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Author Topic: Larsen C Ice Shelf  (Read 185917 times)

Shared Humanity

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Re: Rift in Larsen C
« Reply #350 on: November 08, 2018, 08:08:44 PM »
What are all of the gray spots on the ice berg? Those can't be melt ponds.

Tor Bejnar

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Re: Rift in Larsen C
« Reply #351 on: November 08, 2018, 09:29:59 PM »
Definitely not melt ponds!  It is very cold (from a human perspective) there, I'm sure.  PolarView doesn't create visual-light-like images, but uses other parts of the electro-magnetic spectrums. (These details are not my forte.)  I like these because they "see" through clouds and night's (and winter's) darkness.
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Tor Bejnar

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Re: Rift in Larsen C
« Reply #352 on: November 21, 2018, 05:49:47 PM »
The Ice Island A68-A has rotated about 115º in 6 months, but the pivot end hasn't moved diddlysquat. 

The largest iceberg in the area (current, lower, image) has, in the meantime, moved about 125 km northwards, squeezing through what I'll call an ice-strait.  (1st image from a May 18 post by johnm33 [conveniently at the top of this thread's page 7]; 2nd image from PolarView on November 20.) 

I'm going to postulate, now, that intermittent grounding keeps A68-A where it is; no point is clearly 'actually' stuck in one place for any length of time.  A GIF covering multiple images might show if any spot does get stuck (becoming a fixed (if temporary) pivot point).
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FredBear

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Re: Rift in Larsen C
« Reply #353 on: November 30, 2018, 09:33:37 PM »
The Ice Island A68-A continues to inch(!?) northwards over the last 10 days, with the narrower 'tail' getting further from the coastal ice shelf.

Perhaps more importantly for the region, there is now a blue tint to the remains of the Larsen A & B shelves and also the ice further off-shore (From arctic.io 2018-11-30). Caused by adiabatic winds from the peninsula melting snow from the surface (+insolation)?

Tor Bejnar

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Re: Rift in Larsen C
« Reply #354 on: December 17, 2018, 06:11:58 PM »
A-68A has move about 20 km northward this past month.  I'm guessing it will escape the shoals near Bawden Ice Rise soon (if it hasn't already).
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litesong

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Re: Rift in Larsen C
« Reply #355 on: December 17, 2018, 06:42:31 PM »
A-68A has move about 20 km northward this past month.  I'm guessing it will escape the shoals near Bawden Ice Rise soon (if it hasn't already).
........ as the yearly Antarctic sea ice decrease loses its ability to hold onto A-68A.

FredBear

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Re: Rift in Larsen C
« Reply #356 on: December 18, 2018, 01:49:40 PM »
So far the sea ice has been building to the south and east of A68A (which seems to be blocking the gyre) , but dispersing to the NW.
On 13/12/2018 A68B had drifted in line with the point of the peninsular between Larsens A & B, just SE of a large sea ice floe. Also appears about level with the north of A68A.

The iceberg B15AA has been drifting across the Weddell Sea, but it is moving more slowly than the surrounding sea ice (which is now breaking up quite rapidly.). This demonstrates that icebergs can impede the movement of the  thinner sea ice.

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Re: Rift in Larsen C
« Reply #357 on: December 18, 2018, 04:35:14 PM »
I wonder if it will break into at least 3 chunks soon, especially if the bottom end stays grounded....

johnm33

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Re: Rift in Larsen C
« Reply #358 on: December 18, 2018, 06:29:06 PM »
Wouldn't be surprised if it 1/2 rotated again before breaking.

Tor Bejnar

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Re: Rift in Larsen C
« Reply #359 on: December 18, 2018, 10:56:05 PM »
From an enlargement of the left side of A-68A (PolarView) dated 2018-12-17@23:41:58 (just over 18 minutes before midnight), I don't see current support for the cracks propagating in the suggested manner. Am I missing something?  But for sure, we'll see one way or another.  I do see the possibility of a ~2 x ~10 km scab braking off in this area. (sea ice on the left; A-68A on the right and top)
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Tor Bejnar

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Re: Rift in Larsen C
« Reply #360 on: December 27, 2018, 04:41:28 PM »
The southern end of Bawden Ice Rise has a rim of open water now, per this Sentinel-hub image from December 19. The first image (of mp4) is the NDWI rendering of bands (to identify the setting), and the second image is the 'Natural Color' rendering (exact same area).
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Tealight

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Re: Rift in Larsen C
« Reply #361 on: January 21, 2019, 09:51:18 PM »
A nice view of A-68A today showing the rifts in a lot more detail than usual. A small part even broke off since the last picture posted here in December.

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Re: Rift in Larsen C
« Reply #362 on: February 04, 2019, 02:59:41 PM »
Apparently new cracks appeared recently on the shelf, a big chunk seems ready to get loose.

Shared Humanity

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Re: Rift in Larsen C
« Reply #363 on: February 04, 2019, 05:43:26 PM »
Those fractures are getting dangerously close to the Bawden Ice Rise. Would hate to see the ice shelf lose this pinning point.

Tor Bejnar

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Re: Rift in Larsen C
« Reply #364 on: February 21, 2019, 02:16:23 AM »
IceConcerned,
You will notice the cracks are little changed from one and two years ago, see here, for example.  Yes, they look ready to let loose!
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Tor Bejnar

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Re: Rift in Larsen C
« Reply #365 on: February 25, 2019, 05:06:49 PM »
I'll probably start posting A68A images in the Antarctic Icebergs thread (after this one).  I estimate (with the assistance of a small protractor) the ice island has rotated 175º since forming.  That's quite a bite where it is (was?) grounded.  PolarView image from yesterday.
« Last Edit: February 25, 2019, 05:14:12 PM by Tor Bejnar »
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FredBear

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Re: Rift in Larsen C
« Reply #366 on: February 25, 2019, 11:26:25 PM »
If A68A drifts into the bay it could replace the old Larsen B ice shelf!?? - no, the ice in the shelf area still seems to flush out.
This berg hasn't moved much this summer (but more than last year) - maybe it will move more in the winter.
The sea ice/icebergs in the Weddell Sea have been concentrated on the western & southern sides (apart from the lee of A68A) this year, but their location is very variable from year to year. Rather less total ice than recent years?

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Re: Rift in Larsen C
« Reply #367 on: July 14, 2019, 07:14:41 AM »
BBC has a nice animation of the past 6 months, it has moved north quite a lot in the interim:

https://www.bbc.com/news/science-environment-48920168

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Re: Rift in Larsen C
« Reply #368 on: July 21, 2019, 05:37:04 PM »
Turbulence Observations beneath Larsen C Ice Shelf, Antarctica

Peter E.D. Davis  Keith W. Nicholls
First published: 16 July 2019 https://doi.org/10.1029/2019JC015164 .. paywall

Abstract

Increased ocean‐driven basal melting beneath Antarctic ice shelves causes grounded ice to flow into the ocean at an accelerated rate, with consequences for global sea level. The turbulent transfer of heat through the ice shelf‐ocean boundary layer is critical in setting the basal melt rate, yet the processes controlling this transfer are poorly understood and inadequately represented in global climate models. This creates large uncertainties in predictions of future sea‐level rise. Using a hot‐water drilled access hole, two turbulence instrument clusters (TICs) were deployed 2.5 and 13.5 meters beneath Larsen C Ice Shelf in December 2011. Both instruments returned a year‐long record of turbulent velocity fluctuations, providing a unique opportunity to explore the turbulent processes within the ice shelf‐ocean boundary layer. Although the scaling between the turbulent kinetic energy (TKE) dissipation rate and mean flow speed varies with distance from the ice shelf base, at both TICs the TKE dissipation rate is balanced entirely by the rate of shear production. The freshwater released by basal melting plays no role in the TKE balance. When the upper TIC is within the log‐layer, we derive an under‐ice drag coefficient of 0.0022 and a roughness length of 0.44 mm, indicating that the ice base is smooth. Finally, we demonstrate that although the canonical three‐equation melt rate parameterization can accurately predict the melt rate for this example of smooth ice underlain by a cold, tidally‐forced boundary layer, the law of the wall assumption employed by the parameterization does not hold at low flow speeds.

bligh

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Re: Rift in Larsen C
« Reply #369 on: July 21, 2019, 08:31:11 PM »
Cool article!!

Tor Bejnar

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Re: Rift in Larsen C
« Reply #370 on: October 10, 2019, 08:05:31 PM »
A look at part of Larsen C today... GIF from Sentinel-hub Playground from 2019-10-10 and 2018-12-10 (The October 2018 image just wasn't as clear as the December one.)  Area (annotated) screen print from 2019-10-10.

This part of the Larsen C shelf has moved ~0.6 km in one year (October to October).  The most major cracks appear unchanged, but minor cracks appear to be more developed.  I think it's cute that the shore-bound iceberg broke in half at some point this past year.
« Last Edit: October 10, 2019, 08:15:56 PM by Tor Bejnar »
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Tor Bejnar

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Re: Rift in Larsen C
« Reply #371 on: March 05, 2020, 09:04:43 PM »
Comparing IceConcerned's January 2019 Larsen Shelf images with a PolarView from yesterday, precious little has changed.  One 'iceberg' that has been stuck to the shelf lost half its area (orange arrow).  The crack next to the red line matches what can  be seen in the 2019 post's enlargement (above).  I tried to 'pin' the Bawden Rise 'finger tips' and a point where A68 came from, but I'll not claim exact size or placement.
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kassy

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Re: Rift in Larsen C
« Reply #372 on: May 29, 2020, 07:03:35 PM »
Ancient Antarctic Ice Sheet Loss Dwarfs Modern Melting, Study Finds

More than a year ago, researcher Julian Dowdeswell boarded a research vessel at the edge of the Fimbul ice shelf to the east of the Antarctic Peninsula. He and six other scientists with the University of Cambridge were setting off as part of an expedition to study the ancient patterns of ice sheet retreat along the peninsula, what is one of today’s most vulnerable ice shelves.

The team analyzed the data gathered on that trip and has published a study in Science on Thursday. They have found that ice sheet retreat rates 10,000 years ago make today’s rate of retreat look like baby steps. This period saw ice shelves retreat more than 10 kilometers (6 miles) each year along the Larsen C shelf. That’s three to five times greater than the rates we’ve seen via satellite data over the last 25 years. These findings can improve how scientists’ model the future of ice and what it means for sea level rise.

...

What’s going on under ice shelves—floating extensions of inland ice sheets that feed them—is of utmost importance. When warm water cuts under them, it thins the ice shelf by melting it from below. As the ice thins, it can lift off the seafloor and begin to bob with the tide. That up and down motion can form ridges on the seafloor close to where ice meets the seafloor, an area known as the grounding line. The team identified up to 90 ridges to paint an incredible story of ice melt over the past 10,000 years.

“The grounding zone of ice sheets and the processes that occur there are ‘holy grails’ for glaciologists and glacial geologists because they are so hard to access and image,” Graham said. “The methodology is robust.”

The space between the ridges helps scientists determine how old they are. Using the ridges as a proxy for ice shelf retreat, the study estimates that ice could have pulled back up to 40 to 50 meters (131 to 164 feet) per day. That has profound implications for what the future could hold. Warm water is currently wreaking havoc both along the Antarctic Peninsula and West Antarctic. The risk of rapid ice shelf collapse could raise sea levels 10 feet or more, and researchers are trying to understand just how fast the retreat could be.

https://earther.gizmodo.com/ancient-antarctic-ice-sheet-loss-dwarfs-modern-melting-1843732617

Antarctic Ice Sheets Capable of Retreating Up to 50 Meters per Day

In the ASLR thread has another news article plus the links to the research.
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Re: Rift in Larsen C
« Reply #373 on: September 09, 2020, 09:11:22 AM »
When did these icebergs -larsen d form?

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Re: Rift in Larsen C
« Reply #374 on: September 09, 2020, 09:53:49 PM »
The main break was between june 24th and 29th

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Re: Rift in Larsen C
« Reply #375 on: September 09, 2020, 09:59:04 PM »
A researcher made a media announcement in anticipation in March but it was mostly ignored.

Tor Bejnar

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Re: Rift in Larsen C
« Reply #376 on: September 09, 2020, 11:06:07 PM »
Here is a 2017 image that includes of the area south of Gipps Ice Rise from before Ice Island A-68 broke off just north of it.  The icebergs Grygory showed us are from the south-of-Gipps Ice Rise area.  (Image was posted by Shared Humanity in this thread on March 19, 2018.)



Just a reminder: 
Quote
An ice rise is a clearly defined elevation of the otherwise very much flatter ice shelf, typically dome-shaped and rising several hundreds of metres above the surrounding ice shelf [1] . An ice rise forms where the ice shelf touches the seabed due to a locally increased elevation of the seabed, which however remains below sea level. ...
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FredBear

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Re: Rift in Larsen C
« Reply #377 on: May 20, 2021, 12:05:24 PM »
The BBC has published an article saying that Larsen C has had a couple of retreats in the last 10,000 years but has not collapsed like Larsens A & B:-

https://www.bbc.co.uk/news/science-environment-57169003

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Re: Larsen C Ice Shelf
« Reply #378 on: March 25, 2022, 03:45:44 PM »
The aricle states that 80 percent of the buttressing provided by the Larsen C ice shelf is from the first 5 kms downstream of the grounding line.

The glaciers / shelves that have a similar geography presumably have the same sort of concentration of buttressing. Very significant?

https://tc.copernicus.org/articles/16/883/2022/
The instantaneous impact of calving and thinning on the Larsen C Ice Shelf
Quote
Abstract
The Antarctic Peninsula has seen rapid and widespread changes in the extent of its ice shelves in recent decades, including the collapse of the Larsen A and B ice shelves in 1995 and 2002, respectively. In 2017 the Larsen C Ice Shelf (LCIS) lost around 10 % of its area by calving one of the largest icebergs ever recorded (A68). This has raised questions about the structural integrity of the shelf and the impact of any changes in its extent on the flow of its tributary glaciers. In this work, we used an ice flow model to study the instantaneous impact of changes in the thickness and extent of the LCIS on ice dynamics and in particular on changes in the grounding line flux (GLF). We initialised the model to a pre-A68 calving state and first replicated the calving of the A68 iceberg. We found that there was a limited instantaneous impact on upstream flow – with speeds increasing by less than 10 % across almost all of the shelf – and a 0.28 % increase in GLF. This result is supported by observations of ice velocity made before and after the calving event. We then perturbed the ice-shelf geometry through a series of instantaneous, idealised calving and thinning experiments of increasing magnitude. We found that significant changes to the geometry of the ice shelf, through both calving and thinning, resulted in limited instantaneous changes in GLF. For example, to produce a doubling of GLF from calving, the new calving front needed to be moved to 5 km from the grounding line, removing almost the entire ice shelf. For thinning, over 200 m of the ice-shelf thickness had to be removed across the whole shelf to produce a doubling of GLF.

Calculating the instantaneous increase in GLF (607 %) after removing the entire ice shelf allowed us to quantify the total amount of buttressing provided by the LCIS. From this, we identified that the region of the ice shelf in the first 5 km downstream of the grounding line provided over 80 % of the buttressing capacity of the shelf. This is due to the large resistive stresses generated in the narrow, local embayments downstream of the largest tributary glaciers.
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Re: Larsen C Ice Shelf
« Reply #379 on: March 25, 2022, 04:58:00 PM »
I think we've seen this in other ice shelves, it matters little until it suddenly matters. When two merging flows are no longer merging due to calving front recession, the shelf can speed up and/or further collapse much more easily.  This happened in many locations, the most obvious example that springs to mind is the confluence of the PIG and the SWT.
In Larsen C it appears this point is further ahead in the future, as the major confluence is near the grounding line and far from the calving front.

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Re: Larsen C Ice Shelf
« Reply #380 on: March 25, 2022, 08:54:17 PM »
For iceberg hunters who haven't noticed it yet, one of these days some nice ice cubes will break off the Larsen C (just south of Bawden Ice Rise).
I publish the image of 28/02 because the presence of water in the surface makes the fractures particularly visible.
Since then they don't seem to have moved, but one of these days ...

Click twice to enlarge the image completely

vox_mundi

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Re: Larsen C Ice Shelf
« Reply #381 on: July 13, 2022, 05:25:35 PM »
Tropical Storms Trigger Antarctic Ice Melt
https://phys.org/news/2022-07-tropical-storms-trigger-antarctic-ice.html

New factors have been identified which contribute to record-high temperatures and ice melt over the eastern Antarctic Peninsula and Larsen C Ice Shelf.


Fig. 1: Study area and interannual climate variability governing Larsen C surface melt.

The study, published today (13 July) in the journal Nature Communications, describes how distinct patterns of air circulation over the tropical Pacific Ocean can lead to the formation of intense atmospheric rivers. These long stretches of warm, moist air result in extreme high temperature events and ice melt when they move over the Antarctic Peninsula.

A team, including British Antarctic Survey researchers, used advanced modeling techniques to determine that these anomalous air circulation patterns are caused by thunderstorms and weather patterns resulting from hot air rising in the atmosphere in the central tropical Pacific east of Fiji. Variability in activity over this region was found to explain 40% of the year-to-year variability in melt over the summer period on the Larsen C ice self. It was also likely to be the cause of the two recent record-high Antarctic temperature events in March 2015 and February 2020, both of which led to record-high surface melting on the Larsen ice shelf.

These results suggest the future stability of the Larsen C ice shelf, and the associated contribution of the Antarctic Peninsula to global sea level rise, is closely tied to future variability in central tropical Pacific weather patterns.

... "In our study, we show for the first time that a pattern, which results from the convection near Fiji, leads to a large and deep area of low pressure off the coast of West Antarctica and a strong high-pressure system north of the peninsula over Drake Passage. Together, these features transport very warm and moist air from middle and sub-tropical latitudes of the eastern South Pacific to the Antarctic Peninsula in the form of intense atmospheric rivers. This is another mechanism which may help us predict what might happen with the Larsen C ice shelf."


Fig. 3: The CPAC-forced circulation anomalies driving extreme high temperatures on the eastern Antarctic Peninsula.


Fig. 6: CPAC convection a key driver of extreme atmospheric rivers on the Antarctic Peninsula.

Kyle R. Clem et al, Central tropical Pacific convection drives extreme high temperatures and surface melt on the Larsen C Ice Shelf, Antarctic Peninsula, Nature Communications (2022)
https://www.nature.com/articles/s41467-022-31119-4
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