Discovery of an unrecognized pathway carrying overflow waters toward the Faroe Bank Channel

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial climate regulator, transporting warm, saline Atlantic waters to higher latitudes where they cool, sink, and return as dense overflow waters. The Faroe Bank Channel Overflow (FBCO) is a significant component of this system, transporting deep water from the Nordic Seas to the North Atlantic Ocean through a narrow passage. While the FBCO's volume transport has been monitored since 1995, the source regions and transport routes remain poorly understood. Most studies assume the FBCO originates solely from the western Nordic Seas, flowing north of the Faroes and directly into the Faroe-Shetland Channel (FSC). However, some modeling studies have hinted at a second branch originating from the Norwegian slope. This study aims to investigate the existence, origin, and mechanisms driving this potential eastern overflow pathway using a combination of observational data from moorings, vessel-mounted ADCPs, and an eddy-resolving ocean model. Understanding these pathways is crucial because the production of dense overflow water in the Nordic Seas serves as an important indicator of AMOC stability. Any changes in the strength of these overflows could signify disruptions in the North Atlantic current systems and have significant implications for our climate.

Previous research on the Nordic Seas overflow has focused on wind and thermohaline processes influencing its variability. Studies utilizing observations and models have investigated these factors, but the source regions and transport routes of the FBCO have received less attention. A float study by Søiland et al. (2008) suggested that only water within the 1750 m depth contour northwest of the Faroe plateau could be exclusively routed to the FBC via the FSC. However, modeling studies by Serra et al. (2010) and Köhl (2010) proposed a two-branch system: one from the Norwegian slope and another along the Jan Mayen Ridge, with their relative strengths depending on wind forcing. These models suggested the Norwegian slope as the main source, but with a potential shift towards a two-branch system under strong cyclonic wind forcing. No dedicated observational study, prior to this research, had confirmed the existence of the eastern pathway from the Norwegian slope or identified the primary transport route within the FSC.

This study combined various observational data and modeling techniques. Current measurements were collected from moorings and vessel-mounted Acoustic Doppler Current Profilers (ADCPs) in the Faroe Bank Channel and the Faroe Current. A high-resolution eddy-resolving ocean model (NEMO, version 3.6) with a global grid of nominally 1/12° horizontal resolution and 75 vertical z-star levels was used. The model was forced by the DRAKKAR forcing set v5.2, based on ERA-40 and ERA-interim reanalysis data. The model outputs included three-dimensional velocity, temperature, and salinity fields. Backward trajectory simulations, using the TRACMASS Lagrangian trajectory code, were performed to track water masses backward in time from the FSC, focusing on particles colder than 2°C and tracing them until they reached a latitudinal section corresponding to ~65°N or for a maximum of five years. Satellite altimetry data were used to examine sea-surface height patterns associated with FBCO transport. The study also analyzed volume flux calculations from the model, distinguishing between western and eastern boundary currents in the FSC. Finally, data from moored ADCPs across the FSC provided additional insights into the deep circulation.

The study revealed a strong anticorrelation between the deep eastward flow in the Faroe Current (FCdeep) and the FBCO transport on both interannual and seasonal timescales. This inverse relationship suggests that the FBCO cannot solely originate from the western approach. Backward trajectory simulations demonstrated two distinct pathways feeding the FBCO: a direct western path and an indirect eastern path from the Norwegian slope. The prevalence of each path depends on the atmospheric circulation regime. Anticyclonic (cyclonic) atmospheric circulation anomalies over the Nordic Seas lead to stronger (weaker) FBCO transport and weaker (stronger) eastward FCdeep. Regardless of the upstream pathway, the strongest modeled deep velocities and the bulk of FBCO transport are consistently found along the eastern boundary of the FSC, within a previously unrecognized deep current jet termed the Faroe-Shetland Channel Jet (FSCJ). This jet is bottom-intensified and has a secondary core at intermediate depths. The FSCJ's volume transport is significantly larger than that of the western boundary current in the FSC. Analysis shows that the eastern pathway is associated with colder and denser waters compared to the western path, suggesting an increased supply of water originating from the Jan Mayen Ridge during periods of strong eastern flow.

The findings address the research question by identifying a previously unrecognized pathway for dense overflow waters into the Faroe Bank Channel. The inverse relationship between the FCdeep and FBCO transport highlights the importance of considering multiple pathways and the influence of atmospheric circulation. The discovery of the FSCJ as the main conduit for FBCO transport fundamentally alters our understanding of flow dynamics in the FSC. The observed bimodality of deep water pathways has implications for the Iceland-Scotland overflow water and the subpolar overturning circulation. The similarity between the FSCJ and the North Icelandic Jet suggests a common underlying dynamic mechanism. The study's results emphasize the need for further research to understand the dynamics of this two-branch overflow system and its sensitivity to future climate changes.

This study provides compelling evidence for an eastern pathway feeding the Faroe Bank Channel overflow, supported by observations and an eddy-resolving ocean model. The Faroe-Shetland Channel Jet (FSCJ), a previously unrecognized deep current, is identified as the primary conduit for this overflow, regardless of its upstream origin. The influence of atmospheric circulation on activating this eastern pathway is significant, particularly under anticyclonic conditions. Future research should focus on the detailed dynamics of the FSCJ, its sensitivity to climate change, and its broader implications for the AMOC and water mass properties.

While this study provides strong evidence for the eastern pathway and the dominance of the FSCJ, further investigation is needed to fully quantify the relative contributions of the western and eastern pathways under various atmospheric conditions. The model's resolution, while relatively high, may still limit the representation of certain small-scale processes. Additionally, the observational data spans a limited period, preventing conclusions on long-term trends in overflow pathways.