Hydraulic jump

(by Cristina Arumi Planas, Elise Madeleine Colette Brunet, Haley Okun)

In order to observe Lee Waves and their related phenomenon, an experiment was conducted in a large water tank with a stratified two layer system. The two layer system was constructed with fresh water sitting atop colder salt water. The fresh water had a salinity of about 0‰, with a density of 1000 kg/m3 while the pink-dyed salt water had a salinity of about 35‰, and a density of 1028 kg/m3. In order to force Lee Waves to propagate, a mountain was moved along the bottom at two different speeds, fast and slow. While conducting an experiment to visualize Lee Waves, the phenomenon of the hydraulic jump can be observed. This event can be visualized when water flows over rocks or even in one’s kitchen sink. This occurs when water flowing over a surface goes from subcritical to supercritical, which is calculated through the Froude number. To calculate this, the velocity of the flow is divided by the phase speed of the shallow water gravity waves. The square root of this fraction is then taken to provide a unitless value called the Froude number. The result is either greater than one (supercritical) or less than one (subcritical). Supercritical Froude numbers indicate that waves cannot propagate upstream. This can physically be visualized when the flow over the observed surface goes from smooth and rather thin, to turbulent and rough. As we pushed the mountains through the stratified water, the denser saltwater (shown with pink dye) was forced up and over the mountain, resulting in turbulent motion just behind the surface anomaly. As the thinner flowing water moved from the downhill slope of the mountain to just downstream and onto the bottom of the tank, the flow went from smooth to rather chaotic. The interface where the flow becomes turbulent is the hydraulic jump. The smoother water flowing over the mountain is supercritical while the more mixed water just downstream is the subcritical flow. When the mountain was moved at the faster speed, this hydraulic jump was shifted accordingly. Instead of the hydraulic jump occurring just behind the mountain, the waves seemed to lag with the more turbulent flow occurring farther downstream than with the slower mountain speed.

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To build or not to build, that is the question

Antarctic ice shelves are thinnening at an accelerating rate, and they do so because oceanic currents bring warm water (heat) into their cavities causing them to melt from below. When they melt away, the ice sheet upstream accelerates, ice (mass) is moved from land to the ocean and the sea level rises, threatening to flood vast and densely populated areas. A part from cutting down our CO2 emissions there’s nothing we can do about it, or is there? We can off course build walls, or dikes, to keep the rising seas out like they do in Holland, but what if we built a wall that stopped (or at least slowed it down) the sea from rising?

Recently scientists from Princeton and Bejing Normal University suggested in the Cryosphere that in order to put  off a potential collapse of e.g. Thwaites ice shelf (which is thought to be unstable and which is feed by a huge, marine based ice stream and thus has a potentially large impact on the sea level) further into the future we should simply build a large wall – not in Holland, but at the seafloor in Antarctica, in order to block the warm water and keep it from entering the ice shelf cavity. It sounds like science fiction – and the thought of constructing such a wall is more frightening than anything Hollywood can produce. The risks are huge, the environmental impact is enormous (and thats sort of the point of it) – but the consequences of not doing it may be likewise. Enormous areas would be flooded, millions of people would have to leave their homes as the sea keeps rising. A wall would only buy us time, it would postphone, not stop, what is happening around Antarctica. And it would have no effect at all on other consequences of global warming. We still need to cut down our emissions, sooner rather than later, to make sure that our children and grandchildren – and their children and grandchildren – can live on the planet we call home.

So, if to build or not to build is the question, what would be your answer?

I was interviewed by a journalist from Ekko, a Norwegian radio program, about the proposed wall and the melting Antarctic ice last week. You can listen to the program here (in Norwegian/Svorsk). I’m right at the end!

 

Lee waves

(by: Jori Neteland-Kyte, Sara Elisabeth Holen Sælen , Susanne Moen Olsen)

Lee waves are a type of internal gravity waves, which is generated as fluid moves over an obstacle. The fluid needs to be stably stratified for this to occur. These waves can occur in both the atmosphere and in the ocean. (Cushman-Roisin and Beckers, 2011, page 412) To show this phenomenon it is convenient to perform a simple experiment, where a long tank is used. The tank is filled with stratified water, the bottom layer is denser than the layer above. A purple color is added to the denser water at the bottom layer, as seen in Figure 1.  This is done to distinguish between the two layers. The tank is also equipped with a moving obstacle which is possible to move at different constant velocities across the bottom of the tank.

Figure 1:The initial state of the two-layered stratified fluid.

When the obstacle is moved across the tank, waves are generated in the interface between the layers as seen in the figure 2.

Figure 2:Wave are generated when the obstacle is moved with the lowest speed.

Figure 3. displays how the Lee waves propagates, with the positions for the supercritical area, the hydraulic jump and subcritical area marked by the arrows. The supercritical area is positioned directly above the moving obstacle, which appears as one smooth wave. In the transition between the supercritical and the subcritical area, the hydraulic jump is found. This occurs at the end of the descending side of the moving obstacle. Following behind the
hydraulic jump is the subcritical area, this is where a train of waves are generated. These waves decays with time.

Figure 3: Position of sub- and super critical flow and the hydrualic jump.

 

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Long time no seen…

There hasn’t been much happening on this blog lately, since I’ve been busy, busy, busy teaching geophysical fluid dynamics to the third year bachelor students here at GFI. My job description as an associate professor at UiB includes 50% teaching – however giving the (somewhat equation heavy) course for the first time it felt more like 150%… but now the last lecture is given, the students are happily (?) preparing for the exam, and I finally have time to do some science… and to update the blog!

First I’ll have the students tell and show you what they did with Mirjam when she was visiting the Bjerknes centre and Bergen in October. One flight of stairs down from my office, in the basement of GFI, there is a 6 m long tank. It is not round and it is not rotating (like the tank in Grenoble), but it can move mountains! Or rather, it holds a mountain that can be moved. Why would one want to move a mountain in a tank? Well, as Arne Foldvik, the professor emiritus who built the tank a few decades ago realized, if you want to study flow over topography in the lab, then it is easier to have the fluid move beneath the fluid than to make the fluid move over the mountain… and the physics are the same.

Arne (who later left the lab and became on of the Norwegian pioneers in Antarctic oceanography) spent years with the tank – my student only spent an hour but they did some really nice stuff! Thank you again Mirjam for setting it all up – and thanks to the students for handing in the (non-compluslory) assignments that you’ll be able to read (and watch) in the days to come!

Below you see Arne Foldvik showing off his results – and inspecting his old tank.

Arne Foldvik inspecting his tank
All Arne’s experiments were documented by GFI’s in house (!) photographer!
Detailed logbooks…

 

 

Polar 2018

This week in Davos (Switzerland) about 2000 people are gathering to talk about Polar Sciences!

I (Lucie Vignes) am here to listen to a lot of talk about ocean dynamics, ocean-ice interactions but also talks about sciences-policy issues and women’s perspectives on Polar research! I came with a poster a well, speaking about both data from the Weddell Sea and our experiments in Grenoble. It was a good occasion to meet very interesting people and to share my research. This is my first big conference!

In front of my poster at Polar 2018. Photo: Lucie Vignes

 

Scary reading…

Antarctica has been in the headlines the last week – see e.g. the Guardian or Bergens Tidene – as a large group of scientists concluded in Nature that the Antarctic ice sheet has lost 2720 billion of tons of ice since 1992. 2720 billion tons… that’s enough ice to cover all of Norway with almost 8 m of ice… or to rise the mean sea level with 8 mm.

The uncertainty is large, especially for East Antarctica, because it is not easy an easy task to quantify the mass change of Antarctica. Over the years three main techniques have been developed, either building on satellite altimetry (measurements of the height of the ice sheet), gravimetry (measurements of the gravitational pull on satellites)  or budget calculations (combining estimates of snowfall with estimates of ice loss at the boundary of the continent)  – each with it’s own set of challenges and uncertainties. The author’s have combined results from 24 independent studies, using different methods and models, and the results are unambiguous: Antarctica has been losing mass and the rate of ice loss is accelerating.

Climate is changing; the ice loss is likely to continue and the sea level will continue to rise. It’s scary. I can go back into my office and try to understand more about what role the ocean is playing and about what is happening down south – but I cannot stop it. Not on my own. But maybe, hopefully, we can still do it together, all of us.

Ice berg in the Weddell Sea Photo: E. Darelius

 

 

 

Congratulations, Kjersti!

Yesterday Kjersti successfully defended her thesis “Exchange of water masses between the Southern Weddell Sea continental shelf and the deep ocean”!! Hipp hurray for Kjersti! Kjersti is my first PhD-student who finishes – so I admittedly was a bit nervous… but not as nervous as Kjersti… But she did  (as usual!) an excellent job presenting her work to relatives and colleagues her at GFI – and she responded nicely to all the questions from the opponents: Karen Hayewood and Angelica Renner. We had the chance to have three excellent female oceanographers at the stage at GFI – that’s does not happen that often!

Supervisors, opponents and PhD. Kjersti Daae! Photo: Ellen Grong

 

While finishing off her thesis Kjersti had found the time to knit mittens to us all (see photo and note the Penguins!) – thank you Kjersti!

Kjersti’s Penguin mittens – will definitely join me on my NeXT trip Down south!

Hip, Hip…..

…Hurray! We got money from the university to send Nadine (and some instrumentation) onboard “Kronprins Håkon” (KPH amongst friends) to Antarctica next season! KPH is the brand new Norwegian icebreaker and she will sail down to Dronning Maud Land and Fimbullisen in February, 2019.

Fimbullisen is a relatively small ice shelf that overhangs the continental slope in the eastern Weddell Sea. The Norwegian Polar Institute (NPI) has three sub-ice shelf moorings installed there, and two years ago we added an APRES (a handy little thing that you place on top of the ice to measure time series of ice shelf thickness from which one can infer the basal melt rate) to one of their sites. The plan is now to – in collaboration with NPI – also measure what happens outside of the ice shelf cavity.

Map over Antarctica with the Fimbull ice shelf marked in red. From npolar.no.

Seasonal outflow of Ice Shelf Water from the Filchner Ice Shelf

It took a bit longer than I expected – but here we go – my* latest “baby” is available online!

You can read the full version of the paper here:

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017GL076320

or a summary below!

In February last year we recovered a mooring at the Filchner Ice Shelf front (See map below) that we since long had consider lost. The large German ice-breaker Polarstern had failed to reach it twice due to sea ice, and it had now been in the water for more than four years. When we reached the location with (the much smaller) JCR last year, the mooring was only a few hundred meters from the advancing ice shelf front, and the captain was somewhat hesitant to go there – but he did, and the acoustic release on the mooring SA responded and released as promptly as if it had been deployed the day before! Most of the instruments had run out of battery and thus stopped recording – but one of them were still running, providing a four year long data record!

The mooring had several temperature and salinity sensors, and the records from them showed that there is a pulse of very cold (-2.3C!) ice shelf water (see explanation below) leaving the cavity during late summer and autumn each year. The water has been cooled down so much through interaction with the ice shelf base at depth, that there are ice crystals forming within it as it rises and leaves the cavity (I’ll write about what the ice crystals did to our instruments in a later post). The salinity of the cold water was relatively high – telling us that the water most likely entered the ice shelf cavity in the Ronne Depression, west of Berkner Island (see map).

In an earlier paper**, we had shown (using a numerical model) that ice shelf water flowing northward along the Berkner island would turn east when it reaches the ice shelf front (because conservation of potential vorticity hinders water to flow across the ice shelf front where the water depth suddenly changes by hundreds of meters) and exit the cavity in the east. But now the data showed that water was exiting the cavity in the west anyway?! What about the potential vorticity?? Our data also show that when cold water is flowing out of the cavity in the west during late summer, there is layer of less dense (and warmer) water present above it. In the paper we suggest that the presence of the upper, lighter layer breaks the potential vorticity constraint. The layer of less dense water reaches down roughly as deep as the ice shelf itself – and you can imagine that to the outflow it acts as a continuation of the ice shelf.

We now know that water leaves the ice shelf cavity also in the west – but where does it go then? Is there a flow of dense ice shelf water also along the western part of the Filchner trough?

Map over the southern Weddell Sea. The yellow dots show where our moorings were deployed, and the blue arrow show the path of the ice shelf water. From Darelius & Sallée, 2018.
Temperatures at the front of the Filchner ice shelf. Note that the temperature scale goes down to -2.3C! At the surface seawater can not be colder than -1.9C, then it freezes. Modified from Darelius & Sallée, 2018.
Density profiles at the ice shelf front. Red and green profiles are from periods with outflow – you see that the density decreases around 400 meters, roughly at the level of the ice shelf base. The black profiles are from a period without outflow – the density does not change at the depth of the ice shelf base. From Darelius & Sallée, 2018.

Ice shelf water: We define water that has a temperature below the surface freezing point (which is about -1,9C for sea water) as “ice shelf water”. The water leaving the cavity was as cold as -2.3C (See figure 2 above)! How can it be so cold? It is a combination of two physical facts: 1) The freezing point decreases as pressure increases and 2) water in contact with ice will have a temperature equal to the freezing point. In an ice shelf cavity we have ice in contact with water at large depth ( i.e. at large pressure) and the water will then be cooled down (the heat given off by the water is used to melt ice) to the local freezing point – and voila, you’ve got ice shelf water!

* I say my, but it’s a team effort: many thanks to J.B. Sallée who co-authored the paper and to all the people involved in deploying and recovering the moorings!

**Darelius, E., Makinson, K., Daae, K., Fer, I., Holland, P. R., & Nicholls, K. W. (2014). Circulation and hydrography in the Filchner Depression. Journal of Geophyscial Research, 119, 1–18. http://doi.org/10.1002/2014JC010225