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!

Wind Stress Mediated Variability of the Filchner Trough Overflow, Weddell Sea

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 )

Figure 1. Map of the southeastern Weddell Sea. Moorings on the Filchner Sill are shown with yellow markers. The red arrow indicates the slope current, with a thin recirculationg branch over the Filchner sill. The blue dashed arrow indicate the northward flow of ISW in the Filchner Trough. Upstream wind is calculated from model reanalysis products in the area inside the thick black border. The green star shows the Halley research station where also observational wind data is available.

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).

Figure 2. Along-slope wind from Halley Research station (gray) and Filchner overflow velocity from a mooring at the Sill (yellow) in 1977. High correlation is found between February and September, when the wind-forcing along the continental slope (245o) is strong.

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.

Introducing: Kjersti Daae

 I am a PhD-student (defending my thesis now in June – puh!) working with Elin at the Geophysical Institute, University of Bergen, working on the exchange of water masses between the continental shelf and the deep ocean in the Southern Weddell Sea, Antarctica. 

Based on idealized modeling and moored observations, I study mechanisms that can bring warm water of oceanic origin onto the continental shelf and contribute to basal melting of the ice shelves. Furthermore, I study the production and export of cold and dense shelf waters, which overflows the Filchner sill, mixes with off-shore water masses and forms Antarctic bottom water. Antarctic bottom water is an important driver of the global thermohaline circulation, and is found near the bottom, in the large oceans.

Tomorrow I’ll tell you about one of the articles in my PhD-thesis that just got published!

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.

Getting prepared for emergencies

Going on a scientific cruise is mostly exciting, interesting and fun, but it is also linked to risks.
I participated in a survival suit course that prepares us for emergencies on the ship during which we have to use floating suites and life rafts to survive in cold waters far away from help. During the training we learned how to handle the suits, swim in them, build formations to stay close together and how to enter a life raft that contains all survival equipment. It was a lot of fun and we are now well prepared for our next cruise!

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

 

 

Going to EGU—a huge European geoscience conference

As scientists it’s not enough to only do research alone in our little office, but we also need to get among other scientists to find out what new research is being done, to network and to make other’s aware of our important piece of work. Each year, there is a big European geoscience conference (EGU) in Vienna which again took place last week. 15000 scientists attended from more than 100 different countries and I (Nadine) was among them! In my luggage: A poster presenting our experiments at the Coriolis platform and some first results. This was very exciting, because it is a unique opportunity to talk to other scientists doing similar research. I was even co-convener in a session about the Southern Ocean which means I got to pick the talks and helped preparing everything in advance! After that week I was very exhausted, but happy! The program was stuffed with interesting talks and posters about Antarctica, oceanography, ice shelves, ice shelf—ocean interaction and glaciers and I got to talk to many skilled, interested and motivated scientists. All of them loved our experiments on the water-filled mary-go-round!

Happily presenting our poster at this year’s European geoscience conference (EGU) in Vienna!