The subsurface circulation of the Iceland Sea observed with RAFOS floats

Abstract

The pathways of dense waters located above the sill depth of Denmark Strait were investigated in the Iceland Sea using 52 acoustically tracked RAFOS floats. These floats were deployed in summer 2013 and 2014, with a target depth of 500 m, resulting in a total of 40.9 float-years of track data covering the Iceland Sea basin. In the interior Iceland Sea basin, the float tracks showed a double gyre circulation, out of which floats eventually escaped towards the Norwegian Sea in the East Icelandic Current, with some appearing to be en route to the Faroe Bank Channel. Four floats exited through Denmark Strait and surfaced in the Labrador and Irminger Seas. Four other floats deployed west of the Kolbeinsey Ridge at 70˚N show the connection between the East Greenland Current and the East Icelandic Current. Floats deployed east of the Kolbeinsey Ridge along the Icelandic slope were captured in a region with no clear main flow. Eddy motions, mainly small scale (radii of 0.5 to 3 km), are seen throughout the Iceland Sea. Several floats were grounded on the Icelandic slope both east and west of the Kolbeinsey Ridge due to upslope currents, which created a rim of cold water along the slope. While this water was indicative of the presence of the North Icelandic Jet, no connection between the eastern Iceland Sea and Denmark Strait sill was found. Our investigation of wind stress curl fields from atmospheric reanalysis data suggests that high wind stress curl conditions may have been unfavorable for a westward connection by the North Icelandic Jet at the time of the float observations.

Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

“…Köhl (), using the MIT/GCM, found that pathways to the overflows depend strongly on wind forcing patterns. For example, strong wind stress curl led to a dominance of the East Greenland Current over the NIJ as the feeder source for DSOW, consistent with the observations of de Jong et al (). Behrens et al () simulated particle motion in the Iceland Sea using the high‐resolution VIKING20 model and also associate variability in deep pathways to interannual wind stress curl changes.…”

“…In one of the first observational subsurface Lagrangian studies in the Nordic Seas, Søiland et al () released 22 acoustically tracked floats at 800 m over the northern slope of the Iceland‐Faroes Ridge and found that the source of the dense overflow through the Faroe Bank Channel at the time of the observations was from the west, and not from the interior Norwegian Sea, although the latter path may be favored under different wind forcing conditions (Köhl, ). Some of the 52 deep floats deployed farther west in the Iceland Sea by de Jong et al () show a path southeastward with the East Icelandic Current and then, for two floats, along the northern slope of the Iceland‐Faroes Ridge toward a probable escape through the Faroe Bank Channel.…”

“…Other floats in the de Jong et al () study reveal a dominance of the East Greenland Current, compared to the North Icelandic Jet (NIJ; Jonsson & Valdimarsson, ; Våge et al, ), as the primary source of the dense overflow through Denmark Strait. The authors argue that this may be the result of “NIJ‐unfavorable” wind stress curl over the Iceland Sea during the float observational period.…”

Sources and upstream pathways of the densest overflow water in the Nordic Seas

“…Previous studies give differing views regarding the origin of the NIJ. Model simulations have suggested both the central Iceland Sea 3 and the southern Iceland Sea 25 as source regions, while Lagrangian measurements imply an eastern source 26 . With regard to the Faroe Bank Channel overflow, a recent study has revealed a bottom intensified current of dense overflow water progressing eastward along the north side of the Iceland–Faroe Ridge, presumably supplying the overflow 27 .…”

The Iceland-Faroe Slope Jet: a conduit for dense water toward the Faroe Bank Channel overflow

“…Deep currents in this region are thus suggested to be strongly guided by the bathymetry, which is further supported by estimates from a simplified dynamical model 39 . Moreover, a subset of RAFOS floats deployed at 600–800 m depth northeast of Iceland in 2013 and 2014 40 , and near the Faroe Islands in 2004 25 , drifted southeastward along the slope between Iceland and the Faroe Islands. In the latter case, all but one of the nine floats deployed over isobaths shallower than 1750 m followed the bathymetry southeastward into the Faroe-Shetland Channel, where the floats’ trajectories became more chaotic before approaching the Shetland slope and exiting across the sill into the North Atlantic.…”