Sixteen years of moored observations from the core of the Denmark Strait Overflow (DSO) on the Greenland slope in the Irminger Basin are used to examine monthly to seasonal hydrographic signals. Our analysis reveals the presence of an annual salinity cycle, with freshening in the first half of the year and an increase in salinity in the second half. The magnitude of freshening exceeds 0.04 in 1999, 2004, 2005 and 2014. There is no evidence of this signal upstream in the deepest part of the Denmark Strait Sill, which is fed exclusively by the North Icelandic Jet. Instead, we argue that the signal originates from a lighter source of DSO – either the East Greenland Current or the Irminger Current. Results from a case study in 2011-12 indicate that the East Greenland Current is the more likely origin. Specifically, we show the propagation of two freshening signals from the East Greenland Current 200 km north of Denmark Strait to the core of the DSO at the downstream mooring array (700km downstream), with a transit time of 10 weeks. Previous research has linked remote wind forcing (at Denmark Strait and to the north) with DSO salinity in the Irminger Basin. Here, we use ERA-5 reanalysis output in tandem with the full 16 years of mooring observations – a longer time frame than any previous study – to determine the nature of this relationship. A correlation analysis between a variety of atmospheric forcing metrics and our oceanographic time series are presented, and the implications for the structure and stability of the deep overflow are discussed.
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Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges
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.
more » « less- NSF-PAR ID:
- 10361165
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 4
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Abstract The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–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). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 10 6 m 3 s −1 ), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.more » « less
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A high-resolution numerical model, together with in situ and satellite observations, is used to explore the nature and dynamics of the dominant high-frequency (from one day to one week) variability in Denmark Strait. Mooring measurements in the center of the strait reveal that warm water “flooding events” occur, whereby the North Icelandic Irminger Current (NIIC) propagates offshore and advects subtropical-origin water northward through the deepest part of the sill. Two other types of mesoscale processes in Denmark Strait have been described previously in the literature, known as “boluses” and “pulses,” associated with a raising and lowering of the overflow water interface. Our measurements reveal that flooding events occur in conjunction with especially pronounced pulses. The model indicates that the NIIC hydrographic front is maintained by a balance between frontogenesis by the large-scale flow and frontolysis by baroclinic instability. Specifically, the temperature and salinity tendency equations demonstrate that the eddies act to relax the front, while the mean flow acts to sharpen it. Furthermore, the model reveals that the two dense water processes—boluses and pulses (and hence flooding events)—are dynamically related to each other and tied to the meandering of the hydrographic front in the strait. Our study thus provides a general framework for interpreting the short-time-scale variability of Denmark Strait Overflow Water entering the Irminger Sea.
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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.more » « less
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