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The Denmark Strait Overflow (DSO) is an important contributor to the lower limb of the Atlantic meridional overturning circulation (AMOC). Determining DSO formation and its pathways is not only important for local oceanography but also critical to estimating the state and variability of the AMOC. Despite prior attempts to understand the DSO sources, its upstream pathways and circulation remain uncertain due to short-term (3–5 days) variability. This makes it challenging to study the DSO from observations. Given this complexity, this study maps the upstream pathways and along-pathway changes in its water properties, using Lagrangian backtracking of the DSO sources in a realistic numerical ocean simulation. The Lagrangian pathways confirm that several branches contribute to the DSO from the north such as the East Greenland Current (EGC), the separated EGC (sEGC), and the North Icelandic Jet (NIJ). Moreover, the model results reveal additional pathways from south of Iceland, which supplied over 16% of the DSO annually and over 25% of the DSO during winter of 2008, when the NAO index was positive. The southern contribution is about 34% by the end of March. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the North Atlantic Deep Water (NADW), and needs to be explored further, with in situ observations.

The Lagrangian method-where current location and intensity are determined by tracking the movement of flow along its path-is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid-20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice-free oceans and begin to construct basin-scale, and eventually global-scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC.

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 0.5–3 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.