Semper Stefanie

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  • Article
    Evolution and transformation of the North Icelandic Irminger Current along the North Iceland Shelf
    (American Geophysical Union, 2022-02-20) Semper, Stefanie ; Våge, Kjetil ; Pickart, Robert S. ; Jónsson, Steingrímur ; Valdimarsson, Héðinn
    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.
  • Article
    Water mass transformation in the Iceland Sea: contrasting two winters separated by four decades
    (Elsevier, 2022-06-22) Våge, Kjetil ; Semper, Stefanie ; Valdimarsson, Héðinn ; Jónsson, Steingrímur ; Pickart, Robert S. ; Moore, G. W. K.
    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.
  • Article
    The emergence of the North Icelandic Jet and its evolution from northeast Iceland to Denmark Strait
    (American Meteorological Society, 2019-09-19) Semper, Stefanie ; Våge, Kjetil ; Pickart, Robert S. ; Valdimarsson, Héðinn ; Torres, Daniel J. ; Jónsson, Steingrímur
    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 ≡ 106 m3 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.
  • Article
    The Iceland-Faroe slope jet: a conduit for dense water toward the Faroe Bank Channel overflow
    (Nature Research, 2020-10-23) Semper, Stefanie ; Pickart, Robert S. ; Våge, Kjetil ; Larsen, Karin Margretha H. ; Hátún, Hjálmar ; Hansen, Bogi
    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.
  • Article
    The Iceland Greenland Seas Project
    (American Meteorological Society, 2019-09-27) Renfrew, Ian A. ; Pickart, Robert S. ; Vage, Kjetil ; Moore, G. W. K. ; Bracegirde, Thomas J. ; Elvidge, Andrew D. ; Jeansson, Emil ; Lachlan-Cope, Thomas ; McRaven, Leah T. ; Papritz, Lukas ; Reuder, Joachim ; Sodemann, Harald ; Terpstra, Annick ; Waterman, Stephanie N. ; Valdimarsson, Héðinn ; Weiss, Albert ; Almansi, Mattia ; Bahr, Frank B. ; Brakstad, Ailin ; Barrell, Christopher ; Brooke, Jennifer K. ; Brooks, Barbara J. ; Brooks, Ian M. ; Brooks, Malcolm E. ; Bruvik, Erik Magnus ; Duscha, Christiane ; Fer, Ilker ; Golid, H. M. ; Hallerstig, M. ; Hessevik, Idar ; Huang, Jie ; Houghton, Leah A. ; Jonsson, Steingrimur ; Jonassen, Marius ; Jackson, K. ; Kvalsund, K. ; Kolstad, Erik W. ; Konstali, K. ; Kristiansen, Jorn ; Ladkin, Russell ; Lin, Peigen ; Macrander, Andreas ; Mitchell, Alexandra ; Olafsson, H. ; Pacini, Astrid ; Payne, Chris ; Palmason, Bolli ; Perez-Hernandez, M. Dolores ; Peterson, Algot K. ; Petersen, Guðrún N. ; Pisareva, Maria N. ; Pope, James O. ; Seidl, Andrew D. ; Semper, Stefanie ; Sergeev, Denis ; Skjelsvik, Silje ; Søiland, Henrik ; Smith, D. ; Spall, Michael A. ; Spengler, Thomas ; Touzeau, Alexandra ; Tupper, George H. ; Weng, Y. ; Williams, Keith D. ; Yang, Xiaohau ; Zhou, Shenjie
    The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway.
  • Article
    A numerical study of interannual variability in the North Icelandic Irminger Current
    (American Geophysical Union, 2018-10-11) Zhao, Jian ; Yang, Jiayan ; Semper, Stefanie ; Pickart, Robert S. ; Våge, Kjetil ; Valdimarsson, Héðinn ; Jónsson, Steingrímur
    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.
  • Article
    Coupled atmosphere–ocean observations of a cold‐air outbreak and its impact on the Iceland Sea
    (Royal Meteorological Society, 2022-12-24) Renfrew, Ian A. ; Huang, Jie ; Semper, Stefanie ; Barrell, Christopher ; Terpstra, Annick ; Pickart, Robert S. ; Våge, Kjetil ; Elvidge, Andrew D. ; Spengler, Thomas ; Strehl, Anna‐Marie ; Weiss, Alexandra
    Marine cold‐air outbreaks (CAOs) are vigorous equatorward excursions of cold air over the ocean, responsible for the majority of wintertime oceanic heat loss from the subpolar seas of the North Atlantic. However, the impact of individual CAO events on the ocean is poorly understood. Here we present the first coupled observations of the atmosphere and ocean during a wintertime CAO event, between 28 February and 13 March 2018, in the subpolar North Atlantic region. Comprehensive observations are presented from five aircraft flights, a research vessel, a meteorological buoy, a subsurface mooring, an ocean glider, and an Argo float. The CAO event starts abruptly with substantial changes in temperature, humidity and wind throughout the atmospheric boundary layer. The CAO is well mixed vertically and, away from the sea‐ice edge, relatively homogeneous spatially. During the CAO peak, higher sensible heat fluxes occupy at least the lowest 200 m of the atmospheric boundary layer, while higher latent heat fluxes are confined to the surface layer. The response of the ocean to the CAO is spatially dependent. In the interior of the Iceland Sea the mixed layer cools, while in the boundary current region it warms. In both locations, the mixed layer deepens and becomes more saline. Combining our observations with one‐dimensional mixed‐layer modelling, we show that in the interior of the Iceland Sea, atmospheric forcing dominates the ocean response. In contrast, in the boundary current region lateral advection and mixing counteract the short‐term impact of the atmospheric forcing. Time series observations of the late‐winter period illustrate a highly variable ocean mixed layer, with lateral advection and mixing often masking the ocean's general cooling and deepening response to individual CAO events.Simultaneous observations of the atmosphere and ocean during a cold‐air outbreak over the Iceland Sea. The top panel shows near‐surface potential temperature from two research flights and a research vessel at the onset of the event; the distribution is relatively homogeneous, although affected by the sea‐ice off the East Greenland coast. The bottom panel shows observed ocean mixed‐layer depth during the first part of the event; the response of the ocean is spatially dependent due to counteracting vertical and lateral processes.
  • Article
    How warm Gulf Stream water sustains a cold underwater waterfall
    (Frontiers Media, 2022-07-06) Semper, Stefanie ; Glessmer, Mirjam S. ; Våge, Kjetil ; Pickart, Robert S.
    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.