Antarctic Icebergs

[baking]

The latest Sentinel-1 image shows that A68A hasn't moved much in the last 8 hours.

[baking]

Latest GIF from Sentinel-1 images shows many small pieces of iceberg A68-A breaking off at the grounding point.  No question that it has hit ground, but what will happen next is anyone's guess.

Click image below to see animation.

[blumenkraft]

GIF 1: The crash Baking mentioned.

GIF 2: A comparison of 01.12. and 21.12. to see the grounded icebergs.

GIF 3: Beautiful, very old ice.

[blumenkraft]

BTW, the thickness, at least at this part of the iceberg, is nowhere near the 300-335m figure.

[gerontocrat]

BTW, the thickness, at least at this part of the iceberg, is nowhere near the 300-335m figure.

When A-68 first broke away, the size was said to be 5,800 km2, and the mass 1 trillion tons, i.e. an average thickness of just over 170m. On average freeboard is about 10%, i.e. 17 metres, meaning on average around 155 metres is below the sea surface.

The iceberg is also very flat, so maybe the average is pretty close to the thickness at the edge.

[blumenkraft]

OK, but this is the side that was facing the ocean once it was a shelf still. So we can assume it's thinner on this side than the other, right?

If that's a correct assumption, the 170m must be wrong.

[blumenkraft]

Amazing, how you can see layers of different ice.

I get why it's white on top, but why has it this darker blue middle layer?

[HapHazard]

Perhaps that bit has pretty much always been underwater? (From past flips.) Anyway, definitely neat.

[gerontocrat]

OK, but this is the side that was facing the ocean once it was a shelf still. So we can assume it's thinner on this side than the other, right?

If that's a correct assumption, the 170m must be wrong.

My ignorance on the extent of thinning on the ocean side of an ice shelf is profound. The edges are also chipped and bashed from is journey, presumably mostly on its underside.

So who knows?

[vox_mundi]

... I get why it's white on top, but why has it this darker blue middle layer? 

The clearer (darker) the ice - the fewer trapped air bubbles.

Perphaps those clearer layers represent periods (... tens of thousands of years) of lighter (or heavier snowfall) when that portion of the glacier was much further inland.

This ice shelf has been traveling towards the coast for many 21,000 and 41,000 year precessional and orbital cycles and probably a few 100,000 year cycles. Things change.

[blumenkraft]

Thanks, Hap and Vox! Fascinating stuff.

So who knows?

We are here to find out, aren't we?

[blumenkraft]

A-68A still marching!

23. vs 24.

[FredBear]

Do I see the tail of A-68A (southern end ) breaking up? (arctic.io.   2019-12-24, antarctic true color)

[Tor Bejnar]

Here's the latest available PolarView image of A68-A from 12/23 showing the tail looking just fine.  (I look forward to seeing the next 'radar' image => 12/24.)

Over half the world is into 12/25, so Merry Christmas, y'all!  (I have to wait three and a half hours for the soot-repellent-suit wearing elf to fly down our imaginary chimney and fill the oversized stockings presumptuously laid out on the sofa, eager to be filled with minor loot. [The stockings have a mercurial mind of their own!  They're so calm most of the year.])

[blumenkraft]

Do I see the tail of A-68A (southern end ) breaking up?

Nope. Today's Sentinel-1 just came in.

But looks like it had another bump on the side.

[blumenkraft]

Between yesterday and today, A-68A has moved ~ 10km north-east.

Surprising to me is that it's turning clockwise! 

[baking]

Surprising to me is that it's turning clockwise!

Not all that surprising considering the Northeastern end has run aground.  But it also means a strong likelihood that the Southwestern end will also be grounded.

[blumenkraft]

I find it hard to wrap my head around this massive thing being stoped by grounding though. That kind of momentum needs a lot of friction to slow down. Likely it will rather break in pieces, would you agree Baking?

[baking]

I'm not sure how strong the currents are there, but I do know they would be much stronger than anything it experienced in the past two years.  Also, icebergs that are much longer than they are wide tend to breakup pretty readily.  And there is lots of evidence of cracks and other weaknesses.  If it stays grounded for more than a few days at both ends I think a breakup is likely.

[baking]

Here is a 48-hour GIF of the movement of the SW end of A68-A.  It looks like there may be a possibility of it getting by without grounding.

[baking]

On the other hand, today's Sentinel-1 image of the NE grounding point of A68-A shows that it may have already moved away from that grounding point after a lot of smaller icebergs calved off of the larger one.  (Can the force of calving force the larger berg away from the grounding point?)  See the GIF below.

[blumenkraft]

Today's Sentinel 3 shows it pivoting clockwise furthermore.

[blumenkraft]

More of the same.

The iceburg moved north-east, even though way more east than north, and is rotating clockwise still.

I think it's snaking along.

[Tor Bejnar]

B_,
With undated images, I cannot tell which is older. 

I've always wondered about "clockwise", because from the clock's perspective, the hands go from the top of the dial to the left and downwards.  Now I'm wondering which way time goes:  numbers go up as we increase years before the present.  The future is just a belief.
 
Documentation would keep my mind from wondering. 

[blumenkraft]

Oh dear, that poor confused mind of yours and it's all my fault. :-/

I will do better my friend.

To help you out with this one, the older one is the one where the iceburg is further on the left. Since this is via EO-Browser, north is up.

With the clockwise thing, you confused me btw!

[blumenkraft]

With timestamp, i promised.

More of the same i guess. No signs of grounding.

[FredBear]

Still think that the tail (SE corner, lower left in above images) has lost lumps of ice recently!
(Possibly only sea ice sticking to the 'berg?)

[blumenkraft]

Yes, it lost almost all the fast-ice and a few small pieces at its tail(ish) around the 24th.

[baking]

Yes, it lost almost all the fast-ice and a few small pieces at its tail(ish) around the 24th.

One of those larger pieces is an iceberg that was visible on December 23 in the GIF from my reply #119 and is probably not from A68-A.  Not sure about the other one, or whether it is an iceberg or sea ice.

Edit:  Here is an image from Dec. 18 showing where the iceberg in question broke off of A68-A.  Still searching for the other one.

Edit2:  I found the other piece in the Dec. 23-25 GIF, to the right of the first piece on Dec 23 and above it to the left on Dec. 25, but I can't find it either loose or attached on Dec. 18, but I am now convinced it did break off because it has a dark line across it visible to radar showing it has broken off from old ice.

[blumenkraft]

Yes Baking, totally forgot about that tale one. Thanks for adding.

A68-A is still not grounded. WestEastwards movement, seemingly circumventing shallow waters.

[baking]

Westwards movement

Eastward.  A68-A is almost halfway past the grounding point.

[blumenkraft]

Fixed, thank you, Baking!

What now? Can we declare it free now?

[gerontocrat]

Being lazy, I am asking you lot for the exact lat/long of A-68.

By the looks of it, recently there was a strong wind assisting A-68's northward movement. That is now dead calm or even in reverse. (see gif)

Presumably the prevailing currents will still help to nudge it north, though how strong those currents are I do not know. (see Weddell Gyre and Antarctic Currents images).

[blumenkraft]

Iceberg A68A
Size: 82X26 nm
Coordinates: -54.82, -64.08
Remarks: belle
Date recorded: 2019-12-27
Source: US National Ice Center

[baking]

Coordinates: -54.82, -64.08
Date recorded: 2019-12-27

Based on this morning's radar image, I don't think I would change those coordinates too much, although it has moved a little to the East.  The problem is in finding the center of mass or whatever they are targeting.  The iceberg itself actually stretches from 56W to 53W so maybe 54.5W is more current estimate.

[Tor Bejnar]

Back on December 16 (annotated image), the leading edge of A68-A was just south of Joinville Island (functionally the 'northern' tip - actually eastern tip - of the Antarctic Peninsula).

Baking's image (just above) includes parts of Joinville Island ('above') and Dundee Island ('below' - North is towards the upper left corner of the image).  About 2/3^rds of A68-A is east of Joinville Island.

Attached image (map from here) has approximate July 2018 ice island location and approximate December 2019 location.  (The shaps of the ice island are approximate, and have changed some over these 18 months.) 

Gerontocrat, your green circle on the wind map is way far from where the ice island floats. 

[gerontocrat]

Good heavens. I had no idea it was almost at the open sea. I thought it would take years and years to meander north.

It would be a hoot if it went across the Weddell Sea a bit too far south and got caught by the gyre  and sent south go around the whole coastline.

[blumenkraft]

The trajectory of A68A has clearly changed now. We are headed east (~7km since yesterday), the clockwise rotation seems to have stopped.

[gerontocrat]

I wonder how much of the movement of A-68 is determined by wind and how much by ocean currents.

The wind is currently from the North at A68's location

[blumenkraft]

Since it's pretty constantly snaking along the (underwater) coastline i would assume the wind has only a little influence. My humble opinion based on observation only, no knowledge of the physics involved.

[Stephan]

What about B-22 in Amundsen Sea?

I chose five different remarkable points A, B, C, D, and E and calculated the changes of their position between 10.01.2019 and 27.09.2019 (Shift 1) as well as the changes between 10.01.2019 and 25.12.2019 (Shift 2) from clear Sentinel images.
B-22 lies north of Haynes/Smith Glacier and NW of Thwaites Ice Tongue, from which it broke off almost 10 years ago. It is not perfectly grounded, but almost stuck in shallow waters. But it moves slowly into NW direction. I present the position changes in the following table (numbers in km, direction indicated)

Point  Shift 1 [km]  - Shift 2 [km]
A       2,7 W 2,0 N  -  3,6 W 2,5 N
B       2,5 W 1,9 N  -  4,4 W 2,9 N
C       2,7 W 1,6 N  -  3,8 W 4,6 N
D       2,9 W 1,6 N  -  2,7 W 4,1 N
E       2,8 W 1,8 N  -  3,3 W 3,3 N
_____________________________
F       0,7 W 2,2 N  -  7,9 W 7,7 N (the "cork", see text below)

The westward movement, predominant in the first part (Shift 1) has turned into a more northward movement in the latter part (Shift 2), especially at the eastern end of B-22.
The western end is slowly eroding by calving. It loses maybe 100 m per year at some points. B-22`s length is around 85 km - please calculate by yourself how long it might take for a complete calving...

South of B-22 the compact sea ice has opened (area circled in blue). One of the larger icebergs south of B-22 has floated into NW direction (8 km in Shift 2). Now it seems to be stuck in shallow waters. It blocks the outflow of the sea ice in the blue circled region. Therefore it acts as a cork. I have no idea whether it will stay there for longer or whether it will be pushed through by the currents. Until that happens a complete clear-out of sea ice south of B-22 seems unlikely.

See attached picture.

[baking]

it broke off almost 10 years ago.

Actually, 2002.

[Stephan]

OK, thank you. I should have checked that...

[blumenkraft]

Yesterdays Sentinel 3 came in very late, so i didn't manage to make a GIF.

Here is 3 days instead.

Note the stranded icebergs in the north.

[blumenkraft]

Another 7/8 km of dodging underwater mountains.

Click to play.

[blumenkraft]

A68A 'only' made 4-5 km between the two shots. This can either mean a different time frame than usual between the shots, or it has slowed down. I would guess the further.

The trajectory is still eastwards.

[Tor Bejnar]

Further to my post on December 30 (with Antarctic Peninsula map), here is a Polar View image from 18 hours ago showing A68-A (lower right, mostly off the image, darker grey - 64S and 54W go through the ice island) in relation to the tip of the Antarctic Peninsula (actually, the islands just east of the tip (in blue) - Joinville Island in the middle.  (North is to the upper left. 56N goes through the land islands; 63S brushes the northern one. Lines are every 1 degree N-S and 2 degrees E-W.)

[blumenkraft]

3-4 km eastwards movement only between yesterdays shot and today. That makes it two slow(ish) days. Also, we have a counter-clockwise movement.

I suspect we are scratching along an underwater mountain.

[Stephan]

I wonder whether A-68 will be able to join the westeastward drift around Antarctica or whether it will be caught by the Weddell gyreand remain in that area for a while.

[blumenkraft]

Yeah, me too! That will be interesting to watch.