Circulation along the northern slope of the Greenland-Scotland Ridge

The Greenland-Scotland Ridge separates the subpolar North Atlantic from the Nordic Seas and constrains the flow of the upper and lower branches of the northern extremity of the Atlantic Meridional Overturning Circulation (AMOC). Warm, saline Atlantic Water flowing northward across the Greenland-Scotland Ridge into the Nordic Seas is transformed into cold, dense water, which returns to the south as overflow plumes through gaps in the ridge. The exchange flows across the ridge have been monitored for several decades, but gaps in our knowledge remain about where and how the dense waters are formed and transported toward the overflows. In this thesis, observational data are used to clarify the upstream pathways of the densest overflow waters and to examine the transformation of the Atlantic Water inflow through Denmark Strait. Paper I focuses on the North Icelandic Jet (NIJ), which supplies the densest 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 along the slope north of Iceland, using 13 high-resolution hydrographic/velocity surveys conducted between 2004 and 2018. The comprehensive data set reveals that the current originates northeast of Iceland and that its volume transport increases toward Denmark Strait. The bulk of the NIJ transport is confined to a small area in temperature-salinity space, and these hydrographic properties are not significantly modified along the NIJ's pathway. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8±0.3 Sv (1 Sv ≡10^6 m^3/s), which implies a more substantial contribution from the NIJ to the overflow plume than previously envisaged. In paper II we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel, using a high-resolution hydrographic/velocity survey from 2011 along with long-term hydrographic and velocity measurements north of the Faroe Islands. We refer to this current as the Iceland-Faroe Slope Jet (IFSJ). The bulk of the IFSJ's volume transport occupies a small area in temperature-salinity space. The similarity of the hydrographic properties of the eastward-flowing IFSJ and the westward-flowing NIJ suggests that the densest components of the two major overflows across the Greenland-Scotland Ridge have a common source. We estimate that the IFSJ transports approximately 1.0±0.1 Sv, which can account for roughly half of the total overflow transport through the Faroe Bank Channel. As such, the IFSJ is a significant component of the overturning circulation in the Nordic Seas. In paper III we quantify the along-stream evolution of the North Icelandic Irminger Current (NIIC) as it progresses along the shelf break north of Iceland, using a high-resolution shipboard hydrographic/velocity survey, satellite and surface drifter data, and historical hydrographic measurements. The NIIC cools and freshens along its pathway, predominantly due to mixing with cold, fresh offshore waters. Dense-water formation on the shelf is limited, occurring sporadically in only 7% of all historical winter profiles. The hydrographic properties of this locally formed water match the lighter, shallower portion of the NIJ. Along the northeast Iceland slope, enhanced eddy activity and variability in sea surface temperature indicate that locally formed eddies due to instability of the NIIC divert heat and salt into the interior Iceland Sea. The emergence of the NIJ in the same region suggests that there may be a dynamical link to the formation of the NIJ. As such, our results indicate that while the NIIC rarely supplies the NIJ directly, it may be dynamically important for the overturning circulation in the Nordic Seas. The three papers advance our knowledge about the circulation along the northern slope of the Greenland-Scotland Ridge and highlight its significance for water mass transformation in the Nordic Seas and our understanding of the Nordic Seas–North Atlantic exchange. In particular, my results contribute to an improved understanding of the pathways of dense water feeding the overflows, which is imperative to accurately predict how the AMOC will respond to a changing climate.Doktorgradsavhandlin

How warm Gulf Stream water sustains a cold underwater waterfall

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Semper, S., Glessmer, M., Våge, K., & Pickart, R. How warm Gulf Stream water sustains a cold underwater waterfall. Frontiers for Young Minds, 10, (2022): 765740, https://doi.org/10.3389/frym.2022.765740.The most famous ocean current, the Gulf Stream, is part of a large system of currents that brings warm water from Florida to Europe. It is a main reason for northwestern Europe’s mild climate. What happens to the warm water that flows northward, since it cannot just pile up? It turns out that the characteristics of the water change: in winter, the ocean warms the cold air above it, and the water becomes colder. Cold seawater, which is heavier than warm seawater, sinks down to greater depths. But what happens to the cold water that disappears from the surface? While on a research ship, we discovered a new ocean current that solves this riddle. The current brings the cold water to an underwater mountain ridge. The water spills over the ridge as an underwater waterfall before it continues its journey, deep in the ocean, back toward the equator.Support for this work was provided by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101022251 (SS), the Trond Mohn Foundation Grant BFS2016REK01 (SS and KV), and the U.S. National Science Foundation Grants OCE-1558742 and OCE-1259618 (RP)

Evolution and Transformation of the North Icelandic Irminger Current Along the North Iceland Shelf

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−1 per 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.publishedVersio

Evolution and transformation of the North Icelandic Irminger Current along the North Iceland Shelf

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Semper, S., Våge, K., Pickart, R., Jónsson, S., & Valdimarsson, H. Evolution and transformation of the North Icelandic Irminger Current along the North Iceland Shelf. Journal of Geophysical Research: Oceans, 127(3), (2022): e2021JC017700, https://doi.org/10.1029/2021jc017700.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−1 per 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.This research was supported by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 101022251 (S. Semper), the Trond Mohn Foundation Grant BFS2016REK01 (S. Semper and K. Våge), and the U.S. National Science Foundation Grants OCE-1558742 and OCE-1259618 (R. S. Pickart)

Water mass transformation in the Iceland Sea: Contrasting two winters separated by four decades

Dense water masses formed in the Nordic Seas flow across the Greenland–Scotland Ridge and contribute substantially to the lower limb of the Atlantic Meridional Overturning Circulation. Originally considered an important source of dense water, the Iceland Sea gained renewed interest when the North Icelandic Jet — a current transporting dense water from the Iceland Sea into Denmark Strait — was discovered in the early 2000s. Here we use recent hydrographic data to quantify water mass transformation in the Iceland Sea and contrast the present conditions with measurements from hydrographic surveys conducted four decades earlier. We demonstrate that the large-scale hydrographic structure of the central Iceland Sea has changed significantly over this period and that the locally transformed water has become less dense, in concert with a retreating sea-ice edge and diminished ocean-to-atmosphere heat fluxes. This has reduced the available supply of dense water to the North Icelandic Jet, but also permitted densification of the East Greenland Current during its transit through the presently ice-free western Iceland Sea in winter. Together, these changes have significantly altered the contribution from the Iceland Sea to the overturning in the Nordic Seas over the four decade period.publishedVersio

A numerical study of interannual variability in the North Icelandic Irminger Current

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 123(12), (2018): 8994-9009, doi:10.1029/2018JC013800.The North Icelandic Irminger Current (NIIC) is an important component of the Atlantic Water (AW) inflow to the Nordic Seas. In this study, both observations and a high‐resolution (1/12°) numerical model are used to investigate the seasonal to interannual variability of the NIIC and its forcing mechanisms. The model‐simulated velocity and hydrographic fields compare well with the available observations. The water mass over the entire north Icelandic shelf exhibits strong seasonal variations in both temperature and salinity, and such variations are closely tied to the AW seasonality in the NIIC. In addition to seasonal variability, there is considerable variation on interannual time scales, including a prominent event in 2003 when the AW volume transport increased by about 0.5 Sv. To identify and examine key forcing mechanisms for this event, we analyzed outputs from two additional numerical experiments: using only the seasonal climatology for buoyancy flux (the momentum case) and using only the seasonal climatology for wind stress (the buoyancy case). It is found that changes in the wind stress are predominantly responsible for the interannual variations in the AW volume transport, AW fraction in the NIIC water, and salinity. Temperature changes on the shelf, however, are equally attributable to the buoyancy flux and wind forcing. Correlational analyses indicate that the AW volume transport is most sensitive to the wind stress southwest of Iceland.This work is supported by the U.S. National Science Foundation (NSF) under grants OCE‐1634886 (J. Zhao and J. Yang) and OCE‐1558742 (R. Pickart), and by the Bergen Research Foundation grant BFS2016REK01 (K. Våge and S. Semper). We thank Xiaobiao Xu at Florida State University for providing the initial model configuration. Comments from anonymous reviewers help to improve the manuscript. The altimeter products are produced and distributed by the Copernicus Marine and Environment Monitoring Service (CMEMS, http://www.marine.copernicus.eu). The hydrographic maps along the Hornbanki section are available at http://www.hafro.is/Sjora/.2019-04-1

The Iceland-Faroe slope jet: a conduit for dense water toward the Faroe Bank Channel overflow

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Semper, S., Pickart, R. S., Vage, K., Larsen, K. M. H., Hatun, H., & Hansen, B. The Iceland-Faroe slope jet: a conduit for dense water toward the Faroe Bank Channel overflow. Nature Communications, 11(1), (2020): 5390, doi:10.1038/s41467-020-19049-5.Dense water from the Nordic Seas passes through the Faroe Bank Channel and supplies the lower limb of the Atlantic Meridional Overturning Circulation, a critical component of the climate system. Yet, the upstream pathways of this water are not fully known. Here we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel using high-resolution, synoptic shipboard observations and long-term measurements north of the Faroe Islands. The bulk of the volume transport of the current, named the Iceland-Faroe Slope Jet (IFSJ), is relatively uniform in hydrographic properties, very similar to the North Icelandic Jet flowing westward along the slope north of Iceland toward Denmark Strait. This suggests a common source for the two major overflows across the Greenland-Scotland Ridge. The IFSJ can account for approximately half of the total overflow transport through the Faroe Bank Channel, thus constituting a significant component of the overturning circulation in the Nordic Seas.Support for this work was provided by the Bergen Research Foundation Grant BFS2016REK01 (S.S. and K.V.), the U.S. National Science Foundation Grants OCE-1558742 and OCE-1259618 (R.S.P.), the Danish Ministry of Climate, Energy and Utilities (K.M.H.L., H.H., and B.H.) and the European Union’s Horizon 2020 research and innovation programme under grant agreement 727852 (Blue-Action) (K.M.H.L., H.H., and B.H.)

Water mass transformation in the Iceland Sea : Contrasting two winters separated by four decades

Dense water masses formed in the Nordic Seas flow across the Greenland–Scotland Ridge and contribute substantially to the lower limb of the Atlantic Meridional Overturning Circulation. Originally considered an important source of dense water, the Iceland Sea gained renewed interest when the North Icelandic Jet — a current transporting dense water from the Iceland Sea into Denmark Strait — was discovered in the early 2000s. Here we use recent hydrographic data to quantify water mass transformation in the Iceland Sea and contrast the present conditions with measurements from hydrographic surveys conducted four decades earlier. We demonstrate that the large-scale hydrographic structure of the central Iceland Sea has changed significantly over this period and that the locally transformed water has become less dense, in concert with a retreating sea-ice edge and diminished ocean-to-atmosphere heat fluxes. This has reduced the available supply of dense water to the North Icelandic Jet, but also permitted densification of the East Greenland Current during its transit through the presently ice-free western Iceland Sea in winter. Together, these changes have significantly altered the contribution from the Iceland Sea to the overturning in the Nordic Seas over the four decade period.Peer reviewe

Circulation along the northern slope of the Greenland-Scotland Ridge

The Greenland-Scotland Ridge separates the subpolar North Atlantic from the Nordic Seas and constrains the flow of the upper and lower branches of the northern extremity of the Atlantic Meridional Overturning Circulation (AMOC). Warm, saline Atlantic Water flowing northward across the Greenland-Scotland Ridge into the Nordic Seas is transformed into cold, dense water, which returns to the south as overflow plumes through gaps in the ridge. The exchange flows across the ridge have been monitored for several decades, but gaps in our knowledge remain about where and how the dense waters are formed and transported toward the overflows. In this thesis, observational data are used to clarify the upstream pathways of the densest overflow waters and to examine the transformation of the Atlantic Water inflow through Denmark Strait. Paper I focuses on the North Icelandic Jet (NIJ), which supplies the densest 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 along the slope north of Iceland, using 13 high-resolution hydrographic/velocity surveys conducted between 2004 and 2018. The comprehensive data set reveals that the current originates northeast of Iceland and that its volume transport increases toward Denmark Strait. The bulk of the NIJ transport is confined to a small area in temperature-salinity space, and these hydrographic properties are not significantly modified along the NIJ's pathway. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8±0.3 Sv (1 Sv ≡10^6 m^3/s), which implies a more substantial contribution from the NIJ to the overflow plume than previously envisaged. In paper II we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel, using a high-resolution hydrographic/velocity survey from 2011 along with long-term hydrographic and velocity measurements north of the Faroe Islands. We refer to this current as the Iceland-Faroe Slope Jet (IFSJ). The bulk of the IFSJ's volume transport occupies a small area in temperature-salinity space. The similarity of the hydrographic properties of the eastward-flowing IFSJ and the westward-flowing NIJ suggests that the densest components of the two major overflows across the Greenland-Scotland Ridge have a common source. We estimate that the IFSJ transports approximately 1.0±0.1 Sv, which can account for roughly half of the total overflow transport through the Faroe Bank Channel. As such, the IFSJ is a significant component of the overturning circulation in the Nordic Seas. In paper III we quantify the along-stream evolution of the North Icelandic Irminger Current (NIIC) as it progresses along the shelf break north of Iceland, using a high-resolution shipboard hydrographic/velocity survey, satellite and surface drifter data, and historical hydrographic measurements. The NIIC cools and freshens along its pathway, predominantly due to mixing with cold, fresh offshore waters. Dense-water formation on the shelf is limited, occurring sporadically in only 7% of all historical winter profiles. The hydrographic properties of this locally formed water match the lighter, shallower portion of the NIJ. Along the northeast Iceland slope, enhanced eddy activity and variability in sea surface temperature indicate that locally formed eddies due to instability of the NIIC divert heat and salt into the interior Iceland Sea. The emergence of the NIJ in the same region suggests that there may be a dynamical link to the formation of the NIJ. As such, our results indicate that while the NIIC rarely supplies the NIJ directly, it may be dynamically important for the overturning circulation in the Nordic Seas. The three papers advance our knowledge about the circulation along the northern slope of the Greenland-Scotland Ridge and highlight its significance for water mass transformation in the Nordic Seas and our understanding of the Nordic Seas–North Atlantic exchange. In particular, my results contribute to an improved understanding of the pathways of dense water feeding the overflows, which is imperative to accurately predict how the AMOC will respond to a changing climate