1. Is the Transport of Atlantic Water in the Faroe Shetland Channel changing? – A summary of 20 years of observations
Bee Berx1, Bogi Hansen2, Svein Østerhus3, Karin Margaretha Larsen2, Toby Sherwin4 and Kerstin Jochumsen5
1 Marine Scotland Science 2 Faroe Marine Research Institute; 3 University of Bergen; 4 Scottish Association for Marine Science; 5 Universität Hamburg
Ocean Sci., 9,  , 2013
Good morning, my name is Bee Berx, I am from Marine Scotland Science, and today, I would like to present some of the research I’ve been doing in the Faroe Shetland Channel together with Bogi Hansen, Svein Osterhus, Karin Margretha Larsen, Toby Sherwin and Kerstin Jochumsen. This is work which formed part of the THOR project, and is currently contributing to the NACLIM project.
Before introducing you to the area, I just wanted to spend some time highlighting why we’re interested in circulation in the Faroe Shetland Channel.

2. Why are we interested in the Faroe Shetland Channel (FSC)?
So, if we look at a close up of the North Atlantic, we can see that the underwater Greenland Scotland Ridge forms a barrier to the exchange of water between the Nordic seas and Atlantic basin. There are three gaps in the ridge, where Atlantic water crosses northward into the nordic seas, and where deep cold waters outflow from the arctic back into the atlantic basin. These are important areas where we can observe the exchange between the two basins and any potential changes in future. The black symbols show observations of atlantic inflow into the nordic seas, and the white markers are where overflow observations have been made. So let’s take a closer look at the Faroe Shetland Channel…

3. Our knowledge of the North Atlantic Ocean circulation is not new!
Benjamin Franklin (1770)
JB Tait (1937)

4. Circulation in the Faroe Shetland Channel
So this is the Faroe Shetland Channel, on the Scottish side of the channel, we have the European continental shelf edge, and the Wyville Thomson Ridge which forms an underwater sill leading to the Faroe Bank. The Faroe Islands themselves are surrounded by a narrow continental shelf. So let’s take some time to investigate circulation of water masses of arctic and atlantic origin in the Faroe Shetland Channel.

5. Circulation in the Faroe Shetland Channel
NSDW
Deep
Water
MEIW
NSAIW
Intermediate Water
NAW
MNAW
Atlantic Water

6. Circulation in the Faroe Shetland Channel
NSDW
Deep
Water
At depth in the Faroe Shetland Channel Norwegian sea deep water flows from the norwegian sea in close to the sea bed, and exits through the faroe bank channel, where together with some of the other overflow waters, it will form north atlantic deep water.

7. Circulation in the Faroe Shetland Channel
MEIW
NSAIW
Intermediate Water
In the intermediate layers between about 500 and 800 m depth, the waters of the faroe shetland channel are occupied by norwegian sea arctic intermediate water, a intermediate water mass originating in the norwegian sea which contributes to the overflow, and modified east icelandic water (a water mass originating off the shelf edge near iceland and re-circulating in the faroe shetland channel: some of it joins the overflow, and some of it re-circulates and flows northwards into the nordic seas.

8. Circulation in the Faroe Shetland Channel
NAW
MNAW
Atlantic Water
Two water masses form part of the surface circulation in the faroe shetland channel: north atlantic water is the warmest, most saline water mass in the faroe shetland channel and flows along the continental shelf edge. It’s water which originates from the north atlantic basin, together with water from further south along the continental shelf. The modified north atlantic water is water from further west in the north atlantic basin, which has flowed across the iceland faroes ridge, some of this water enters the faroe shetland channel and recirculates, joining the north atlantic water into the nordic seas, while some of it flows into the atlantic basin and around the faroe islands.
Atlantic Water is defined as warmer than 5°C.

9. Observing Circulation in the FSC
So how do we monitor the circulation of atlantic water in the faroe shetland channel? Traditionally, this was done based solely on the hydrography (i.e. the ship-based salinity and temperature observations), but since 1992, we have also deployed acoustic profiling current meters to measure current speeds, these send an acoustic signal into the water column and based on the change in frequency of the backscatter from small particles (including zooplankton) in the water column, the instrument can calculate the velocity at a number of depth levels in the water column. Most recently we’ve also included sea surface height observations to calculate the amount of atlantic water, the volume transport, through the faroe shetland channel. Before I give a short overview of how we do this, we can have a look at the cross-section of water properties on the section above. Although hydrographic monitoring occurs at other sections, I’m going to focus on the Fair Isle Munken section.

10. Temperature and Salinity in the FSC on the FIM section
These four panels show the cross-section distribution of average temperature, salinity density and along-channel current speed. We can clearly see the different water masses, especially the surface north atlantic and modified north atlantic waters. These are identified by the high saline core and warm surface waters on the shetland shelf edge, and the modified north atlantic water on the faroese side which is generally cooler and less saline. at depth we find the fairly fresh and cold (so the densest water mass) deep water from the nordic seas, which forms the faroe bank overflow. On the bottom right panel, we can see the average velocities of these waters (the black contours) with the temperature colour plot underneath. We can see the strongest currents coincide with the core of the north atlatnic water, although the surface waters across the channel move northwards, on the left hand side (the faroese side) the surface waters have a generally slower speed and are moving southward, this is the recirculation from north of faroe islands part of which continues in to the atlantic, and part which veers and joins the north atlantic water northwards along the norwegian shelf edge in to the arctic.

11. The assumption of geostrophy
How to estimate Atlantic Water transport in the Faroe Shetland Channel?
The assumption of geostrophy
[where large ocean currents are an equilibrium of the pressure gradient and Coriolis]
allows us to convert sea level height measurements into current speeds.
SEOS Project
So how can we combine the knowledge from these three observational data sets to estimate the quantity of water flowing into the nordic seas, and how much heat is transported. We can use the assumption of geostrophy, where large currents are approximated to a balance between the pressure gradient and the Coriolis force. This means that we can relate sea level height measurements from the altimeter into surface velocity. The reason why we can extend this below the surface is that there is good agreement between the geostrophic velocity profiles based on the hydrography and the ADCP averages, and because these profiles show the surface water velocity to be fairly uniform in the Atlantic water layer.
Regression of Atlantic Water velocities from the current meter against sea surface height allows estimation of altimetry-based AW velocity.

12. Average volume, relative heat and salt transports in the FSCNE SW
Net (Atl. Inflow)
Surface-to-bottom
3.5 Sv
-3.1 Sv
0.4 Sv
Warmer than 5°C
-0.8 Sv
2.7 Sv
Colder than 5°C
0.0 Sv
-2.3 Sv
Relative Heat Transport
131 TW
-24 TW
107 TW
Salt Transport
125x106 kg s-1
-27x106 kg s-1
98x106 kg s-1
Average transport of Atlantic water in the 2.7 =1,100 Olympic-sized swimming pools per second
Compared to the total transport of Atlatnic Water across the GSR = 7.0 Sv
Or 2,800 swimming pools/second
cross the Greenland-Scotland Ridge
1 Sv = 106 m3 s-1

13. Average volume, relative heat and salt transports in the FSCNE SW
Net (Atl. Inflow)
Surface-to-bottom
3.5 Sv
-3.1 Sv
0.4 Sv
Warmer than 5°C
-0.8 Sv
2.7 Sv
Colder than 5°C
0.0 Sv
-2.3 Sv
Relative Heat Transport
131 TW
-24 TW
107 TW
Salt Transport
125x106 kg s-1
-27x106 kg s-1
98x106 kg s-1
Average transport of Atlantic water in the 2.7 =1,100 Olympic-sized swimming pools per second
Compared to the total transport of Atlatnic Water across the GSR = 7.0 Sv
Or 2,800 swimming pools/second
cross the Greenland-Scotland Ridge
Gritter capacity - 10,120kgs
DAF
1TW = 10^12 W = J/s
Mars = Energy kJ 1873kJ per 100g = 1086 kJ per 58 g bar
1 Sv = 106 m3 s-1
© All rights reserved by Thomas_Ashley (via Flickr)
Net (Atl. Inflow)
Surface-to-bottom (OSSP)
~160
Warmer than 5°C (OSSP)
~1,100
Colder than 5°C (OSSP)
~940
Relative Energy Transport (Mars)
~100 million
Salt Transport (Gritter)
~9700
1 OSSP = 1 Olympic Size Swimming Pool
DAF Gritter
(CPD Engineering)
1 standard Mars bar (58g)

14. Transport of Atlantic water in the FSC
The graphs show our observations of the transport of Atlantic Water. As can be seen, we have considerable variability in the time series. Part of the THOR project is to tease out the different factors which influence the transport and make it so variable.

15. Seasonal cycle in volume transport
Seasonal amplitude of Atlantic water transport in FSC
~ Sv
This is ~ 25% of the average transport
Maximum transport in winter-time, lowest transport in summer.
Different amplitude based on calculation method
If we plot the data by year or month, the two bottom panels here, we can see that this variability is not only on seasonal timescales, but there is also a considerable amount of changes between the years. It is important to understand what drives these changes, as numerical modellers are trying to predict changes on decadal and longer timescales. Under THOR, we have been investigating whether our observational strategy needs improving in order to better quantify this seasonal, inter-annual and decadal variability.

16. Transport of Atlantic Water in the FSC 1992-2012
No long term trend !
An analysis of the long-term change shows no significant positive or negative trend. With approximately 20 years of observations, we’re . Similar conclusions were found by people observing the atlantic inflow east of iceland and north of faroe islands, and those observing the deep water overflow in faroe bank channel and denmark strait.

17. Conclusions
Combined temperature and salinity measurements with current meter and sea elevation observations to estimate transport of Atlantic water through the FSC.
On average 2.7±0.5 Sv is transported into the Nordic Seas.
Although variable, the net volume transport shows consistent seasonality with maximum Dec-Jan, and an amplitude of 0.7 Sv.
No significant long-term trend in volume transport between 1992 and 2011.
We have observed increases in temperature and salinity, and may therefore expect trends in relative heat and salt transports (but difficult to verify statistically based on this time series).
Currently studying whether less variable observations can be made in an area to the south-west of the FIM section
By combining ADCP, CTD and altimetry data from the
1995–2009 period, we estimate the average net inflow of
Atlantic water (warmer than 5 C) through the FSC to
2.7±0.5 Sv. The average heat transport relative to 0 C was
estimated to 107±21TW and the average salt import to
98±20×106 kg s−1. Although highly variable, the net volume
transport seems to have a consistent seasonality with
maximum flow around the turn of the year and a seasonal amplitude
of 0.7 Sv. We find no significant trend in the volume
transport between December 1992 and September 2011, but
increasing temperatures and salinities may have induced positive
trends in the relative heat and salt transports, although
this is difficult to verify statistically.
When the ADCP mooring system was designed in the
early 1990s, it was decided to locate the ADCPs on a section
that had been monitored for temperature and salinity
for more than a century. As the measurements progressed,
it became clear that the high variability on that section would
make short-term transport variations difficult to monitor with
the established system.
The comprehensive ADCP and CTD data set has allowed
us to calibrate altimetry data to provide volume transport
time series and, for the future, altimetry data will be an essential
component of a monitoring system, but continued in situ
measurements will be needed to monitor water mass properties
and the subsurface velocity field. Whether this monitoring
should remain on the established section (Fair Isle–
Munken), or be moved to a section farther southwest depends
on results from an experiment carried out in 2011-
2012 within the EU-funded THOR project and on on-going
measurements in the framework of EU-NACLIM.
Ocean Sci., 9,  , 2013

18. Thank you. For questions/collaborations, please email b. berx@marlab
Thank you! For questions/collaborations, please
The research leading to these results has received funding from the European Union 7th Framework Programme (FP ), under grant agreement n NACLIM