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Cryospheric Sciences

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Image of the Week — Allez Halley!

Image of the Week — Allez Halley!

On the Brunt Ice Shelf, Antarctica, a never-observed-before migration has just begun. As the pale summer sun allows the slow ballet of the supply vessels to restart, men and machines alike must make the most of the short clement season. It is time. At last, the Halley VI research station is on the move!


Halley, sixth of its name

Since 1956, the British Antarctic Survey (BAS) has maintained a research station on the south eastern coast of the Weddell Sea. Named after the 17th century British astronomer Edmond Halley (also the namesake of Halley’s comet), this atmospheric research station is, amongst other things, famous for the measurements that led to the discovery of the ozone hole (Farman et al., 1985).

Due to the inhospitable nature of Antarctica, there have been six successive Halley research stations:

  • Halley I to IV had to be abandoned and replaced when they got buried too deeply beneath the snow that accumulated over their lifetimes (up to ten years per station).
  • Halley V was built on steel platforms that were raised periodically, so the station did not end up buried under snow. However, Halley V was flowing towards the ocean along with the ice shelf when a crack in the ice formed. To avoid finishing up as an iceberg, the station was demolished in 2012.
  • Halley VI, active since 2012, can be raised above the snow and also features skis, so that it can be towed to a safer location if the ice shelf again threatens to crack. However, no one expected that this would have to be put in practice less than 5 years after the station’s opening…

The relocation project, featuring the new October crack. Inset, timeline of the awakening of Chasm 1. The ice shelf flows approximately from right to left. [Credit: British Antarctic Survey].

The awakening of the cracks

The project of moving Halley VI was announced a year ago. A very deep crack in the ice (“Chasm 1” in the map above) upstream of the station and dormant for 35 years, started growing again barely a year after the opening of Halley VI. The risk of losing the station if this part of the ice shelf broke off as an iceberg became obvious, and it was decided to move the station upstream – beyond the crack.

Additionally, there is another problem, or rather another crack, which appeared last October. This one is located north of the station and runs across a route used to resupply Halley VI. This means that of the two locations where a supply ship would normally dock, one is no longer connected to the research station and hence rather useless. Not only is the station now encircled by deep cracks, now it also has only one resupply route remaining; to bring equipment, personnel and food and fuel supplies to the station – all of which are needed to successfully pull off the station relocation.

Bringing Halley VI to its new location before the end of the short Antarctic summer season will be a challenge. We shall certainly keep you up-to-date with Halley news as well as with news about the rapid changes of the Brunt Ice Shelf (because we’re the Cryosphere blog after all!). In the meantime, you can feel like a polar explorer and enjoy this (virtual) visit of Halley VI.

References and further reading

Edited by Clara Burgard, Sophie Berger and Emma Smith

Image of the Week – What an ice hole!

Image of the Week – What an ice hole!

Over the summer, I got excited… the Weddell Polynya was seemingly re-opening! ”The what?” asked my new colleagues. So today, after brief mentions in past posts, it is time to explain what a polynya is.


Put it simply, a polynya, from the Russian word for “ice hole”, is a hole in the sea-ice cover. That means that in the middle of winter, the sea ice locally and naturally opens and reveals the ocean.

There are two types of polynyas

  • coastal polynyas, also known as latent heat polynyas, open because strong winds push the sea ice away from the coast.
    The ocean being way warmer than the winter-polar night atmosphere, there is a strong heat loss to the atmosphere. New sea ice also forms,  rejecting brine (salt) and forming a very cold and salty surface water layer, which is so dense that it sinks to the bottom of the ocean. This type of polynya can close back when the wind stops.
  • open ocean polynyas, sometimes called sensible heat polynyas, open because the sea ice is locally melted by the ocean. In normal conditions, a cold and fresh layer of water sits above a comparatively warm and salty layer. But mixing can occur which would bring this warm water up, directly in contact with the sea ice, which then melts. Similar to the coastal one, once the polynya has open heat loss and sea ice processes form dense water that will sink. But in this case, the sinking sustains the polynya: it further destabilises the water column, so more warm water has to be mixed up, which prevents sea ice from reforming…
What a polynya looks like, from MODIS satellite: (https://modis.gsfc.nasa.gov/)

What a polynya looks like, from MODIS satellite: (https://modis.gsfc.nasa.gov/) [Credit: David Fuglestad for Wikimedia Commons]

Some polynyas worth mentioning

  • the North Water Polynya, between Greenland and Canada in Baffin Bay, is the largest in the Arctic with 85 000 km2 (Dunbar 1969) and was officially discovered as early as 1616 by William Baffin. In fact, Inuit communities have lived in its vicinity for thousands of years (e.g. Riewe 1991), since this hole in the ice is extremely rich in marine life (e.g. Stirling, 1980).
  • Hell Gate Polynya, in the Canadian archipelago which owes its name to a dramatic event…  but this is a story for later as today we would like to leave you,  reader, with a positive impression about polynyas!
  • the Weddell Polynya, in the Weddell Sea, was discovered as we started monitoring sea ice by satellites in the 1970s. It was a huge open ocean polynya, reaching 200-300 000 km2 and lasting three winters (Carsey 1980), and it is so famous because it has not re-opened since. Although this year, the signs are here… it may happen again! It is also my personal favourite because I spent my PhD studying its representation in climate models, which wrongly simulate its opening every winter, for reasons that are still not totally clear…

Polynyas are a fascinating feature of the cryosphere, not least because they occur in the middle of winter in harsh environments and cannot be instrumented easily. They are a key spot where the ocean, the ice and the atmosphere interact directly. Their opening has a large range of consequences from plankton bloom to deep water formation. And we still struggle to represent them in models, so there is lots of work to do for early career scientists!

References and further reading

  • Carsey, F. D (1980). “Microwave observation of the Weddell Polynya.” Monthly Weather Review 108.12: 2032-2044.
  • Dunbar, M (1969). “The geographical position of the North Water”. Arctic. 22: 438–441. doi:10.14430/arctic3235
  • Riewe, R (1991). “Inuit use of the sea ice.” Arctic and Alpine Research 1:3-10. doi:10.2307/1551431
  • Smith Jr, W. O., and D. Barber, eds (2007). “Polynyas: Windows to the world”. Vol. 74. Elsevier.
    Stirling, I. A. N. (1980). “The biological importance of polynyas in the Canadian Arctic.” Arctic: 303-315, http://www.jstor.org/stable/40509029

Edited by Sophie Berger and Emma Smith

Image of the Week – Inside a Patagonian Glacier

Image of the Week – Inside a Patagonian Glacier

Chilean Patagonia hosts many of the most inhospitable glaciers on the planet – in areas of extreme rainfall and strong winds. These glaciers are also home to some of the most spectacular glacier caves on Earth, with dazzlingly blue ice and huge vertical shafts (moulins). These caves give us access to the heart of the glaciers and provide an opportunity to study the microbiology and water drainage in these areas; in particular how this is changing in relation to climate variations. Our image of this week shows the entrance to one of these caves on Grey Glacier in the Torres del Paine National Park.


“Glacier karstification”

Glaciers in Patagonia are “temperate”, which means that the ice temperature is close to the melting point. As glacial melt-water runs over the surface of this “warm” ice it can easily carve features into ice, which are similar to those formed by limestone dissolution in karstic landscapes. Hence, this phenomenon is called Glacier karstification. It is this process that forms many of the caves and sinkholes that are typically found on temperate glaciers.

From the morphological (structural) point of view, glaciers actually behave like karstic areas, which is rather interesting for a speleologist (scientific cave explorer). Besides caves and sinkholes one often finds other shapes similar to karstic landscapes. For example, small depressions on the ice surface formed by water gathering in puddles, whose appearance resembles small kartisic basins (depressions). Of all the features formed by glacier karstification glacier caves are the most important from a glaciological perspective.

Glacier caves can be divided in two main categories:

  • Contact caves – formed between the glacier and bed underneath; or at the contact between extremely cold and temperate ice by sublimation processes (Fig. 2a)
  • Englacial caves – form inside the glacier – as shown in our image of the week today. Most of these caves are formed by runoff, where water enters the glacier through a moulin (vertical shaft) and are the most interesting for exploration and research (Fig. 2b)
Figure 2: Two different types of caves explored on the Grey Glacier. A- Contact formed between the glacier bed and overlying ice [Credit: Tommaso Santagata]. B- Entrance to an englacial cave [Credit: Alessio Romeo/La Venta].

Figure 2: Two different types of caves explored on the Grey Glacier. A- Contact formed between the glacier bed and overlying ice [Credit: Tommaso Santagata]. B- Entrance to an englacial cave [Credit: Alessio Romeo/La Venta].

Exploring the moulins of a Patagonian glacier

Located in the Torres del Paine National Park area (see Fig. 3), the Grey glacier was first explored in 2004 by the association La Venta Esplorazioni Geografiche. In April of this year, a team of speleologists went back to the glacier to survey the evolution of the glacier.

Figure 3: Map of Grey Glacier with survey site of 2004 and 2016 indicated by red dot [Adapted from: Instituto Geografico Militar de Chile ]

Figure 3: Map of Grey Glacier with survey site of 2004 and 2016 indicated by red dot [Adapted from: Instituto Geografico Militar de Chile ]

Grey glacier begins in the Andes and flows down to it’s terminus in Grey Lake, where it has three “tongues” which float out into the water (Fig, 3). As with many other glaciers, Grey Glacier is retreating, though mass loss is less catastrophic than some of Patagonia’s other glaciers (such as the Upsala – which is glaciologically very similar to the Grey Glacier). Grey Glacier has retreated by about 6 km over the last 20 years and has thinned by an average of 40 m since 1970.

In 2004 research was concentrated on the tongue at the east of this Grey Glacier (Fig. 3 – red dot), which is characterised by a surface drainage network with small-size surface channels that run into wide moulin shafts, burying into the glacier. In this latest expedition, the same area was re-examined to see how it had changed in the last 12 years.

Several moulins were explored during the 2016 expedition, including a shaft of more than 90 m deep and some horizontal contact caves (Fig 2). The glacier has clearly retreated and the surface has lowered a lot from the 2004 expedition. The extent of the thinning in recent years can be easily measured on the wall of the mountains around the glacier. Interestingly the entrance to the caves which were explored in 2004 and in 2016 was in the same position as 12 years ago, although the reasons for this are not yet clear.

The entrance of two of the main moulins which were explored were also mapped in 3D using photogrammetry techniques (see video below). The 3D models produced help us to better understand the shape and size of these caves and to study their evolution by repeating this mapping in the future. For more information about the outcome of this expedition, please follow the Inside the Glaciers Blog.

 

 

Further Reading:

Books on the subject:

  • Caves of the Sky: A Journey in the Heart of Glaciers, 2004, Badino G., De Vivo A., Piccini L.
  • Encyclopaedia of Caves and Karst Science, 2004, Editor: Gunn J.

Edited by Emma Smith and Sophie Berger


tom_picTommaso Santagata is a survey technician and geology student at the University of Modena and Reggio Emilia. As speleologist and member of the Italian association La Venta Esplorazioni Geografiche, he carries out research projects on glaciers using UAV’s, terrestrial laser scanning and 3D photogrammetry techniques to study the ice caves of Patagonia, the in-cave glacier of the Cenote Abyss (Dolomiti Mountains, Italy), the moulins of Gorner Glacier (Switzerland) and other underground environments as the lava tunnels of Mount Etna. He tweets as @tommysgeo

Image of the Week — FRISP 2016

Image of the Week — FRISP 2016

The Forum for Research into Ice Shelf Processes, aka FRISP, is an international meeting bringing together glaciologists and oceanographers. There are no parallel sessions; everyone attends everyone else’s talk and comment on their results, and the numerous breaks and long dinners encourage new and interdisciplinary collaborations. In fact, each year, a few presentations are the result of a previous year’s question!

The location changes every year, moving around the institutions that are involved with Arctic and Antarctic research. The 2016 edition just occurred this week, 3rd – 6th October, in a marine research station of the University of Gothenburg, in the beautiful Gullmarn Fjord.

Each year, a few presentations are the result of a previous year’s question!

Fjord at the sunset [Credit: Céline Heuzé]

Gullmarn fjord at the sunset [Credit: Céline Heuzé]

70 participants from 37 institutions:

  • Attended 49 talks on model results, new observation techniques, and everything in between;

  • Spent more than 15h discussing these results, including 2h around 15 posters;

  • Drank 50 L of coffee, 60 L of tea, 20 L of lingon juice… and a fair amount of wine!

Poster session at the FRISP 2016 meeting. [Credit: Céline Heuzé]

Poster session at the FRISP 2016 meeting. [Credit: Céline Heuzé]

I can’t really choose THE highlight of the conference.
As an organiser, it was a real pleasure to simply see it happen after all the long hours of planning.
As a scientist, it was a great and productive meeting, giving me new ideas and the opportunity to discuss my recent work with the big names of the field in a friendly environment.
And as a human, I enjoyed most the under-ice footages, and in particular the general ”ooooh” that came from the audience.

It was a bit sad to say goodbye to the participants, old friends and new collaborators. But I know that I will see them again during FRISP 2017… and I hope to see you there as well!

 Edited by Sophie Berger and Emma Smith

Image of the Week – Satellite Measurements of Arctic Sea Ice

Image of the Week – Satellite Measurements of Arctic Sea Ice

Sea ice is an important part of the Earth’s climate system. When sea ice forms, it releases heat and salt. When sea ice melts, it takes up heat and adds freshwater to the salty ocean water. It is also important for the exchange of energy between the atmosphere and the ocean surface, and for the ocean currents that transport warm and cold water from the equator to the poles and back.

The main route of sea ice moving south from the Arctic Ocean is through Fram Strait. This is the passage between  Greenland and Svalbard around 75-80°N latitude (see map). Today’s “Image of the Week” shows the amount of sea ice that flows  through this pathway each day (known as the mean observed volume flux) for October-November 2006.


What is sea ice volume flux and how is it computed?

Sea ice volume flux sounds a bit wordy – so what is it and why do we use it? Let’s start with the sea ice volume  – this simply tells us how much sea ice there is. In order to calculate the volume, you need to know not only the concentration of sea ice (i.e. the fraction of ocean covered by ice) but also its thickness. The flux parts comes in when we want to talk about the amount of sea ice moving through an area (e.g. Fram Strait). In order to calculate sea ice volume flux, you need to know both sea ice volume and drift. The latter is the displacement of sea ice over a certain period of time, typically measured in kilometres per day. So the volume flux is basically a measure of the volume of ice that moves through a certain area over a given time! Simple!?

An aerial view of the helicopter taking data of the sea ice below.

An aerial view of the helicopter taking data of the sea ice below. Photo credit: Alice O’Connor.

What does the “Image of the Week” show?

The  map was constructed by Spreen et al. (2009) from satellite data and provided one of the first estimates of  sea ice volume flux through Fram Strait exclusively using satellites.  However, it is  challenging to accurately measure sea ice thickness in the summer using satellites because of the  melt ponds (pools of open water forming on sea ice when temperatures are higher, i.e. summer) that interfere with the satellite signals. A few measurements of sea ice thickness have been collected in the summer from British and US submarines in the 1980s and 1990s but only in the region near the pole.  A recent study presents a way to compute sea ice volume fluxes through Fram Strait in the summer using a new dataset of ground-based and airborne electromagnetic ice thickness measurements (Krumpen et al., 2016).

In this new study, the concentration of sea ice is found from satellite measurements (passive microwave sensors to be specific). The sea ice drift can be found by using both satellite data and observations from buoys. Finally, the thickness of the sea ice comes from surveys using radar instruments on the ground or from an aircraft. This data was collected during 5 summers, by combining the three of these measurements, Krumpen et al., (2016) calculated the sea ice volume flux during the summer!

In the figure below, the red lines show the sea ice area flux using the concentration and drift only through Fram Strait in July and August from 1980 to 2012. Positive values mean that sea ice moves out of the Arctic Basin, while negative values represent sea ice that goes into the Arctic Basin. The figure shows that there are large variations in sea ice area flux from year to year.

The black dots are the volume fluxes (area flux times thickness) for the 5 summer seasons where the ice thickness was measured. The dots show that in some summers sea ice volume flux is negative, meaning that sea ice moves into the Arctic Basin. In other summers, for example in 2010, sea ice moves out of the Arctic Basin.

July (black, gray shading for uncertainty) and August (red, light red shading for uncertainty) sea ice area fluxes through Fram Strait (left axis). Black dots show sea ice volume flux for the 5 campaigns during which ice thickness is computed using electromagnetic measurements (right axis). Positive values mean that sea ice is moving out out of the Arctic Ocean while negative values mean that sea ice is flowing into the Arctic Ocean. [Figure 8 of Krumpen et al. (2016)]

July (black, gray shading for uncertainty) and August (red, light red shading for uncertainty) sea ice area fluxes through Fram Strait (left axis). Black dots show sea ice volume flux for the 5 campaigns during which ice thickness is computed using electromagnetic measurements (right axis). Positive values mean that sea ice is moving out out of the Arctic Ocean while negative values mean that sea ice is flowing into the Arctic Ocean. [Figure 8 of Krumpen et al. (2016)]

What comes next…

If field campaigns in the future continue to measure sea ice thickness in the summer, the calculations can be continued and we will learn more about the volume of sea ice that is transported through Fram Strait in the summer. This is important if we are to measure the balance of sea ice in the Arctic Ocean.

Edited by Nanna B. Karlsson


DavidDavid Docquier is a post-doctoral researcher at the Earth and Life Institute of Université catholique de Louvain (UCL) in Belgium. He works on the development of processed-based sea-ice metrics in order to improve the evaluation of global climate models (GCMs). His study is embedded within the EU Horizon 2020 PRIMAVERA project, which aims at developing a new generation of high-resolution GCMs to better represent the climate.

Water Masses “For Dummies”

Water Masses “For Dummies”

Polar surface water, circumpolar deep water, dense shelf water, North Atlantic deep water, Antarctic bottom water… These names pop in most discussions about the ice-ocean interaction and how this will change in a warming climate, but what do they refer to?

In our second “For Dummies” article, we shall give you a brief introduction to the concept of “water mass”, explain how to differentiate water from more water, and why you would even need to do so.


Global heat budget and the need for an ocean circulation

The global climate is driven by differences between the incoming shortwave radiation and the outgoing longwave radiation (Fig. 1):

  • In the tropics, there is a surplus of energy: the Sun brings more heat, all year-round, than what is radiated out;
  • At the poles in contrast, there is a net deficit: more energy is leaving than is coming from the Sun (who is absent in winter).

The global ocean and atmosphere circulations act to reduce this imbalance, by transporting the excess heat from the tropics to the pole. Here we will focus on the global ocean circulation only, since this post is written by an oceanographer, but similar principles also apply to atmospheric circulation.

Fig 1 :Earth’s latitudinal radiation bugdet, The tropics show a surplus of energy that compensates the Poles’ deficit[Credit: National Oceanograpy Center

Fig 1 :Earth’s latitudinal radiation bugdet, The tropics show a surplus of energy that compensates the Poles’ deficit [Credit: National Oceanograpy Center].

The global ocean circulation

In a nutshell, surface waters bring heat towards the poles where they cool down, sink to the abyss, and return towards the tropics as deep waters where they can go back to the surface..…

We talk about “the global ocean circulation” because although the Earth officially has five oceans, they are not totally separate bodies of water. In fact, the Arctic, Atlantic, Indian, Pacific and Southern oceans are interconnected, with water circulating and moving between them. How does this happen?

The global ocean circulation has two components:

  • The wind-driven circulation, fast but limited to a few hundred metres below the surface of the ocean (read more about it here for example);
  • And the thermohaline circulation (shown on Fig. 2), slower but which affects the whole depth of the ocean.

Today’s post focuses on the latter, since we will talk about water properties. The thermohaline circulation, also called density-driven circulation, depends on two water properties:

  • The temperature (‘thermo’) is mostly controlled by heat exchange with the atmosphere or the ice. Cold water has a high density.
  • The salinity (‘haline’) can be modified by evaporation, precipitation, or addition of fresh water from melted glaciers/ice sheets or rivers. Salty water has a high density.
Fig 2- The global thermohaline circulation shows warm surface currents in red, cold deep currents in blue. Deep waters form in the North Atlantic and Southern oceans. [Credit: NASA]

Fig 2- The global thermohaline circulation shows warm surface currents in red, cold deep currents in blue. Deep waters form in the North Atlantic and Southern oceans [Credit: NASA].

Roughly speaking, a water mass is any drop of the ocean within a specific range of temperature and salinity, and hence specific density. Some water masses are found at particular locations or seasons, while others can be found all around the globe, all the time. Since density sets the depth (density MUST always increase with depth), water masses will lie and travel at particular depth levels.

A quick and dirty oceanography guide

Water masses are formed.

Some are the result of the mixing of other water masses. The others start at the water surface, where they exchange gas (notably oxygen and carbon) with the atmosphere. When a water mass becomes denser than the waters below it , for example, if it is cooled by the wind or ice, it sinks to its corresponding depth within the ocean.

Fig 3- The bathymetry of the Arctic Ocean forces dense (deep) water masses to enter the region via Fram Strait whereas lighter (shallower) waters can go through the Barents Sea [Credit: adapted from IBCAO bathymetry map, Jakobsson et al., 2012 ].

Water masses move all around the globe…

…provided their density allows it. The vertical distribution of density in the ocean must be “stably stratified”, which means that the density increases with depth. In practice, that means that dense waters cannot climb up a shallow bathymetric feature but have to find a way around it. For example to enter the Arctic Ocean (Fig 3), a dense water mass has no choice but to go via Fram Strait, whereas a less dense one can go via the Barents Trough. Similarly, there is a depth limit of about 500 m to reach the northwestern Greenland glaciers.

Water masses retain their properties

Or rather, not all these properties change considerably with space and time. We are not talking only about temperature and salinity, but also about gas and chemical concentrations. It is then possible to track a water mass as it travels around the globe or watch its evolution with time.

You should use T-S diagrams

Visualising water properties can either be done with one graph showing how the temperature varies with depth plus another one for the salinity (multiplied by the number of locations to be observed at the same time); or all of this information can be combined on one image (as done on Fig. 4). This image is called a T-S diagram it and shows how the temperature (T) varies as a function of the salinity (S). It is customary to also draw the lines of constant density (the ‘isopycnals’, black on Fig. 3). These isopycnals give information about the types of mixing happening and the stratification, but we will talk about that in another post.

Fig 4 - an example of how to combine several profiles (top) into a T-S diagram, for one of the randomly selected Arctic historical points that I work with.[Credit: C. Heuzé]

Fig 4 – an example of how to combine several profiles (top) into a T-S diagram, for one of the randomly selected Arctic historical points that I work with [Credit: C. Heuzé].

Because each water mass occupies a very specific region of the T-S diagram (see Fig 5 for an example in the Atlantic), identifying them is relatively easy once you have plotted your data on such diagrams.

Fig 5 – example of a reference T-S diagram with the different water masses of the Atlantic Ocean. Water massed are labelled by their acronym (e.g. AABW= Antarctic Bottom Water) [Credit: after Emery and Meincke (1986)]

Why do ocean water masses matter to the cryosphere?

  • Marine ice sheet instability, and more generally basal melting, is caused by warm dense waters melting floating glaciers from below; how dense the water mass is determines whether it can even reach the glacier.
  • Sea ice formation and melting can be largely affected by water masses moving up and down, especially is those going up are warm.

But there’s a reason why we always talk about “ice-ocean” interactions: it’s not just the ocean acting on the ice, but also the ice impacting the ocean:

  • The densest water mass in the world, Antarctic Bottom Water, forms in the middle of winter if a hole in the sea-ice cover opens (that is called a polynya), suddenly exposing the relatively warm ocean to the extremely cold atmosphere. The resulting strong heat loss and the increased salinity as sea ice reforms make this water sink straight to the bottom;
  • On the other hand, deep water formation can be stopped by the cryosphere: paleorecord evidence showed that it happened in the North Atlantic due to surging ice sheet / marine ice sheet instability (so called Heinrich events) or meltwater floods (Younger Dryas);
  • Less dramatically, icebergs, ice shelves or even sea ice, can cool or freshen water masses they meet, forming “modified” water masses (for example “modified Atlantic Water”),

Each aspect of these interactions is already experiencing climate change and is much more complex than this brief overview… but that will be the topic of another post!

Further reading

 Edited by Sophie Berger and Emma Smith

Image of the week – The winds of summer (and surface fluxes of winter)

Image of the week – The winds of summer (and surface fluxes of winter)

Antarctica is separated from the deep Southern Ocean by a shallow continental shelf. Waters are exchanged between the deep ocean and the shallow shelf, forming the Antarctic cross-shelf circulation:

  • Very dense waters leave the shelf as Antarctic Bottom Water (AABW) that will then flow at the bottom of all oceans.
  • Meanwhile, relatively warm water from the Southern Ocean, Modified Circumpolar Deep Water (MCDW*) comes on the continental shelf and brings heat to the ice shelves.

That is, Antarctic cross-shelf circulation influences the water mass that transports heat, carbon and nutrient all around the globe in very large volumes (Purkey and Johnson 2013), and the basal melting of Antarctic floating ice (Hellmer et al. 2012), hence the stability of the whole Antarctic ice sheet.

Although critical for both the ocean and the cryosphere, very little is known about the mechanisms behind cross-shelf circulation. We know that the mechanisms that control it vary on a seasonal time scale (Snow et al. 2016b). However, most hydrographic observations around Antarctica are taken in summer, when there is less sea ice and when the Southern Ocean is the least stormy. This means that we have very few measurements of the seasonal variations of the cross-shelf circulation itself.

Why does it matter that the cross-shelf circulation varies between summer and winter?

Three words: sea level rise.
Nearly half of the world’s population lives in coastal areas (
UN report). Antarctica contains enough ice to raise the sea level by 60 m, and although a total melting is very unlikely, current rates could raise the sea level by 1m by 2100 (read more about it on AntarcticGlaciers.org). To project future sea level rise and design relevant coastal defences, we need models to predict when and where the Antarctic ice will melt.

However, models are only as good as the observations that were used to constrain them. Having only summer observations in an area of Antarctica that has notable differences between summer and winter ocean circulation means that until now, models could not represent accurately the transfer of heat from the ocean to the ice shelves

A better observation strategy is needed if we want our models to correctly represent Antarctic basal melting and the global ocean circulation.

Antarctic cross-shelf circulation: summer vs winter

In summer, the circulation is mostly controlled by the strong katabatic winds blowing from the interior of the Antarctic continent towards the ocean. All the surface water masses go in the same direction, simply following the Antarctic coastal current. Nothing really happens at depth.

In winter, the circulation is also controlled by buoyancy forcings, that is changes in temperature or salinity at the surface of the ocean. Here, these mostly occur in a polynya (a hole in the sea ice cover) where the “warm” ocean is cooled by the very cold atmosphere, and where the surface becomes very salty as sea ice reforms (a process called “brine rejection”: salt is expelled from the new ice as water freezes). These buoyancy forcings form dense water (DSW), which sinks to the abyss and off the shelf as AABW. Mass conservation means that something else (here MCDW*) needs to come to the shelf to compensate for that outflow. You can notice that MCDW now flows in the opposite direction than it did in summer.

Take home message

Summer data is better than no data. But always be aware of the limitations of your model if you don’t have the datasets to test it– you may have a surprise when you do!

Reference

Snow, K., B. M. Sloyan, S. R. Rintoul, A. McC. Hogg, and S. M. Downes (2016), Controls on circulation, cross-shelf exchange, and dense water formation in an Antarctic polynya, Geophys. Res. Lett., 43, doi:10.1002/2016GL069479.

Edited by Sophie Berger and Emma Smith

Marine Ice Sheet Instability “For Dummies”

Marine Ice Sheet Instability “For Dummies”

MISI is a term that is often thrown into dicussions and papers which talk about the contribution of Antarctica to sea-level rise but what does it actually mean and why do we care about it?

MISI stands for Marine Ice Sheet Instability. In this article, we are going to attempt to explain this term to you and also show you why it is so important.


Background

The Antarctic Ice Sheet represents the largest potential source of future sea-level rise: if all its ice melted, sea level would rise by about 60 m (Vaughan et al., 2013). According to satellite observations, the Antarctic Ice Sheet has lost 1350 Gt (gigatonnes) of ice between 1992 and 2011 (1 Gt = 1000 million tonnes), equivalent to an increase in sea level of 3.75 mm or 0.00375 m (Shepherd et al., 2012). 3.75 mm does not seem a lot but imagine that this sea-level rise is evenly spread over all the oceans on Earth, i.e. over a surface of about 360 million km², leading to a total volume of about 1350 km³, i.e. 1350 Gt of water… The loss over this period is mainly due to increased ice discharge into the ocean in two rapidly changing regions: West Antarctica and the Antarctic Peninsula (Figure 1, blue and orange curves respectively).

Figure 1: Cumulative ice mass changes (left axis) and equivalent sea-level contribution (right axis) of the different Antarctic regions based on different satellite observations (ERS-1/2, Envisat, ICESat, GRACE) from 1992 to 2011 (source: adapted from Fig. 5 of Shepherd et al., 2012 ) with addition of inset: Antarctic map showing the different regions ( source )

What are the projections for the future?

Figure 2: Ice velocity of the glaciers in the Amundsen Sea Embayment, West Antarctica, using ERS-1/2 radar data in winter 1996. The grounding line (boundary between ice sheet and ice shelf) is shown for 1992, 1994, 1996, 2000 and 2011 (source: Fig. 1 of Rignot et al., 2014 ).

Figure 2: Ice velocity of the glaciers in the Amundsen Sea Embayment, West Antarctica, using ERS-1/2 radar data in winter 1996. The grounding line (boundary between ice sheet and ice shelf) is shown for 1992, 1994, 1996, 2000 and 2011 (source: Fig. 1 of Rignot et al., 2014 ).

According to model projections from the Intergovernmental Panel on Climate Change (IPCC), global mean sea level will rise by 0.26 to 0.82 m during the twenty-first century (Church et al., 2013). The contribution from the Antarctic Ice Sheet in those projections will be about 0.05 m (or 50 mm) sea-level equivalent, i.e. 10% of the global projected sea-level rise, with other contributions coming from thermal expansion (40 %), glaciers (25 %), Greenland Ice Sheet (17 %) and land water storage (8 %).

The contribution from Antarctica compared to other contributions does not seem huge, however there is a high uncertainty coming from the possible instability of the West Antarctic Ice Sheet. According to theoretical (Weertman, 1974; Schoof, 2007) and recent modeling results (e.g. Favier et al., 2014; Joughin et al., 2014), this region could be subject to marine ice sheet instability (MISI), which could lead to considerable and rapid ice discharge from Antarctica. Satellite observations show that MISI may be under way in the Amundsen Sea Embayment (Rignot et al., 2014), where some of the fastest flowing glaciers on Earth are located, e.g. Pine Island and Thwaites glaciers (Figure 2). So what exactly is MISI?

What is marine ice sheet instability (MISI)?

 

Figure 3: Antarctic map of ice sheet (blue), ice shelves (orange) and islands/ice rises (green) based on satellite data (ICESat and MODIS). The grounding line is the separation between the ice sheet and the ice shelves. Units on X and Y axes are km (source: NASA ).

Figure 3: Antarctic map of ice sheet (blue), ice shelves (orange) and islands/ice rises (green) based on satellite data (ICESat and MODIS). The grounding line is the separation between the ice sheet and the ice shelves. Units on X and Y axes are km (source: NASA ).

To understand the concept of MISI, it is important to define both ‘marine ice sheet’ and ‘grounding line’:

 

  • A marine ice sheet is an ice sheet sitting on a bedrock that is below sea level, for example the West Antarctic Ice Sheet.
  • The grounding line is the boundary between the ice sheet, sitting on land, and the floating ice shelves (Figure 3 for a view from above and Figure 4 for a side view). The position and migration of this grounding line control the stability of a marine ice sheet.

 

 

The MISI hypothesis states that when the bedrock slopes down from the coast towards the interior of the marine ice sheet, which is the case in large parts of West Antarctica, the grounding line is not stable (in the absence of back forces provided by ice shelves, see next section for more details). To explain this concept, let us take the schematic example shown in Figure 4:

  1. The grounding line is initially located on a bedrock sill (Figure 4a). This position is stable: the ice flux at the grounding line, which is the amount of ice passing through the grounding line per unit time, matches the total upstream accumulation.
  2. A perturbation is applied at the grounding line, e.g. through the incursion of warm Circumpolar Deep Water (CDW, red arrow in Figure 4) below the ice shelf as observed in the Amundsen Sea Embayment.
  3. These warm waters lead to basal melting at the grounding line, ice-shelf thinning and glacier acceleration, resulting in an inland retreat of the grounding line.
  4. The grounding line is then located on a bedrock that slopes downward inland (Figure 4b), i.e. an unstable position where the ice column at the grounding line is thicker than previously (Figure 4a). The theory shows that ice flux at the grounding line is strongly dependent on ice thickness there (Weertman, 1974; Schoof, 2007), so a thicker ice leads to a higher ice flux.
  5. Then, the grounding line is forced to retreat since the ice flux at the grounding line is higher than the upstream accumulation.
  6. This is a positive feedback and the retreat only stops once a new stable position is reached (e.g. a bedrock high), where both ice flux at the grounding line and upstream accumulation match.
Figure 4: Schematic representation of the marine ice sheet instability (MISI) with (a) an initial stable grounding-line position and (b) an unstable grounding-line position after the incursion of warm Circumpolar Deep Water (CDW) below the ice shelf (source: Fig. 3 of Hanna et al., 2013 ).

Figure 4: Schematic representation of the marine ice sheet instability (MISI) with (a) an initial stable grounding-line position and (b) an unstable grounding-line position after the incursion of warm Circumpolar Deep Water (CDW) below the ice shelf (source: Fig. 3 of Hanna et al., 2013 ).

  • In summary, the MISI hypothesis describes the condition where a marine ice sheet is unstable due to being grounded below sea level on land that is sloping downward from the coast to the interior of the ice sheet.
  • This configuration leads to potential rapid retreat of the grounding line and speed up of ice flow from the interior of the continent into the oceans.

Is there evidence that MISI is happening right now?

 

Figure 5: Buttressing provided by Larsen C ice shelf, Antarctic Peninsula, based on a model simulation (Elmer/Ice). Buttressing values range between 0 (no buttressing) and 1 (high buttressing). The red contour shows the buttressing=0.3 isoline. Observed ice velocity is also shown (source: Fig. 2 of Fürst et al., 2016 ).

Figure 5: Buttressing provided by Larsen C ice shelf, Antarctic Peninsula, based on a model simulation (Elmer/Ice). Buttressing values range between 0 (no buttressing) and 1 (high buttressing). The red contour shows the buttressing=0.3 isoline. Observed ice velocity is also shown (source: Fig. 2 of Fürst et al., 2016 ).

In reality, the grounding line is often stabilized by an ice shelf that is laterally confined by side walls (see Figure 5, where Bawden and Gipps ice rises confine Larsen C ice shelf) or by an ice shelf that has a contact with a locally grounded feature (Figure 6). Both cases transmit a back force towards the ice sheet, the ‘buttressing effect’, which stabilizes the grounding line (Goldberg et al., 2009; Gudmundsson, 2013) even if the configuration is unstable, i.e. in the case of a grounding line located on a bedrock sloping down towards the interior (Figure 4b).

 

However, in the last two decades, the grounding lines of the glaciers in the Amundsen Sea Embayment (Pine Island and Thwaites glaciers for example) retreated with rates of 1 to 2 km per year, in regions of bedrock sloping down towards the ice sheet interior (Rignot et al., 2014). The trigger of these grounding-line retreats is the incursion of warm CDW penetrating deeply into cavities below the ice shelves (Jacobs et al., 2011), carrying important amounts of heat that melt the base of ice shelves (Figure 4). Increased basal melt rates have led to ice-shelf thinning, which has reduced the ice-shelf buttressing effect and increased ice discharge. All of this has led to grounding-line retreat. The exact cause of CDW changes is not clearly known but these incursions are probably linked to changes in local wind stress (Steig et al., 2012) rather than an actual warming of CDW.

 

 

Figure 6: Schematic representation of ice-shelf buttressing by a local pinning point (source: courtesy of R. Drews ).

Figure 6: Schematic representation of ice-shelf buttressing by a local pinning point (source: courtesy of R. Drews ).

There is currently no major obstacle to these grounding line retreats. Therefore, the Amundsen Sea Embayment is probably experiencing MISI and glaciers will continue to retreat if no stabilization is reached. This sector of West Antarctica contains enough ice to raise global sea level by 1.2 m.

 

What can we do about it?

MISI is probably ongoing in some parts of Antarctica and sea level could rise more than previously estimated if the grounding lines of the glaciers in the Amundsen Sea Embayment continue to retreat so fast. This could have catastrophic impacts on populations living close to the coasts, for example more frequent flooding of coastal cities, enhanced coastal erosion or changes in water quality.

Thus, it is important to continue monitoring the changes happening in Antarctica, and particularly in West Antarctica. This will allow us to better understand and project future sea-level rise from this region, as well as better adapt the cities of tomorrow.

Edited by Clara Burgard and Emma Smith


DavidDavid Docquier is a post-doctoral researcher at the Earth and Life Institute of Université catholique de Louvain (UCL) in Belgium. He works on the development of processed-based sea-ice metrics in order to improve the evaluation of global climate models (GCMs). His study is embedded within the EU Horizon 2020 PRIMAVERA project, which aims at developing a new generation of high-resolution GCMs to better represent the climate.