1. 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

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 -0.5°C to 0.5°C; 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.

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., Østerhus, 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 1968–1982. 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. & Østerhus, S. (2000). North Atlantic-Nordic Seas exchanges. Progress in Oceanography, 45(2), pp.109-208. 6.Hansen, B., Østerhus, S., Hátún, 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., & Østerhus, 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 Procès-Verbaux des Réunions du Conseil International pour l’Exploration 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., Hátún, 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.Østerhus, S., Turrell, W.R., Jónsson, 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., & Østerhus, S. (2008). Wind ‐ driven monthly variations in transport and the flow field in the Faroe–Shetland 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 δ 18 O, all nutrients 142.74405455.09352 (2) No δ 18 O, limited nutrients 91.0588-- (3) With δ 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 δ 18 O data. Case FIMNOL NAWMNAWMEI W NSAI W NSD W NAWMNA W MEIWNSAI W NSDW (1) No δ 18 O, all nutrients 5.27.03.95.04.65.37.13.14.23.6 (2) No δ 18 O, limited nutrients 8.511.03.95.65.9----- (3) With δ 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 ( δ 18 O). Even though δ 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