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1. Kinematic Structure and Dynamics of the Denmark Strait Overflow

   Lin, P; Pickart, R; Jochumsen, K; Moritz, M; Moore, K; Valdimarsson, H; Fistedt, T (January 2020, Ocean science)

   The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic Meridional Overturning Circulation. While its hydrographic structure is well documented, a full description of the velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic and velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993-2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the Separated EGC and the North Icelandic Jet (NIJ), and the northward flowing North Icelandic Irminger Current (NIIC). We deconstruct the dense overflow in terms of water mass constituents and flow components, demonstrating that the combined EGC branches and NIJ transport comparable amounts. A strong cyclonic structure was present in two-thirds of the occupations, which is thought to be due to the combined effect of eddies and wind. Strong negative wind stress curl north of the strait intensifies the separated EGC, while the enhanced northerly winds under these conditions strengthen the NIIC and cause it to shift the west. Both the cyclonic and non-cyclonic flow states can be super-critical in different parts of the strait, leading to symmetric instability and enhanced mixing. A proxy is used to assess this condition in a larger set of shipboard crossings with hydrography only, elucidating the degree to which mesoscale features drive such mixing. 

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2. The Emergence of the North Icelandic Jet and Its Evolution from Northeast Iceland to Denmark Strait

   https://doi.org/10.1175/JPO-D-19-0088.1  

   Semper, Stefanie; Våge, Kjetil; Pickart, Robert S.; Valdimarsson, Héðinn; Torres, Daniel J.; Jónsson, Steingrímur (October 2019, Journal of Physical Oceanography)

   The North Icelandic Jet (NIJ) is an important source of dense 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 following the continental slope north of Iceland, using 13 hydrographic/velocity surveys of high spatial resolution conducted between 2004 and 2018. The comprehensive dataset reveals that the current originates northeast of Iceland and increases in volume transport by roughly 0.4 Sv (1 Sv ≡ 10 6 m 3 s −1 ) per 100 km until 300 km upstream of Denmark Strait, at which point the highest transport is reached. The bulk of the NIJ transport is confined to a small area in Θ– S space centered near −0.29° ± 0.16°C in Conservative Temperature and 35.075 ± 0.006 g kg −1 in Absolute Salinity. While the hydrographic properties of this transport mode are not significantly modified along the NIJ’s pathway, the transport estimates vary considerably between and within the surveys. Neither a clear seasonal signal nor a consistent link to atmospheric forcing was found, but barotropic and/or baroclinic instability is likely active in the current. The NIJ displays a double-core structure in roughly 50% of the occupations, with the two cores centered at the 600- and 800-m isobaths, respectively. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8 ± 0.3 Sv, which is substantially larger than estimates from a year-long mooring array and hydrographic/velocity surveys closer to the strait, where the NIJ merges with the separated East Greenland Current. This implies a more substantial contribution of the NIJ to the Denmark Strait overflow plume than previously envisaged. 

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3. High frequency variability in the North Icelandic Jet

   Harden, B.E.; Pickart, R.S. (April 2018, Journal of marine research)

   We describe the high-frequency variability in the North Icelandic Jet (NIJ) on the Iceland Slope using data from the densely instrumented Kögur mooring array deployed upstream of the Denmark Strait sill from September 2011 to July 2012. Significant sub-8-day variability is ubiquitous in all moorings from the Iceland slope with a dominant period of 3.6 days. We attribute this variability to topographic Rossby waves on the Iceland slope with a wavelength of 62 ± 3 km and a phase velocity of 17.3 ± 0.8 km/day−1 directed downslope (−9◦ relative to true-north). We test the theoretical dispersion relation for these waves against our observations and find good agreement between the predicted and measured direction of phase propagation. We additionally calculate a theoretical group velocity of 36 km day−1 directed almost directly up-slope (106◦ relative to true-north) that agrees well with the propagation speed and direction of observed energy pulses. We use an inverse wave tracing model to show that this wave energy is generated locally, offshore of the array, and does not emanate from the upstream or downstream directions along the Iceland slope. It is hypothesized that either the meandering Separated East Greenland Current located seaward of the NIJ or intermittent aspiration of dense water into the Denmark Strait Overflow are the drivers of the topographic waves. 

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4. Reduced Transport of Overflow Water in the West Greenland Boundary Current System: The Role of Upstream Entrainment

   https://doi.org/10.1175/JPO-D-25-0024.1  

   Sun, Yan; Pickart, Robert S; Lin, Peigen; Pacini, Astrid; Macrander, Andreas; Larsen, Karin-Margretha H; Hátún, Hjálmar; Frajka-Williams, Eleanor; Lan, Jian; Dilmahamod, Ahmad Fehmi (November 2025, Journal of Physical Oceanography)

   Abstract A mooring array has been maintained across the West Greenland shelf and slope since 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). Here, we use the first 8 years of data to investigate the interannual variability of the two overflow water components of the deep western boundary current (DWBC): the Denmark Strait Overflow Water (DSOW) and the Northeast Atlantic Deep Water (NEADW). While the velocity structure has remained similar throughout the record, both water masses have freshened considerably, especially the NEADW salinity core. Using revised density criteria to define these two components, their transports decreased significantly between 2014 and 2022: from 6.2 to 3.8 Sv (1 Sv ≡ 10⁶m³s⁻¹) (−0.33 Sv yr⁻¹) for the DSOW and from 5.4 to 4.1 Sv (−0.19 Sv yr⁻¹) for the NEADW. Since the overflows across the Denmark Strait and the Faroe Bank Channel have remained steady over this period, this points to decreased entrainment downstream of the sills as a possible mechanism for the observed transport reduction south of Greenland. Using shipboard and mooring data from the two sills, and a hydrographic database for the surrounding region, we predict the downstream transport of the two DWBC components via the framework of a streamtube model. The predicted transport explains 94% of the observed DSOW trend and 63% of the observed NEADW trend. This implies that further entrainment of the NEADW must occur during its long pathlength, which would also help explain the fresher-than-predicted NEADW salinity observed at the OSNAP array. 

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5. Evidence of a Slope Jet Transporting Dense Water toward the Faroe Bank Channel

   Semper, S; Pickart, R; Vage, K; Larsen, K; Hansen, B; Hatun, H (January 2020, Ocean science)

   The densest overflow water from the Nordic Seas passes through the Faroe Bank Channel and contributes to the headwaters to the lower limb of the Atlantic Meridional Overturning Circulation. The upstream pathways of this dense overflow water are not well known. Using data from a high-resolution hydrographic/velocity survey in 2011, as well as long-term moored velocity and shipboard hydrographic measurements north of the Faroe Islands, we present evidence of a current following the continental slope from Iceland toward the Faroe Bank Channel. This narrow current, which we call the Iceland-Faroe Slope Jet (IFSJ), is bottom-intensified and associated with dense water banked up on the slope. North of the Faroe Islands the IFSJ is situated beneath the Faroe Current, and its variability is tightly linked to the flow of Atlantic Water above. The bulk of the IFSJ’s volume transport is confined to a small area in ϴ-S space centered near a potential density anomaly of 28.06 kg m-3. This is slightly denser than the transport mode of the North Icelandic Jet, which follows shallower isobaths along the slope north of Iceland in the opposite direction and feeds the Denmark Strait overflow. However, the similarity of the hydrographic properties suggests that the two currents have a common source. The average transport of water denser than σϴ = 27.8 kg m-3 in the IFSJ is on the order of 1 Sv, which may account for roughly 50% of the overflow through the Faroe Bank Channel. 

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