Föhn winds: New insight into what weakens Antarctic ice shelves

New research describes for the first time the role that warm, dry winds (katabatic winds) play in influencing the behaviour of Antarctic ice shelves. Presenting this week at a European conference scientists from British Antarctic Survey (BAS) explain how spring and summer winds, known as föhn winds, are prevalent on the Larsen C Ice Shelf, West Antarctica and creating melt pools. The Larsen C Ice Shelf is of particular interest to scientists because it of the collapse of Larsen A in 1995 and Larsen B in 2002.

The researchers observed the föhn winds, which blow around 65% of the spring and summer period, extend further south and are more frequent than previously thought, and are likely to be a contributing factor that weakens ice shelves before a collapse. The results are presented this week (Tuesday 25 April) at the European Geosciences Union General Assembly (EGU) in Vienna.

In 1995 and 2002, the Larsen A and B ice shelves collapsed, depositing an area the size of Shropshire into the Weddell Sea. Whilst ice shelf collapse doesn’t directly contribute to sea level rise, the glaciers which fed into the ice shelves accelerated, leading to the loss of land ice, and subsequently indirect sea level rise. The processes responsible for the collapse of these ice shelves were largely debated, and it is now thought that crevasses on the ice shelf were widened and deepened by water draining into the cracks. Föhn winds are thought to be responsible for melting the ice shelf surface and supplying the water.

The findings describe when and where the warm, dry winds occur over the Larsen C Ice Shelf, the largest remaining ice shelf on the Antarctic Peninsula (roughly the size of Wales). Föhn winds were measured from near-surface weather stations and regional climate model data over a five year period and observed all year-round, but were most frequent in spring.

PhD student and lead scientist on this project from British Antarctic Survey (BAS) and Leeds University, Jenny Turton says:

“What’s new and surprising from this study is that föhn winds occur around 65% of the time during the spring and summer. And we didn’t know how much they influence the creation of melt pools and therefore are likely to weaken the ice shelf. Whilst a high number occur in spring, the combined warming over a number of days leads to much more surface melting than was experienced during days without föhn winds. This is important, as melting during summer and re-freezing during winter weakens the ice surface, and makes it more at risk of melting again the following season.

“We know the ice shelf often melts a little during summer, however we have found that when föhn events occur as early as September (three months earlier than the start of the summer melt season), the ice shelf surface is melting. Now that we know how prevalent and spatially extensive these winds are, we can look further into the effect they are having on the ice shelf.”

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62 thoughts on “Föhn winds: New insight into what weakens Antarctic ice shelves

    • It’s a lot more complicated than that.
      It’s warm winds that are capable of melting ice even before the traditional melt season begins.
      It’s also water flowing into the ice and weakening it so that it can break more easily at a time in the future.

      • ice is like concrete; it has no tensile strength, and hence no flexural strength either, so if you try to bend it, it breaks, just like concrete does.

        There’s this phenomenon call the tides, caused by the earth rotating under the influence of the moon’s and the sun’s gravity forces. This results in a noticeable bulge in the water on both sides of the planet.

        down in Antarctica, you have the gap between the Antarctic Peninsula, and Terra del Fuego, and the Atlantic, and Pacific oceans go sloshing through that gap back and forth twice a day.

        So that water rises underneath the ice shelves, and since water is nearly incompressible, it can easily buckle the ice sheet, and break it.

        The mystery is why the ice shelves are there at all, with the daily tides trying to break them up all the time.

        Breaking the ice shelves is the null hypothesis in this case. They are supposed to break.

        G

    • No surprise that there are some people angry enough to shoot at John’s office.
      This is ballsy, I like it.

    • It’s not floating ice, it’s attached to the land, until the tidal flow breaks it like it is supposed to.

      G

      • George – “It’s not floating ice…”

        Oh, I disagree. Ice is buoyant (it floats on it’s own) and from where the bottom of the ice separates from the sea floor to where it is ending at sea (sometimes hundreds of miles), it is floating. If it were not buoyant, it would continue & follow the sea floor.

        J

      • “It’s not floating ice, it’s attached to the land”

        So in Your view a ship in harbor isn’t floating?

      • We are talking about ice ….. shelves ….. They are attached to the land like cantilevers.

        The shelves in my kitchen are NOT floating on air, but if I put a fan underneath them and blow upwards, they will bend upwards, but they don’t break because wood has flexural strength.

        And the ….. SEA ice ….. that IS floating, will break up when the sea gets rough enough so that the wave length of the principal swells is a lot shorter than the size of the ice floe. Even steel ships will break in half in heavy seas with a wavelength much shorter than the ship. That actually happened in 1995 during the Americas Cup race between Australia One, and NZ Black Magic.

        The seas off San Diego got very rough and Australia One broke in half and sank in the Pacific. Luckily they were able to get all of the crew off, before she went under. Black Magic abandoned the race, which they had well in hand, to assist the Aussie Crew.

        And we are talking about man made structures that have much greater flexural strength than either concrete or ice shelves.

        The shelves are NOT grounded (at the ends) and they are NOT floating; and they do buckle and break from tidal water underneath.

        A little homework is required before firing off your tweaks.

        G

      • Well tty, I never even mentioned a ship in the harbor.

        But since you chose to bring it up, there are plenty of famous photographs and paintings of ships in harbors; even tide up to the dock, that are NOT floating; they are sitting on the mud after the tide went out. And when the tide comes back in they will be floating again.

        And when some of them are winched out of the water and put on trailers, they will again be not floating, but will be resting on a trailer on dry land.

        Do you have any more idiotic scenarios that are not cantilevered ice shelves that flowed off the land out into the sea, and just sit there above the water, waiting for the tidal change to break them off ??

        G

  1. What I find surprising is that, after having researchers in Antarctica for so long, they only just realized these fohn winds blow 65% of the time. Or did they just recently set up a station on the Larsen shelf?

  2. How large is Shropshire anyway? Should I know? Over here we say “… the size of Rhode Island”.

    Are you sure the spelling isn’t “föehn”? I was told by my college meteorology professor that the word is from Switzerland. (pronounced “feen” in English.)

    FWIW an allied term is “föehn gap” (from AMS Glossary):

    A break in an extensive cloud deck or cloud shield, usually a band parallel to and downwind of the mountain ridge line. Especially visible in satellite pictures, this cloud-free zone results from the strong sinking motion on the lee side of a mountain barrier during a föehn or chinook.

    Where I live in Reno, NV, on the lee base of the Sierra Nevada, a föehn gap is often positioned right over the city during typical lenticular (“mountain wave”) cloud conditions.

    • Ok, spelling cleared up: “foehn” is anglicized spelling of “föhn” without the special character.

      • Problem is it’s pronounced “feen”. The “oe” is a long ‘e’ as in “Phoenix” and “Oedipus”. (The latter is commonly mispronounced. Sad!)

      • Hold on: The German pronunciation is close to “fern”. I was taught “feen” in college. In English the “oe” is pronounced “ee” as in Phoenix. But the word isn’t English, so …

      • Set your lips to say “oo” (cool) and your tongue to say “er” (perk) in standard British English pronunciation, and you will get it almost right.

        If you don’t use standard British pronunciation, you never get anything right anyway, so don’t bother.

      • And words become English when we adopt them and use them; and we will pronounce them as we see fit.

        Cholmondeley DOES NOT sound like it looks, and we don’t care how they say it wherever it came from originally.

        G

    • I was about to write that I’m British and I don’t know how big Shropshire is either.
      It’s smaller than Wales though.

    • Rhode Island — The Geo-measurement State — is 1,214 square miles in area of which 1,045 is land area. Unoffiicial motto: You Count With Us.

      • Shropshire is a little bigger than the State of Rhode Island, at 1346 sq mi.

        It’s a lot smaller than Wales (8023 sq mi), which itself covers less area than the State of New Jersey.

      • Geez!

        Quick, what is it in square acre seconds per square fathom months? Or beer bottles, the standard SI unit?

    • Umlaut over “o” is a vowel sound that doesn’t exist in English. It’s made by forming your tongue for something like “ueh” but with lips pursed.

    • The size of Rhode Island is 1/20th the size of the Arctic National Wildlife refuge, that is tucked away in a small corner of Alaska.
      Well Delaware can only fit in there 12 times.

      G

      • That’s fine, but it is still [fö:n].

        Even if you think you say ü right, it probably isn’t true. Same for me in English. Phonemes don’t map well from language to language. English speakers often give speech hints with ‘said like’ a word, but this does not really work for any foreign language.

        Saying the word based on what it looks like is not that bad. As long as you remember Bei3jing1 does not really have a [b] sound. And it does have tones.

      • The spelling in the title is correct. If one doesnt have the ö available you’d use oe instead.

        As for pronounciation… Switzerland doesn’t have “a” language, only dialects and in so far you will most probably find some region whose pronounciation fits whatever pronounciation you choose.

  3. …I do not understand why people, especially in the North, want the “Frozen North” to stay FROZEN !

  4. Lucky for the BAS they use the Halley VI station-

    so what happened to Halle 1-4

    “There have been five previous bases at Halley. Various construction methods have been tried, from unprotected wooden huts to steel tunnels. The first four were all buried by snow accumulation and crushed until they were uninhabitable”

    Halley VI
    “It is a structure which, like Halley V, is jacked up on hydraulic legs to keep it above the accumulation of snow”

    Good to know the snow is still accumulating.

  5. “The Larsen C Ice Shelf is of particular interest to scientists because it of the collapse of Larsen A in 1995 and Larsen B in 2002.”

    Remove “it” after “because”?

  6. The article correctly points out that ice floating on the seawater does not increase sea level if it melts, but then says “the glaciers which fed into the ice shelves sped up”. Glaciers speed up or slow down mostly based on accululation of snow/ice in the head zone, the zone of accumulation of snowfall. Removal of some ice at the distal portion produces a very small change in the overall topographic gradient and would be difficult to view as a factor in overall glacier behavior. If the glacier sped up it was probably due to accumulating load at the head zone. There are a lot of pictures from Antartica showing cultural relics buried in deep snow/ice, suggesting that the glaciers are undergoing net increase. What happens around the perimeter with sea-water interaction is another matter, but remember, it is only snow/ice accumulating or melting on a continental mass that affects sea level.

    • Yup, ron.

      What drives the movement of glacial ice is (just as with liquid water) gravity. Ice accumulation in the central highlands of the continent will always tend to move downhill in a plastic flow. Antarctica is the world’s highest continent on average with respect to its rock surficial elevations, including two main mountain ranges. As everyone knows, high ground (mountains) accumulate much more snowfall mass than low ground.

      So all the panic over various ice-shelves cracking and eventually falling into the sea was just the usual hysterical global warmism at work … despite major ice-falls, sea level did NOT make an significant or sudden increases. That’s because what is forced out to sea comes from up high on the land in Antarctica.

    • If the sea ice at the end of the glacier was grounded on something, then there would be a small amount of back pressure that could slow down the rate at which ice was flowing out of the glacier. However,
      1) The difference would be small
      2) The glacier in question isn’t grounded anyway

    • Actually the sea level goes down when the floating ice melts, because the heat to melt the floating ice comes out of the sea. Almost all of the surface area of the floating ice is under water, along with 9/10 or 1o/11 of the volume. So you have to get 80 calories per gram out of the sea water, to melt one gram of ice, and that cools the sea water, which then shrinks. Sea water is salty enough that it’s density increases all the way down to the freezing Temperature.

      So the water level goes down when the floating ice melts.

      G

  7. “Föhn winds were measured from near-surface weather stations and regional climate model data.”

    They measured the winds model data? It’s probably a typo but indicative of the over emphasis on models in modern climate studies. One can’t “measure” a physical phenomenon with a model.

    • Did I read that they have five years of records? If so, how can they develop a model from such a short time frame? Of course they could have developed the model first – without data. They are, however, dealing with real data on a natural phenomenon – kudos.

  8. http://soral.as.arizona.edu/heat/

    The origin of the katabatic winds is on the high plateau “where the stratosphere reaches all the way to the ground“. Increased GHGs cool the stratosphere and therefore also ought to increase the strength of katabatic winds… Bye bye ice shelf?

    • First, BI2HS, increased GHG’s would have to warm the troposphere.

      The data says: that has not happened.

      End of story.

      • Hi Janice-

        To be clear, both UAH and RSS show the lower troposphere warming and the lower stratosphere cooling.

        Anyway, it’s an interesting twist: Antarctic Ice Shelf Threatened by GHG Cooling!

      • “warm the troposphere.”

        Wrong. tropical troposphere..

        If you look at the more accurate data, you will see a hotspot.

        look at old UAH stuff, you’ll never find it until they fix their errrors

    • Outer space reaches all the way to the ground also; so sometimes outer space, comes all the way, and makes a big hole in the ground (or oceans)>

      G

  9. Looks like a melt-induced BIAs:
    Blue (bare) ice area (BIA) is an ice-exposed area where ablation exposes the underlying glacier ice. Blue ice areas have a light blue appearance, in sharp contrast to the vast white snow plains in Antarctica.

    Winther et al (2001) classified the BIAs in Antarctica into two different types: melt-induced and wind-induced,depending on the dominant climate process. They pointed out that melt-induced BIAs are located on slopes near coastal areas where surface and near surface melt occurs,and wind-induced BIAs are distributed near mountains or on outlet glaciers where snow is eroded by constant winds year-round.

    http://www.academia.edu/3971650/Blue_ice_areas_and_their_topographical_properties_in_the_Lambert_glacier_Amery_Iceshelf_system_using_Landsat_ETM_ICESat_laser_altimetry_and_ASTER_GDEM_data

  10. Likely a melt-induced BIA:

    Blue (bare) ice area(BIA) is an ice-exposed area where ablation exposes the underlying glacier ice. Winther
    et al (2001) classified the BIAs in Antarctica into two different types: melt-induced and wind-induced,depending on the dominant climate process. They pointed out that melt-induced BIAs are located on slopes near coastal areas where surface and near surface melt occurs,and wind-induced BIAs are distributed near mountains or on outlet glaciers where snow is eroded by constant winds year-round.

    http://www.academia.edu/3971650/Blue_ice_areas_and_their_topographical_properties_in_the_Lambert_glacier_Amery_Iceshelf_system_using_Landsat_ETM_ICESat_laser_altimetry_and_ASTER_GDEM_data

  11. I’d like the empirical evidence that water added and refrozen to a shelf is less strong than a packed ice shelf that has broken before. They simple state is as fact but a reference is in order.

    It is kinda funny:
    a warmer world was suppose to reduce Antarctic Ice (didn’t
    a warmer world was suppose to reduce Penguins. (didn’t)
    a warmer world was suppose to wipe out the Antartctic Pennisula (didn’t)
    a warmer world was suppose lauch all these mega-size icebergs (didn’t)
    a warmer Antartcic pennisula was suppose to warm the entire Antarctic (Stieg trick: didn’t work)

    Now winds that have been know of centuries are suppose to be weaking the ice shelf.
    The ice these winds help melt will refreeze and this refrozen ice is sufficiently different that
    “it makes is more at risk of remelting.”

    What? once thawed the next time around more prone to melting?
    Hey, I got some 30C (or 60X) homeopathic alcohol to get you drunk on too.

  12. Seeing recent a NASA space photo sequence of Antarctic sea ice loss anyone can see that see that the sea ice is first lost close to the Antarctic coast, well before the outer edge. Finally some scientists were allowed to research and publish even – Antropogenic CO2 be damned for a minute.

    • Yes. These ice-free areas along the coast caused by katabatic winds are known as “polynyas” (a russian term). They have a large impact on Southern ocean circulation and are well-known to anybody interested in Antarctic climate (except climate scientists apparently).

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