Thwaites Glacier Discussion

[Stephan]

New melting and breaking off at Thwaites Glacier. Today I analysed the outer ice field which continues Thwaites Ice tongue to the northwest. I compared the EOSDIS data from Jan 12 and Jan 30, 2019. I marked lost fast sea ice positions in pale magenta and newly formed cracks in orange. I wonder whether this outer ice field, which now has almost completely lost its connection to the Thwaites Ice tongue, will survive this fading austral summer.
See attached picture.

[sidd]

Big hole under Thwaites: melt rate much higher than models find

Apparently CDW induced melt is much faster in cavities when bed slope is prograde than retrograde.

"The newly formed cavity at B is thin, however, which does not favor warm CDW intrusion from geostrophic flow and efficient vertical mixing (19, 24) and explains the low ice shelf melt rates. In contrast, the prograde bed at A favors an efficient opening of a new ice shelf cavity, stronger CDW intrusion, and efficient mixing, with melt rates 20 times higher than those at B. "


"Ice shelf melt at A exceeds values used in numerical ice sheet/ocean models by factors of 2 to 3"

open access, read the whole thing: DOI: 10.1126/sciadv.aau3433

sidd

[solartim27]

Good summary and pictures of above here:
https://www.jpl.nasa.gov/news/news.php?feature=7322

[vox_mundi]

From the jpl article

[b_lumenkraft]

Comparison Thwaites Glacier - Nighttime Imagery (Day/Night band, enhanced contrasts via Suomi NPP VIIRS)

Left hand 2019-01-30
Right hand 2019-01-06

[Stephan]

In addition to the posting I have just made and in addition to my post from yesterday I have to report that a further new crack in the fast ice / ice mélange of Thwaites has developed yesterday.
See attached image.
Colour code: orange is the crack I have just mentioned, yellowish green is the crack I reported of yesterday and the new crack which connects both cracks is painted in pale magenta.
It seems that the whole thing is about to collapse.

Update 31 Jan 2019: The crack in pale magenta has massively widened. The whole eastern part moves to the southeast. The crack in yellowish green has not changed significantly.

[Andreas T]

Looking through past years i noticed there has been a swathe of grounded icebergs in the past which seemed to follow the same line as some of the present sea ice.
To have a better comparison I have overlayed an image of 21 feb 2008 in a purple tint over 22 jan 2019
This shows that the stranded icebergs were mostly further west than the curved piece of sea ice. In the same location there are still icebergs which don't move when other bits of ice move around them.
We will probably see soon how much the mobility of the large chunk of sea ice is constrained by frozen in icebergs.

So, my hunch was a bit off, but I thought it might be worth sharing because it tells us something about water depth in that area.

[Stephan]

I have also noticed that many of the ice bergs west of the Thwaites Ice Tongue might be grounded. Comparing day-by-day or week-by-week some of them just turn around at their position, others seem to be completely immobile. The waters must be shallow there.
If I look at the outer ice field I mentioned some days ago, it contains only few ice bergs originated from Thwaites. Therefore I think that - if sea temperature and currents are "right" - it will further disappear or move, and some of the grounded bergs in it will just stay where they are at the moment...

[Tealight]

New melting and breaking off at Thwaites Glacier. Today I analysed the outer ice field which continues Thwaites Ice tongue to the northwest. I compared the EOSDIS data from Jan 12 and Jan 30, 2019. I marked lost fast sea ice positions in pale magenta and newly formed cracks in orange. I wonder whether this outer ice field, which now has almost completely lost its connection to the Thwaites Ice tongue, will survive this fading austral summer.
See attached picture.

What you have marked as "Thwaites Ice tongue" has been named iceberg B22A for several years., since it is not connected to the main glacier anymore.

Looking through past years i noticed there has been a swathe of grounded icebergs in the past which seemed to follow the same line as some of the present sea ice.
To have a better comparison I have overlayed an image of 21 feb 2008 in a purple tint over 22 jan 2019
This shows that the stranded icebergs were mostly further west than the curved piece of sea ice. In the same location there are still icebergs which don't move when other bits of ice move around them.
We will probably see soon how much the mobility of the large chunk of sea ice is constrained by frozen in icebergs.

So, my hunch was a bit off, but I thought it might be worth sharing because it tells us something about water depth in that area.

The water depth is known relatively well and available on the bedmap2 from the British Antarctic Survey.

https://www.bas.ac.uk/project/bedmap-2/#data

The whole ice field is over a roughly 300-400m deep part.This is about half the depth of the surrounding areas. I attached an image of the bedrock overlayed with a coastline mask. The mask is maybe from 2012 when the bedmap2 was created and doesn't have the newest glacier front positions.

[Andreas T]

thank you very much, Tealight.
This is really helpful, I guess that means B22A would need to move north or north east if it becomes mobile, westwards its path is blocked. The small amount of movement since it arrived in its present position is probably due to some bottom melting. I guess there is ice thickness data for B22A somewhere? I hope it isn't too cheeky to ask, instead of searching for it myself, but if somebody has done the work already, or can tell me where to look?

[dingojoe]

In addition to the posting I have just made and in addition to my post from yesterday I have to report that a further new crack in the fast ice / ice mélange of Thwaites has developed yesterday.
See attached image.
Colour code: orange is the crack I have just mentioned, yellowish green is the crack I reported of yesterday and the new crack which connects both cracks is painted in pale magenta.
It seems that the whole thing is about to collapse.

Hi, I rarely post and am not sure how skilled I am at linking, but the tongue did fracture along your yellow and pink line back in 2016

https://go.nasa.gov/2S6IUoa

However, the fractured ice really didn't have a means of exiting the area due to the large block/iceberg/grounded ice north of the yellow line.  It does seem like that block has slowly ground it's way westward which may be making it easier for fractured ice on the east side of the tongue to flow out of the area. 

[b_lumenkraft]

Hey dingojoe,

you linked well. Good find!

[Tealight]

I guess there is ice thickness data for B22A somewhere? I hope it isn't too cheeky to ask, instead of searching for it myself, but if somebody has done the work already, or can tell me where to look?

The only two instruments that could measure thickness is the not yet in science phase ICESat-2 and CryosSat, but CryosSat doesn't have a public thickness product for icebergs.

The offical CryosSat data ditribution only has Arctic Sea Ice and Greenland /Antarctic Ice sheets elevations.
http://www.cpom.ucl.ac.uk/csopr/index.html

If you are lucky there a published scientific papers about iceberg thickness, if not then you have to request the raw data yourself, find the icebergs in the data and calculate the thickness yourself.

[Stephan]

Further retreat of the ice mélange west of Thwaites ice tongue. The distance between open water and the calving front of Thwaites glacier is reduced to 15-18 km. You can also recognize that iceberg B22A is slightly moving westward - this was not visible the last months. May melting from below have thinned it so that it lost connection to some of the pinning points?
See the link (comparison of Jan 22 with Feb 02, 2019) to EOSDIS worldview
https://worldview.earthdata.nasa.gov/?ca=false&cm=swipe&cv=94&p=antarctic&l=VIIRS_SNPP_CorrectedReflectance_TrueColor(hidden),MODIS_Aqua_CorrectedReflectance_TrueColor(hidden),MODIS_Terra_CorrectedReflectance_TrueColor,Coastlines&l1=VIIRS_SNPP_CorrectedReflectance_TrueColor(hidden),MODIS_Aqua_CorrectedReflectance_TrueColor(hidden),MODIS_Terra_CorrectedReflectance_TrueColor,Coastlines&t=2019-02-02-T00%3A00%3A00Z&z=3&t1=2019-01-22-T00%3A00%3A00Z&v=-1699945.223835678,-608838.4314290967,-1487465.223835678,-490822.43142909673

[Andreas T]

from an earlier post by ASLR

I found the attached radar soundings which give an idea of the (initial) thickness of B22A. The line RS is along the ice tongue from which B22A broke off a few years before.
source https://www.the-cryosphere.net/11/1283/2017/tc-11-1283-2017.pdf

How much bottom melting has occurred since then is hard to know but we know that it is the deeper water which is melting the glacier and that melting rates become smaller as ice shelves thin towards the seaward end.

Stephan, I think there is slight pivoting of B22A but no westward (i.e. down in the worlview image) movement. But the key point is that it is now clear that it is not held in place by sea ice, since that has now cracked across its width on the landward side.
For comparison the movement since march 2012 when it arrived in its present vicinity https://go.nasa.gov/2HOnreJ as seen on worldview

[Andreas T]

I have found some more information on thickness of Thwaites ice tongue, although it does not tell me more because it again is the tongue after separation of B22A (I am guessing this from the shape, no date is given).
It is very low resolution because it comes from a small picture in a slide show type PDF
but it shows that data is around, I am guessing that this comes from the 1km elevation model

[b_lumenkraft]

B22A moved a bit!

[Gray-Wolf]

https://www.theguardian.com/environment/2019/feb/07/cavity-two-thirds-the-size-of-manhattan-discovered-under-antarctic-glacier

Looks like the recent flip flip in Antarctic sea ice behaviour coincides with this uptick in melt and the potential for rapid collapse as the ice thins further?

[b_lumenkraft]

The ice tongue behind B22A is breaking up now.

[Bernard]

I've been following this thread for a while, but it's often very hard in this complex zone to figure out scale of things, and what is what, default explicit scale and orientation of pictures. Would it be difficult to have some reference map, with scale and orientation, on which background the different pictures posted here could be localized? Or are things so mobile there that the very notion of such a map is impossible?

[b_lumenkraft]

Sorry, Bernard,

you are right. Just a screenshot is confusing without context.

Picture was taken from >> https://www.polarview.aq/antarctic
Link to source (yellow box)>> http://bslmagb.nerc-bas.ac.uk/iwsviewer/?image=DataPolarview/111_S1jpeg2000_201902/S1A_EW_GRDM_1SSH_20190207T041138_0C53_S_1.8bit.jp2

[b_lumenkraft]

Bernard,

here you can find the position of iceberg B22A >> https://worldview.earthdata.nasa.gov/?p=antarctic&l=VIIRS_SNPP_CorrectedReflectance_TrueColor(hidden),MODIS_Aqua_CorrectedReflectance_TrueColor(hidden),MODIS_Terra_CorrectedReflectance_TrueColor,MODIS_Terra_Sea_Ice(hidden),MODIS_Aqua_Sea_Ice(hidden),MODIS_Aqua_Brightness_Temp_Band31_Night(hidden),MODIS_Aqua_Brightness_Temp_Band31_Day(hidden),MODIS_Terra_Brightness_Temp_Band31_Night(hidden),MODIS_Terra_Brightness_Temp_Band31_Day(hidden),VIIRS_SNPP_DayNightBand_ENCC(hidden),Coastlines&t=2011-08-30-T00%3A00%3A00Z&z=3&v=-2023193.48151667,-770746.8934159877,-1252121.48151667,-326842.8934159877&e=EONET_2736,2011-08-30

[Bernard]

Thanks a lot! Will try to make sense of all this.

Makes me wonder, BTW, how to make people aware that understanding what's going on behind those difficult-to-grasp images and data has a critical impact on their future?

[b_lumenkraft]

You are welcome. If you have any problems with the tools i linked, feel free to PM me Bernard.

[b_lumenkraft]

how to make people aware that understanding what's going on behind those difficult-to-grasp images and data has a critical impact on their future?

You don't. If people don't ask independently they will not listen most likely anyway. So spear your breath i say.

When people ask you, you have already won. In this case, encourage them to stay curious.

[Bernard]

how to make people aware that understanding what's going on behind those difficult-to-grasp images and data has a critical impact on their future?

You don't. If people don't ask independently they will not listen most likely anyway. So spear your breath i say.

When people ask you, you have already won. In this case, encourage them to stay curious.

Wisdom indeed. But now that Thwaites news are flowing in mainstream media, maybe time has come to be a bit more proactive.

[Stephan]

I carefully compared Thwaites Glacier cracks Dec 15, 2018 with Feb 09, 2019. A new one, around 15 km long, has formed within the last 8 weeks.
See attached figure.
I indicated some of the features so you have a better orientation. Thanks Bernard for letting us know that we should give some additional information, where exactly all these things are going on...

[Stephan]

New development at the fast ice east of Bear Peninsula and South of Iceberg B-25 B-22-A.
In-between the coloured lines (I used the same colours as in Reply #49 earlier in this thread) there are hundreds of cracks. So one might expect a further degradation and dis-integration of the fast ice.
But summer is fading rapidly and some features already show a thin ice cover, for example parts of the pale magenta coloured crack, or the blue circled area close to the open ocean.
So the late austral summer may prevent the fast ice from a complete collapse.
See attached picture.

[Bernard]

Thanks Stephan for the annotations on the images! Added value, much appreciated 

[oren]

Thanks Stephan for the annotations on the images! Added value, much appreciated 

+1

[Andreas T]

Sorry for being pedantic but to avoid confusion, the large iceberg is named B22A by the US National Ice Centre https://www.natice.noaa.gov/pub/icebergs/Iceberg_Tabular.pdf

[Stephan]

OK, I will keep that in mind. Sorry for not having it thoroughly checked.

[b_lumenkraft]

From 2017 but i think it belongs here.

CryoSat data have been processed in a new way and reveal that, in 2013, four interlinked lakes under Thwaites drained into the ocean. The image shows how the surface of the ice over one of the lakes sank by as much as six metres as a result of the drainage. The ice sank in a similar way over the other three lakes. Subglacial lakes have been found in many parts of Antarctica and are, indeed, commonly associated with fast-flowing glaciers. However, this is the first time that they have been found and observed draining into the Amundsen Sea. Also, this draining process is thought to happen only every 20–80 years.

Link >> https://www.esa.int/spaceinimages/Images/2017/02/One_of_four_lakes_under_the_glacier

[Often Distant]

One day. Strong wind. A significant breaking apart of a large floe.

[Tor Bejnar]

The break apart of two large floes/bergs (triangular one just left of center of image, and slightly-crooked-finger-shaped one just below center of image)

[Stephan]

In addition to my post #50 in this thread I state that most of the "outer ice field" has dis-integrated into many pieces of smaller sea ice.
I tried to cover the original area of this ice field in orange. It seems to have "grown" in the last days, but this extra sea ice derives from the massive clean up at Pine Island Bay and Thwaites (circled in light blue) I wrote about the last days in the "PIG has calved" thread.
Below (in magenta) you see the iceberg B-22-A which moves slightly westward, on some days more in a counterclockwise manner than in a direct flow. Unfortunately this Sentinel picture is partly cloudy - but there is no better view available. The latest picture before this is from Jan 30. It shows a more or less intact outer ice field with a lot of cracks, that indicated the future collapse which has now occurred.
In the NE part of the picture is Burke Island for a better orientation.
The picture covers an area of ~ 320x175 km, so it is huge!

[Andreas T]

This (marked by X) is where the Nathaniel B Palmer is today according to sailwx, S 74°54' W 107°18'. It probably be a while before we hear details of the research there, but the blog posts have a lot of information.
North marked for orientation.
https://thwaitesglacier.org/blog

[Stephan]

Thank you very much for the link to the "Snow on Ice" expedition.

[b_lumenkraft]

4 years of @ThwaitesGlacier ice tongue from @CopernicusEU #sentinel1 satellite imagery showing how this chaotic ice tongue has become a collection of icebergs glued together by sea ice.

Link >> https://twitter.com/StefLhermitte/status/1101870380623511552

[Stephan]

Thank you very much for this linked video.
It is intersting to see that the ice in the upper left corner is so slowly moving compared to the Thwaites iteself.

[Andreas T]

this observation is confirmed by the ice velocities for the thwaites ice tongue posted further upthread
The reason lies in the shallower depth below the ice surface both upstream and downstream of the grounding line shown in the bathymetry posted by tealight

...

...

The water depth is known relatively well and available on the bedmap2 from the British Antarctic Survey.

https://www.bas.ac.uk/project/bedmap-2/#data

The whole ice field is over a roughly 300-400m deep part.This is about half the depth of the surrounding areas. I attached an image of the bedrock overlayed with a coastline mask. The mask is maybe from 2012 when the bedmap2 was created and doesn't have the newest glacier front positions.

[Stephan]

As for the Pine Island Bay (see my posting in that thread) also the Thwaites area was completely clear today which offered an analysis of the EOSDIS picture.
The limit between ice shelf and ice mélange is roughly indicated by the orange line. The distance to the calving front is now < 10 km. Its continuation to NNW (in blue) shows the same "crack" I reported about two weeks ago. It also seems to be the boundary between ice shelf/fast ice and the more moblie ice mélange NE of it.
Iceberg B-22-A has moved very slowly west-/northwestward (compared to its position on Feb. 04). And it has lost small pieces off its western shore (not on the picture). Many of the icebergs lost there stay in that area and seem to be grounded.
Another bigger grounded iceberg N of the tip of Thwaites Ice Tongue is circled in pale magenta. Imo these grounded bergs can cause floating sea ice to be stopped and help the formation of new sea ice/fast ice in the next freezing season around them.
The "outer ice field" has completely disintegrated, but the melting season is probably over, so that a melt-out of the sea ice between the icebergs will not take place anymore.
The green line differs between very slowly (or not moving at all) moving ice of the Thwaites Ice Tongue E of it and the area of faster movement of the many icebergs that Thwaites glacier endlessly produces (W of this line).
Comparing Thwaites and Pope the latter is much slower moving (a month difference almost does not show a change of position of individual features).

See attached picture

[Andreas T]

This is from the Thor cruise blog https://thwaitesglacieroffshoreresearch.org/news/2019/3/12/thwaites-glacier-there-and-gone and shows some detailed bathymetry off the Twaites ice tongue and some pretty impressive sea floor coring sites

[Stephan]

Thank you for that useful information!

[Andreas T]

from the same report come this photo which shows some pretty thick icefloes. That freeboard looks like 2m in places. I guess it is some of the ice which has been sitting between the icebergs of the Thwaites ice cube maker for years. a large part of that freeboard will of course be snow (multiyear firn I guess) of lower density so not a guide to below waterline thickness

[solartim27]

I wonder if the expeditions got a close up of this good sized calving and breakup.  Shots from Polarview Mar 3 to 18th, though the breakup happened a bit earlier.

[Sleepy]

solartim, I haven't seen any pictures but according to Anna Wåhlin at the University of Gothenburg who took part with HUGIN on N.B. Palmer, they had a bit of drama just after retrieving the AUV when the ice suddenly pushed out a couple of kilometres in just a couple of hours. From an interview on Swedish radio last thursday.

She also stated that they wasn't sure about what this meant and that it could be a natural cycle, they only had data twenty years back.

I fail to understand that last part of her comment.

[Andreas T]

This is from the Thor cruise blog https://thwaitesglacieroffshoreresearch.org/news/2019/3/12/thwaites-glacier-there-and-gone and shows some detailed bathymetry off the Twaites ice tongue and some pretty impressive sea floor coring sites

The ice position in this  looks like the last clear world view image on Feb 17. The ice edge probably stayed in that position enabling the NBP to reach all the places shown in the track. On the Mar 2. icebergs started to come off the mix of begs and sea ice and 3rd  and 4th this loosening spread further towards the actual calving front of the glacier. The icebergs set loose by this spread into the water crisscrossed by the NBP in the trackmap posted above (without dates which is a bit disappointing) I dont think any ship would want to be among this stuff when they would have to dodge multiple icebergs and thick sea ice shifting rapidly.
A clear out of the bergs has happened fairly recently so her comment was probably a general statement for an audience with isn't expected to know much about this topic.

[Sleepy]

She also said that they were very careful about where the ice edge was but was still surprised by the surge.

Edit; the recovery of HUGIN took place on March 1st.

Edit2: ASLR just posted an article from the RollingStone that included an image with N.B. Palmers locations around that event:
https://forum.arctic-sea-ice.net/index.php/topic,2205.msg192455.html#msg192455
The "drama" was obviously not that dramatic aboard.
I like Alley's analogy: “if Thwaites were a car, you could say that it has lost part of its bumper. And, while that’s not hugely important, it is part of a pattern that is pointing toward larger changes to come.”

[Stephan]

One consequence of this breakout is visible on the latest EOSDIS picture.
After nearly half a month of mostly complete cloud coverage, a glimpse between the clouds reveals that open water has almost reached the calving front of the Thwaites glacier (it is approx. parallel to the grey line ("2010 grounding line")) and marked in pale magenta.
I wonder whether the next days will allow a deeper look onto what happened there and whether the calving front is really affected.
See attached picture.