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Stephan

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Re: PIG has calved
« Reply #1250 on: December 07, 2019, 02:43:42 PM »
I wish PIIS were as stable as many other ice shelves around Antarctica. But it isn't.
But in this case this instability it may help the climate scientists on the IPCC conference having actual arguments to limit global warming. From this point of view a major calving event would be helpful before the conference ends. I guess that the participants in Madrid are aware of the actual situation.
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paolo

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Re: PIG has calved
« Reply #1251 on: December 07, 2019, 04:40:53 PM »
Stephan, we are all concerned, even very concerned, but let's remain cautious, let's try to see the elements we can have, satellite images (the observation) yes, but also the articles (the analyses and the reasons, even if there is always a gap between what is happening and the publications).
Only in this way can we achieve reasonable (short-term) forecasts. ;)

I also think that PIG has become unstable and the calving line, which in the past has remained rather stable, is no longer stable and that it is likely to fall back in the next 10 years by several kilometres, but to move from a feeling to reasonable arguments there is work to be done, and that is why we are investing in this blog.  :)

It should be remembered that PIG advances more than 4 km per year and that without new rifts the calving line can advance again. :o

Stephan

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Re: PIG has calved
« Reply #1252 on: December 07, 2019, 04:54:05 PM »
Yes, the calving line will eventually advance again. PIG has speeded up and new ice will fill the gap of the lost ice after calving events.
But it will be different after the "cork" has gone. I expect a row of icebergs from the "zone of destruction" moving with the currents in WNW direction one after another, leading to an extension of the polynya. These icebergs already wait to be liberated and never before (after the "cork" has gone) has a structure like this been accessed by open water. Look at PIIS three years ago. A block of solid ice almost without cracks. Until 2019 only big (dozens of km²) icebergs calved from time to time. This will change now dramatically.
The reason for this zone of destruction in my opinion is the different speeds of the PIG/PIIS main trunk on the one hand (fast, WNW-headed) and the southwestern part (slow, NNE-headed). The general destruction, thinning, speeding-up has weakened the ice in general. Therefore the different speeds and movement directions now come into play.
« Last Edit: December 07, 2019, 05:12:42 PM by Stephan »
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paolo

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Re: PIG has calved
« Reply #1253 on: December 07, 2019, 06:35:24 PM »
The rapid emptying of the SSM is not obvious without analysis: on the one hand there is currently a rather dense mixture of large icebergs with the creation of tensions at the SIS and the PIG itself and the fact that it can empty quickly is only a hypothesis based on images and on the other hand the SIS advances towards the PIG and thus tends to recreate the shear zone.

And even in the event of a rapid emptying, this only implies that for the PIG there would be no more shear in the south and that it would remain only in the north (NSM).
This can create tensions in the PIG, but not rapid destruction.

Calving requires rifts, not at the sides, but in the center and asymmetric shear, is it enough to create many?

By absurdity, let us suppose that there are no more shear zones, neither in the south nor in the north, the consequence would be an advancement of the PIG!

I also think it will have to step back, but it must be supported by analyses and then we will be able to make hypotheses about its entity and its speed.

paolo

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Re: PIG has calved
« Reply #1254 on: December 07, 2019, 06:42:29 PM »
The analysis takes time, but in the months following this calving we will have it. :)

Moving on to something else, I think a reminder of the calvings of 2017, 2018 and the one that begins could be useful, hence the attached gif (note the fast frequency which is new for the PIG)

baking

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Re: PIG has calved
« Reply #1255 on: December 07, 2019, 07:22:00 PM »
I also think it will have to step back, but it must be supported by analyses and then we will be able to make hypotheses about its entity and its speed.

I think the long-term pattern has been alternating pinning points on the Northern and Southern Shear Margins and the rifting we have been seeing upstream on both margins implies this will continue for a while.

See the image below.  For a long time the glacier was pinned at about point 1 on the NSM, then about 2010-2011 it switched to point 2 on the SSM.  Recently it has switched to point 3.  With the collapse of the SSM downstream of point 4, that will likely become the new pinning point in 5+ years time.  Points 5 and 6 are just off the image but are pretty much at opposite points on the glacier so it is hard to predict where it might go from there, but that is hopefully many years away.

Stephan

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Re: PIG has calved
« Reply #1256 on: December 07, 2019, 09:38:36 PM »
The rapid emptying of the SSM is not obvious without analysis: on the one hand there is currently a rather dense mixture of large icebergs with the creation of tensions at the SIS and the PIG itself and the fact that it can empty quickly is only a hypothesis based on images and on the other hand the SIS advances towards the PIG and thus tends to recreate the shear zone.

And even in the event of a rapid emptying, this only implies that for the PIG there would be no more shear in the south and that it would remain only in the north (NSM).
This can create tensions in the PIG, but not rapid destruction.

Calving requires rifts, not at the sides, but in the center and asymmetric shear, is it enough to create many?

By absurdity, let us suppose that there are no more shear zones, neither in the south nor in the north, the consequence would be an advancement of the PIG!

I also think it will have to step back, but it must be supported by analyses and then we will be able to make hypotheses about its entity and its speed.
Thank you for your thoughts and opinions. I am an interested layman, not a glaciologist, so maybe it is not correct what I am going to write here.

If the "zone of destruction" between the SIS and the PIIS will be once cleared out and a 1-2 km wide and maybe 10 km long area of open water is in-between them, wouldn't PIIS try to expand into that open space? And would it do it evenly by just thinning and spreading out or wouldn't occur this by new cracks parallel to the flow direction? And, if so, wouldn't these cracks widen and give room for new calvings?

Finally, the speeding-up of PIIS will continue. Further upstream there will be new shear margins between the SIS and the PIIS, leading to new "zones of destruction" which will then be vulnerable to further minor calvings in the future.

In the end, speeding-up, thinning and new cracks should also lead to a further retreat of the grounding line. Are there actual measurements of the grounding line evolution in PIG underway?
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paolo

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Re: PIG has calved
« Reply #1257 on: December 07, 2019, 10:01:05 PM »
Banking,
Let's try to understand us: I alerted you recently, if I'm not mistaken, to the fact that the shear caused by the dense mixture of icebergs had just created two new rifts (3/4 km) in the south side PIG and on a separation between the PIG and the SIS with a probable increase in speed on the south side.

I am well aware of the poor state of the PIG, which is very unstable, and I believe I have already mentioned the risk of its gradual and almost inevitable decline. But I would like to discuss, with the means at our disposal, all the factual elements that can slow down or even accelerate this decline, and we will have time to do so.

A quick remark: in recent years, historically unusual for the PIG, all calvings have started from the extension of the central rift extending towards the edges and for the moment we do not see any new central rifts (they will arrive, I know)

To finish a picture of a glacier currently in agony  :'(, in 15/20 years there will be nothing left and for centuries in the best of hypotheses (in the dolomites many of the small glaciers I had known they no longer exist; forgive me for this small personal parenthesis)

paolo

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Re: PIG has calved
« Reply #1258 on: December 07, 2019, 10:31:07 PM »
New image, not very big innovations: to the NE, between the 23/11 and today, the R1 rift has been extended by 500m and the R2 rift by 300m, but by turning 90° towards R1.

I've already bothered you enough today, I'm just adding a zoom on the "Cork"/PIG joint

baking

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Re: PIG has calved
« Reply #1259 on: December 08, 2019, 03:37:44 AM »
A quick remark: in recent years, historically unusual for the PIG, all calvings have started from the extension of the central rift extending towards the edges and for the moment we do not see any new central rifts (they will arrive, I know)

Central rifts seem like they are important, but they are only a symptom.  They are usually caused when the downstream ice is moving faster than the upstream ice.  The real question is, why is the ice moving so fast.

There are three things that slow down a glacier: 1) Ice shelves in front of the glacier (whether small ones like the Thwaites Eastern Ice Shelf, or larger ones like the Larsen C or Ross ice shelves,) 2) Grounding on the bottom surface, and 3) friction at the shear margins.

The story with the Pine Island Glacier has always been "It has no ice shelf in front to stabilize it, its grounding line is back from the ice front, BUT THANKFULLY it is longer than it is wide and the friction at the shear margins on the sides of the glacier will keep it from a rapid collapse.

UNFORTUNATELY, as the Pine Island Glacier has begun to speed up, the shear margins have started to rift.  It usually starts upstream and spreads downstream.  Rifting in on the Southern Shear Margin was first observed in 1999 and it is just now reaching the calving front.  What it means is that there are now large parts of the Southern Shear Margin that provide only minimal friction to slow down the glacier.

And recently rifting has been observed upstream on the Northern Shear Margin.  If it continues to spread downstream, the same thing will happen on that side.

In short, there is a positive feedback cycle where an increase in the speed of the glacier leads to thinner and weaker ice which will mean even faster speeds in the future.

This does not go on indefinitely.  There will probably a new equilibrium reached at some point, probably when the calving front reaches they upstream point of the current rifts and is much closer to the grounding line.  Of course what this new equilibrium will look like or what it will mean is pure speculation at this point.  Maybe with better data and models someone will be able to make an educated guess, but the path up until that point seems pretty clear to me based on recent history.

paolo

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Re: PIG has calved
« Reply #1260 on: December 08, 2019, 05:08:45 PM »
Banking,
I see that we are on the same wavelength and we will talk about it again soon.

Returning to the current calving, here is a zoom of the PIG-Cork with an alignment on the future iceberg
The animation is based on the images of 17/11 and 07/12

Double click!!!

Stephan

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Re: PIG has calved
« Reply #1261 on: December 08, 2019, 06:10:30 PM »
A new Sentinel picture requires a new detailed analysis of the Cork and its surroundings. I compared the 2019-12-07 woth the 2019-11-03 picture.
The crack in the SW tributary has widened markedly (circled in green). Some icebergs S and E of the cork have turned over (circled in red). Some of the smaller icebergs were smashed between the bigger ones (circled in blue). New or widened cracks are marked in red. The bigger floe S of the cork (called "Cork II") has a less structural integrity as the "original" cork. I guess it will break further apart as the movement of the PIIS increases after the calving. It has already separated from the SIS, but it is still linked to the SW tributary on its western edge.
I also think there is open water (yellow arrow) SE of Cork II. But this may be a mis-interpretation.

See attached picture.
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blumenkraft

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Re: PIG has calved
« Reply #1262 on: December 08, 2019, 07:47:28 PM »
And here is an update on the upstream cracks.

Since i am the only one reporting on them, feel free to call them blumencracks. :P  ;D
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Wipneus

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Re: PIG has calved
« Reply #1263 on: December 08, 2019, 08:02:03 PM »
The new crack is getting longer and better visible in this Sentinel 2 image (contrast enhanced, 20m/pix).

baking

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Re: PIG has calved
« Reply #1264 on: December 08, 2019, 08:11:52 PM »
When I saw this tweet this morning, I knew we'd be seeing some nice Sentinel-2 pics today.

https://twitter.com/geologicalJo/status/1203591078403825664

baking

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Re: PIG has calved
« Reply #1265 on: December 08, 2019, 08:26:11 PM »
This is the part of the crack (northern end) I have been watching closely.

oren

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Re: PIG has calved
« Reply #1266 on: December 08, 2019, 10:29:00 PM »
This can't hold on much longer. I only wonder which will go first, the main crack or the cork. Or both together.

paolo

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Re: PIG has calved
« Reply #1267 on: December 09, 2019, 12:20:02 AM »
Banking,
I don't know if I can reassure you, I can't do it for myself, because of the state of the SSM, but the NSN for now is holding.

Attached are two GIF with images of the northern pinning point and the upper part of the NSM on the dates of 03/12/2016, 21/01/2018, 18/12/2018 and 07/12/2019.

The northern pinning point seems to still hold and one should only witness an anti-clockwise rotation of the PIG calving line and the condition of the MSN at the moment is not much different from the past.

The problem is the future in the south....

baking

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Re: PIG has calved
« Reply #1268 on: December 09, 2019, 02:02:35 AM »
paolo,

Maybe we agree in principle, but only differ on our wording, but this is how I see it.

1.  The collapse in the South is a foregone conclusion.  The margin will be just a melange of icebergs in a month or two.  How quickly the rest of the collapse happens is not important, but it will happen.

2.  The pinning point in the North is the current anchor and should be for a number of years.  We need to cross our fingers and hope it holds.  This is where I say the future lies.

3.  But those rifts coming from upstream on the northern margin will eventually work their way down and weaken whatever pinning point remains in the North.

4.  Then the focus will be on the next pinning point in the South, etc.

paolo

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Re: PIG has calved
« Reply #1269 on: December 09, 2019, 03:42:44 PM »
 “Merde” (nice French word), even side NSM it does not go too well!!

What to watch

Click to move !

kassy

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Re: PIG has calved
« Reply #1270 on: December 09, 2019, 04:12:39 PM »
How likely would the sideways paths be? I would think the crack should go forward assuming all else being equal and of course it is not but i do not see a reason why they would go into that location. Then again i really know nothing about these things except from what i pick up from your discussions.

There is a weakness on the left?
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paolo

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Re: PIG has calved
« Reply #1271 on: December 09, 2019, 06:10:02 PM »
Kassy,

For several reasons:
1. The pinning point exerts a back stress to the glacier.
2. A rift already exists towards the calving line (and others further inland were forming; see for example post 1267)
3. The shear zone is a more fragile area: lower thickness and basal channels parallels to flow

For these reasons one might have expected a more or less cross-cutting junction between the central rift and the lateral rift.

Several animations of the recent calving have been posted recently (e.g. post 1254).
« Last Edit: December 09, 2019, 06:22:11 PM by paolo »

baking

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Re: PIG has calved
« Reply #1272 on: December 09, 2019, 06:31:34 PM »
I have a great picture of the crack, but every time I post it here it looks like a compression artifact.  Very frustrating!

baking

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Re: PIG has calved
« Reply #1273 on: December 09, 2019, 06:38:00 PM »
Try again:

blumenkraft

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Re: PIG has calved
« Reply #1274 on: December 09, 2019, 06:44:19 PM »
Baking, this is a phenomenon i've experienced myself way too often also...

(If it helps, i see what you see.)
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baking

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Re: PIG has calved
« Reply #1275 on: December 09, 2019, 07:35:03 PM »
Just to be clear, what paolo saw in the low-res image from December 9 is close to, but not what I saw in the image from December 1.

My "crack" is parallel to the existing marginal rifts in the shear margin.  We also see a turning ("hook") of the main transverse rift in line with the marginal rifts that implies there is some connection between two cracks.

What paolo has identified seems to be closer to perpendicular to the margin and what he may be seeing are some bands in the marginal ice seen in the Sentinel-2 image below from December 7.

In response to kassy, I have identified a zone of thinner ice and I feel once the crack reaches there the calving will follow within a matter of days if not hours.

paolo

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Re: PIG has calved
« Reply #1276 on: December 09, 2019, 09:35:44 PM »
Baking,

Let us hope that the white line on today's Sentinel1 image is a "false alert" and that the difference between the images of 07 and of 09 is due solely to the fact that these correspond at two passages of the satellite with different characteristics (this is the problem you can have when you need day-to-day information). The next Sentinel1 images will give the answer.

To move on, attached a test, a gif animation with:

-A recent image of bathymetry with the usually displayed grounding line that date from 1996!!

-An image of elevations of the PIG and PIIS with a 2011 grounding line

-A Sentinel2 image of 11/17/2019

Before finalizing and an official presentation with all the information I wanted to have an opinion

Click to move

baking

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Re: PIG has calved
« Reply #1277 on: December 09, 2019, 10:19:02 PM »
paolo,

Thanks for the overlays.  I don't want to steal your thunder, but if you are making the case that the Northern pinning "point" is substantial and the front would have to retreat quite a bit to reach the ice shelf above the pining point, I cede your point.

BUT, when the southern side retreats significantly, the northern point will become much more vulnerable AND think of the upstream rifts slowly but surely moving downstream.

Stephan

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Re: PIG has calved
« Reply #1278 on: December 09, 2019, 11:06:06 PM »
Paolo,

thank you for that gif. I'd like to see it playing somewhat slower so that I can focus more on the details of bathymetry and elevation. Would it be possible to put a scale on the elevation profile?  Thank you.
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paolo

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Re: PIG has calved
« Reply #1279 on: December 09, 2019, 11:38:44 PM »
Stephan,

It was a test version that will be quickly:

-completed with the information’s relative to the images
-completed with indications, even if vague, of the current positioning of the calving line (in articles one could found indications on the speed in past years of the move of the calving line)
-slowed down to allow you to position the particular elements of each image in your head
-accompanied by the original images (more extensive where possible: bathymetry)
-accompanied, as and when, other images for which we can now position the elements using the calving line displayed (the one of 1996 that now we will know how to put in context)

I would use them for analysis and begin with a first examination of the problem of pinning points of the PIG

paolo

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Re: PIG has calved
« Reply #1280 on: December 10, 2019, 03:05:17 PM »
For today only an inconclusive low-resolution Sentinel 1 image  >:(

On this subject I limit myself to publishing the image of Baking retouched  ;)

Tonight I should publish the promised images  :)
« Last Edit: December 10, 2019, 03:20:12 PM by paolo »

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Re: PIG has calved
« Reply #1281 on: December 10, 2019, 04:19:46 PM »
For today only an inconclusive low-resolution Sentinel 1 image  >:(

I see what you see in this one, Paolo.
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Stephan

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Re: PIG has calved
« Reply #1282 on: December 10, 2019, 10:58:41 PM »
I followed the track of the two new little icebergs (circled in orange) generated by the calving event last weekend. They traveled up to 37 km in the last three days - a sign for a strong current in Pine Island Bay. The third little iceberg (circled in yellow) that had been "almost broken off" on Dec 7 is now also floating freely close to the calving front.

See attached picture.
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paolo

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Re: PIG has calved
« Reply #1283 on: December 11, 2019, 05:07:41 PM »
In a previous post on SWT I had already used a bathymetric image from an article for which I had given a brief bibliographical indication. Baking had used it in relation to the TG and had given a full bibliographical indication of it. I can only invite those interested in monitoring the fate of the ESA (Admunsen Sea Embayment) glaciers to consult it.

At least consult the image relating to bathymetry (Figure 2),
(a) it is the most up-to-date currently available,
(b) it covers the entire ESA glacier: from the "Cosgrove Ice Shelf" to the "Dotson Ice Shelf" and there is also bathymetry from inside the ice pack,
c) it is very detailed (it can be zoomed in at will),
but also the technique used and especially the important elements brought by this update!

I recall the bibliographical reference:
R. Millan, Rignot, V. Bernier, M. Morlighem, and P. Dutrieux (2017), Bathymetry of the Amundsen Sea Embayment sector of West Antarctica from Operation IceBridge gravity and other data, Geo-phys. Res. Lett. 44, 1360-1368, doi:10.1002/2016GL072071.

I join here:
1) a small complete image of Figure 2 on bathymetry to give you the extension of its coverage,
2) a small zoom on the PIIS: NIS, PIG and SIS with the SWT

Note: the level line highlighted are those relating to -300m, -700m (this height is very important in relation to the intrusions of the "hot" ones of the CDW), -1100m

Note: The grunding line is the one for 1996 and a post will follow (as promised) with a comparison with the grunding line for 2011

Click to zoom in

Translated with www.DeepL.com/Translator (free version)
« Last Edit: December 11, 2019, 05:13:32 PM by paolo »

paolo

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Re: PIG has calved
« Reply #1284 on: December 11, 2019, 07:40:43 PM »
Before passing the animation related to the grunded line I would like to present the article from where I had taken the figure with the grunded line of 2011:
 "Ice shelf basal melt melt rates from a high-resolution digital elevation model (DEM) record for Pine Island Glacier, Antarctica" David E. Shean, Ian R. Joughin, Pierre Dutrieux, Benjamin E. Smith, and Etienne Berthier
The Cryosphere, 13, 2633-2656, 2019https://doi.org/10.5194/tc-13-2633-2019

In this article, based on shelf data: altimetry and motion, they deduce the basal melt-rate of the shelf

It contains many interesting elements

Attached :
1) the image of the PIG with the elevation and grunding line of 2011
2) a zoom of this image
3) the image of the PIG with the movements ("surface velocity magnitude with the color ramp saturated at 1 km yr−1to show detail over tributaries")
4) the image with the calculated quantities of the basal melt

Click to zoom in

paolo

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Re: PIG has calved
« Reply #1285 on: December 11, 2019, 08:28:50 PM »
Relating to the grunding line:

1) In many of the images that can be found in the articles it is displayed the 1996 grunding line and to position the data of these images relative to the Sentinel images it is necessary to establish a link between this line and the Sentinel images

2) This line allows us to better understand what is happening and it seems to me that having its most recent positioning is important

Hence the attached animation relating to the 1996 grunding line and the 2011 grunding line

I have decided not to add satellite images to the animation for several reasons:
1) Sentinel 1 images do not really provide, at this scale, elements allowing, from the animation, to be positioned when viewed at normal magnification
2) For Sentinel 2 images it's a little better, but they use another projection, which means that not only do you have to turn them over slightly, but, more importantly, there are deformations
3) The elevation image, used here, already contains the elements to better position itself

The animation is therefore generated from:
1) The image relating to bathymetry with the 1996 grunding line
2) From the image with the elevations with the 2011 grunding line

2.5s between the images, click for animation

paolo

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Re: PIG has calved
« Reply #1286 on: December 11, 2019, 08:45:37 PM »
Next post what remains of the NSM (40 km) between the last pinning point and a small unnamed lateral ice shelf.
NSM that is already starting to tear at its beginning upstream

paolo

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Re: PIG has calved
« Reply #1287 on: December 12, 2019, 03:03:25 PM »
Last Sentinel 1 image of 12/12

blumenkraft

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Re: PIG has calved
« Reply #1288 on: December 12, 2019, 04:50:21 PM »
How are the chances this pinning point is the reason it hasn't already calved?
The apocalypse is already here; it's just not very evenly distributed.

baking

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Re: PIG has calved
« Reply #1289 on: December 12, 2019, 05:04:38 PM »
How are the chances this pinning point is the reason it hasn't already calved?

It's the only thing that's holding it back.  I put no faith in the "cork" or in the tiny piece of ice between the two cracks.

blumenkraft

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Re: PIG has calved
« Reply #1290 on: December 12, 2019, 05:14:20 PM »
That's what i thought. Thank you, Baking.
The apocalypse is already here; it's just not very evenly distributed.

paolo

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Re: PIG has calved
« Reply #1291 on: December 12, 2019, 06:44:21 PM »
In view of a future discussion on the next future of the PIIS I would like to recall an analysis of the calving of PIG made in the article : S.Jeong et al. 2016 (references below)

“Recent observations of continued acceleration, retreat, and thinning of Pine Island Glacier affirm its dynamic instability, suggesting that irreversible retreat has already begun. Observational analysis and ice flow models suggest that current degenerative change of Pine Island Glacier will persist for a century or more. The triggers for the ongoing changes remain poorly understood. Recent studies, however, have increasingly pointed toward ice-ocean interaction as the dominant driver. In addition to increased thinning and grounding line retreat, over the past several decades, the Pine Island Glacier has undergone increased rifting and expansion of the lateral shear zones flanking the fast-flowing ice shelf. Since the late 1990s, both the southern and northern ice shelf shear margins have become increasingly fractured and the northern margin has progressively opened, reducing the area of contact between the ice shelf and the shear margins. Because shearing along the ice shelf edges generates a stress that resists flow while ice thinning reduces the amount of stress that can be generated, fracturing and disintegration of the shear margins can cause ice flow acceleration, creating a potential positive feedback between shear margin rifting and acceleration leading to unstable disintegration. Additionally, three times since 2000, marginal rifts have propagated from the northern shear zone through the width of the ice shelf, resulting in the calving of large tabular icebergs. Similar to rifting and calving events observed on other ice shelves, these transverse rifts initiate where the shelf flank loses contact with the margin and thus may be related to the resulting loss in lateral shear stresses that resists outward flow of the shelf. The reason(s) that a particular rift propagates laterally across the entire shelf is unknown, although structural heterogeneities, such as basal crevasses or suture zones between merging ice streams, remain a possibility. While the most recent (2011) rifting and calving event initiated further inland than the two prior events, the net change in ice front position was small, with little resulting change to the ice shelf’s structure.”

“However, two anomalous rifts appeared in late 2014 and early 2015 that, in contrast to previous events, initiated in the center of the ice shelf and propagated toward the margins.”

“The style of ice shelf rifting currently underway at the Pine Island Glacier is fundamentally different from previous episodes of rifting and calving in the last decade, in which preexisting, marginal rifts propagated from the seaward end of the northern shear margin across the tongue, perpendicular to the mean flow direction. The initiation of multiple rifts in the center of a fast-flowing (faster than 1 km/yr) ice shelf is unusual. Basal crevassing near the grounding line has been observed at the Pine Island Glacier and rifts transverse to flow are observed at the grounding line of neighboring Thwaites Glacier, which is undergoing similar acceleration and thinning. The series of basal crevasses observed upstream of rifts R1 and R2 suggest that, in contrast to previous rifting events, basal crevasses have initiated at the grounding line and have widened and deepened as they advect downstream. Recent studies show that the presence of wide channels at the bottom of the ice can be sufficient to trigger full thickness ice fracture. Moreover, melting within basal crevasses can widen them, enabling penetration through an increasingly large fraction of the ice thickness as they propagate downstream. At the Pine Island Glacier, basal crevasses may have advected with the ice shelf and then propagated through the full ice thickness due to a combination of increased bottom melting, ice thinning and increased deviatoric stresses. The cause of the initial formation of the basal crevasses are uncertain but may be the result of periods of enhanced basal melt due to episodic intrusions of warm deep water and/or subglacial meltwater discharge associated with lake drainage.
Rather than being transverse to flow, these rifts are oriented oblique to the average flow direction and approximately perpendicular to the strike of marginal rifts on the northern side. Thus, their formation was not likely caused by increasing along-flow deviatoric stresses as the ice reaches the front, as is typical. Instead, we suggest that the evolution of rifts is accelerated by the same overall stress regime that is causing both northward rotation of the terminus and northern migration of the southern shear margin. An explanation for this northward migration is the retreat of the highly rifted portion of the northern shear margin. The removal of this section of the ice shelf has now completely decoupled the ice shelf from the northern coast, resulting in a nearly unconfined ice tongue. While this mélange-filled and highly rifted shear margin was likely weak, providing small resistance along flow, it may have still acted to confine the shelf on the northern flank and provide resistive stresses transverse to flow similar to the way that mélange-filled fjords in Greenland have been found to provide a backstress large enough to inhibit calving of fast flowing outlet glaciers.”

To which I add the subsequent changes:

NSM : end of the zone of ice mélange and flowing on a new pinning point (hope that it will hold for a few years)

SSM : end of pinning point created by the joint with the SWT and  zone of ice mélange, (no pinning point before 40km)

Reference :
S. Jeong, I. M. Howat, and J. N. Bassis(2016),
Accelerated ice shelf rifting and retreat at Pine Island Glacier, West Antarctica,
Geophys. Res. Lett.,43,11,720–11,725, doi:10.1002/2016GL071360.

baking

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Re: PIG has calved
« Reply #1292 on: December 12, 2019, 08:35:34 PM »
Imagine this paper is written in 2026 and just replace "northern margin" with "southern margin" and you will get my prediction for the near future.

paolo

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Re: PIG has calved
« Reply #1293 on: December 12, 2019, 10:39:15 PM »
40 km in 6 years (an article is written at least a year later) it would mean every year an iceberg of 11 km 7 (=40/6) + 4 (PIG speed which in reality is higher), it is indeed a possibility, but withdraw could be a little slower (the expectations of a line of folds temporarily stable before 40 km seems to me very difficult, not to say impossible, no miracles).

But PIG could before that find itself in a new configuration: the pinning point will not hold until the goal and we would no longer have SM neither on the left neither on the right and the behavior could become more difficult to predict and could form a tongue.

But there are other concerns, the NIS now isolated will live its life and does not worry me, in the south it is another discourse, the SIS with its two tributaries will be completely destabilized and behind there is TG (it is especially the SWT which will become worrying, it is no longer retained, already is melting (post 1284), the grunding line is withdrawing (post 1285), looks at the bathymetry and especially the -700 line...)

We have a lot of work to do to analyze all this.....

Translated with www.DeepL.com/Translator (free version)
« Last Edit: December 12, 2019, 11:10:15 PM by paolo »

paolo

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Re: PIG has calved
« Reply #1294 on: December 13, 2019, 01:23:12 PM »

new Sentinel image1 => new animations   ;)

click to move

paolo

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Re: PIG has calved
« Reply #1295 on: December 13, 2019, 01:37:27 PM »
Beyond the iceberg all this is not very solid...
Even the pinning point (see top left of the first animation[east]), many cracks (white marks) seem to me much more marked on the 13 >:(

paolo

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Re: PIG has calved
« Reply #1296 on: December 13, 2019, 01:46:23 PM »
It's not ice anymore, it's porcelain. :o

baking

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Re: PIG has calved
« Reply #1297 on: December 13, 2019, 04:19:38 PM »
Today's Sentinel-1 image (at least as processed by PolarView) has a higher contrast than the comparable one from December 7.  Therefore I've used the previous image from December 1 in the GIF below to show the detail on the northern end of the main crack.  Today's image confirms at least some portion of the crack seen on December 1, but not the part that crosses into the margin.

paolo

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Re: PIG has calved
« Reply #1298 on: December 13, 2019, 06:48:34 PM »
All the elements reported in my position (1294):
   - Opening of transverse rifts (with respect to flow) in the iceberg
   - Detaching, from the part between the two rifts, of a new piece
   - The fact that the longitudinal rifts expected in the NSM do not open and that the rift extends towards the pinning point
   - Signs of weakening in the pinning point with many cracks orthogonal to the flow (reported in post 1295)
all are expressions of the same cause: the pressure force due to the action of the Cork (to simplify what when everyone wants to pass through a door at the same time).

This pressure will stop as soon as it breaks, but the damage is there and in the months following calving I expect a piece of the pinning point to be destroyed, see attached picture.

Translated with www.DeepL.com/Translator (free version)
« Last Edit: December 13, 2019, 07:21:44 PM by paolo »

blumenkraft

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Re: PIG has calved
« Reply #1299 on: December 14, 2019, 06:11:47 AM »
Damit! Yesterdays Sentinel-2 is cloudy! :(
The apocalypse is already here; it's just not very evenly distributed.