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, 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

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!