A decomposition of the Faroe-Shetland Channel water masses
using POMP analysis Mckenna, C. 1, Berx, B. 2, and Austin, W. 1,3 1
School of Geography and Geosciences, University of St Andrews, St
Andrews, Fife, KY16 9AL, Scotland 2 Marine Scotland Science,
Scottish Government, Marine Laboratory, 375 Victoria Road,
Aberdeen, AB11 9DB, Scotland 3 Scottish Marine Institute, Scottish
Association for Marine Science, Oban, PA37 1QA, Scotland 1 2.
Current state of the art Since the 1970s, water properties have
been monitored regularly across 2 sections of the FSC (Fig. 1),
which has established that 5 water masses flow through the channel
(Fig. 2). Their contrasting origins gives them distinct temperature
and salinity signatures, which have been used in the past to detect
their presence in the FSC 11,15. But, while an absent signature
implies the absence of a water mass, this could also arise from
intense mixing. Thus, the nature and extentnd mixingextent
1.Background The poleward flow of warm and saline Atlantic water
through the Faroe-Shetland Channel (FSC) accounts for a large
fraction of the total Atlantic inflow into the Nordic Seas 1,12.
Therefore, the FSC is an important conduit for the poleward
transport of salt, heat and nutrients, which creates favourable
conditions for the economically important Nordic fish stocks 10.
This transport of salt also enhances the formation of intermediate
and deep waters in the Arctic 6. These waters then flow back
towards the south, transporting a total ~5.6Sv of water into the
North Atlantic 13, of which which ~2.1Sv passes through the FSC 7.
The FSC is thus an integral gateway in the present operation of the
thermohaline circulation (see video) and, as such, research into
the nature of mixing and circulation in the channel is important.
Video showing the important role that the FSC (highlighted in box)
plays in the North Atlantic thermohaline circulation. Source:
https://www.youtube.com/watch?v=c- GOFHPkf6Q Figure 1: Map of the
Faroe-Shetland Channel and the 2 standard hydrographic sections:
Fair-Isle-Munken (FIM) and Nolso-Flugga (NOL). and extent of mixing
between the FSC water masses is currently uncertain. Simple
empirical mixing models have been used to calculate water mass
ratios in the FSC 6,8, but due to a limited number of known
variables they can only model up to 3 of the FSC water masses with
statistical significance. We therefore propose using the novel
method of Parametric Optimum Multi- Parameter (POMP) analysis 2,
which can model all 5 FSC water masses while maintaining
statistical significance. Figure 2: The distribution of the FSC
water masses across the NOL section, established from temperature
and salinity measurements. NAW North Atlantic Water; MNAW Modified
North Atlantic Water; MEIW Modified East Icelandic Water; NSAIW
Norwegian Sea Arctic Intermediate Water; NSDW Norwegian Sea Deep
Water. 5
Slide 2
4. Results in May 2013 2 Figure 3: Plot of temperature vs.
salinity using data collected from the FSC over the period May
2009-2013. The source water properties were identified from
prominent end members e.g. NAW most saline point; NSAIW least
saline point in the region -0.5C to 0.5C; NSDW densest point.
Dotted lines are density contours. is likely to be in high
proportions at adjacent points. This places an extra constraint on
the problem, therefore reducing the number of unknown variables and
improving the statistical significance of the solution. Generally,
the water masses enter the channel as well defined cores, but
spread out and sink as they flow through the channel. The POMP
analysis captures these mixing processes well, as well as correctly
predicting the general distribution of the water masses (Fig. 2).
The interesting shape of MNAW implies its recirculation at FIM, as
is thought in the literature 14,16. The POMP analysis suggests this
also causes increased vertical mixing of NAW, MEIW and NSAIW in the
centre of FIM, ~300m. POMP confuses MEIW and MNAW in surface waters
on the Faroese side of the channel, especially at FIM. This
probably reflects that MNAW contributes to the MEIW formation
process 5. Internal solitary waves are thought to propagate up the
Shetland side of the FSC, causing mixing between deeper and upper
water layers 4,9. The mixing fractions manage to capture this
process and suggest this mixing may be significant on NOL.
Slide 3
5. Conclusions POMP currently works well in the FSC with
temperature, salinity and nutrient data. Even though the model is
empirical, it tell us a lot about which mixing processes and
relationships are significant on each section of the channel.
Acknowledgements This work received funding from the MASTS pooling
initiative (The Marine Alliance for Science and Technology for
Scotland) and their support is gratefully acknowledged. MASTS is
funded by the Scottish Funding Council (grant reference HR09011)
and contributing institutions. Salinity analyses were conducted by
members of the Oceanography Group at Marine Scotland Science: we
would like to thank David Lee, Matthew Geldart, Dougal Lichtman and
George Slesser for their efforts. Nutrient analyses were performed
by the Analytical Chemistry Group at Marine Scotland Science: we
are grateful to Pamela Walsham, Alison Taylor and Lynda Webster.
Oxygen isotope analyses were conducted by WENA and Angus Calder at
the University of St Andrews; we acknowledge helpful discussions of
these data with Lauren Gillespie. The POMP analysis computer code
was kindly given to us by Anouk de Brauwere, for which we are very
grateful. References 1.Berx, B., Hansen, B., sterhus, S., Larsen,
K.M., Sherwin, T., & Jochumsen, K. (2013). Combining in-situ
measurements and altimetry to estimate volume, heat and salt
transport variability through the Faroe Shetland Channel. Ocean
Science Discussions, 10(1), pp.153-195. 2.de Brauwere, A., Jacquet,
S.H., De Ridder, F., Dehairs, F., Pintelon, R., Schoukens, J.,
& Baeyens, W. (2007). Water mass distributions in the Southern
Ocean derived from a parametric analysis of mixing water masses.
Journal of Geophysical Research: Oceans (1978-2012), 112(C2).
3.Dickson, R.R., Meincke, J., Malmberg, S.A., & Lee, A.J.
(1988). The great salinity anomaly in the northern North Atlantic
19681982. Progress in Oceanography, 20(2), pp.103-151. 4.Hall,
R.A., Huthnance, J.M., & Williams, R.G. (2011). Internal tides,
nonlinear internal wave trains, and mixing in the Faroe-Shetland
Channel. Journal of Geophysical Research: Oceans (1978-2012),
116(C3). 5.Hansen, B. & sterhus, S. (2000). North
Atlantic-Nordic Seas exchanges. Progress in Oceanography, 45(2),
pp.109-208. 6.Hansen, B., sterhus, S., Htn, H., Kristiansen, R.,
& Larsen, K.M.H. (2003). The Iceland- Faroe inflow of Atlantic
water to the Nordic Seas. Progress in Oceanography, 59(4),
pp.443-474. 7.Hansen, B., & sterhus, S. (2007). Faroe Bank
Channel overflow 1995-2005. Progress in Oceanography, 75(4),
pp.817-856. 8.Hermann, F. (1967). The TS Diagram Analysis of the
Water Masses over the Iceland-Faroe Ridge and in the Faroe Bank
Channel (Overflow 60). Rapports et Procs-Verbaux des Runions du
Conseil International pour lExploration de la Mer, 157, pp.139-149.
9.Hosegood, P., van Haren, H., & Veth, C. (2005). Mixing within
the interior of the Faeroe- Shetland Channel. Journal of Marine
Research, 63(3), pp.529-561. 10.Larsen, K.M.H., Htn, H., Hansen,
B., & Kristiansen, R. (2012). Atlantic water in the Faroe area:
sources and variability. ICES Journal of Marine Science: Journal du
Conseil, 69(5), pp.802-808. 11.Martin, J.H.A (1993). Norwegian Sea
intermediate water in the Faroe-Shetland Channel. ICES Journal of
Marine Science: Journal du Conseil, 50(2), pp.195-201. 12.sterhus,
S., Turrell, W.R., Jnsson, S., & Hansen, B. (2005). Measured
volume, heat, and salt fluxes from the Atlantic to the Arctic
Mediterranean. Geophysical Research Letters, 32(7). 13.Sherwin,
T.J., Griffiths, C.R., Inall, M.E., & Turrell, W.R. (2008).
Quantifying the overflow across the Wyville Thomson Ridge into the
Rockall Trough. Deep Sea Research Part I: Oceanographic Research
Papers, 55(4), pp.396-404. 14.Sherwin, T.J., Hughes, S.L., Turrell,
W.R., Hansen, B., & sterhus, S. (2008). Wind driven monthly
variations in transport and the flow field in the FaroeShetland
Channel. Polar Research, 27(1), pp.7-22. 15.Turrell, W.R., Slesser,
G., Adams, R.D., Payne, R., & Gillibrand, P.A. (1999). Decadal
variability in the composition of Faroe Shetland Channel bottom
water. Deep-Sea Research Part I: Oceanographic Research Papers,
46(1), pp.1-25. 16.van Aken, H.M. (1988). Transports of water
masses through the Faroese Channels determined by an inverse
method. Deep Sea Research Part A. Oceanographic Research Papers,
35(4), pp.595-617. Case FIMNOL CFDFCFDF (1) No 18 O, all nutrients
142.74405455.09352 (2) No 18 O, limited nutrients 91.0588-- (3)
With 18 O, limited nutrients 102.71112-- Table 1: A comparison of
the cost function (CF) for different data combinations. CF =
distance between the modelled fractions and measurements. DF =
degrees of freedom; if the CF is larger in magnitude than DF, then
there are potentially model errors present. Case 1 used the full
resolution nutrient dataset, whereas cases 2 and 3 only used
nutrient data where there was 18 O data. Case FIMNOL NAWMNAWMEI W
NSAI W NSD W NAWMNA W MEIWNSAI W NSDW (1) No 18 O, all nutrients
5.27.03.95.04.65.37.13.14.23.6 (2) No 18 O, limited nutrients
8.511.03.95.65.9----- (3) With 18 O, limited nutrients
8.110.63.95.65.9----- Table 2: A comparison of the uncertainties
associated with the mixing fractions (in %) for each case tested
(calculated from 1000 Montecarlo simulations in each case). 6.
Future perspectives Our May 2013 results highlight the mixing
processes between the water masses. However, we need to investigate
different seasons and years to explore the temporal variability of
these processes. Changes in the properties of one water mass could
propagate into another, with wider implications for the
thermohaline circulation. Indeed, mixing in the FSC between
freshening Atlantic waters and intermediate waters may have helped
wide- scale freshening of the northern North Atlantic in the 1960s,
70s and 80s, which is thought to have inhibited convective
overturning 3. The results could be more accurate if we used an
additional variable in the model: stable oxygen isotopes ( 18 O).
Even though 18 O is currently sampled at a relatively low
resolution in the FSC, it still improves the results (compare cases
2 and 3 in Tables 1 and 2). Samples will be collected at a higher
resolution in October 2014. POMP may help us to identify NAW in the
FSC and, so, improve estimates of the volume transport of the
Atlantic inflow into the Nordic Seas (i.e. the strength of the
thermohaline circulation). 3