On the origin and pathway of the saline inflow to the Nordic Seas: insights from models

The behaviours of three high-resolution ocean circulation models of the Nor th Atlantic, differing chiefly in their description of the vertical coordin ate, are investigated in order to elucidate the routes and mechanisms by wh ich saline water masses of southern origin provide inflows to the Nordic Se as. An existing hypothesis is that Mediterranean Overflow Water (MOW) is ca rried polewards in an eastern boundary undercurrent, and provides a deep so urce for these inflows. This study, however, provides an alternative view t hat the inflows are derived from shallow sources, and are comprised of wate r masses of western origin, carried by branches of the North Atlantic Curre nt (NAC), and also more saline Eastern North Atlantic Water (ENAW), transpo rted northwards from the Bay of Biscay region via a 'Shelf Edge Current' (S EC) flowing around the continental margins. In two of the models, the MOW f lows northwards, but reaches only as far as the Porcupine Bank (53 degreesN ). In third model, the MOW also invades the Rockall Trough (extending to 60 degreesN). However, none of the models allows the MOW to flow northwards i nto the Nordic Seas. Instead, they all support the hypothesis of there bein g shallow pathways, and that the saline inflows to the Nordic Seas result f rom NAG-derived and ENAW water masses, which meet and partially mix in the Rockall Trough. Volume and salinity transports into the southern Rockall Tr ough via the SEC are, in the various models, between 25 and 100% of those i mported by the NAG, and are also a similarly significant proportion (20-75% ) of the transports into the Nordic Seas. Moreover, the highest salinities are carried northwards by the SEC (these being between 0.13 and 0.19 psu mo re saline at the southern entrance to the Trough than those in the NAG-deri ved waters). This reveals for the first time the importance of the SEC in c arrying saline water masses through the Rockall Trough and into the Nordic Seas. Furthermore, the high salinities found on density surfaces appropriat e to the MOW in the Nordic Seas are shown to result from the wintertime mix ing of the saline near-surface waters advected northwards by the SEC/NAG sy stem. Throughout, we have attempted to demonstrate the extent to which the models agree or disagree with interpretations derived from observations, so that the study also contributes to an ongoing community effort to assess t he realism of our current generation of ocean models. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.