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Abstract The Deep Western Boundary Current (DWBC) – the primary component of the lower limb of the Atlantic Meridional Overturning Circulation – flows along the eastern flank of Greenland from a combination of Denmark Strait Overflow Water and Iceland Scotland Overflow Water. The Overturning in the Subpolar North Atlantic Program (OSNAP) has continuously measured the DWBC since 2014 using current meters, temperature/salinity sensors, and acoustic doppler current profilers. This mooring array located near Cape Farewell also incorporates data from the Ocean Observatories Initiative’s Global Irminger Sea Array to create the longest continuous observations of the DWBC closest to where Iceland Scotland Overflow Water and Denmark Strait Overflow water first merge. This study reveals that the DWBC has decreased by 26% over the first six years of OSNAP observations primarily due to a thinning of the traditionally defined DWBC layer (σθ > 27.8 kg m-3) due to a known freshening signal moving through the subpolar region. Despite this decrease, the Atlantic Meridional Overturning Circulation as calculated by OSNAP has remained relatively steady over the same period. Ultimately, the reason for this difference is due to the methods used to define these two circulations. Finding such notably different trends for two seemingly dependent circulations raises the question of how to best define these transports.
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The nature and origin of monthly to seasonal hydrographic signals in the Denmark Strait Overflow
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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- Award ID(s):
- 1558742
- PAR ID:
- 10163034
- Date Published:
- Journal Name:
- Ocean science
- ISSN:
- 1812-0784
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Abstract The boundary current system in the Labrador Sea plays an integral role in modulating convection in the interior basin. Four years of mooring data from the eastern Labrador Sea reveal persistent mesoscale variability in the West Greenland boundary current. Between 2014 and 2018, 197 mid-depth intensified cyclones were identified that passed the array near the 2000 m isobath. In this study, we quantify these features and show that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. A composite cyclone is constructed revealing an average radius of 9 km, maximum azimuthal speed of 24 cm/s, and a core propagation velocity of 27 cm/s. The core propagation velocity is significantly smaller than upstream near Denmark Strait, allowing them to trap more water. The cyclones transport a 200-m thick lens of dense water at the bottom of the water column, and increase the transport of DSOW in the West Greenland boundary current by 17% relative to the background flow. Only a portion of the features generated at Denmark Strait make it to the Labrador Sea, implying that the remainder are shed into the interior Irminger Sea, are retroflected at Cape Farewell, or dissipate. A synoptic shipboard survey east of Cape Farewell, conducted in summer 2020, captured two of these features which shed further light on their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea—a discovery that could have important implications for interior stratification.more » « less
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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.more » « less
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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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