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Stephan

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The Ross Ice Shelf Thread
« on: February 17, 2019, 09:04:01 PM »
I just wondered that the largest Antarctic Ice Shelf - the Ross Ice Shelf - does not have its own thread. You can see it constantly growing, almost no cracks at its northern edge, apart from two or three seemingly elder ones. And so it didn't show larger calving events for many years or even decades.
Yesterday the northeasternmost corner of it showed a minor calving, an area of around 10 x 2 km broke off, in an area already full of cracks, see attached picture.
« Last Edit: February 17, 2019, 09:15:04 PM by Stephan »
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oren

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Re: The Ross Ice Shelf Thread
« Reply #1 on: February 19, 2019, 12:51:07 PM »
ASLR had a combined RIS/FRIS thread.

Gray-Wolf

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Re: The Ross Ice Shelf Thread
« Reply #2 on: February 19, 2019, 01:31:43 PM »
Sorry Oren when I search on that all I get is your post above coming back?

My interest in the cryosphere was piqued with a huge crevice on Ross ( from roosevelt to mid shelf) to the point that I spoke with a few guys down at McMurdo who were ,that season, putting sensors along the length of it to monitor movement there?

https://worldview.earthdata.nasa.gov/?p=antarctic&l=VIIRS_SNPP_CorrectedReflectance_TrueColor(hidden),MODIS_Aqua_CorrectedReflectance_TrueColor(hidden),MODIS_Terra_CorrectedReflectance_TrueColor,Coastlines&t=2019-02-17-T00%3A00%3A00Z&z=3&v=-440576,-1281664,-90880,-1121664

I've watched it for over a decade now so the last couple of years of lower sea ice might be allowing more mechanical strains into the shelf hastening its eventual calve?

Through the noughties we saw warm basal water travelling down from the peninsula after it snuck under the circumpolar current (through valleys on the ocean bed) . this water arrived at Ross up to 5 years ago so impacting the grounding line of the shelf?

How close to the crack is this undercut now?
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Tor Bejnar

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Re: The Ross Ice Shelf Thread
« Reply #3 on: February 19, 2019, 04:21:09 PM »
I believe Oren was referring to the thread called
Hazard Analysis for the FRIS/RIS in the 2012 to 2060 Timeframe
I think using the 'search' tool only looks in the folder you are in. When searching for "RIS" in "Antarctica", it finds "FRIS/RIS", if in "The Ross Ice Shelf Thread", you only find Oren's post.

ASLR had a combined RIS/FRIS thread.
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gerontocrat

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Re: The Ross Ice Shelf Thread
« Reply #4 on: February 19, 2019, 04:49:15 PM »
I believe Oren was referring to the thread called
Hazard Analysis for the FRIS/RIS in the 2012 to 2060 Timeframe
I think using the 'search' tool only looks in the folder you are in. When searching for "RIS" in "Antarctica", it finds "FRIS/RIS", if in "The Ross Ice Shelf Thread", you only find Oren's post.

ASLR had a combined RIS/FRIS thread.
You can broaden or narrow your search
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FredBear

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Re: The Ross Ice Shelf Thread
« Reply #5 on: February 19, 2019, 04:59:40 PM »
The big Ross calving was Mar.14-17 2000, the Ross "coastline" in EODIS appears to be set in 2011 and the shelf has been growing outwards since then.
Most of the Ross ice shelf is not grounded, so can be melted by any available 'warm' water.
I attach an image from:-

Ice Sheet Stability and Sea-Level Rise

    John B. Anderson*

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Science  30 Mar 2007:
Vol. 315, Issue 5820, pp. 1803-1804
DOI: 10.1126/science.1140766
 "PHOTO CREDIT: BASEMAP FROM GOOGLE EARTH. ORIGINAL IMAGE FROM NASA"
« Last Edit: February 19, 2019, 05:06:37 PM by FredBear »

maga

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Re: The Ross Ice Shelf Thread
« Reply #6 on: February 19, 2019, 10:50:53 PM »
Just a comment to Stephans post above: My impression is that it is a piece of old fast ice that broke off, not a real ice berg. But note as well that the major crack directly below the date has expanded several kilometers to the west this year. However, I still expect the next major calving to occur along the even bigger crack behind within the next few years.

Stephan

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Re: The Ross Ice Shelf Thread
« Reply #7 on: February 21, 2019, 08:38:59 PM »
Looking at the shadow this iceberg produces om the water you may be right. It is probably much thinner than a "real ice berg" deriving from the shelf itself and therefore it is likely just a piece of older fast ice closely joint to the ice shelf.
This takes me to the question about the (average) thickness of the Ross Ice Shelf at its sea front, and whether this thickness (which is obviously stable) has been taken into account when it comes to possible MICI instabilities that demand a keel board of > 90-100 m discussed in other topics of this forum?
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Andreas T

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Re: The Ross Ice Shelf Thread
« Reply #8 on: February 21, 2019, 10:49:57 PM »
Marine Ice Cliff Instability is a process which could affect grounded ice as far as I know. This would mean that a floating ice shelf (like Ross) may be affected by Marine Ice Shelf Instability but not MICI if its edge retreats to thicker parts of the shelf, which at the present is not happening.
I also think it is not the keel (below water) but the height above water (freeboard) which is critical. Iceshelves with a ice thickness of more than 300m are calving as tabular icebergs in Antarctica and in Greenland.

Stephan

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Re: The Ross Ice Shelf Thread
« Reply #9 on: February 22, 2019, 04:37:12 PM »
Thank you for that information.
I meant freeboard and not keel, sorry for having it mixed up.
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bligh8

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Re: The Ross Ice Shelf Thread
« Reply #10 on: June 21, 2019, 02:05:43 PM »
Tidal and Thermal Stresses Drive Seismicity Along a Major Ross Ice Shelf Rift

First published: 23 May 2019 https://doi.org/10.1029/2019GL082842

Abstract
Understanding deformation in ice shelves is necessary to evaluate the response of ice shelves to thinning. We study microseismicity associated with ice shelf deformation using nine broadband seismographs deployed near a rift on the Ross Ice Shelf. From December 2014 to November 2016, we detect 5,948 icequakes generated by rift deformation. Locations were determined for 2,515 events using a least squares grid‐search and double‐difference algorithms. Ocean swell, infragravity waves, and a significant tsunami arrival do not affect seismicity. Instead, seismicity correlates with tidal phase on diurnal time scales and inversely correlates with air temperature on multiday and seasonal time scales. Spatial variability in tidal elevation tilts the ice shelf, and seismicity is concentrated while the shelf slopes downward toward the ice front. During especially cold periods, thermal stress and embrittlement enhance fracture along the rift. We propose that thermal stress and tidally driven gravitational stress produce rift seismicity with peak activity in the winter.

Plain Language Summary
In Antarctica, large bodies of floating ice called ice shelves help prevent ice on land from sliding into the ocean. To predict how Antarctica might respond to climate change, we need to understand how ice shelves interact with the environment, including the atmosphere and the ocean. The largest ice shelf, the Ross Ice Shelf, is over 500,000 km2 in area, making it the largest body of floating ice in the world. In this study, we deployed nine seismographs, the same instruments used to study earthquakes, to monitor vibrations and cracking within the Ross Ice Shelf over a 2‐year period. During that time, the instruments detected nearly 6,000 fracture events along a 120‐km‐long crack in the ice shelf. We compared the timing of the cracking to air temperature data, ocean wave activity, and tides to see whether these factors influenced the crack's behavior. We found that fracture occurs most frequently just after high tide during winter and when the air is very cold. We also found that fracture at the rift is not triggered by ocean waves. This work demonstrates that Antarctic ice shelves are very sensitive to the environment and highlights the need to continue studying them.

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL082842

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pietkuip

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Re: The Ross Ice Shelf Thread
« Reply #11 on: June 22, 2019, 03:57:40 PM »
The Ross ice shelf melting ten times faster than previously thought, according to Nina Kirchner in an item on Swedish television: https://www.svt.se/nyheter/vetenskap/varldens-storsta-flytande-glaciar-smalter-10-ganger-snabbare-an-man-trott

From the abstract of the linked article (available in fulltext):
"We show that basal melt rates beneath a thin and structurally important part of the shelf are an order of magnitude higher than the shelf-wide average. This melting is strongly influenced by a seasonal inflow of solar-heated surface water from the adjacent Ross Sea Polynya that downwells into the ice shelf cavity, nearly tripling basal melt rates during summer. Melting driven by this frequently overlooked process is expected to increase with predicted surface warming."


solartim27

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Re: The Ross Ice Shelf Thread
« Reply #12 on: July 23, 2019, 04:32:20 PM »
Interesting article showing that the Ross melting is from local effects, mostly solar.  Good description of the equipment used.

Quote
The Rosetta scientists took a new approach to gather data from the Ross Sea. They deployed six profiling floats called Air-Launched Autonomous Micro Observer, or ALAMO, floats. They fastened parachutes to the floats and launched them out of a New York Air National Guard airplane from 2,500 feet above the icy waters below. The instruments were programmed to avoid sea ice that could damage their external sensors and antennae. In addition, the team took a novel approach by "parking" the floats on the sea floor between profiling so as to limit their drifting on ocean currents.
https://phys.org/news/2019-07-robots-roaming-antarctic-reveal-ross.amp

Quote
More information: David F. Porter et al, Evolution of the Seasonal Surface Mixed Layer of the Ross Sea, Antarctica, Observed With Autonomous Profiling Floats, Journal of Geophysical Research: Oceans (2019). DOI: 10.1029/2018JC014683
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bligh8

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Re: The Ross Ice Shelf Thread
« Reply #13 on: August 02, 2019, 06:05:53 PM »
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL084123

Ross Ice Shelf Icequakes Associated with Ocean Gravity Wave Activity


Z. Chen P.D. Bromirski P. Gerstoft R.A. Stephen W.S. Lee S. Yun S.D. Olinger R.C. Aster
D.A. Wiens A.A. Nyblade
First published: 01 August 2019
https://doi.org/10.1029/2019GL084123


Abstract

Gravity waves impacting ice shelves illicit a suite of responses that can affect ice shelf integrity. Broadband seismometers deployed on the Ross Ice Shelf (RIS), complemented by a near‐icefront seafloor hydrophone, establish the association of strong icequake activity with ocean gravity wave amplitudes (AG) below 0.04 Hz. The RIS‐front seismic vertical displacement amplitudes (ASV) are well‐correlated with AG, allowing estimating the frequency‐dependent transfer function from gravity wave amplitude to icefront vertical displacement amplitude (TGSV (f)). TGSV (f) is 0.6‐0.7 at 0.001‐0.01 Hz, but decreases rapidly at higher frequencies. Seismicity of strong icequakes exhibits spatial and seasonal associations with different gravity wave frequency bands, with the strongest icequakes observed at the icefront primarily during the austral summer when sea ice is minimal and swell impacts are strongest.

see also..pdf.. open access...Annals of Glaciology 53(60) 2012  doi: 10.3189/2012AoG60A058
Response of the Ross Ice Shelf, Antarctica, to oceangravity-wave forcing
This is a much more complete paper with lotsof purty images and graphs.

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maga

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Re: The Ross Ice Shelf Thread
« Reply #14 on: September 25, 2019, 09:21:51 PM »
New and very important crack on the east side - expect major calving relatively soon!