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Author Topic: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland  (Read 751048 times)

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2000 on: January 11, 2019, 03:02:19 PM »
Glacier-ice is malleable at depths of 30m and deeper. Therefore the ice-stream channel must be full of ice until the grounding-line (which for Jakobshavn is estimated to reside within ~1km of the calving front depending on the season) even if the depth of the channel varies. That being said, meltwater channels do form under the ice-stream.

Quote
Surface meltwater-induced velocity variation is a quasi-diurnal signal. Podrasky et al. (2012) detected an amplitude of up to 0.1 m d−1 diurnal signal 20–50 km upstream from the terminus of Jakobshavn Isbræ

https://search.proquest.com/openview/2b991b9bb1ea4dbf97777598550b275e/1?pq-origsite=gscholar&cbl=105732

Espen

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2001 on: January 12, 2019, 02:04:41 AM »
Glacier-ice is malleable at depths of 30m and deeper. Therefore the ice-stream channel must be full of ice until the grounding-line (which for Jakobshavn is estimated to reside within ~1km of the calving front depending on the season) even if the depth of the channel varies. That being said, meltwater channels do form under the ice-stream.

Quote
Surface meltwater-induced velocity variation is a quasi-diurnal signal. Podrasky et al. (2012) detected an amplitude of up to 0.1 m d−1 diurnal signal 20–50 km upstream from the terminus of Jakobshavn Isbræ

https://search.proquest.com/openview/2b991b9bb1ea4dbf97777598550b275e/1?pq-origsite=gscholar&cbl=105732

From what I read it doesnt explain why we have tabular icebergs showing up again?
Have a ice day!

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2002 on: January 13, 2019, 12:28:13 AM »
The grounding line in that paper is what I think of as the strand line, where the wall of ice moving down the fjord actually touches down on bedrock, beyond there upstream I'm thinking it's supported by bouyant dense ice, a melange if you will that fills the chasm. The tabular bergs slide/grind on that melange and are floated west by tidal movement.

johnm33

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johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2004 on: January 15, 2019, 10:53:50 PM »
Some calving taking place, looking at 12:00 the north stream has pushed into the bay, and to the right there's some outflow from area north of what will become the island, the main glacier releases some more tabular bergs some of which have turned.

from Polarveiw
 Grounding line/Strand line, if the glacier is grounded then some real retreat is possible, if it's current calving front is a strand line then the fluidity of the base upstream will inhibit retreat and advance will only be possible if the bergs at the face are small enough to float pass the shallow strand line. I don't think that's likely soon but when it happens it could be quite spectacular, probably needs a clear fjord for the best show. I guess.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2005 on: January 28, 2019, 10:58:47 AM »
Some recent calving, detail from polarview
There's also a feature midstream at the bottom of the image which is what I imagine a giant berg would look like moving in the stream.


Espen

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2006 on: January 28, 2019, 08:02:22 PM »
The tabs are now gone, back to normal calving activity:

Have a ice day!

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2007 on: January 29, 2019, 02:01:24 PM »
There's also a feature midstream at the bottom of the image which is what I imagine a giant berg would look like moving in the stream.
The channel is full of ice upstream from from the grounding-line, so certainly in that position. Thick ice acts like a viscous fluid (like honey).

iwantatr8

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2008 on: February 08, 2019, 01:40:53 PM »
There's an interesting article from cryosphere looking at using machine learning to track calving faces through SAR products - which has the potential to support the overall tracking of major calvings in greenland and antarctica.

https://www.the-cryosphere-discuss.net/tc-2019-14/tc-2019-14.pdf

Stephan

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2009 on: February 08, 2019, 07:51:48 PM »
Thanks for sharing.
And a 'like' earned

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2010 on: February 14, 2019, 04:21:20 PM »
No calving to speak of but about 1km advance polarview



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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2011 on: February 14, 2019, 07:40:42 PM »
If you look at that Polarview image you can see a lot of frozen summer melt lakes on the ice sheet further inland. They haven't drowned through the moulins.

Espen

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2012 on: February 26, 2019, 10:08:27 PM »
Another big calving:
Have a ice day!


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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2014 on: March 25, 2019, 05:00:21 PM »
Greenland's Jakobshavn Glacier Reacts to Changing Ocean Temperatures


vox_mundi

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2015 on: March 25, 2019, 09:43:44 PM »
Related

Cold Water Currently Slowing Fastest Greenland Glacier
https://www.jpl.nasa.gov/news/news.php?feature=7356

A major Greenland glacier that was one of the fastest shrinking ice and snow masses on Earth is growing again, a new NASA study finds.

The Jakobshavn glacier around 2012 was retreating about 1.8 miles (3 kilometers) and thinning nearly 130 feet (almost 40 meters) annually. But it started growing again at about the same rate in the past two years, according to a study in Monday's Nature Geoscience . Study authors and outside scientists think this is temporary.

A natural cyclical cooling of North Atlantic waters likely caused the glacier to reverse course, said study lead author Ala Khazendar, a NASA glaciologist on the Oceans Melting Greenland (OMG) project. Khazendar and colleagues say this coincides with a flip of the North Atlantic Oscillation—a natural and temporary cooling and warming of parts of the ocean that is like a distant cousin to El Nino in the Pacific.

The water in Disko Bay, where Jakobshavn hits the ocean, is about 3.6 degrees cooler (2 degrees Celsius) than a few years ago, study authors said.

While this is "good news" on a temporary basis, this is bad news on the long term because it tells scientists that ocean temperature is a bigger player in glacier retreats and advances than previously thought, said NASA climate scientist Josh Willis, a study co-author. Over the decades the water has been and will be warming from man-made climate change, he said, noting that about 90 percent of the heat trapped by greenhouse gases goes into the oceans.

"In the long run we'll probably have to raise our predictions of sea level rise again," Willis said.

Interruption of two decades of Jakobshavn Isbrae acceleration and thinning as regional ocean cools, Nature Geoscience (2019)
“There are three classes of people: those who see. Those who see when they are shown. Those who do not see.” ― Leonardo da Vinci

Insensible before the wave so soon released by callous fate. Affected most, they understand the least, and understanding, when it comes, invariably arrives too late

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2016 on: March 25, 2019, 11:06:22 PM »
"Tracing the origin of the cold waters in front of Jakobshavn was a challenge," explained Ian Fenty of JPL, a co-author of the study. "There are enough observations to see the cooling but not really enough to figure out where it came from."
Not that I'd bet on my guess against NASA's but I have been expecting saline waters to emerge from beneath the ice for quite some time, I have long suspected that sea water has penetrated as far as the round feature upstream, more or less central in this image from Polar View. Also that it's very likely that some salt deposits will sit in the deepest troughs/valleys just as it does beneath ice yedoma elsewhere, if there's any merit in my idea then the salt will continue to dissolve and the resultant slush will continue to lubricate the basal ice's passage. We'll see.
added- Link to supplemental pdf with salinity/temp charts etc.
« Last Edit: March 25, 2019, 11:19:16 PM by johnm33 »

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2017 on: March 29, 2019, 09:56:53 AM »
I have long suspected that sea water has penetrated as far as the round feature upstream,
...not sure if there is any evidence of that:

https://kuscholarworks.ku.edu/bitstream/handle/1808/21964/Braaten_2014.pdf

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2018 on: March 29, 2019, 11:28:16 AM »
Thanks for the link, I've speed read it and will try to digest it later.
I'm not sure either, but once the calving front began to retreat into the southern branch we began to see huge bergs with keels being calved which, for me, the simplest source would be deep ice freed from the chasms, and the simplest mechanism would be sea-water penetration beyond the cill in this vicinity. Once that started it's density would would keep the saline water in the deeps filling any voids, and moving upstream into the deeper cuts. These are so far below sea level it's hard to think of a mechanism which would clear them of salt-water so there may be deposits of salt in them, just as below ice yedoma elsewhere. There have been enough keel shaped bergs to account for most of the length of these deep cuts, so my guess is that the fjord is now lubricated by saline waters and there's so much ice falling into the troughs it almost fills them causing a build-up of ice at the 'southern front' cill, liberated now in pulses by tidal action.
This process probably began arond 2004, but once the first basin was cleared of ice the retreat moved swiftly to the next cill around 2012 from there, there are no barriers to the saline water penetrating deep in to the interior [page18 of the pdf]. Also it took a while but the ice on the NE side of the fjord began to speed up it's fall which continues apace, when that slows the flow lines visible on polarwiew images will shift direction to a more southern direction.
 IF my guess is good then the flow of ice will continue to 'flood' the fjord preventing sustained retreat from the first cill of the southern front and on the other hand the ice will be so smashed up when it breaks out over that cill that there will be no sustained advance either apart from winter 'logjams'.

I entertain the possibility that the cold waters in Disko are the basal waters flowing out beneath the glacier.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2019 on: April 05, 2019, 12:07:10 AM »
The image below is no.9 from the paper above. My interpretation of it is that the 'zebra' layers are the mobile ice, below that is ice/boulder yedoma, and the interfaces below that already show signs of water penetration. At that depth I believe that even when these images were generated were indications of salt/sea-water penetration. Given that the glacier sheds about 17km of ice every year and that we've had 14 years of tidal penetration since the grounding line moved east of the cill below the current calving front, the eastern penetration of saline water [to the round feature] doesn't seem unreasonable. That all of the yedoma ice has been lifted into the outflow I'm much less confident about, but still consider it possible.
 So I'm guessing the build up of ice at the calving front is just a logjam of bergs waiting to be melted/lifted above the cill hence the pulsed release of bergs associated with tidal extremes.


nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2020 on: April 05, 2019, 11:38:28 AM »
So I'm guessing the build up of ice at the calving front is just a logjam of bergs waiting to be melted/lifted above the cill hence the pulsed release of bergs associated with tidal extremes.
AFAIK the grounding line of Jakobshavn is at the calving front, or very close to it. That would mean that the fjord waters are not penetrating under it. If they did, the tidal signal would be detectable far upstream from the calving front, and I don't think that is the case.

The following paper states:
Quote
We hypothesize that Jakobshavn Isbræ maintains a short floating tongue from winter to early summer, when ice flow exceeds ice loss by calving and the glacier front advances. In summer, iceberg calving surpasses ice flow, and the glacier front retreats, becoming nearly grounded by late summer.
https://search.proquest.com/openview/2b991b9bb1ea4dbf97777598550b275e/
« Last Edit: April 05, 2019, 11:44:40 AM by nukefix »

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2021 on: April 11, 2019, 11:24:01 AM »
Looking back this is why I formed the view that seawater penetration once past the first cill, at 5K, was more or less unstoppable, the more saline water will always pass over the inner cills and move upstream before it becomes dilute enough to move back towards Disko. The recorded calvings of giant icebergs I suspect are the result of the ice in the deepest part of the trough being floated and breaking through the weight of ice above freeing that space for seawater penetration. There have been enough of these giant icebergs to convince me that some of them have come from inland of sills#1 and #2 from the first link. These blocks of coherent ice have been replace by ice moving downstream, so now in place of a smooth ice surface to grind over there is a melange of broken ice pressed to the depths of the chasms by the weight of the ice above it suffused with saline water.
 Were any cryopegs present beneath these giants? are there more upstream? as would seem probable if they froze from the top down.
 Tidal forces act on landmasses as well as oceans so they will act on ice bodies too the more fluid the ice the greater the potential effect, so it's not just a matter of the tidal waters penetrating and lifting the ice, and all I'm suggesting here is that the calvings and advances occur close to the extreme tidal range at new/full moons. The ice height of 200m ft. above the waters surface means there can't be any actual lifting from tidal penetration but increased melt is another matter.
See this comment too, https://forum.arctic-sea-ice.net/index.php/topic,154.msg56298.html#msg56298
So I'm still of the opinion that the cool saline water in Disko is discharge from the glacier mixed with tidal and possibly cryopeg waters. Is it correct to call it a grounding line if the ice is a composite of melange and seawater? if so ok.
« Last Edit: May 04, 2019, 04:56:05 PM by johnm33 »

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2022 on: April 11, 2019, 11:43:54 AM »
this is why I formed the view that seawater penetration once past the first cill, at 5K, was more or less unstoppable, the more saline water will always pass over the inner cills and move upstream before it becomes dilute enough to move back towards Disko.
Above the grounding-zone (thai is close to the calving front in this case) the ice-stream is grounded, in other words not floating but sliding on bedrock/till. So your theory is that despite this there is sea-water penetration far inland? I guess that is possible and there are these deep blue icebergs coming out of Jakobshaven that could be a sign of ice made from freezing water (instead of compaction of snow).

Quote
The recorded calvings of giant icebergs I suspect are the result of the ice in the deepest part of the trough being floated and breaking through the weight of ice above freeing that space for seawater penetration.
No. If they were floating there would be a clear tidal signal there, and there isn't.

Quote
Is it correct to call it a grounding line if the ice is a composite of melange and seawater? if so ok.
The grounded ice is too thick to float, so no...

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2023 on: April 15, 2019, 10:31:45 PM »
An interesting image from polarview today, it looks like there's been a serious flood of basal water break out from beneath the southern branch, which has advanced and calved and this before a big seasonal tide at the weekend. The second collapse feature on the northeast face of the southern branch, just before the bend, is not evident in the image so I await a clear day on sentinel.
  Coincident with this the northern glacier, not branch, which had a 'fan' calving this winter has begun to move too.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2024 on: April 16, 2019, 11:28:06 AM »
I took a walk along the beach today at low tide, the tide was as far out as I would expect for a full or new moon low, and the nights high had swept the whole beach clean, so the above post is more than likely wrong. It's more probable that polarview caught the full flood of the receding tide, but why such extreme tides?

Neven

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2025 on: April 16, 2019, 11:55:51 AM »
Il faut comparer, comparer, comparer, et cultiver notre jardin

lifeblack

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2026 on: April 17, 2019, 01:10:56 AM »
this is why I formed the view that seawater penetration once past the first cill, at 5K, was more or less unstoppable, the more saline water will always pass over the inner cills and move upstream before it becomes dilute enough to move back towards Disko.
Above the grounding-zone (thai is close to the calving front in this case) the ice-stream is grounded, in other words not floating but sliding on bedrock/till. So your theory is that despite this there is sea-water penetration far inland? I guess that is possible and there are these deep blue icebergs coming out of Jakobshaven that could be a sign of ice made from freezing water (instead of compaction of snow).

Quote
The recorded calvings of giant icebergs I suspect are the result of the ice in the deepest part of the trough being floated and breaking through the weight of ice above freeing that space for seawater penetration.
No. If they were floating there would be a clear tidal signal there, and there isn't.

Quote
Is it correct to call it a grounding line if the ice is a composite of melange and seawater? if so ok.
The grounded ice is too thick to float, so no...

Questions...  The way I understood the original statement of hypothesized seawater penetration was that some amount of seawater made it over the sill into the deeper parts of the bed, and that the duration of water flowing over the sill was relatively short due to the glacier advancing and regrounding itself on the sill, sealing off any inflow.

Question 1: Without a connection to the ocean, would a tidal signal be seen from any seawater pooled at the deep point just upstream?  (I think the answer is no?)

Question 2: If any seawater found a small gap over the sill, is it physically possible for the water to slide downhill displacing any ice on the way?  I think the answer could be yes, but only because I recall reading that a crevasse filled with water beyond a certain depth (I think 100 meters?) ends up with the water forcing open a crack to the bottom of the glacier.  With the difference in geometry here, and the water already resting on gently sloped rock I think it's doubtful for the amount of water I was picturing (but since I only have a vague-ish conceptual idea of what's happening, I'd love to learn from people who know what's actually involved)

Question 3:  How rapidly can the glacier flow at depth around obstructions / fill deep parts w/o much surface elevation change, and how much of the ice filling the deep part of the glacier bed rises up to pass over the sill?  Is the pressure sufficient at depth to keep it from jumbling into a melange at these flow rates? 


sidd

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2027 on: April 17, 2019, 01:50:55 AM »
1) you are correct, the tidal signal is tiny. Techically, there is a tidal signal in the solid earth as well. The ocean tide is the difference between the earth response and ocean response along the coastline. But in an isolated pool, the signal is not visible.

2) Water flow under glaciers is complicated. But a simple calculation will tell you that water can flow uphill along the bedrock over slopes 10 times the (negative of) ice surface slope. i.e. say the surface is sloping down toward the coast . But there is a sill in the bedrock  that goes upward as you approach the coast. Water can flow coastward uphill over the sill unless the upward slope of the sill is more than 10 times the magnitude of ice surface slope down.

3) the lower part of Jacoshawn is all a melange, to granular flow might be one way to look at it. As to timescale for flow, we can see what the surface speed is, so something of that magnitude  for the full depth might be a good guess for the average across depth and width.

sidd

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2028 on: April 17, 2019, 02:03:23 AM »

2) Water flow under glaciers is complicated. But a simple calculation will tell you that water can flow uphill along the bedrock over slopes 10 times the (negative of) ice surface slope. i.e. say the surface is sloping down toward the coast . But there is a sill in the bedrock  that goes upward as you approach the coast. Water can flow coastward uphill over the sill unless the upward slope of the sill is more than 10 times the magnitude of ice surface slope down.


Aha!  Thanks Sidd, I had not considered the factor of ice surface slope - the water gets squirted back out, assuming the correct geometry.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2029 on: April 18, 2019, 12:19:29 AM »
lifeblack read this I'd be interested in your opinion it's mostly about tidal effects on the glacier it's movement and calving, bear in mind that due to it's geographical position the tides in Jacobshvn fjord are about 3-4hrs behind the 'natural' tide time so in some sense out of synch with any effects on the glacier. I found nothing in it that confirmed or challenged my view, apart from the step change of melange height above the cill [fig2] which, to me, suggests a deep logjam of melange, and if so then voids enough for tidal seawater/meltwater exchange.

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2030 on: April 18, 2019, 10:04:21 AM »
There is no water under Jakobshavn upstream from the grounding zone, which is very close to the calving front (part of the year they are at the same spot, so no floating tongue).

As far as I know there is zero evidence of sea-water penetration under Jakobshavn. Tides influence the back-pressure in the glacial plumbing system that should be full of glacial meltwater.

This thesis says

Quote
For this investigation I re-processed a 10 km-long high-resolution reflection seismic line at Jakobshavn Isbræ, Greenland, using an iterative velocity determination approach for optimizing sub-glacier imaging. The resultant line imaged a sub-glacier sediment layer ranging in thickness between 35 and 200 meters. I interpret three distinct seismic facies based on the geometry of the reflectors as a basal till layer, accreted sediments and re-worked till. The basal till and accreted sediments vary in thickness between 4 and 93 meters and are thought to be water-saturated actively-deforming sub-glacier sediments. A polarity reversal observed at one location along the ice-sediment interface suggests the presence of water saturated sediments or water ponding 2-4 m thick spanning approximately 240 m across.

https://kuscholarworks.ku.edu/handle/1808/19476
https://kuscholarworks.ku.edu/bitstream/handle/1808/19476/VelezGonzalez_ku_0099D_13964_DATA_1.pdf

A seismic image plus legend are attached.



nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2031 on: April 18, 2019, 10:13:42 AM »
I found nothing in it that confirmed or challenged my view, apart from the step change of melange height above the cill [fig2] which, to me, suggests a deep logjam of melange, and if so then voids enough for tidal seawater/meltwater exchange.
johnm33 it's not "melange" but solid deforming ice all the way to the glacial till according to both seismic and ground penetrating radar studies. There is a changing network of glacial drainage but are there studies arguing seawater can be found upstream of the grounding-zone?

Adam Ash

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2032 on: April 18, 2019, 12:38:19 PM »
Well, since (virtually) all rivers near coasts have salt wedges running up them as the tide comes in, I would expect similar tide-synchronised intrusions of denser salt water below the outflowing meltwater beneath the fragmented glacier by the cil.  Why wouldn't it?

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2033 on: April 18, 2019, 12:58:21 PM »
Well, since (virtually) all rivers near coasts have salt wedges running up them as the tide comes in, I would expect similar tide-synchronised intrusions of denser salt water below the outflowing meltwater beneath the fragmented glacier by the cil.  Why wouldn't it?
I don't think the ice is fragmented as the pressure is around 100bar at 1km depth, but there are drainage channels. Tides increase back-pressure and can be detected far upstream but how far does the salt-water intrude? dunno

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2034 on: April 18, 2019, 02:59:00 PM »
According to this paper:

https://www.cambridge.org/core/journals/annals-of-glaciology/article/characteristics-of-meltwater-export-from-jakobshavn-isbrae-and-ilulissat-icefjord/1DEDDDC0AAD910F83213AEBAFD7A31E6/core-reader

...the salinity of the Subglacial discharge, Submarine meltwater and Basal Submarine meltwater is zero, which would not be possible if saline waters were penetrating into and under the ice-stream. Case closed?


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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2035 on: April 18, 2019, 06:29:54 PM »
lifeblack read this I'd be interested in your opinion it's mostly about tidal effects on the glacier it's movement and calving, bear in mind that due to it's geographical position the tides in Jacobshvn fjord are about 3-4hrs behind the 'natural' tide time so in some sense out of synch with any effects on the glacier. I found nothing in it that confirmed or challenged my view, apart from the step change of melange height above the cill [fig2] which, to me, suggests a deep logjam of melange, and if so then voids enough for tidal seawater/meltwater exchange.
So, here's my take on the paper:  It's a derivation of the location of the grounding line of the glacier, and whether the glacier front coincides with the grounding line.  The technique used is flow rate variation along the line of sight from a vantage point to various locations over a period of several days (1).  The flow rate variations are (via math) transformed to a spectral distribution and plotted, the three major tide components are also marked in these plots.
 A phase offset is seen at the glacier front, and some vertical motion is found during early summer (2).  A step change is seen in the flow rate of the melange of the ice floating in the fjord, along with a decrease in height above surface that coincides with this point.
Paper concludes with logical argument consistent with presented evidence that a short tongue of ice exists in early summer (3) and that the glacier calves it off by the end of summer and the front is grounded.  At no point in the paper does it suggest that seawater is penetrating past the grounding line - in fact, given the height increase at the grounding line (and according to Sidd's explanation) it appears that seawater will NOT be moving upstream past the sill until the glacier is thin enough to float at that location (or pretty darn close to it) (4)
1. I'm going to trust that this is done correctly.  I got a little bit confused with the description and how topography affects things.
2. I'm not entirely clear on how they extract vertical movements from their LOS measurements.  See 1.
3. There's also vertical movement associated with what they think is the tongue, and the glacier height is low enough to support being floated at that location.
4. Seawater at the base of the glacier will definitely help melt it, but I don't think it's capable of generating a cavity extending upstream until the height of the glacier at that point is low enough to be floated (and not pressing down hard on the bed).

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2036 on: April 19, 2019, 12:54:52 AM »
Cased closed? no the data is from 2014 outside the fjord they've no way of knowing where the mixing they acknowledge took place.
this image shows that whilst it's possible there may have been some seawater past the cill as early as 2006, the calving front retreated beyond the current 'grounding' line in 2009, my take is that it then took three years for sufficient seawater ingress to float the basal ice in the first trough and have that break through the overburden of glacial discharge. Then there was a major breakthrough and a period of rapid retreat followed, once the second trough was clear, by an ongoing steady avalanche of ice into the fjord, pushing the calving front back to the cill.
This shows the retreat +/- 4k beyond the current 'grounding' line/cill the ice is floating in?



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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2037 on: April 19, 2019, 12:59:10 PM »
my take is that it then took three years for sufficient seawater ingress to float the basal ice in the first trough and have that break through the overburden of glacial discharge.
The glacier is too thick to float and both GPR & seismic indicate it's solid ice until water-permeated glacial till at the bottom. The meltwater is fresh and it's being dumped into the fjord continuously. Can you find a scientific publication where the authors argue that the saltwater gets to penetrate upstream?

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2038 on: April 30, 2019, 12:31:43 PM »
Yesterdays cloudless image from Sentinel shows 2+km movement of the previous calving group, with minimal retreat of the glacier. Looking further down the fjord the next cluster of bergs has moved about 10km putting some distance between the two, and the whole of the winters fast ice is on it's way out the fjord. 
effects:- atmospheric correction, .4 gain, 1.7gamma

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2039 on: May 04, 2019, 06:08:17 PM »
my take is that it then took three years for sufficient seawater ingress to float the basal ice in the first trough and have that break through the overburden of glacial discharge.
The glacier is too thick to float and both GPR & seismic indicate it's solid ice until water-permeated glacial till at the bottom. The meltwater is fresh and it's being dumped into the fjord continuously. Can you find a scientific publication where the authors argue that the saltwater gets to penetrate upstream?
"The glacier is too thick to float" agreed, but here I was only referring to the solid basal ice being lifted/floated from the base of the chasm,  which was very different in character to the granular ice above it.
"publication" no but then in 2014 A-Team was lamenting the fact that none of the data post 2008 had been processed and I assume that's still the case, until it's "established" that there's any space for salt water penetration to occur in why would anyone look for it, this notwithstanding the fact that the calving front remained several km. beyond the cill for more than 7 years, personally I'd regard seawater not making some ingress in that amount of time a near miracle.
I came across the following image which shows a shift in the flow path of the glacier post 2011, which ties in with what I would expect from a 'softening' of the ice in the deepest parts of the channel, and 2011 is apposite since that is the year the ice retreated 4km[?] beyond the cill for the first time. From https://www.nature.com/articles/s41561-019-0329-3

huge changes in the last month or so. Polarview


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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2040 on: May 05, 2019, 01:45:05 PM »
The calving and discharge of water continue


Sentinel with ac 0.4gain 1.7gamma

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2041 on: May 05, 2019, 06:35:22 PM »
johnm33

There is no change in the flow-path of the main trunk as it is confined to a narrow canyon. The "paths" plotted in the image are the flight-paths NASA did with their Airborne Topographic Mapper (ATM):

https://icebridge.gsfc.nasa.gov/?page_id=118

The Sentinel-1 images you posted are from ascending and descending orbits so their geometry differs. Also the presence of wet snow darkens the image by a great deal and I assume PolarView does some adaptive contrast stretching that complicated side-by-side comparisons when large contrast differences are present.

I still have no reason to believe salt water penetrates very far above the grounding zone but if you find a study arguing so I'd be delighted to read it!

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2042 on: May 05, 2019, 07:08:58 PM »
The calving and discharge of water continue


Sentinel with ac 0.4gain 1.7gamma

The north branch of the calving looks really odd with a tongue of ice sticking out as evidenced by the shadows and a stretch of open water southeast of it.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2043 on: May 06, 2019, 01:24:15 AM »
"North branch" this image is from 28:03 shows the extent of the winters advance, before the recent calvings. The western side is a different ice stream to the eastern side, which seems to have a variety of contributions to it's flow. It appears to me that the western side is also discharging considerable amounts of meltwater.

 

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2044 on: May 10, 2019, 01:02:42 AM »
The output of the glacier continues, a detail from the latest Polarview


 It may be that sufficient ice is flowing down the main channel/falling down from the heights of the ice sheet that we are witnessing the squeezing out of the deep waters present, it seems a lttle early for melt water to be flooding out, or it could be that saline water has kept last seaons meltwater liquid and possibly eased the flow properties of the granular ice in the channel. If the latter is true then as the ice flow above is eased by melt we could witness a very abrupt advance of the calving front which could only be sustained by a widespread local slump of the ice sheet. On the other hand if we are witnessing the consolidation of the ice in the main channel then we may see little change over the next few years, just small advances, calvings and retreats.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2045 on: May 14, 2019, 03:06:29 PM »


“There are three classes of people: those who see. Those who see when they are shown. Those who do not see.” ― Leonardo da Vinci

Insensible before the wave so soon released by callous fate. Affected most, they understand the least, and understanding, when it comes, invariably arrives too late

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2046 on: May 15, 2019, 10:07:27 AM »
https://www.bbc.com/news/science-environment-48265217

(Quote ish) Slams on brakes, gone from thinning 20m a year to thickening by 20m a year[/quote]

Wow quite some thickening!

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2047 on: May 15, 2019, 01:58:45 PM »
In this post on the Greenland melt season thread one of the links has ice-bridge saying the calving front is now 100m high as the cill hereabouts is 8900m below sea level that suggests to me that the front is bouyant, just.
The front continues to calve and release meltwater[?] in the most recent Polarview the large iceberg just past the northern branch has moved out 2-3km. as a consequence, that leaves the calving front exposed, only slightly upstream of the cill apex, if with the full moon tides seawater penetrates beyond that point then both the bouyancy and melt rate of the submerged ice will increase.
« Last Edit: May 15, 2019, 03:06:47 PM by johnm33 »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2048 on: May 15, 2019, 11:19:55 PM »
More recent and far better image from polarview I think the distinctive berg is by the bay on the north side, so 12+km and still enough melange being shed to fill the fjord.

I've been reviewing what I think, and realised the more important cill is the 700+m one which lies roughly between the two icebergs in the previous posts image, the 800m contour runs back from here both sides of the channel way upstream.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #2049 on: May 17, 2019, 04:39:36 PM »
 You need to zoom in on this gif from hycom to see the impressive outburst of freshwater from Jakobshavn around the 11th
Looks like the granular ice in the channel is being finely ground, from sentinel