Frontogenesis and Variability in Denmark Strait and Its Influence on Overflow Water

Spall, Michael A.; Pickart, Robert S.; Lin, Peigen; Appen, Wilken-Jon von; Mastropole, Dana; Valdimarsson, Héðinn; Haine, Thomas W. N.; Almansi, Mattia

doi:10.1175/jpo-d-19-0053.1

Cited by 16 publications

(30 citation statements)

References 27 publications

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“…Further north of the sill, similarly high temperatures are not found below approximately 350 m (Jónsson & Valdimarsson, , ; Smith, ). In agreement with recent findings by Spall et al (), we conclude that the warming events are caused by the meandering of the NIIC front, as the westward movement of the cold overflow plume allows the warm water of Atlantic origin to temporarily occupy the array (Jochumsen et al, ). The shift of the NIIC front is connected to the occurrence of pulses (Spall et al, ; von Appen et al, ) and anticyclonic patterns in the velocity field.…”

Section: Discussionsupporting

confidence: 93%

Mesoscale Eddies Observed at the Denmark Strait Sill

et al. 2019

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The Denmark Strait overflow is the major export route of dense water from the Arctic Mediterranean into the North Atlantic. At the Strait's shallow sill, the overflow is a bottom-intensified cold and dense plume, bound to the east by a thermal front formed with the warmer, northward flowing North Icelandic Irminger Current. More than two decades of observations at the sill show strong fluctuations of volume flux on daily time scales. To better understand the source of this variability, a five-mooring array was installed at the sill, capturing nearly 1 year of velocity and bottom temperature measurements at a high temporal and spatial resolution. Bottom temperature fluctuations that exceed 4 • C indicate a meandering of the front between the plume and the North Icelandic Irminger Current. Current vector rotation shows trains of alternating cyclones and anticyclones at the sill. An eddy crosses the sill every 3 to 6 days with a mean velocity of 0.4 m/s and a typical diameter of 30 to 40 km. The results suggest that anticyclones, with centers passing through the deepest part of the sill, may be responsible for periods of increased volume flux-also referred to as boluses and pulses in previous studies. Although the relationship between eddies, pulses, and boluses is still unclear, the results show that eddies are directly linked to fluctuations in the strength, thickness, and position of the overflow plume. Plain Language SummaryThe southward flow of dense water from the Arctic ocean plays a crucial role in global ocean circulation but is almost immediately constrained on its way south by a submarine ridge that connects Greenland, Iceland, the Faroe Islands, and Scotland. The southward flow is therefore forced to pass through several straits and up and over relatively shallow sills. Most of the flow passes over a sill in the Denmark Strait, located between Greenland and Iceland. In this study, we present observations from an array of instruments, which measure the southward flow as it passes over the Denmark Strait sill. The flow is characterised by trains of eddies (vortices), with an alternating sense of rotation; meaning a counterclockwise eddy is usually followed by a clockwise eddy. The eddies are 30 to 40 km wide and need about 1 day to pass over the sill. These eddies help to explain pronounced changes in the flow across the sill, as they can help either to speed up the flow or slow it down. The results of this study contribute to understanding mesoscale fluctuations, which influence local mixing processes and water mass transports.

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Section: Discussionsupporting

confidence: 93%

Mesoscale Eddies Observed at the Denmark Strait Sill

et al. 2019

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“…Furthermore, it demonstrates that the baroclinic conversion in the region of the NIJ is significantly stronger when the NIJ is in its dominant state (the type3 scenario). The model simulation of Spall et al () also has a region of strong baroclinic eddy conversion north of Denmark Strait in the region of the NIIC hydrographic front, as we find here. They demonstrated that closer to the strait, this variability is coupled to the dense overflow water variability, which occurs throughout the year.…”

Section: Resultssupporting

confidence: 86%

Structure and Variability of the North Icelandic Jet From Two Years of Mooring Data

et al. 2019

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Mooring data from September 2011 to July 2013 on the Iceland slope north of Denmark Strait are analyzed to better understand the structure and variability of the North Icelandic Jet (NIJ). Three basic configurations of the flow were identified: (1) a strong separated East Greenland Current (EGC) on the mid‐Iceland slope coincident with a weak NIJ on the upper slope, (2) a merged separated EGC and NIJ, and (3) a strong NIJ located at its climatological mean position, coincident with a weak signature of the separated EGC at the base of the Iceland slope. Our study reveals that the NIJ‐dominant scenario was present during different times of the year for the two successive mooring deployments—appearing mainly from September to February the first year and from January to July the second year. Furthermore, when this scenario was active it varied on short timescales. An energetics analysis demonstrates that the high‐frequency variability is driven by mean‐to‐eddy baroclinic conversion at the shoreward edge of the NIJ, consistent with previous modeling work. The seasonal timing of the NIJ dominant scenario is investigated in relation to the atmospheric forcing upstream of Denmark Strait. The resulting lagged correlations imply that strong turbulent heat fluxes in a localized region on the continental slope of Iceland, south of the Spar Fracture zone, lead to a stronger NIJ dominant state with a two‐month lag. This can be explained dynamically in terms of previous modeling work addressing the circulation response to dense water formation near an island.

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“…During these events, the NIIC migrates westward, and the deepest part of the sill is filled with subtropical-origin water. Boluses, pulses, and flooding events are all associated with the meandering of the hydrographic front in Denmark Strait (Spall et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges

Almansi

Haine

Gelderloos

et al. 2020

Geophysical Research Letters

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Mesoscale features present at the Denmark Strait sill regularly enhance the volume transport of the Denmark Strait overflow (DSO). They are important for the Atlantic Meridional Overturning Circulation and ultimately, for the global climate system. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high‐transport events) and DSO cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high‐transport events start collapsing. Regardless of their formation mechanism, DSO cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.

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Frontogenesis and Variability in Denmark Strait and Its Influence on Overflow Water

Cited by 16 publications

References 27 publications

Mesoscale Eddies Observed at the Denmark Strait Sill

Mesoscale Eddies Observed at the Denmark Strait Sill

Structure and Variability of the North Icelandic Jet From Two Years of Mooring Data

Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges

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