The Santa Claus at the Norwegian Research Council distributes his gifts already during the first weeks of December… and this year one of them landed on our desk! Our Project iMelt (which is short for “Ocean-ice shelf Interaction and channelized Melting in Dronning Maud Land”) led by Laura de Steur at NPI got funded!
10 MNoK to service moorings and installations on the Fimbull ice shelf, to hire a PhD-student and posdocs to analyse all the data we are and will be collecting and to numerically model the system we are studying 🙂
This is the Project summary form the Application:
The recent increase in the Antarctic contribution to global sea-level rise is a major concern given that the majority of the world’s population lives along the coastlines. This increase, which is now thought to be irreversible in West Antarctica, is triggered by ocean-induced melting beneath the floating parts of the ice sheet known as ice shelves. Most basal melting occur near the ice-sheet grounding lines and the ice-shelf fronts, as well as within basal channels underneath the ice shelves. This project will quantify the processes and importance of ocean-ice shelf interactions and channelized basal melting in Dronning Maud Land, East Antarctica. The main focus will be on Fimbulisen ice shelf which has a complex network of basal channels in the central part of the ice shelf and a tongue that extends seaward of the continental shelf. Under-ice shelf data has been collected at Fimbulisen since 2010 and new, planned infrastructure along the coast of Dronning Maud Land will allow us to investigate ocean processes outside the ice shelf. Three autonomous radars are also deployed on Fimbulisen and Nivlisen ice shelves to monitor ice-shelf basal melting directly. The Project will quantify the relationship between far-field ocean dynamics, ocean-ice interactions and basal melt rates through these concurrent oceanographic and under-ice shelf measurements. This interdisciplinary research combines in-situ measurements, satellite remote sensing, and high-resolution modeling of ice-ocean interaction in Dronning Maud Land and will provide fundamental new knowledge on processes related to basal melting, essential for a better understanding of the stability of the Antarctic ice sheet.
In 1893, during the Fram expedition, Fridtjof Nansen and his crew encountered a phenomenon where the velocity of the ship was reduced significantly, even though the engine was working at full speed. Nansen described this phenomenon as “dead water” (Brady, 2014). This dead water effect can happen when the ship creates an internal wave as it moves through water. The water must be stratified, meaning that the top layer is less dense than the bottom layer. At the same time, the draught of the ship must have the same depth as the top layer. The internal wave produces a drag, reducing the velocity of the ship. The speed of the wave is only dependent of densities and depth of the layers, not the velocity of the ship. (Grue, 2018).
We performed an experiment (as seen in the video) where we recreated the ocean conditions and created an internal wave. Then we explored how and when the internal wave could influence the velocity of the ship. To simulate the conditions Nansen experienced, a wooden boat was pulled with constant force across a tank filled with water. The water had two layers, one fresh layer on top (clear), and one saline underneath (purple). The depth of the saline layer must be much greater than the depth of the fresh layer.
The experiment was performed several times with the boat being pulled with constant, but different, force. We expect that if the speed of the boat is larger than the speed of the internal wave, the boat will not feel the wave because it moves faster than the internal wave. If the speed of the boat is smaller than the speed of the internal wave (as seen in the video), the wave will catch up with the boat, and the speed of the boat will be much reduced.
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.
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!
(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.
When the obstacle is moved across the tank, waves are generated in the interface between the layers as seen in the figure 2.
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.
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.
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!
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.
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!
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!
This semester has been very busy. I have been working simultaneously on two papers, as well as written and submitted my doctoral thesis. Two days before I submitted my PhD-thesis I received some very good news. My paper on the Filchner overflow was accepted! I was very pleased to include the acceptance status in my thesis.
You can read the full version of the paper if you click here … or read the summary below:
During a large part of my PhD, I have been studying processes associated with the production and pathways of cold Ice Shelf Water (ISW) in the Weddell Sea (see map in Figure 1). ISW is formed under the Filchner-Ronne ice shelf and is flowing northward along the Filchner Trough. The ISW overflows the Filchner sill, mixes with warmer water masses and form Antarctic Bottom Water.
(You can read more about ISW here )
At the Filchner Sill, several year-long records of current velocity exist between 1977 and 2017. The records show large fluctuations in the Filchner overflow velocity. However, no previous studies have been able to figure out which mechanisms contribute to the strong current fluctuations. Most of the current records contain about one year of data, and are therefore too short to capture long-term variations that may be related to climate change or long-term variability. We focused instead on monthly time scales, and found a link between the variability of the Filchner overflow and the wind forcing. Strong wind along the continental slope leads to higher Filchner overflow velocity (Figure 2).
So how can the along-slope wind upstream of the Filchner Trough influence the Filchner overflow?
We think that the slope current, which is flowing westward along the continental slope, may hold the key to answering this question. In a previous model study (Daae et.al, 2017 ), we found that parts of the slope current takes a detour, and circulates over the Filchner trough mouth region during strong wind-forcing (indicated by the thin red arrow in Figure 1). This circulation may interact with the Filchner overflow and lead to enhanced overflow. Although the existing data set is insufficient to prove that this is what happens, we present measurements at different locations which are consistent with this hypothesis.
Elin was part of a team that deployed several moorings across the Filchner sill and the continental slope in 2017. We hope that the data from these moorings, will contribute to increase our understanding of the Filchner overflow variability and out hypothesis of interaction between the slope current and the Filchner overflow.