Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland

[johnm33]

I think of myself as an expert in misinterpretting polarview images, here I have the impression the main channel flow has changed. Where once there was an oveturning wave joining the flow now that stream has overwhelmed the wave and flows as one with the main stream.

[NotaDenier]

Some pictures. Do I see a crack/separation on the north side of the main channel?

[johnm33]

Do I see a crack/separation on the north side of the main channel?

There's a deep steep sided trough right there and that's a slow motion overturning wave collapsing into it.

[NotaDenier]

Do I see a crack/separation on the north side of the main channel?

There's a deep steep sided trough right there and that's a slow motion overturning wave collapsing into it.

Agreed but I’m talking a little to the west of the trough.

[johnm33]

By the twin bergs? from the headland back to the main calving front has been calving independently, as far as I can tell, for a short while now.
No giants have appeared for some time so I'm guessing we could see a big release and retreat in the next ten days or so. I hope someone is there to see it.

[johnm33]

Calving and retreat, I don't think we're done yet, 12-16-17^th added a frame 19^th replaced the gif

[Stephan]

Thank you for keeping an eye on it.

[NotaDenier]

By the twin bergs? from the headland back to the main calving front has been calving independently, as far as I can tell, for a short while now.
No giants have appeared for some time so I'm guessing we could see a big release and retreat in the next ten days or so. I hope someone is there to see it.

Yes by the twin bergs and then it did calve. That must have been nearly 5 cubic kilometers of ice. Calving front is 5km wide and about 1km tall.

Not sure I’ve seen it calve on the north side very often.
Thank you for the gif! It must be getting pretty close to record retreat?

[johnm33]

The retreat and advance continue I've added a few frames. On polarview the largest of the cluster of bergs comes up white so I assume it's overturning. I'll remove the big gif tomorrow.

[johnm33]

The first image is from upstream, here what I suspect I'm looking at are waves orthogonal to the movement downstream and these are propagating through the water of a saturated base. The wave pattern/harmonic resonance appears to have changed somewhat to a lower frequency whether this is so and what it implies as to the movement of the glacier???
Then.
An update with overlap. [click]
and the latest polarview of the southern front

[johnm33]

A little more misinterpretation to consider. Being used to the passive radar system of the light spectrum it's difficult to know quite what an active radar image means but here I'm assuming lighter means 'noise'/movement/coarse and dark/black means smooth. There are some new bergs calved one of which is caught overturning, so some retreat and advance, but around the bend a light area that could indicate a collapse or less likely movement over a cill, probably a reaction to the space created by the advance, first time I've seen this and an interesting piece of confirmation bias, since I've been looking for something like this. Further up the wave form appears to have been partially collapsed by movement too, no causal relationship implied between the two.
both from PolarView

[IceConcerned]

Jakobshavn glacier has retreated almost 3km since last April (blue trace on the picture)

[Mr. Ä]

A melt water lake on the south side has drained. It has no effect on calving but I found it interesting and decided to try Copernicus explorer create timeseries tool.

[RoxTheGeologist]

Wow! Look at the channel it cut. I bet that was a very impressive amount of water.

[Espen]

A melt water lake on the south side has drained. It has no effect on calving but I found it interesting and decided to try Copernicus explorer create timeseries tool.

Nice catch!

[NotaDenier]

As I look at this picture the low sun angle really accentuates the vertical drop on the south side of the main channel canyon wall. Is that south canyon wall above sea level?

Once I found a topography map it confirms where the large shadows are rises about 300-400 meters above sea level. As the glacier continues to slump and retreat we’ll eventually see the rock be exposed there. I’m guessing this will take a century or more however.


https://www.cambridge.org/core/services/aop-cambridge-core/content/view/63F80A130C190B69F732123CD1779F0B/S0022143000202086a.pdf/div-class-title-bed-topography-of-jakobshavn-isbrae-greenland-and-byrd-glacier-antarctica-div.pdf

[oren]

As I look at this picture the low sun angle really accentuates the vertical drop on the south side of the main channel canyon wall. Is that south canyon wall above sea level?

Your question is not clear.
First, all the walls of the canyon are above sea level, as they top the ice flowing down the main channel which is itself ~130m above sea level at the calving front.
Second, do you mean the region south of the main channel? That is an exposed rocky region, from which was filmed the famous "Chasing Ice" film in 2012.

[Mr. Ä]

As I look at this picture the low sun angle really accentuates the vertical drop on the south side of the main channel canyon wall. Is that south canyon wall above sea level?

Once I found a topography map it confirms where the large shadows are rises about 300-400 meters above sea level. As the glacier continues to slump and retreat we’ll eventually see the rock be exposed there. I’m guessing this will take a century or more however.

At the rate new rock has been exposed on that south side I think it will be more like decades.

[NotaDenier]

Oren I meant the part I have circled in red.

[Mr. Ä]

That whole area starting from the exposed rock is quite lumpy and you can see big shadows at this time of year. That would mean that the ground is uneven with hills, canyons and fjords and the ice is thin enough for it show through.

[oren]

Thanks for the clarification, Mr A.
A-Team posted very long animations in this thread many years ago, which showed the ice flowing over the circled area. I remember them to be quite impressive, they are somewhere upthread.

[Mr. Ä]

It might also be ice pushing against ice in the channel that is flowing perpendicular. I'm not sure if ice piles up at these scales like it does at smaller scales. I remember seeing some topograhical maps of the are, but those were not very high resolution.

edit: https://www.cambridge.org/core/journals/journal-of-glaciology/article/bed-topography-of-jakobshavn-isbrae-greenland-and-byrd-glacier-antarctica/63F80A130C190B69F732123CD1779F0B Too bad the image files there are so low resolution even the numbers are hard to read. But there does seem to be a large peak in that spot. It's going to be a gorgeous fjord in a century or two.

[Espen]

Too bad the image files there are so low resolution even the numbers are hard to read.

That's typically the problem with these science reports, their visual documentation is pretty much always lousy, don't know the reason maybe on purpose. Visual documentation here in the forum is strangely significantly better?

[oren]

Here's a pair of topo maps posted very far upthread by Espen and A-Team

[oren]

Here is the final version of movement of Jakobshavn near and above the elbow region. Two tiles are shown that cover the elbow region and to its east; these should animate without a click. After quite a bit of experimentation, these are fairly consistent in contrast which is tricky with early season and cloud shadows on some dates.

2015    43    0    9   11   LC80090112015043LGN00   12 Feb 15
2015    50    7   10   11   LC80100112015050LGN00   19 Feb 15
2015    52    2    8   11   LC80080112015052LGN00   21 Feb 15
2015    59    7    9   11   LC80090112015059LGN00   28 Feb 15
2015    75   16    9   11   LC80090112015075LGN00   16 Mar 15
2015    91   16    9   11   LC80090112015091LGN00   01 Apr 15
2015   107   16    9   11   LC80090112015107LGN00   17 Apr 15
2015   114    7   10   11   LC80100112015114LGN00   24 Apr 15
2015   123    9    9   11   LC80090112015123LGN00   03 May 15
2015   146   14   10   11   LC80100112015146LGN00   26 May 15
2015   155    7    9   11   LC80090112015155LGN00   04 Jun 15
2015   162    7   10   11   LC80100112015162LGN00   11 Jun 15
2015   178   16   10   11   LC80100112015178LGN00   27 Jun 15
2015   187    7    9   11   LC80090112015187LGN00   06 Jul 15
2015   194    7   10   11   LC80100112015194LGN00   13 Jul 15
2015   210   14   10   11   LC80100112015210LGN00   29 Jul 15
2015   212    2    8   11   LC80080112015212LGN00   31 Jul 15
2015   226   14   10   11   LC80100112015226LGN00   14 Aug 15
2015   235    7    9   11   LC80090112015235LGN00   23 Aug 15
2015   251   16    9   11   LC80090112015251LGN00   08 Sep 15
2015   267   16    9   11   LC80090112015267LGN00   24 Sep 15
2015   283   16    9   11   LC80090112015283LGN00   10 Oct 15

And here are the long animations I remembered. Took a long time to find, this thread was extremely active in 2015-2017. Click.

[Espen]

Here's a pair of topo maps posted very far upthread by Espen and A-Team

Yes you are moderating a gold mine of documentation, we just need to get it reviewed by some Nvidia machinery!

[oren]

Jacobshavn during the dark season in 16 Sentinel 1B images.

Another long animation of JH, this one from 2018.
What I found the most interesting when I watched these animations is how the whole terrain is flowing, and quite fast. It's not just the ice in the main channel.
Also of note is that the calving front has not moved since ~2013. I guess the flow is too fast, so that any calving is quickly replaced with new ice. The current front is in a narrow chokepoint, in order to recede further it needs a location where the channel widens, which is what brought about the retreat in the 2000s.
I do wonder if the forward speed has changed during the last decade with the static front.

[NotaDenier]

Thanks guys. I like to keep watching this glacier. As the ice on the margins of Greenland continues to slump I would expect the sides of the hidden side of fjord to be exposed. I wonder if it will speed up then?

I agree it will be a lovely fjord with a train of icebergs being exported in a century or two.