Pine Island Glacier (PIG) Calving and Discussion

[baking]

This video of the Amundsen Sea Embayment shows that it was frozen throughout 2018.  Also, there doesn't seem to be a strong seasonal component, with half of the winters never freezing over.  Since 2015, it froze solid in the winter of 2017 through the winter of 2018 and again in 2020.

All I can say for sure is that once it freezes, it stays frozen for a while and once it breaks up it takes a while to refreeze.  I don't know that it is a cycle as much as a hysteresis effect.

[interstitial]

Thanks baking more to think about.

[paolo]

I think that we have to differentiate between the two problems: the presence of the polynias opposite the PIG and the clearing of the sea ice of the Admunsen Bay opposite the PIG.
Indeed, if the second phenomenon clearly and mainly depends on the outgoing currents (ex CDW), the first is influenced by these currents, but it depends above all on many other factors: weather conditions and currents in the Admunsen Bay.
I had already presented statistics from 2001 to 2019 based on the Modis archives, but to make them more readable I present them graphically: for each month, if there is at least one readable image, I display the status of the image with the most ice free, the status being :
> Not polynyas
> Presence of one to three polynyas
> Presence of a unique and widespread polynya from NSM to SSM
> Admunsen ice-free bay in front of the PIG
I also give examples for these statutes

In a future post I will present the analysis of this year's polynyas.


Click to enlarge 

EDIT: As I mentioned in the previous post, the first decade is the one in which the CDW and the retreat of the grounding line is strongest.

EDIT2 the white boxes correspond to the months for which I don't have any pictures.

[paolo]

I have already presented summary statistics on the complete clearing of sea ice in front of the PIG front or, alternatively, on the presence of polynyas.
We have seen that the presence of sea ice all year round in front of the PIG is not exceptional, far from it. This presence depends on meteorological factors related to Amundsen Bay: atmospheric conditions, sea surface conditions: sea currents, temperature, salinity, etc., factors which largely escape us: no information on the current situation, not to mention their history ...

But the presence of sea ice has the advantage for us to highlight the polynias which inform us about the outgoing currents: the warm waters of the CDW which, after melting the ice platform in the grounding line zone, rise to the surface by licking the bottom of the ice shelf and continuing in their melting action (clearly their temperature decreases during the return trip, their thermal energy being spent in the melting of the ice shelf.
Clearly in the analysis of the images it is also necessary to take into account elements that can hinder the formation of polynyas:
> presence of thick sea ice, or even worse, presence of a dense mélange of ice and icebergs (no melting possible, nor displacement)
> calvings refilling existing polynyas with an ice mélange (the creation of polynyas always has to be started again)
These two elements are particularly present this year:
> NSM: high frequency of calving (Note: the ice mélange is then moved by surface currents towards the centre where it will contrast the formation of polynyas also in this sector)
> SSM: large calvings that have saturated the sea in front of a thick mélange of ice and icebergs

Using the Sentinel1 images I made statistics on the presence of polynyas from 04/07/20 to 23/02/21 (see first image).

NSM side (see second image) : Following the retreat of the front we currently have two distinct areas of polynya formation :
> NIS: the formation zone is located west of the Ice Rise Evan's knoll: the zone is invaded from time to time in the west by sea ice pushed by the currents and in the east by frequent calving on the MNS side, but the polynyas are very present in time.
> NSM: the action of the outgoing current is indeed there, but it is thwarted by calving and by the presence of a thick mixture of ice and icebergs, but polynyas manage to form and even merge with the polynya on the NIS side, in some cases forming a single large polynya.

Central side (see third image): even if the formation of polynyas is hindered by the presence of an ice mélange of NSM origin (carried by the surface current), their presence remains well identified.  One may also wonder whether the longitudinal rifts which are being revealed in the satellite images in the "future iceberg" are not due to the action of outgoing currents channelled by pre-existing longitudinal crevasses in the bottom of the ice shelf.

SSM side (see fourth image): Following the retreat of the front, there are currently two distinct areas of polynya formation:
> SWT: the zone of formation is located east of the SM (shear margin) between the SWT and the SIS: the zone is invaded from time to time by calving on the SSM side, but the polynyas, even if not very extensive, are present over time.
> SSM: the action of the outgoing current seems to be there, but it is very upset by the calvings and especially by the presence of a very thick mixture of ice and icebergs.

Click to enlarge

EDIT: I have corrected the statistics (first image) by differentiating between the two cases: no polynyas and polynyas of weather origin (winds). In the latter case there may have been a contribution from outgoing currents, but this contribution is drowned in the weather action.

[kassy]

Antarctic seals reveal worrying threats to disappearing glaciers

...

The researchers found a highly variable meltwater distribution with two meltwater-rich layers - one in the upper 250 metres and another at around 450 metres deep - connected by scattered meltwater-rich columns. The hydrographic signature of meltwater is clearest in winter, when its presence can be unambiguously mapped; this analysis is only possible in winter.

https://www.eurekalert.org/pub_releases/2021-03/uoea-asr030321.php

Winter seal-based observations reveal glacial meltwater surfacing in the southeastern Amundsen Sea
https://www.nature.com/articles/s43247-021-00111-z

Some PIG specific background research (the data is from 2014).

[grixm]

Is it just me or is there a new extension to the outer crack in the south, at a very vulnerable place? Is the calving approaching?

[solartim27]

Isn't the calving at the tip of the Trib new as well?

[paolo]

I don't know if the crack indicated is spreading, but in any case the movement of the central part of the front line is impressive.
Below is a 6 days animation (from 01/03 to 07/03) aligned on SM (Shear Margin) between the SWT and the SIS, more precisely aligned on its southern part (upstream), the SM itself being deforming.

Click to animate, the image being very large: 500x1300

EDIT :
PS: I am astonished at the resistance offered by the SIS to the forces induced by the SWT movements, it will break; but it was solid!

[paolo]

solartim27,
If you are referring to the tip of the SWT, this is an old calving that has remained in place moving only a few tens of metres (it has slid very slightly northwards along the fracture).
Moreover, the next point of the SWT (upstream) also calved without moving, being held back by the remaining parts of the tributary (the rift is clearly visible in the Sentinel2 images).

[baking]

Here is a 10-day Sentinel2 GIF of the Southern Ice Shelf.

(Highlight Optimized Natural Color Script.)

[paolo]

New Sentinel2 image, I take the opportunity to make two animations related to the SIS-SWT: East side and West side.
The animation is based on the two images of 07/03 and 11/03 and they highlight the widening of the main rifts in only 4 days!
It is also interesting to notice that the dense mixture of ice and small iceberg is crossed by a "fault" separating clearly a part attached to the SIS and a part attached to the PIG, the latter sliding smoothly and apparently without strain on the former!

Click to animate

Edit: the movement of the SIS front, in solidarity with the SWT, is also very clear

[grixm]

New s-2 images.

[Stephan]

Thanks for that animation, grixm.
It makes me wonder who will be the first for a major calving. SWT or SIS? I would vote for the latter, but maybe if SIS calves this will not let SWT unaffected.
Second question: Will we be able to watch it before the sun goes down?

[baking]

Second question: Will we be able to watch it before the sun goes down?

I think you have about 4 days.

[interstitial]

Polarview is year round it doesn't have quite the resolution but it can see through clouds so it is more consistent.

[grixm]

New crack in the south. Looks increasingly precarious.

[paolo]

Grixm, Eye of linx
Yes, it is an explosive extension of a very small rift. This extension is not visible, not even partially, neither in the low resolution S1 image of yesterday, nor in the high resolution S1 image of the day before yesterday, nor in the S2 image of 17/03!

Now we have the high resolution S1 image and I have built an animation based on the collage of two high resolution S1 images from today and the high resolution image from the day before yesterday.

It's a bit of a tinkering: collage of two images + two different orbits, but it shows the sudden extension of the rift and the important movement of the central part of the front (the movement is over two days!)

I think that indeed calving can start at any time.

Click to animate and enlarge 

[interstitial]

While unexpectedly little calving occured during this summer glacier flow rates are at or near all time highs.

[grixm]

Another possible rift extension in the last couple of weeks. In the north this time, from a rift that has been dormant for a while.

The gif is from two different angles because I couldn't find the same shot on polarview, just from a different orbit.. But the difference is so clear that I don't think it's just a shadow. It doesn't show up on the last S-2 shot either.

[paolo]

Grixm,
Sorry, but using the high resolution Sentinel1 images of 19/03 and 25/03 (same orbit) I don't see significant differences
I take the opportunity to show also an animation of the SIS/SWT still with Sentinel1 images of 19/03 and 25/03

Click to animate

Correction: the alignment point, drawn on the image is not correct, in reality the images are aligned on the other end (all to the east) of the same rift.
That's good, we can see the SWT compression on the SIS!

PS : I have the impression that in the next few days it is better not to be in the vicinity of the SIS, the calving should be of the explosive type, navigation forbidden 

[paolo]

I think that an animation relating to the NDZd (Northern Domages Zone downstream) could also be interesting, indeed calvings in this sector should not be long in coming (still with the Sentinel1 images of 19/03 and 25/03)

[grixm]

Grixm,
Sorry, but using the high resolution Sentinel1 images of 19/03 and 25/03 (same orbit) I don't see significant differences

Really? I definitely still see it in that comparison. It's faint enough to maybe just be a compression artifact but the line definitely has a longer section.

By the way, are there no more sentinel 2 images this season?

[paolo]

Grixm,
We have never had any Sentinel2 images beyond 20 March, even bad ones!
We'll have to wait until September 

But of course we still have Sentinel1 and never clouds

[paolo]

New high resolution image, I take the opportunity to present :
> A new animation (6 day interval) of the SIS
> An animation of the future iceberg: the rifts are not moving, but I can detect a small relative movement of the southern part relatively to the northern part (movement that could be linked to the longitudinal rifts that have recently appeared)

Click to animate

[Stephan]

There is a sharp line that distinguishes between the ESE-WNW current of the sea (the same direction the Main Ice Shelf of PIIS has) and the SSW-NNE flow direction of the SIS. This line is close north of the actual calving front and clearly visible in the animations (thank you paolo as always).
My question is: Will this flow direction distinguishing line stay where it is after the "soon to come" calving or will the sea current direction take over to the new calving front? And if so, what damage will the newly formed icebergs have when they will be pushed against the eastern calving front of the SWT (left low corner in the latest animation)?

[paolo]

Stephan,
What you see is not the sea current, but the mixture of sea ice, ice shelf detritus and icebergs, a rigid mélange with a fault line between the MIS (Main Ice Shelf of PIIS) and the SIS, a fault line starting upstream of the dSDZ (downstream South Domage Zone).  This mélange is completely rigid and is not carried along by the current which flows clockwise in the bay (eastwards on the NIS side, southwards on the MIS side and westwards on the SWT side), and in fact the sea ice in the bay does not move as well.
This mixing should be heavily impacted at calving time, after which the fault line should be reconstituted with the current movements, waiting for the sea current induced loop to recover. The collision of this mixture with the SWT should not impact the latter except for the two icebergs in its NE corner, icebergs already detached, but still in place.
What can impact the SWT is rather the calving itself: directly at the time of calving and in the following days following the modifications of the induced tensions.

[gerontocrat]

A recent research paper for the technical guys

https://tc.copernicus.org/articles/15/1501/2021/tc-15-1501-2021.pdf

The tipping points and early warning indicators for Pine Island Glacier, West Antarctica

Abstract
Mass loss from the Antarctic Ice Sheet is the main source of uncertainty in projections of future sea-level rise, with important implications for coastal regions worldwide. Central to ongoing and future changes is the marine ice sheet instability: once a critical threshold, or tipping point, is crossed, ice internal dynamics can drive a self-sustaining retreat committing a glacier to irreversible, rapid and substantial ice loss. This process might have already been triggered in the Amundsen Sea region, where Pine Island and Thwaites glaciers dominate the current mass loss from Antarctica, but modelling and observational techniques have not been able to establish this rigorously, leading to divergent views on the future mass loss of the West Antarctic Ice Sheet. Here, we aim at closing this knowledge gap by conducting a systematic investigation of the stability regime of Pine Island Glacier. To this end we show that early warning indicators in model simulations robustly detect the onset of the marine ice sheet instability. We are thereby able to identify three distinct tipping points in response to increases in ocean-induced melt. The third and final event, triggered by an ocean warming of approximately 1.2 ∘C from the steady-state model configuration, leads to a retreat of the entire glacier that could initiate a collapse of the West Antarctic Ice Sheet.

How to cite.
Rosier, S. H. R., Reese, R., Donges, J. F., De Rydt, J., Gudmundsson, G. H., and Winkelmann, R.: The tipping points and early warning indicators for Pine Island Glacier, West Antarctica, The Cryosphere, 15, 1501–1516, https://doi.org/10.5194/tc-15-1501-2021, 2021.

[baking]

This mixing should be heavily impacted at calving time, after which the fault line should be reconstituted with the current movements, waiting for the sea current induced loop to recover.

Well, until the sea ice clears again, movements in the debris field will follow the movements of the ice fronts.  The SIS ice front is currently bowing outward so that effect is exaggerated.  Once the SIS calves the normal movement of the front should be barely perceptible compared with the main front of the PIG pushing the ice along.

Don't forget that wind often plays a big factor too.

[RoxTheGeologist]

A recent research paper for the technical guys

https://tc.copernicus.org/articles/15/1501/2021/tc-15-1501-2021.pdf

... To this end we show that early warning indicators in model simulations robustly detect the onset of the marine ice sheet instability

Honestly, from the brilliant animations from everyone on this thread, I think we have all seen this happen blow-by-blow, the glacier accelerating and thinning, the switch from a ductile to brittle rheology on the margins causing the margins to lose cohesion; the subsequent start of the break up of the NIS and SIS. The latter is worrying: It seems to be happening very rapidly. We might find out if Pollard was right back in 2015 sooner than we all hope.

[grixm]

More cracking and bulging.

[interstitial]

Glacier speed is at highest / still increasing at approximately 14 m/day. The exact number is hard to tell from the graph.

[paolo]

Calving finally took place between 4:35 and 8:26

PS: tomorrow we will see the effects ...

Click to animate

[baking]

Calving finally took place between 4:35 and 8:26

That is some dedication, Paolo!

[Stephan]

Thank you paolo.
But it is a little bit unfair. Many of us were waiting for this event to come - and when it finally happened the show is interrupted. Like a cliffhanger...

[paolo]

A first animation of yesterday's calving using the low-resolution Sentinel1 images of 06/04 and 08/04

[paolo]

I would like to add some information about calving

[paolo]

Fix: The new high-resolution Sentinel1 image has arrived and by zooming you can notice that the piece, in green in the image of this post, has already calved, contrary to what I indicated in my previous post

ADD:
I'm adding a closer zoom of the fracture (and without colour)

[Espen]

Just adding some:
Please click on image to enlarge and animate!

[paolo]

Espen pretty images, but always reversed 
I took the liberty of putting them in the right verse 

Click to animate

[paolo]

Espen, I take advantage of your very high definition image to comment on fractures, or extensions of fractures not visible in the Polarview high definition images

Click to enlarge (very large image)

[Stephan]

Thank you paolo and Espen for this update.
There are still some (minor) calvings to be expected soon, so it is still a sort of cliffhanger.

I noticed that the iceberg from Point B, deriving from PIS-MIIS moved a lot into NW direction compared to its "almost no" movement in the last weeks. I guess this calving was sort of explosive nature giving the sea ice and the iceberg a real push.

[paolo]

Stephan,
Yes, as expected it was explosive, between 4:35 and 8:26, the tip of the iceberg that broke off moved a little more than a km and this movement must actually correspond in large part to the moment of rupture

[grixm]

New images, with some highlights of extensions that wasn't pointed out yet

[paolo]

I am taking advantage of today's high resolution image to take stock of the SIS.
I have coloured the following areas:
* Mixture of Icebergs and Ice
* Icebergs already detached without displacement
* Likely calving
* Next calving
* To be watched in the coming months

For this analysis I also used the latest available Sentinel2 image and the very high resolution Sentinel1 image posted by Espen.

Click to enlarge the image

[oren]

PIG 3D timelapse from Google Earth, showing the MIS, SWT and SIS as well as surroundings.
Time period is 1985-2020.

Double-click to maximize.