Tag: asbjørnsen

Variable Nordic Seas Inflow Linked to Shifts in North Atlantic Circulation

Asbjørnsen, H., Johnson, H.L., Årthun, M. 2021: Variable Nordic Seas Inflow Linked to Shifts in North Atlantic Circulation. Journal of Climate. https://doi.org/10.1175/JCLI-D-20-0917.1 .

Summary: The inflow across the Iceland-Scotland Ridge determines the amount of heat supplied to the Nordic Seas from the subpolar North Atlantic (SPNA). Consequently, variable inflow properties and volume transport at the ridge influence marine ecosystems and sea ice extent further north. Here, we identify the upstream pathways of the Nordic Seas inflow, and assess the mechanisms responsible for interannual inflow variability. Using an eddy-permitting ocean model hindcast and a Lagrangian analysis tool, numerical particles are released at the ridge during 1986-2015 and tracked backward in time. We find an inflow that is well-mixed in terms of its properties, where 64% comes from the subtropics and 26% has a subpolar or Arctic origin. The local instantaneous response to the NAO is important for the overall transport of both subtropical and Arctic-origin waters at the ridge. In the years before reaching the ridge, the subtropical particles are influenced by atmospheric circulation anomalies in the gyre boundary region and over the SPNA, forcing shifts in the North Atlantic Current (NAC) and the subpolar front. An equatorward shifted NAC and westward shifted subpolar front correspond to a warmer, more saline inflow. Atmospheric circulation anomalies over the SPNA also affect the amount of Arctic-origin water re-routed from the Labrador Current toward the Nordic Seas. A high transport of Arctic-origin water is associated with a colder, fresher inflow across the Iceland-Scotland Ridge. The results thus demonstrate the importance of gyre dynamics and wind forcing in affecting the Nordic Seas inflow properties and volume transport.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

New EASAC report: “A Sea of Change”

Translated from the Norwegian press release at the Bjerknes Centre for Climate Research

Tor Eldevik leads EASAC report, “A sea of change: Europe’s future in the Atlantic realm”.

In the report an international panel of experts goes through the changes seen until now in the Atlantic Ocean, and what we can expect of climate change. But there is also a potential in being the closest neighbour to our western ocean.

The report is published by EASAC, the European science academy advisory council. The panel of experts is led by Tor Eldevik, Professor at the University of Bergen and the Bjerknes Centre for Climate Research, and Deputy Leader in the BCPU.

A potential in climate prediction

The report shows how fluctuations and trends in the Atlantic Ocean affects the climate in Europe and both the environment and resources in the ocean and on land.

“The report is very clear about future climatic risks, but equally focuses on the future benefits we can harvest from better understanding of the relations between the state of the Atlantic and climatic conditions over Europe that affects everything from the supply of renewable energy to fisheries,” says Tor Eldevik.

He emphasises how this knowledge can be used far better than it is now. Climate predictions developed today have the potential to predict cod movements between years, including movements out of Norwegian fisheries sectors.

To power companies the knowledge of how westerlies in the Atlantic Ocean (NAO index) affect Norwegian hydro power production can also be useful.

Figurtekst: Norsk vasskraftproduksjon svinger saman med vestavindsbeltet i Atlanterhavet, slik tidlegare vist av Helene Asbjørnsen og Noel Keenlyside UiB og Bjerknessenteret. Vasskraftdata frå SSB, styrke på vestavind vinterstid (NAO-indeks) frå climatedataguide.ucar.edu
Figure 4.1 Norwegian hydropower production swings with the westerly winds (wintertime NAO; variance explained 40%). (Source: H. Asbjørnsen and N. Keenlyside, University of Bergen / Bjerknes Climate Prediction Unit; power production and NAO data from https://www.ssb.no/en/statbank/table/08307 and https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlanticoscillation-nao-index-station-based, respectively.)

Climate risk

Tor Eldevik points out how future changes in the ocean are connected to how successful we are at mitigating global warming.

“If we succeed in keeping the average warming to 1.5°C, then Antarctica may continue melting at current rates; but overshooting the 2 °C Paris Agreement target towards 3°C may lead to Antarctic melt alone add 0.5 cm a year by 2100,” he says.

Sea level rise have regional differences, but to the many million people living by the North Sea Basin, accounting for a meter rise in sea level.

Cities along the coast of the Netherlands, Germany, Denmark and Great Britain will be affected greatly.

Figure 2.5 The North Sea coastline with +1 m of global SLR with the flooded areas in blue. Major population centres are marked in circles. (Source: https://sealevel.climatecentral.org/maps/.)
Figure 2.5 The North Sea coastline with +1 m of global SLR with the flooded areas in blue. Major population centres are marked in circles. (Source: https://sealevel.climatecentral.org/maps/.)

Central points in the report

  • Sea level rise
    On average, the sea level has risen 11-16 centimeters in the twentieth century.
    Europe must prepare for up to one meter sea level rise by 2100. Storm surges on a level we now expect every 100 years, could be yearly by 2100 if CO2 emissions continues as today. Ice melts on Greenland and the Antarctic contributes to sea level rise, as well as glacial metling in warmer areas and sea water expanding with heat. There is uncertainty linked to melting on Greenland and the Antarctic which needs to be followed closely.
  • Renewable energy
    Wind, weather and precipitation over Europe, and especially the Norwegian coast, kan be linked to the ocean. The strength of the Gulf Stream and the westerlies over the Atlantic Ocean affects the severity of wind and precipication over Europe, including the Norwegian coast. This knowledge is critical to predict climate fluctuations for the coming years and seasons – which in Norway is especially useful to power companies, both wind and hydro energy production.
  • Ocean acidification
    Temperature increases leads to fish stocks moving, uptake of CO2 makes the ocean more acidic, which changes the living conditions for life in the ocean. If the current emissions of climate gases is kept up, we will reach a level in 2100 that is uninhabitable.
  • Ocean circulation, ocean streams and the Gulf Stream giving us a milder climate
    Speculations that the Gulf Stream will stop are excessive. But the Gulf Stream strength are connected to climate in Europe and Norway. A decline in heat transportation of 20% is expected further South in the Atlantic this century, but as far North as Norway we are likely to see an increase in the stream and a continued heating of the ocean.

Read the report with EASAC

 

 

Mechanisms and pathways of ocean heat anomalies in the Arctic-Atlantic region

Asbjørnsen, Helene (2020-12-10). Mechanisms and pathways of ocean heat anomalies in the Arctic-Atlantic region (PhD thesis, University of Bergen, Bergen, Norway). https://bora.uib.no/bora-xmlui/handle/11250/2712025 .

Summary: Along the Atlantic water pathway, from the Gulf Stream in the south to the Arctic Ocean in the north, variability in ocean heat content is pronounced on interannual to decadal time scales. Ocean heat anomalies in this Arctic-Atlantic sector are known to affect Arctic sea ice extent, marine ecosystems, and continental climate. However, there is at present neither consensus nor any complete understanding of the mechanisms causing such heat anomalies. This dissertation obtains a more robust understanding of regional ocean heat content variability by assessing the mechanisms and pathways of ocean heat anomalies in the Arctic-Atlantic region. The results are presented in three papers.

The first paper investigates the link between a variable Nordic Seas inflow and large- scale ocean circulation changes upstream. Using a global, eddy-permitting ocean hind- cast together with a Lagrangian analysis tool, numerical particles are seeded at the Iceland-Scotland Ridge and tracked backward in time. Water from the subtropics sup- plied by the North Atlantic Current (NAC) is found to be the main component of the Nordic Seas inflow (64%), while 26% of the inflow has a subpolar or Arctic origin. Different atmospheric patterns are seen to affect the circulation strength along the advective pathways, as well as the supply of subtropical and Arctic-origin water to the ridge through shifts in the NAC and the subpolar front. A robust link between a high transport of Arctic-origin water and a cold and fresh inflow is furthermore established, while a high transport of subtropical water leads to higher inflow salinities. The second paper investigates the mechanisms of interannual heat content variability in the Norwegian Sea downstream of the Iceland-Scotland Ridge, using a state-of-the-art ocean state estimate and closed heat budget diagnostics. Ocean advection is found to be the primary contributor to heat content variability in the Atlantic domain of the Norwegian Sea, although local surface fluxes also play an active role. Anomalous heat advection furthermore depends on the strength of the Atlantic water inflow and the conditions upstream of the ridge. Combined, the two papers demonstrate the importance of gyre dynamics and large-scale wind forcing in causing variability at the ridge, while high- lighting the impacts on Norwegian Sea heat content downstream.

For the third paper, warming trends in the Barents Sea and Fram Strait are explored, and, thus, the mechanisms underlying recent Atlantification of the Arctic Ocean. The Barents Sea is seen to transition to a warmer state, with reduced sea ice concentrations and Atlantic water extending further poleward. The mechanisms driving the warming are, however, found to be regionally dependent and not stationary in time. In the ice- free region, ocean advection is found to be a major driver of the warming trend due to increasing inflow temperatures in the late 1990s and early 2000s, while reduced ocean heat loss is contributing to the warming trend from the mid-2000s and onward. A considerable upper-ocean warming and a weakened stratification is seen in the ice- covered northwestern Barents Sea. However, in contrast to what has been previously hypothesized, the results do not point to increased upward heat fluxes from the Atlantic water layer to the Arctic surface layer as the source of the upper-ocean warming.

The supply of Atlantic heat to the Nordic Seas and the Arctic Ocean has been scrutinized using both Lagrangian methods and heat budget diagnostics. Combined, the three papers demonstrate the important role of ocean heat transport in causing regional heat content variability and change in the Arctic-Atlantic region. A better understanding of interannual to decadal ocean heat content variability has implications for future prediction efforts, and for how we understand the ocean’s role in ongoing and future climate change.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Mechanisms underlying recent Arctic Atlantification

Asbjørnsen, H., Årthun, M., Skagseth, Ø., Eldevik, T. 2020: Mechanisms underlying recent Arctic Atlantification. Geophys. Res. Lett. https://doi.org/10.1029/2020GL088036 .
Summary: Recent “Atlantification” of the Arctic is characterized by warmer ocean temperatures and a reduced sea ice cover. The Barents Sea is a “hot spot” for these changes, something which has broad socioeconomic and environmental impacts in the region. However, there is, at present, no complete understanding of what is causing the ocean warming. Here, we determine the relative importance of transport of heat by ocean currents (ocean advection) and heat exchanges between the atmosphere and the ocean (air-sea heat fluxes) in warming the Barents Sea and Fram Strait. In the ice-free region, ocean advection is found to be the main driver of the warming trend due to increasing inflow temperatures between 1996 and 2006. In the marginal ice zone and the ice-covered northern Barents Sea, ocean advection and air-sea heat fluxes are found to be of interchanging importance in driving the warming trend through the 1993–2014 period analyzed. A better understanding of the recent warming trends in the Barents Sea and Fram Strait has implications for how we understand the ocean’s role in ongoing and future Arctic climate change.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Reduced efficiency of the Barents Sea cooling machine

Skagseth, Ø., Eldevik, T., Årthun, M., Asbjørnsen, H., Lien, V.S., Smedsrud, L.H. 2020: Reduced efficiency of the Barents Sea cooling machine. Nature Climate Change. https://doi.org/10.1038/s41558-020-0772-6

Summary: Dense water masses from the Barents Sea are an important part of the Arctic thermohaline system. Here, using hydrographic observations from 1971 to 2018, we show that the Barents Sea climate system has reached a point where ‘the Barents Sea cooling machine’—warmer Atlantic inflow, less sea ice, more regional ocean heat loss—has changed towards less-efficient cooling. Present change is dominated by reduced ocean heat loss over the southern Barents Sea as a result of anomalous southerly winds. The outflows have accordingly become warmer. Outflow densities have nevertheless remained relatively unperturbed as increasing salinity appears to have compensated the warming inflow. However, as the upstream Atlantic Water is now observed to freshen while still relatively warm, we speculate that the Barents Sea within a few years may export water masses of record-low density to the adjacent basins and deep ocean circulation.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Reduced efficiency of the Barents Sea cooling machine

Skagseth, Ø., Eldevik, T., Årthun, M., Asbjørnsen, H., Lien, VS., Smedsrud, LH. 2020: Decreasing efficiency of the Barents Sea cooling machine. Nature Climate Change. https://doi.org/10.1038/s41558-020-0772-6 .

Summary: Dense water masses from the Barents Sea are an important part of the Arctic thermohaline system. Here, using hydrographic observations from 1971 to 2018, we show that the Barents Sea climate system has reached a point where ‘the Barents Sea cooling machine’—warmer Atlantic inflow, less sea ice, more regional ocean heat loss—has changed towards less-efficient cooling. Present change is dominated by reduced ocean heat loss over the southern Barents Sea as a result of anomalous southerly winds. The outflows have accordingly become warmer. Outflow densities have nevertheless remained relatively unperturbed as increasing salinity appears to have compensated the warming inflow. However, as the upstream Atlantic Water is now observed to freshen while still relatively warm, we speculate that the Barents Sea within a few years may export water masses of record-low density to the adjacent basins and deep ocean circulation.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.