Periodic Reporting for period 2 - SUPERFLOW (Supply of dense water to the overflows across the Greenland-Scotland Ridge)
Periodo di rendicontazione: 2023-05-18 al 2024-05-17
The densest water supplying the Atlantic Meridional Overturning Circulation (AMOC), which helps maintain the temperate climate of northwest Europe, stems mainly from the Greenland Sea and crosses the submarine Greenland-Scotland Ridge through deep passages east and west of Iceland. While the exchange flows across the ridge have been monitored for decades, the circulation and partitioning of the upstream flows that are co-located northeast of Iceland are not yet quantified and understood. The EU-funded SUPERFLOW project will identify the sources, quantify the transports, and understand the dynamics of these dense-water currents using observations and high-resolution realistic and idealised ocean models. This knowledge is imperative for an accurate prediction of how the AMOC will respond to a changing climate.
The scientific work in SUPERFLOW is divided into 3 work packages, related to i) observations, ii) high-resolution, realistic numerical simulations, and iii) idealised modelling.
i) I processed and quality-controlled observations of hydrography and ocean currents from moorings that had been deployed northeast of Iceland and in the northwestern Iceland Sea. Together with shipboard hydrographic observations from this area, we investigated where dense water that supplies the lower limb of the AMOC may be formed in the Iceland Sea, what role air-sea interaction plays in determining the density of newly formed water, and by what pathways this dense water is transported toward the Greenland-Scotland Ridge.
ii) I evaluated a high-resolution, realistic numerical simulation for the Nordic Seas against the available moored hydrographic and velocity time series and historical hydrographic observations. Furthermore, we conducted experiments with synthetic floats by releasing particles and following their trajectories to study the upstream and downstream pathways of the dense-water currents north of Iceland in the numerical simulation. From the assessment of the numerical simulation, we concluded that the model does not seem suitable to determine the pathways and volume transports of the deep currents on the slope north of Iceland. However, as surface currents have previously been shown to be represented well in this model, I now co-supervise one PhD and one MSc student who use this simulation to investigate the surface-intensified East Greenland Current in the western Nordic Seas.
iii) To understand the dynamics of dense-water currents north of Iceland, we used a simple, 2-layer idealised model and investigated the formation of dense slope currents and their flow around an island. The island is connected to submarine ridges on its eastern and western side. In our set-up, we introduced a source of dense water in the north of the domain that supplies the deep layer. This results in transport of dense water around the island in the south of the domain and across the submarine ridges. In particular, we are interested in the role of eddies in transporting the dense water toward the slope and forming the boundary currents, and how the deep currents partition to cross the eastern and western ridge.
So far, I have (co-)authored four publications that are related to the project (two additional manuscripts are currently under review). I have presented my work at 10 international conferences and workshops. In particular, the annual Arctic-Subarctic Ocean Fluxes workshop provided an excellent platform for establishing and continuing scientific collaborations in the community for regional, process-oriented studies in the subpolar regions.
For the outreach component of the project, we published our findings in a scientific journal for kids and teenagers. This paper, which had been reviewed by teenagers, formed the basis for a classroom lesson with a DIY-experiment we developed for high-school students. The material for the lesson has been published online and was presented at a teacher’s conference.
i) I processed and quality-controlled observations of hydrography and ocean currents from moorings that had been deployed northeast of Iceland and in the northwestern Iceland Sea. Together with shipboard hydrographic observations from this area, we investigated where dense water that supplies the lower limb of the AMOC may be formed in the Iceland Sea, what role air-sea interaction plays in determining the density of newly formed water, and by what pathways this dense water is transported toward the Greenland-Scotland Ridge.
ii) I evaluated a high-resolution, realistic numerical simulation for the Nordic Seas against the available moored hydrographic and velocity time series and historical hydrographic observations. Furthermore, we conducted experiments with synthetic floats by releasing particles and following their trajectories to study the upstream and downstream pathways of the dense-water currents north of Iceland in the numerical simulation. From the assessment of the numerical simulation, we concluded that the model does not seem suitable to determine the pathways and volume transports of the deep currents on the slope north of Iceland. However, as surface currents have previously been shown to be represented well in this model, I now co-supervise one PhD and one MSc student who use this simulation to investigate the surface-intensified East Greenland Current in the western Nordic Seas.
iii) To understand the dynamics of dense-water currents north of Iceland, we used a simple, 2-layer idealised model and investigated the formation of dense slope currents and their flow around an island. The island is connected to submarine ridges on its eastern and western side. In our set-up, we introduced a source of dense water in the north of the domain that supplies the deep layer. This results in transport of dense water around the island in the south of the domain and across the submarine ridges. In particular, we are interested in the role of eddies in transporting the dense water toward the slope and forming the boundary currents, and how the deep currents partition to cross the eastern and western ridge.
So far, I have (co-)authored four publications that are related to the project (two additional manuscripts are currently under review). I have presented my work at 10 international conferences and workshops. In particular, the annual Arctic-Subarctic Ocean Fluxes workshop provided an excellent platform for establishing and continuing scientific collaborations in the community for regional, process-oriented studies in the subpolar regions.
For the outreach component of the project, we published our findings in a scientific journal for kids and teenagers. This paper, which had been reviewed by teenagers, formed the basis for a classroom lesson with a DIY-experiment we developed for high-school students. The material for the lesson has been published online and was presented at a teacher’s conference.
Using historical hydrographic observations from the north Iceland shelf, we demonstrated that the dense water supplying the two deep slope currents, the North Icelandic Jet and the Iceland-Faroe Slope Jet, cannot be formed on the shelf, refuting a previous hypothesis based on numerical simulations. Furthermore, the observations corroborate that eddy activity northeast of Iceland may play a role in the emergence of the North Icelandic Jet. Using historical hydrographic observations from the interior Iceland Sea, we showed that the dense water supplying the two deep slope currents can currently not be formed in the central Iceland Sea either, in contrast to four decades ago, when the locally formed water masses were denser. However, in recent years, sufficiently dense waters were formed in the northwestern Iceland Sea, where the retreat of the sea-ice edge toward Greenland has exposed large areas to the atmosphere during winter. Water mass transformation in the western Nordic Seas is heavily dependent on air-sea-interaction. In particular, the occurrence of episodic excursions of cold air over the comparatively warm ocean, so-called cold-air outbreaks, account for the major portion of the wintertime oceanic heat loss. These events thus play an important role in the formation of the dense water that supplies the slope currents north of Iceland. We have also documented a cold-air outbreak with measurements both in the atmosphere and the ocean and assessed the direct, regionally-dependent response of the ocean mixed layer to this atmospheric forcing.
Results from our work on a high-resolution, realistic numerical simulation of the Nordic Seas and a 2-layer idealised model for the dense-water circulation around Iceland highlight the role of eddies in transporting dense water toward the Icelandic slope and in forming dense slope currents around the island. Eddies are also important for controlling the freshwater exchange from the Greenland shelf to interior basins of the Nordic Seas in summer. Freshwater diverted into the Iceland and Greenland Sea can inhibit open-ocean convection and thus reduce the formation of dense water.
The use of new and existing observations, high-resolution realistic numerical simulations, and idealised modelling in this project provided different perspectives on where and how dense water is formed in the Nordic Seas. Such an understanding of the sources, pathways, and dynamics of the dense-water flows that supply the lower limb of the AMOC is crucial for accurate predictions of the AMOC’s response to climate change.
Results from our work on a high-resolution, realistic numerical simulation of the Nordic Seas and a 2-layer idealised model for the dense-water circulation around Iceland highlight the role of eddies in transporting dense water toward the Icelandic slope and in forming dense slope currents around the island. Eddies are also important for controlling the freshwater exchange from the Greenland shelf to interior basins of the Nordic Seas in summer. Freshwater diverted into the Iceland and Greenland Sea can inhibit open-ocean convection and thus reduce the formation of dense water.
The use of new and existing observations, high-resolution realistic numerical simulations, and idealised modelling in this project provided different perspectives on where and how dense water is formed in the Nordic Seas. Such an understanding of the sources, pathways, and dynamics of the dense-water flows that supply the lower limb of the AMOC is crucial for accurate predictions of the AMOC’s response to climate change.