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

 

 

Article accepted!

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!

 

 

 

 

Polarstern in the Weddell Sea

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.

Polarstern 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.

 

Trying out triangulation!

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.

Waiting for the ship to get in position (Algot, Vår and Carola)

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!

Transducer going down!
Is there anyone out there? Vår listening for the acoustic release to respond

 

 

 

 

 

 

 

 

 

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.

To the Bjørnafjord with students from GFI!

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!

Leaving Bergen for Bjørnafjorden (Photo: Carola Detring)

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!

Discussing mooring design (photo: Carola Detring)

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?

Map over Lukksundet, with the Bjørnafjord in the north and the Hardangfjord in the south.

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!

On our way to deploy a tidal gauge in Lukksundet (Poto: Carola Detring)

The students have posted photos and a film from the cruise here!

 

 

 

A birthday present from the Weddell Sea!

Amongst the facebook birthday wishes in my inbox I also found this perculiar Message:

Transmit Time: 2018-02-02T13:41:59Z UTC

Iridium Latitude: -77.0799

Iridium Longitude: -33.8234

Data: 2c31362c54454d50313030302c323031372d30312d30312030303a30303a30362c36302c393534302c333630302c2d312e3837312c333233392c332c3630382e33342c3630352e39312c31312e30322c76322e362e382c31363031383b…

which means that at least one of the LoTUS buoys that I deployed last year did what it was told and surfaced today!

A LoTUS buoy waiting to be deployed

LoTUS stands for “Long Term Underwater Sensing” and it is a bottomlander that you more or less through over the side of the ship. It sinks to the bottom, where it measures the tempearture until it is programmed to let og of its weight and come to the surface. Once at the surface, it transmits the data back to us in the office via satellite. Very nice!

The number in the Message above are as in-understandable to me as they are to you – but hopefully the instrument devellopers from KTH will be able to transform them into understandable data… a one year long temperature record from a location just north of the Filchner Ice Shelf front in the Weddell Sea! There was one more buoy that was programmed to come up today and which didn’t yet report home – so keep your finger’s crossed!

A collegaue of mine, Svein Østerhus, is currently onboard Polarstern in the Weddell Sea, and he will deploy more of these buoys for me later during the cruise!

Below are a few Pictures from the LoTUS buoy deployment last year:

Sorting out the ropes…
Waiting for the ship to be in the right position… (I’m in black and yellow)
… and off it goes!

 

 

27/01/2018: On to Plan C!

Guest blog by Karen Assmann

After our outing on to the eastern Getz Ice Shelf, the weather got a bit worse again we continued our way east with more moorings being recovered and deployed and three ocean gliders going in the water. These small autonomous underwater vehicles will spend the next year going in and out of the Dotson Ice Shelf cavity, if everything goes to plan. Getting ocean observations in the ice shelf cavities, rather than along their edges like we do with ships and moorings, is difficult. The few observations that exist come from drilling holes through several hundred meters of ice and deploying instruments through them or from sending bigger unmanned submarines into them. So sending ocean gliders that normally profile the surface ocean into the cavity under the ice shelf is rather daring. If the project by the University of Washington and Columbia University succeeds, it will mean that we will learn a huge amount about what happens in the ocean under the shelves, but the risk that the ocean gliders do not make it back out is high.

Looking across the helicopter deck towards Moore Dome where the weather station was supposed to be deployed. Now that there won’t be any more helicopter operations, other equipment has been moved back on to the helicopter deck.

 

At this point the weather turned good again and we dashed east across the bay to start on the next lot of helicopter work. This was to include the replacement of an automated weather station (AWS), and two days of radar and magnetics surveys and ApRES deployments on Thwaites Glacier and Dotson Ice Shelf. We were crossing fingers for 3 days of good weather. The AWS replacement on the first day ended rather abruptly, when the old weather station couldn’t be found and the responsible scientist decided not to replace it with a new one. Preparations for the second day of air operations came to an abrupt halt when a leak in the hydraulic system of one of the helicopters was found. Despite best efforts and extensive spares kits the leak couldn’t be fixed and this spelled the end of any work that needed a helicopter. Two working helicopters are needed for safe operations as an emergency backup, so the fact that one of them was no longer safe to fly meant the other couldn’t fly either. After a brief pause to re-group, on we went with the ocean work on the schedule while considering what could be done with the time that has suddenly become available.

Attachments

24/01/2018: More moorings…and a birthday in the air and on the ice shelf

Guest blog by Karen Assmann

After we finished our mooring marathon at the western Getz ice shelf we started to make our way east towards the Amundsen polynya. One stop on the way was the last UIB mooring that was placed in one of the smaller openings of the ice shelf. The area where the mooring was located had been covered in fast ice and heavy sea ice since we started monitoring sea ice conditions at the start of the cruise. When we got there the mooring position was located in open water about 100 meters from sea ice that was tightly packed by strong winds. After a bit of deliberation we decided not to release and recover the mooring, because we were worried that the strong winds and surface currents were going to carry it into the sea ice. We did however talk to the acoustic release and worked out that the mooring was still where we had left it two years ago.

Gray, blustery waves and a heavy sea ice cover at the UIB3 mooring site.
OLYMPUS DIGITAL CAMERA

Then we turned further eastward to make use of a weather window that would allow us to complete some of the helicopter work that was on the schedule. As well as a joint KOPRI/University of Bergen/University of Gothenburg effort to deploy autonomous phase-sensitive radars ApRES to measure ice shelf basal melt rates, there was also a group from the University of Texas who had installed radar to measure the ice sheet structure and magnetics equipment to find out about the type of ground under the ice on one of the helicopters. After being windy, snowy and cloudy for a few days the weather turned sunny, clear and calm on 22 January that also happened to be my birthday. After a few delays we loaded our equipment into one of our helicopters, hopped in and away we went over the cliff of the ice shelf edge onto the white expanse of the eastern Getz ice shelf. I have been involved in research around ice shelves for 20 years and two years ago I finally got to stand on one. This time was just as exciting even though at the surface and ice shelf is just very flat and white and big.

An ApRES site during the setup with a central snowhole to hold the actual instrument and battery and two holes for the antenna boxes to transmit and receive the radar signal.

We spent a bit of time setting up our first ApRES site, while the helicopter shuttled out more equipment and fuel drums for the other helicopter to refuel while doing its radar and magnetics surveys. Then we loaded everything into the helicopter and moved to the second site where the setup went much faster, partly because we now had the pilot to help as well, partly because we had now worked out how to do things efficiently. After a photo session we took the scenic route back to the ship.

 

The ApRES team with the mast holding the Iridium and GPS antennas that is all that remains above the surface when the site has been deployed.
Looking towards the western side of the bay at Getz East. One of the icebergs in the distance probably damaged some instruments on one of the KOPRI moorings.

23/01/2018 The Western Getz Mooring Marathon

In some ways it was lucky that Johan only realized quite how full the schedule was going to be at the western side of the Getz ice shelf two days before the cruise. Otherwise he might not have agreed to come on the cruise. The change of schedule had moved this work to the start of the second half of the cruise and there were only two stations before we started our first mooring recovery at 22.00 on 18 January.

After a few pings from the deck unit the release on the first mooring woke up and we got the okay from the bridge to release the mooring. Shortly afterwards we spotted the orange buoys at the surface and the ship inched up to them to catch the top float with grappling hooks and to start pulling the rope, floats and instruments onto the ship over the stern. The whole recovery took about an hour and by about 07.00 in the morning we had recovered two more moorings, the last rather close to the ice shelf edge. Mooring two didn’t tell us that it had released and just popped up to the surface. The top instrument on the third had been pushed to the next one below by an iceberg, but both were fine.

A mooring comes to the surface at the ice shelf edge.

To get the new moorings into water we had to download the data from some of the instruments, change their batteries and give them a service as quickly as possible to not keep the ship waiting too long. Luckily we had help from the glider team and just after lunchtime we were ready to deploy the first of our new moorings. These first two deployments took us to dinner. The transit to the other side of the ice shelf gave us the opportunity to catch a few hours sleep before the last recovery and deployment. Those came and went without any problems as well and both us, and the deck crew were getting so efficient at the whole process that the time we were given on the schedule for each recovery and deployment was reduced from four to two hours. By the time it got to breakfast on 20 January we were finished, 34 hours after starting with all four moorings that we deployed in the area two years ago successfully recovered and three new ones in the water to take over their work.

An ADCP current meter is deployed from the A-frame.
Mooring work with a view – Mount Siple.