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!
…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.
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?
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
Writing a scientific article is a long process – you collect the data, you calibrate them, process them and you analyze them. You plot them, think about them, discuss them, think about them again until hopefully, at some point, the data give you results that you can understand and – publish. So you write the paper – in between meetings and teaching you somehow manage to squeeze your outstanding results and neatly prepared figures into the template provided by the journal. Then you submit – and forget about it all until you hear back from the editor three months later: the REVIEWS are back… sometimes it’s like this:
i.e. you quickly find out that your results were not that outstanding and your figures not that neat… the reviewers have filled page after page with “Did you consider…”, “why didn’t you calculate…” how does this compare to..”, “can you really ignore the effect of….” and “you ought to refer to the paper by mr so and so”…so you start over, you do all the extra analyses that reviewer three asked for, you make new figures, you clarify and expand and include a citation of mr so and so (the reviewer?). You read and write the text over and over and at some point you realize that you’ve done all that they ask for… and that version 6.2 of the paper is indeed much better than version 1.0. So you write a very polite letter to the editor, where you respond to each and every comment from the reviewers and explain what you’ve changed – and then you resubmit. And you wait. Again. For three months.
… but then sometimes, you get three short lines from the editor stating that you paper is accepted! It will be published!!! YES!!!
I received one of these e-mails the other day – and once the paper get online in a couple of days I’ll let you know what it is all about!
I’m still in the Bjørnafjord doing one last section before we head back to Bergen – but I just had a report from Svein Østerhus and Polarstern. They are now just north of the front of the Ronne Ice shelf in the Weddell Sea.
Scientist from British Antarctic Survey are onboard with “Boaty McBoatface” – an unmanned, autonomous (i.e. not attached to a cable) submarine with sensors for just about everything onboard – that they plan to send on a mission beneath the Ronne ice shelf! Truly exciting!!! I’d love to be there…
While being in the vicinity of the ice shelf front, Svein will deploy a couple of temperature recording LoTUS bouys (see previous post) within the ice shelf front polynya* for me. These will remain five years at the bottom before surfacing… so be patient!
*a polynya is an area within otherwise ice covered water. Tidal currents and wind typically keep the area just in front of the ice shelf front ice free during summer, and often also during winter.
A new day in the Bjørnafjord with the fjord oceanography students from GFI has begun – and we decided to check in on one of our moorings. The moorings are equipped with an “acoustic release”, a unit which we can communicate with using acoustic signals. Normally we only talk to it to tell it to release the anchor and come up to the surface, but you can also use it to find out where the mooring actually is… and that was what was on the schedule this morning.
The captain made three stops around the position where we let go of the anchor, and at each position we lowered a transducer down into the water and asked the release to tell us how far away it is*. There was some confusion about what codes to actually use (sorry Kristin for waking you up!), but once we got the right one the release responded promptly!
With three positions and three distances you can draw three circles – and if all is well they ought to cross each other in one location… which is where your mooring is! This time it was well and safe were we thought it was – which is good, because the captain had already reported the position to the navy who will do submarine training here in the weeks to come!
*what actually happens is that the deck unit measures the time it takes between emitting a signal and receiving a response, and knowing the speed of sound in the water you can calculate the distance.
You don’t have to go all the way to Antarctica to do exciting oceanographic fieldwork! This week I’m lucky enough to bring a bunch of enthusiastic students out on Krisitin Bonnevie to explore the fjord “Bjørnafjorden” just South of Bergen. Many of them have never been at sea before, but a week of CTD’s and moorings and they are ready to go just about anywhere!
One of the aims of the cruise is too try to solve the puzzle with the mysterious tidal currents in Lukksundet… Lukksundet is a narrow strait connecting the Bjørnafjord to the Hardangerfjord in the south. The tidal currents are very strong here – nothing strange with that – what’s strange is that they turn every two hours!
The tides along the coast of Norway are semi-diurnal; there are two high tides and two low tides a day. We’d expect the tidal currents to have the same periodicity (i.e. to change direction every sixth hour), but to be shifted in time so that maximum tidal currents occur in between high and low tides. Obviously, something more complicated is happening in Lukksundet! I’ve got an hypothesis about what is going on… do you?
The students have deployed moorings within and around the strait, and hopefully we’ll be able to resolve the riddle when we retrieve the data on another student cruise to the fjord a month from now!
The students have posted photos and a film from the cruise here!