The decomposition of the Faroe-Shetland Channel water masses using Parametric Optimum Multi-Parameter analysis

The Faroe-Shetland Channel (FSC) is an important conduit for the poleward flow of Atlantic water towards the Nordic Seas and, as such, it plays an integral part in the Atlantic's thermohaline circulation. Mixing processes in the FSC are thought to result in an exchange of properties between the channel's inflow and outflow, with wider implications for this circulation; the nature of this mixing in the FSC is, however, uncertain. To constrain this uncertainty, we used a novel empirical method known as Parametric Optimum Multi-Parameter (POMP) analysis to objectively quantify the distribution of water masses in the channel in May 2013. This was achieved by using a combination of temperature and salinity measurements, as well as recently available nutrient and δ18O measurements. The outcomes of POMP analysis are in good agreement with established literature and demonstrate the benefits of representing all five water masses in the FSC. In particular, our results show the recirculation of Modified North Atlantic Water in the surface layers, and the pathways of Norwegian Sea Arctic Intermediate Water and Norwegian Sea Deep Water from north to south for the first time. In a final step, we apply the mixing fractions from POMP analysis to decompose the volume transport through the FSC by water mass. Despite a number of caveats, our study suggests that improved estimates of the volume transport of Atlantic inflow towards the Arctic and, thus, the associated poleward fluxes of salt and heat are possible. A new prospect to more accurately monitor the strength of the FSC branch of the thermohaline circulation emerges from this study.