Description: The upstream sources and pathways of the Denmark Strait Overflow Water and their variability have been investigated using a high-resolution model hindcast. This global simulation covers the period from 1948 to 2009 and uses a fine model mesh (1/20°) to resolve mesoscale features and the complex current structure north of Iceland explicitly. The three sources of the Denmark Strait Overflow, the shelfbreak East Greenland Current (EGC), the separated EGC, and the North Icelandic Jet, have been analyzed using Eulerian and Lagrangian diagnostics. The shelfbreak EGC contributes the largest fraction in terms of volume and freshwater transport to the Denmark Strait Overflow and is the main driver of the overflow variability. The North Icelandic Jet contributes the densest water to the Denmark Strait Overflow and shows only small temporal transport variations. During summer, the net volume and freshwater transports to the south are reduced. On interannual time scales, these transports are highly correlated with the large-scale wind stress curl around Iceland and, to some extent, influenced by the North Atlantic Oscillation, with enhanced southward transports during positive phases. The Lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of Iceland, where water carried by the North Icelandic Irminger Current is transformed and feeds the North Icelandic Jet. Monitoring these two currents and the region north of the Iceland shelfbreak could provide the potential to track long-term changes in the Denmark Strait Overflow and thus also the AMOC.

Description: Between 1965 and 1990, the waters of the Nordic Seas and the subpolar basins of the North Atlantic Ocean freshened substantially1. The Arctic Ocean also became less saline over this time, as a consequence of increasing runoff1, 2, 3, 4, but it is not clear whether flow from the Arctic Ocean was the main source of the Nordic Seas salinity anomaly. As a region of deep-water formation, the Nordic Seas are central to the Atlantic meridional overturning circulation, but this process is inhibited if the surface salinity is too low2. Here we use the instrumental record of Nordic Seas hydrography, along with a global ocean–sea-ice model hindcast simulation, to identify the sources and magnitude of freshwater that has accumulated in the Nordic Seas since 1950. We find that the freshwater anomalies within the Nordic Seas can mostly be explained by less salt entering the southern part of the basin with the relatively saline Atlantic inflow, with seemingly little contribution from the Arctic Ocean. We conclude that hydrographic changes in the Nordic Seas are primarily related to changes in the Atlantic Ocean. We infer that if the Atlantic inflow and Nordic Seas both freshen similarly, this would render the Atlantic meridional overturning circulation relatively insensitive to Nordic Seas freshwater content.

Description: The Denmark Strait overflow water is the largest dense water plume from the Nordic seas to feed the lower limb of the Atlantic Meridional Overturning Circulation. Its primary source is commonly thought to be the East Greenland Current. However, the recent discovery of the North Icelandic Jet—a deep-reaching current that flows along the continental slope of Iceland—has called this view into question. Here we present high-resolution measurements of hydrography and velocity north of Iceland, taken during two shipboard surveys in October 2008 and August 2009. We find that the North Icelandic Jet advects overflow water into the Denmark Strait and constitutes a pathway that is distinct from the East Greenland Current. We estimate that the jet supplies about half of the total overflow transport, and infer that it is the primary source of the densest overflow water. Simulations with an ocean general circulation model suggest that the import of warm, salty water from the North Icelandic Irminger Current and water-mass transformation in the interior Iceland Sea are critical to the formation of the jet. We surmise that the timescale for the renewal of the deepest water in the meridional overturning cell, and its sensitivity to changes in climate, could be different than presently envisaged.

Description: This dataset contains water velocity profile measurements collected in the Slétta transect north of Iceland, during 9 cruises ranging from 2009 to 2018: RV Bjarni Saemundsson B10/2009, B1/2011, B2/2012, B1/2013, B7/2015, and B11/2017; RV Håkon Mosby HM201618, RV Kristine Bonnevie KB2018614 and Knorr KN203-2 (2011). Velocities were measured using an upward- and downward-facing lowered acoustic Doppler current profiler (LADCP) system mounted on the rosette. The LADCP data were processed using the LADCP Processing Software Package from the Lamont-Doherty Earth Observatory (Thurnherr 2010, 2018). Following the processing, the barotropic tides were removed from the velocity data set by applying an updated version of the regional tidal model of Egbert and Erofeeva (2002), which has a resolution of 1/60°.

Description: This data set contains hydrographic and velocity measurements of high spatial resolution which were collected during 13 shipboard surveys between 2004 and 2018. The surveys included seven transects across the northern slope of Iceland. The hydrographic data on all of the cruises were obtained using a Sea-Bird 911+ conductivity-temperature-depth (CTD) instrument. The CTD was mounted on a rosette with Niskin bottles to collect water samples, which were used to calibrate the conductivity sensor. The resulting accuracy of the CTD measurements is 0.3 db for pressure, 0.001 °C for temperature, and 0.002 for salinity. Velocities were measured using acoustic Doppler current profiler (ADCP) instruments. On three of the cruises (RRS James Clark Ross 2004 JR105, R/V Knorr 2008 KN194, NRV Alliance 2018 ALL0118), a vessel-mounted ADCP (VMADCP) was used, while an upward- and downward-facing lowered ADCP (LADCP) system mounted on the rosette was utilized on the remaining surveys. The VMADCP data on the R/V Knorr 2008 (KN194) and NRV Alliance 2018 (ALL0118) cruises were acquired using the University of Hawaii Data Acquisition System (UHDAS) and the VMDAS collection software (Teledyne RDInstruments), respectively. Subsequently, these data were processed using the Common Ocean Data Access System (CODAS; Firing and Hummon 2010). On the RRS James Clark Ross 2004 cruise (JR105), VMADCP data were collected and processed using a custom data acquisition system unique to the ship (Pstar system). The LADCP data were processed using the LADCP Processing Software Package from the Lamont-Doherty Earth Observatory (Thurnherr 2010, 2018). Following the processing, the barotropic tides were removed from all of the velocity data sets by applying an updated version of the regional tidal model of Egbert and Erofeeva (2002), which has a resolution of 1/60°. This data set also includes data from a current meter mooring deployed from 23 August 2005 to 10 August 2006, situated 19 km north of the shelf break at the Hornbanki transect. The instrument was an Aanderaa RCM-7, sampling hourly, which was placed at 365 m depth on the 620 m isobath.

Description: This data set contains hydrographic measurements of high spatial resolution which were collected during a shipboard survey onboard RV Knorr in September 2011. The survey included transects across the northern slope of Iceland and the Iceland-Faroe Ridge. The sections are named A to E starting with the north-westernmost transect. The hydrographic data were obtained using a Sea-Bird 911+ conductivity-temperature-depth (CTD) instrument. The CTD was mounted on a rosette with Niskin bottles to collect water samples, which were used to calibrate the conductivity sensor. The resulting accuracy of the CTD measurements is 0.3 db for pressure, 0.001 °C for temperature, and 0.002 for salinity.