Circulation along the northern slope of the Greenland-Scotland Ridge

Abstract

The Greenland-Scotland Ridge separates the subpolar North Atlantic from the Nordic Seas and constrains the flow of the upper and lower branches of the northern extremity of the Atlantic Meridional Overturning Circulation (AMOC). Warm, saline Atlantic Water flowing northward across the Greenland-Scotland Ridge into the Nordic Seas is transformed into cold, dense water, which returns to the south as overflow plumes through gaps in the ridge. The exchange flows across the ridge have been monitored for several decades, but gaps in our knowledge remain about where and how the dense waters are formed and transported toward the overflows. In this thesis, observational data are used to clarify the upstream pathways of the densest overflow waters and to examine the transformation of the Atlantic Water inflow through Denmark Strait. Paper I focuses on the North Icelandic Jet (NIJ), which supplies the densest water to the overflow plume passing through Denmark Strait. The properties, structure, and transport of the NIJ are investigated for the first time along its entire pathway along the slope north of Iceland, using 13 high-resolution hydrographic/velocity surveys conducted between 2004 and 2018. The comprehensive data set reveals that the current originates northeast of Iceland and that its volume transport increases toward Denmark Strait. The bulk of the NIJ transport is confined to a small area in temperature-salinity space, and these hydrographic properties are not significantly modified along the NIJ's pathway. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8±0.3 Sv (1 Sv ≡10^6 m^3/s), which implies a more substantial contribution from the NIJ to the overflow plume than previously envisaged. In paper II we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel, using a high-resolution hydrographic/velocity survey from 2011 along with long-term hydrographic and velocity measurements north of the Faroe Islands. We refer to this current as the Iceland-Faroe Slope Jet (IFSJ). The bulk of the IFSJ's volume transport occupies a small area in temperature-salinity space. The similarity of the hydrographic properties of the eastward-flowing IFSJ and the westward-flowing NIJ suggests that the densest components of the two major overflows across the Greenland-Scotland Ridge have a common source. We estimate that the IFSJ transports approximately 1.0±0.1 Sv, which can account for roughly half of the total overflow transport through the Faroe Bank Channel. As such, the IFSJ is a significant component of the overturning circulation in the Nordic Seas. In paper III we quantify the along-stream evolution of the North Icelandic Irminger Current (NIIC) as it progresses along the shelf break north of Iceland, using a high-resolution shipboard hydrographic/velocity survey, satellite and surface drifter data, and historical hydrographic measurements. The NIIC cools and freshens along its pathway, predominantly due to mixing with cold, fresh offshore waters. Dense-water formation on the shelf is limited, occurring sporadically in only 7% of all historical winter profiles. The hydrographic properties of this locally formed water match the lighter, shallower portion of the NIJ. Along the northeast Iceland slope, enhanced eddy activity and variability in sea surface temperature indicate that locally formed eddies due to instability of the NIIC divert heat and salt into the interior Iceland Sea. The emergence of the NIJ in the same region suggests that there may be a dynamical link to the formation of the NIJ. As such, our results indicate that while the NIIC rarely supplies the NIJ directly, it may be dynamically important for the overturning circulation in the Nordic Seas. The three papers advance our knowledge about the circulation along the northern slope of the Greenland-Scotland Ridge and highlight its significance for water mass transformation in the Nordic Seas and our understanding of the Nordic Seas–North Atlantic exchange. In particular, my results contribute to an improved understanding of the pathways of dense water feeding the overflows, which is imperative to accurately predict how the AMOC will respond to a changing climate.