Abstract
The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope – at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel.
Highlights
The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC)
Strengthened cyclonicity during the early 1990s of the atmospheric circulation within the Nordic Seas and that of the Norwegian Sea produces an unimodal probability distribution suggesting that under anomalously cyclonic wind forcing the direct path contribution to the Faroe Bank Channel Overflow (FBCO) is dominant. These model results strongly indicate that the prevailing atmospheric circulation via its regulation of the wind-driven basin circulation is able to draw upon different upstream deep water pathways feeding the Faroe-Shetland Channel Jet (FSCJ) and the FBCO: one is routed directly into the Faroe–Shetland Channel (FSC), and one reaching the Norwegian margin before turning south toward the FSC
A key difference between the two periods under investigation is that according to the model during the early 1990s, the flow is strongly constrained to shallower isobaths that connect directly to the FSC, while during the early 2000s an additional flow to the east opens up along deeper isobaths (Fig. 4b)
Summary
Inferring the eastern overflow path from observations. A strong indication that the flow feeding the FBCO does not always a DS: Denmark Strait. Strengthened cyclonicity during the early 1990s (anticyclonicity during the early 2000s) of the atmospheric circulation within the Nordic Seas and that of the Norwegian Sea produces an unimodal (bimodal) probability distribution suggesting that under anomalously cyclonic (anticyclonic) wind forcing the direct (indirect) path contribution to the FBCO is dominant These model results strongly indicate that the prevailing atmospheric circulation via its regulation of the wind-driven basin circulation is able to draw upon different upstream deep water pathways feeding the FSCJ (as discussed below) and the FBCO: one is routed directly into the FSC, and one reaching the Norwegian margin before turning south toward the FSC. We conclude, based on both model simulations and observations from multiple platforms, that the FSCJ is the main conduit of overflow waters through the Faroe Bank Channel regardless of upstream pathways
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