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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Evidence of a Slope Jet Transporting Dense Water toward the Faroe Bank Channel
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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- Award ID(s):
- 1558742
- NSF-PAR ID:
- 10163029
- Date Published:
- Journal Name:
- Ocean science
- ISSN:
- 1812-0784
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. The Arctic Mediterranean (AM) is the collective name forthe Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into thisregion through the Bering Strait (Pacific inflow) and through the passages across theGreenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modifiedwaters leave the AM in several flow branches which are grouped into two differentcategories: (1) overflow of dense water through the deep passages across theGreenland–Scotland Ridge, and (2) outflow of light water – here termed surface outflow– on both sides of Greenland. These exchanges transport heat and salt into and out ofthe AM and are important for conditions in the AM. They are also part of the global oceancirculation and climate system. Attempts to quantify the transports by various methodshave been made for many years, but only recently the observational coverage has becomesufficiently complete to allow an integrated assessment of the AM exchanges based solelyon observations. In this study, we focus on the transport of water and have collecteddata on volume transport for as many AM-exchange branches as possible between 1993 and2015. The total AM import (oceanic inflows plusfreshwater) is found to be 9.1 Sv (sverdrup,1 Sv =106 m3 s−1) with an estimated uncertainty of 0.7 Sv and hasthe amplitude of the seasonal variation close to 1 Sv and maximum import in October.Roughly one-third of the imported water leaves the AM as surface outflow with theremaining two-thirds leaving as overflow. The overflow water is mainly produced frommodified Atlantic inflow and around 70 % of the total Atlantic inflow is convertedinto overflow, indicating a strong coupling between these two exchanges. The surfaceoutflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but isstill approximately two-thirds of modified Atlantic water. For the inflowbranches and the two main overflow branches (Denmark Strait and Faroe Bank Channel),systematic monitoring of volume transport has been established since the mid-1990s, andthis enables us to estimate trends for the AM exchanges as a whole. At the 95 %confidence level, only the inflow of Pacific water through the Bering Strait showed astatistically significant trend, which was positive. Both the total AM inflow and thecombined transport of the two main overflow branches also showed trends consistent withstrengthening, but they were not statistically significant. They do suggest, however,that any significant weakening of these flows during the last two decades is unlikely andthe overall message is that the AM exchanges remained remarkably stable in the periodfrom the mid-1990s to the mid-2010s. The overflows are the densest source water for thedeep limb of the North Atlantic part of the meridional overturning circulation (AMOC),and this conclusion argues that the reported weakening of the AMOC was not due tooverflow weakening or reduced overturning in the AM. Although the combined data set hasmade it possible to establish a consistent budget for the AM exchanges, the observationalcoverage for some of the branches is limited, which introduces considerable uncertainty.This lack of coverage is especially extreme for the surface outflow through the DenmarkStrait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottishshelf. We recommend that more effort is put into observing these flows as well asmaintaining the monitoring systems established for the other exchange branches.
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Abstract The North Icelandic Irminger Current (NIIC) flowing northward through Denmark Strait is the main source of salt and heat to the north Iceland shelf. We quantify its along‐stream evolution using the first high‐resolution hydrographic/velocity survey north of Iceland that spans the entire shelf along with historical hydrographic measurements as well as data from satellites and surface drifters. The NIIC generally follows the shelf break. Portions of the flow recirculate near Denmark Strait and the Kolbeinsey Ridge. The current's volume transport diminishes northeast of Iceland before it merges with the Atlantic Water inflow east of Iceland. The hydrographic properties of the current are modified along its entire pathway, predominantly because of lateral mixing with cold, fresh offshore waters rather than air‐sea interaction. Progressing eastward, the NIIC cools and freshens by approximately 0.3°C and 0.02–0.03 g kg−1per 100 km, respectively, in both summer and winter. Dense‐water formation on the shelf is limited, occurring only sporadically in the historical record. The hydrographic properties of this locally formed water match the lighter portion of the North Icelandic Jet (NIJ), which emerges northeast of Iceland and transports dense water toward Denmark Strait. In the region northeast of Iceland, the NIIC is prone to baroclinic instability. Enhanced eddy kinetic energy over the steep slope there suggests a dynamical link between eddies shed by the NIIC and the formation of the NIJ as previously hypothesized. Thus, while the NIIC rarely supplies the NIJ directly, it may be dynamically important for the overturning circulation in the Nordic Seas.
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Using vessel-mounted acoustic Doppler current profiler data from four different routes between Scotland, Iceland and Greenland, we map out the mean flow of water in the top 400 m of the northeastern North Atlantic. The poleward transport east of the Reykjanes Ridge (RR) decreases from 8.5 to 10 Sv (1 Sverdrup 106 m3 s1) at 59.58N to 618N to 6 Sv crossing the IcelandFaroesScotland Ridge. The two longest 1200 km transport integrals have 1.40.94 Sv uncertainty, respectively. The overall decrease in transport can in large measure be accounted for by a 1.5 Sv flow across the RR into the Irminger Sea north of 59.58N and by a 0.5 Sv overflow of dense water along the IcelandFaroes Ridge. A remaining 0.5 Sv flux divergence is at the edge of detectability, but if real could be accounted for through wintertime convection to 400 m and densification of upper ocean water. The topography of the Iceland Basin and the banks west of Scotland play a fundamental role in controlling flow pathways towards and past Iceland, the Faroes and Scotland. Most water flows north unimpeded through the Iceland Basin, some in the centre of the basin along the Maury Channel, and some along Hatton Bank, turning east along the northern slopes of George Bligh Bank, Lousy Bank and Bill Bailey’s Bank, whereupon the flow splits with 3 Sv turning northwest towards the IcelandFaroes Ridge and the remainder continuing east towards and north of the Wyville-Thomson Ridge (WTR) to the Scotland slope thereby increasing the Slope Current transport from 1.5 Sv south of the WTR to 3.5 Sv in the FaroesShetland Channelmore » « less
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Abstract Data from repeat hydrographic surveys over the 25‐year period 1993 to 2017, together with satellite altimetry data, are used to quantify the temporal and spatial variability of the North Icelandic Irminger Current (NIIC), East Icelandic Current (EIC), and the water masses they advect around northern Iceland. We focus on the warm, salty Atlantic Water (AW) flowing northward through Denmark Strait and the cooler, fresher, denser Atlantic‐origin Overflow Water (AtOW) which has circulated cyclonically around the rim of the Nordic Seas before being advected to the Iceland slope via the EIC. The absolute geostrophic velocities reveal that approximately half of the NIIC recirculates just north of Denmark Strait, while the remaining half merges with the EIC to form a single current that extends to the northeast of Iceland, with no further loss in transport of either component. The AW percentage decreases by 75% over this distance, while the AtOW percentage is higher than that of the AW in the merged current. The NIIC and merged NIIC‐EIC are found to be baroclinically unstable, which causes the flow to become increasingly barotropic as it progresses around Iceland. A seasonal accounting of the water masses within the currents indicates that only in springtime is the NIIC dominated by AW inflow north of Denmark Strait. Overall, there is considerably more seasonal and along‐stream variability in the properties of the flow prior to the merging of the NIIC and EIC. Over the 25‐year time period, the NIIC became warmer, saltier, and increased in volume transport.
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