Revised circulation scheme north of the Denmark Strait

Abstract

The circulation and water mass transports north of the Denmark Strait are investigated using recently collected and historical in-situ data along with an idealized numerical model and atmospheric reanalysis fields. Emphasis is placed on the pathways of dense water feeding the Denmark Strait Overflow Water plume as well as the upper-layer circulation of freshwater. It is found that the East Greenland Current (EGC) bifurcates at the northern end of the Blosseville Basin, some 450 km upstream of the Denmark Strait, advecting overflow water and surface freshwater away from the boundary. This "separated EGC" flows southward adjacent to the previously identified North Icelandic Jet, indicating that approximately 70% of the Denmark Strait Overflow Water approaches the sill along the Iceland continental slope.Roughly a quarter of the freshwater transport of the EGC is diverted offshore via the bifurcation. Two hypotheses are examined to explain the existence of the separated EGC. The atmospheric fields demonstrate that flow distortion due to the orography of Greenland imparts significant vorticity into the ocean in this region. The negative wind stress curl, together with the closed bathymetric contours of the Blosseville Basin, is conducive for spinning up an anti-cyclonic gyre whose offshore branch could represent the separated EGC. An idealized numerical simulation suggests instead that the current is primarily eddy-forced. In particular, baroclinic instability of the model EGC spawns large anticyclones that migrate offshore and coalesce upon reaching the Iceland continental slope, resulting in the separated EGC. Regardless of the formation mechanism, the recently obtained shipboard data and historical hydrography both indicate that the separated EGC is a permanent feature of the circulation north of the Denmark Strait.

The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic meridional overturning circulation

“…Variations in Nordic Seas convection, however, do not necessarily match with variations in the overflows across the GSR, since the source waters of the overflow water masses at the GSR are relatively complex (e.g. Tanhua et al, 2008;Våge et al, 2013). Our study suggests that the Denmark Strait overflow, though its influence is, on multimodel average, smaller than that of subpolar deep water formation, still explains a substantial part of multidecadal AMOC variance.…”

“…Since the source waters of the overflow water masses at the GSR are relatively complex (e.g. Tanhua et al, 2008;Våge et al, 2013), variations in Nordic Seas convection do not necessarily match with variations in the overflows across the GSR. In the CCSM4 (Community Climate System Model) preindustrial control simulation , AMOC variability is primarily driven by deep convection changes in the Labrador Sea.…”

Does the East Greenland Current exist in northern Fram Strait?

“…High instantaneous variability observed in both the synoptic data and the model output is attributed to eddies, the representation of which is crucial as they mediate the westward transport of AW in the recirculation and thus structure the confluence forming the EGC. (Mauritzen, 1996;Rudels et al, 2002;Våge et al, 2013). Some AW enters the Arctic Ocean through the Barents Sea and Fram Strait.…”

The circulation of Icelandic waters – a modelling study

“…Hence, the bulk of the Polar Water, which is mostly ice covered, passes Iceland along the western side of the Denmark Strait whereas smaller parts mix into the NIIC to the east (Logemann and Harms, 2006;Jónsson and Valdimarsson, 2012). This seems to happen mainly in the form of cold and fresh eddies separating from the Polar Front (Våge et al, 2013). Furthermore, the variable wind field north of Denmark Strait may cause events of eastward drift of Polar Water onto the North Icelandic shelf.…”