CR
Cryospheric Sciences

Céline Heuzé

is a VINNOVA Marie Curie research fellow at the University of Gothenburg, in southwest Sweden. She is a polar physical oceanographer currently focusing on one particular water mass, the North Atlantic Water, studying it from its formation in the Labrador Sea to its melting of Greenland floating glaciers. She completed her PhD last year at the University of East Anglia, UK, on the representation of Antarctic Bottom Water in climate models. She tweets as @ClnHz and manages Scisnack, and blogs on PolarFever when at sea.

Image of the Week — The ice blue eye of the Arctic

Image of the Week — The ice blue eye of the Arctic

Positive feedback” is a term that regularly pops up when talking about climate change. It does not mean good news, but rather that climate change causes a phenomenon which it turns exacerbates climate change. The image of this week shows a beautiful melt pond in the Arctic sea ice, which is an example of such positive feedback.


What is a melt pond?

The Arctic sea ice is typically non-smooth, and covered in snow. When, after the long polar night, the sun shines again on the sea ice, a series of events happen (e.g. Fetterer and Untersteiner, 1998):

  • the snow layer melts;

  • the melted snow collects in depressions at the surface of the sea ice to form ponds;

  • these ponds of melted water are darker than the surrounding ice, i.e. they have a lower albedo. As a result they absorb more heat from the Sun, which melts more ice and deepens the pond. Melt ponds are typically 5 to 10 m wide and 15 to 50 cm deep (Perovich et al., 2009);

  • eventually, the water from the ponds ends up in the ocean: either by percolation through the whole sea-ice column or because the bottom of the pond reaches the ocean. Sometimes, it can also simply refreeze, as the air temperatures drop again (Polashenski et al., 2012).

Melt ponds cover 50-60% of the Arctic sea ice each summer (Eicken et al., 2004), and up to 90% of the first year ice (Perovich al., 2011). How do we know these percentages? Mostly, thanks to satellites.

Monitoring melt ponds by satellites

Like most phenomena that we discuss on this blog, continuous in-situ measurements are not feasible at the scale of the whole Arctic, so scientists rely on satellites instead. For melt ponds, spectro-radiometer data are used (Rösel et al., 2012). These measure the surface reflectance of the Earth i.e. the proportion of energy reflected by the surface for wavelengths in the visible and infrared (0.4 to 14.4 μm). The idea is that different types of surfaces reflect the sunlight differently, and we can use these data to then map the types of surfaces over a region.

In particular for the Arctic, sea ice, open ocean and any stage in-between all reflect the sunlight differently (i.e. have different albedos). The way that the albedo changes with the wavelength is also different for each surface, which is why radiometer measurements are taken for a range of wavelengths. With these measurements, not only can we locate the melt ponds in the Arctic, but even assess how mature the pond is (i.e. how long ago it formed) and how deep it extends. These values are key for climate change predictions.

Fig. 2: Melt pond seen by a camera below the sea ice. (The pond is the lighter area) [Credit: NOAA’s climate.gov]

Melt ponds and the climate

Let’s come back to the positive feedback mentioned in the introduction. Solar radiation and warm air temperature create melt ponds. The darker melt ponds have a higher albedo than the white sea ice, so they absorb more heat, and further warm our climate. This extra heat is also transferred to the ocean, so melt pond-covered sea ice melts three times more from below than bare ice (Flocco et al., 2012). This vicious circle heat – less sea ice – more heat absorbed – even less sea ice…, is called the ice-albedo feedback. It is one of the processes responsible for the polar amplification of global warming, i.e. the fact that poles warm way faster than the rest of the world (see also this post for more explanation).

The ice-albedo feedback is one of the processes responsible for the polar amplification of global warming

But it’s not all doom and gloom. For one thing, melt ponds are associated with algae bloom. The sun light can penetrate deeper through the ocean under a melt pond than under bare ice (see Fig. 2), which means that life can develop more easily. And now that we understand better how melt ponds form, and how much area they cover in the Arctic, efforts are being made to include more realistic sea-ice properties and pond parametrisation in climate models (e.g. Holland et al., 2012). That way, we can study more precisely their impact on future climate, and the demise of the Arctic sea ice.

Edited by Sophie Berger

Further reading

Image of the Week – On the tip of Petermann’s (ice) tongue

Image of the Week – On the tip of Petermann’s (ice) tongue

5th August 2015, 10:30 in the morning. The meeting had to be interrupted to take this picture. We were aboard the Swedish icebreaker Oden, and were now closer than anyone before to the terminus of Petermann Glacier in northwestern Greenland. But we had not travelled that far just for pictures…


Petermann’s ice tongue

Petermann is one of Greenland’s largest “marine terminating glaciers”. As the name indicates, this is a glacier, i.e. frozen freshwater, and its terminus floats on the ocean’s surface. Since Petermann is confined within a fjord, the glacier is long and narrow and can be referred to as an “ice tongue”.

Petermann Glacier is famous for its recent calving events. In August 2010, about a quarter of the ice tongue (260 km2) broke off as an iceberg (Fig. 2). In July 2012, Petermann calved again and its ice tongue lost an extra 130 km2.

These are not isolated events. Greenland’s marine terminating glaciers are all thinning and retreating in response to a warming of both air and ocean temperatures (Straneo et al., 2013), and Greenland’s entire ice sheet itself is threatened. Hence, international fieldwork expeditions are needed to understand the dynamics of these glaciers.

Fig. 2: The 2010 calving event of Petermann. Natural-color image from the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite ( August 16, 2010).  [Credit: NASA’s Earth Observatory]

The Petermann 2015 expedition

In summer 2015, a paleoceanography expedition was conducted to study Petermann Fjord and its surroundings, in order to assess how unusual these recent calving events are compared to the glacier’s past. Our small team focused on the present-day ocean, and specifically investigated how much of the glacier is melted from below by the comparatively warm ocean (that process has been described on this blog previously). In fact, this “basal melting” could be responsible for up to 80% of the mass loss of Petermann Glacier (Rignot, 1996). Additionally, we were also the first scientists to take measurements in this region since the calving events.

Our results are now published (Heuzé et al., 2017). We show that the meltwater can be detected and tracked by simply using the temperature and salinity measurements that are routinely taken during expeditions (that, also, has been described on this blog previously). Moreover, we found that the processes happening near the glacier are more complex than we expected and require measurements at a higher temporal resolution, daily to hourly and over several months, than the traditional summer single profiles. Luckily, this is why we deployed new sensors there! And since these have already sent their data, we should report on them soon!

Edited by David Rounce and Sophie Berger

References and further reading

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 — 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

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

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