to more basal melting and a modification of the FRIS ice draft. The corresponding change in cavity shape will affect advective pathways and the spatial distribution of tidal currents, which play important roles in basal melting under FRIS. These feedbacks between heat flux, basal melting, and tides will affect the evolution of FRIS under the influence of a changing climate. We explore these feedbacks with a three-dimensional ocean model of the southern Weddell Sea that is forced by thermodynamic exchange beneath the ice shelf and tides along the open boundaries. Our results show regionally-dependent feedbacks that, in some areas, substantially modify the melt rates near the grounding lines of buttressed ice streams that flow into FRIS. These feedbacks are introduced by variations in meltwater production as well as the circulation of this meltwater within the FRIS cavity; they are influenced locally by sensitivity of tidal currents to water column thickness and non-locally by changes in circulation pathways that transport an integrated history of mixing and meltwater entrainment along flow paths. Our results highlight the importance of including explicit tidal forcing in models of future mass loss from FRIS and from the adjacent grounded ice sheet as individual ice stream grounding zones experience different responses to warming of the ocean inflow."

[AbruptSLR]

The linked open access reference studies fracturing and calving events for the Ronne-Filchner Ice Shelf:

Li, R., Xiao, H., Liu, S., and Tong, X.: A Systematic Study of the Fracturing of Ronne - Filchner Ice Shelf, Antarctica, Using Multisource Satellite Data from 2001 to 2016, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-178, in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-178/

Abstract. We propose a new framework of systematic fracture mapping and major calving event prediction for the large ice shelves in Antarctica using multisource satellite data, including optical imagery, SAR imagery, altimetric data, and stereo mapping imagery. The new framework is implemented and applied for a comprehensive study of the fracturing of Ronne-Filchner Ice Shelf (RFIS), the second largest ice shelf in Antarctica, using a long time dataset dating back to 1957. New remote sensing data that have been made available in the past decade, including Landsat 8, WV-2, ZY-3 and others, greatly enhance our abilities to detect new fractures and monitor large rifts in three dimensions. Two large rifts, Rifts 1 and 2, were newly detected and are comparable to the Grand Chasm that caused a major calving event in the region in 1986. Three-dimensional rift models generated from quasi real-time stereo ZY-3 images revealed important topographic information about the large rifts that can be used to improve the reliability of ice shelf modeling and support enhanced analyses of ice shelf stability. Based on the results of the 2D and 3D fracture mapping, the spatial and temporal analyses of the overall fracture changes and large rift evolutions, i.e., the level of fracturing in RFIS, were slightly increased, particularly at the front of the ice sheet. The overall fracture observations do not seem to suggest immediate significant impacts on the stability of the shelf. However, the most active regional fracturing activities occurred at the front of Filchner Ice Shelf (FIS). A potential upcoming major calving event of FIS is estimated to occur in 2051. The stability of the ice shelf, particularly with regard to the developments of Rifts 1 and 2, should be closely monitored.

[AbruptSLR]

The linked reference provides field evidence of the impacts of tsunami and infragravity waves on the Ross Ice Shelf and concludes that such very long period waves can reduce the stability of Antarctic ice shelves; which could then reduce the buttressing on the associated marine glacial, and thus can serve to accelerate the rate of sea level rise (and the ice-climate feedback mechanism).

P. D. Bromirski et al. (20 July 2017), "Tsunami and infragravity waves impacting Antarctic ice shelves", JGR Oceans, DOI: 10.1002/2017JC012913 

http://onlinelibrary.wiley.com/doi/10.1002/2017JC012913/full

Abstract: "The responses of the Ross Ice Shelf (RIS) to the 16 September 2015 8.3 (Mw) Chilean earthquake tsunami (>75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array deployed on the RIS from November 2014 to November 2016. Here we show that tsunami and IG-generated signals within the RIS propagate at gravity wave speeds (∼70 m/s) as water-ice coupled flexural-gravity waves. IG band signals show measureable attenuation away from the shelf front. The response of the RIS to Chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural-gravity wave excitation by very long period (VLP; >300 s) gravity waves. Displacements across the RIS are affected by gravity wave incident direction, bathymetry under and north of the shelf, and water layer and ice shelf thicknesses. Horizontal displacements are typically about 10 times larger than vertical displacements, producing dynamical extensional motions that may facilitate expansion of existing fractures. VLP excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm. Because VLP flexural-gravity waves exhibit no discernable attenuation, this energy must propagate to the grounding zone. Both IG and VLP band flexural-gravity waves excite mechanical perturbations of the RIS that likely promote tabular iceberg calving, consequently affecting ice shelf evolution. Understanding these ocean-excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise."

Plain Language Summary
"A major source of the uncertainty in the magnitude and rate of global sea level rise is the contribution from Antarctica. Ice shelves buttress land ice, restraining land ice from reaching the sea. We present the analysis of seismic data collected with a broadband seismic array deployed on the Ross Ice Shelf, Antarctica. The characteristics of ocean gravity-wave-induced vibrations, that may expand existing fractures in the ice shelf and/or trigger iceberg calving or ice shelf collapse events, are described. The mechanical dynamic strains induced can potentially affect ice shelf integrity, and ultimately reduce or remove buttressing restraints, accelerating sea level rise."

[AbruptSLR]

The linked reference indicates that basal melt rates for ice for the Ross Ice Shelf has a significant impact on the behavior of the Southern Ocean, including that this consideration needs to be included in ESM projections:

Liu, X.: Modelling Ross Ice Shelf melting effect on the Southern Ocean in quasi-equilibrium, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-228, in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-228/

Abstract. To study the influence of basal melting of Ross Ice Shelf (BMR) on the Southern Ocean (ocean southward of 35° S) in quasi-equilibrium, numerical experiments with and without BMR effect have been performed with a global ocean-sea ice-ice shelf coupled model. In both experiments, the model started from a state of quasi-equilibrium ocean and was integrated for 500 years forced by CORE (Coordinated Ocean-ice Reference Experiment) normal year atmospheric fields. The simulation results of the last 100 years have been analysed. It’s shown that, the melt rate averaged over the entire Ross Ice Shelf is 0.253 m/a, which is associated with a freshwater flux of 3.15 mSv (1 mSv = 103 m3/s). The extra freshwater flux decreases the salinity in the Southern Ocean substantially whereas the effect of concurrent heat flux is not so significant except in the middle layer of water body (roughly from 1500 m to 3000 m). The decreased density due to BMR effect creates local circulation anomalies in the Ross Sea and nearby water with the help of ocean bathymetry. Through advection by the Antarctic Circumpolar Current, the flux anomaly from BMR gives rise to the increase of sea ice thickness and sea ice concentration in the Ross Sea adjacent to the coast and the ocean water westward. The warm advection and downwelling associated with the local circulation anomalies decrease the sea ice concentration in the rim of sea ice cover adjacent to open water in the Ross Sea in September. The decreased density weakens the sub-polar cell as well as the lower cell in the global residual meridional overturning circulation. And, northward meridional heat transport anomaly in most latitudes of the global ocean is accompanied accordingly.

[AbruptSLR]

The linked reference demonstrates that small changes in ice thickness on the edge of Antarctic ice shelves (like RIS) can induce thinning over distances of more than 900km of the rest of the ice shelf (see the attached image), which also reduces the buttressing action on the adjoining marine glacier:

R. Reese et al (2017), "The far reach of ice-shelf thinning in Antarctica", Nature Climate Change, doi:10.1038/s41558-017-0020-x

http://www.nature.com/articles/s41558-017-0020-x

Abstract: "Floating ice shelves, which fringe most of Antarctica’s coastline, regulate ice flow into the Southern Ocean. Their thinning or disintegration can cause upstream acceleration of grounded ice and raise global sea levels. So far the effect has not been quantified in a comprehensive and spatially explicit manner. Here, using a finite-element model, we diagnose the immediate, continent-wide flux response to different spatial patterns of ice-shelf mass loss. We show that highly localized ice-shelf thinning can reach across the entire shelf and accelerate ice flow in regions far from the initial perturbation. As an example, this ‘tele-buttressing’ enhances outflow from Bindschadler Ice Stream in response to thinning near Ross Island more than 900 km away. We further find that the integrated flux response across all grounding lines is highly dependent on the location of imposed changes: the strongest response is caused not only near ice streams and ice rises, but also by thinning, for instance, well-within the Filchner–Ronne and Ross Ice Shelves. The most critical regions in all major ice shelves are often located in regions easily accessible to the intrusion of warm ocean waters, stressing Antarctica’s vulnerability to changes in its surrounding ocean."

See also:
https://www.pik-potsdam.de/news/press-releases/tiny-ice-losses-at-antarctica2019s-fringes-can-accelerate-ice-loss-far-away

Caption: "Ross Ice Shelf: changes in speed resulting from 1m thinning (red: area of thinning, blue shading: resulting change in ice flow speed, ocean in grey). Fig. 2b from Reese et al, 2017"