Tag: årthun

Expanding influence of Atlantic and Pacific Ocean heat transport on winter sea-ice variability in a warming Arctic

Dörr, J., Årthun, M., Eldevik, T., Sandø, A. B. 2024: Expanding influence of Atlantic and Pacific Ocean heat transport on winter sea-ice variability in a warming Arctic. Geophys Res Lett Oceans. https://doi.org/10.1029/2023JC019900

Summary: The gradual anthropogenic-driven retreat of Arctic sea ice is overlaid by large natural (internal) year-to-year variability. In winter, sea-ice loss and variability are currently most pronounced in the Barents Sea. As the loss of winter sea ice continues in a warming world, other regions will experience increased sea-ice variability. In this study, we investigate to what extent this increased winter sea-ice variability in the future is connected to ocean heat transport (OHT). We analyze and contrast the present and future link between Pacific and Atlantic OHT and the winter Arctic sea-ice cover using simulations from seven single-model large ensembles. We find strong model agreement for a poleward expanding impact of OHT through the Bering Strait and the Barents Sea under continued sea-ice retreat. Model differences on the Atlantic side can be explained by the differences in the simulated variance of the Atlantic inflows. Model differences on the Pacific side can be explained by differences in the simulated strength of Pacific Water inflows, and upper-ocean stratification and vertical mixing on the Chukchi shelf. Our work highlights the increasing importance of the Pacific and Atlantic water inflows to the Arctic Ocean and highlights which factors are important to correctly simulate in order to capture the changing impact of OHT in the warming Arctic.

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

Arctic-Atlantic Climate Variability and Predictability in Observations and in a Dynamical Prediction System (PhD thesis)

Goncalves Dos Passos, Leilane (2023-11-03): Arctic-Atlantic Climate Variability and Predictability in Observations and in a Dynamical Prediction System. PhD thesis, University of Bergen, Bergen, Norway. https://bora.uib.no/bora-xmlui/handle/11250/3099594

Summary: The major focus of this thesis is on understanding decadal climate predictability to improve climate models and their predictions. Climate predictions show promising results but are still facing challenges, especially in connecting the ocean and atmosphere. The ocean is the main source of predictability. The ocean’s capacity to store and release heat over long periods of time makes it a thermal memory of the climate system. In the Arctic-Atlantic region, ocean currents transport heat to polar areas, and along this path, the ocean releases the heat to the atmosphere through surface fluxes. From this interaction, both the ocean and the atmosphere change. On the one hand, as the ocean releases heat into the atmosphere, it cools down, increasing its density. The denser water eventually flows southward as part of the Atlantic Meridional Overturning Circulation (AMOC). On the other hand, the atmosphere being warmed by the ocean affects nearby land areas through the winds, influencing the climate variability of Western Europe.
This dynamic ocean-atmosphere interaction is a source of predictability in the Arctic-Atlantic region and is investigated here using observations and a dynamical prediction system, the Norwegian Climate Prediction Model (NorCPM). Dynamical prediction systems are useful tools for investigating and predicting climate variability on decadal timescales. Beginning their development in the early 2000s, these systems are currently the focus of significant efforts by the scientific community to provide operational decadal forecasts with reliable and accurate information. The research of this thesis is aligned with the development of NorCPM while also focusing on investigating key mechanisms that give rise to predictability in the Arctic-Atlantic region.
Climate predictions are initialized in different ways, which affects their performance. The first study of the thesis investigates the best initialization method for the Arctic-Atlantic region using NorCPM. Paper I finds that employing a more complex data assimilation method leads to the improved predictive skill of temperature and salinity in the Subpolar North Atlantic (SPNA) but not in the Norwegian Sea. The loss of skill in the Norwegian Sea is found in regions characterized by intense surface heat fluxes and eddy activity, such as the Norwegian and Lofoten Basins. The warm Atlantic water moving northwards from the SPNA to the Norwegian Sea carries thermohaline anomalies, and it is transformed from light-to-dense waters by surface forcing along the path. These two mechanisms are investigated in observation-based data in Paper II. Their relationship is analyzed, focusing on the decadal timescale in the eastern SPNA. Paper II finds that warm anomalies are associated with surface-forced water mass transformation in the light-density classes, while during cold anomalies, more transformation happens in denser classes. This relationship was disrupted during the Great Salinity Anomaly events of the 70s and 90s. Furthermore, the study highlights a faster propagation of thermohaline anomalies in the SPNA compared to the Norwegian Sea, particularly regarding temperature.
The influence of the ocean on the climate of Europe is investigated in Paper III. This study advances the understanding of how constrained ocean variability impacts the atmosphere of NorCPM. The results show a more realistic thermodynamic component of surface air temperature (SAT) over the ocean and some European regions. Paper III shows that there is potential to improve multi-annual to decadal predictions over Europe, which is currently challenging in prediction systems. The research presented in this Thesis enhances the understanding of climate predictability in the Arctic-Atlantic region. It provides insights into the interactions between the atmosphere and ocean and adds to the development of the Norwegian Climate Prediction Model, contributing to making this prediction system operational in the coming years. Following similar approaches as presented in this thesis for other dynamical prediction systems would be highly recommended.

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

Forced and internal components of observed Arctic sea-ice changes

Dörr, J.S., Bonan, D.B., Årthun, M., Svendsen, L., Wills, R.C.J. 2023: Forced and internal components of observed Arctic sea-ice changes. The Cryosphere. https://doi.org/10.5194/tc-17-4133-2023

Summary: The Arctic sea-ice cover is strongly influenced by internal variability on decadal timescales, affecting both short-term trends and the timing of the first ice-free summer. Several mechanisms of variability have been proposed, but how these mechanisms manifest both spatially and temporally remains unclear. The relative contribution of internal variability to observed Arctic sea-ice changes also remains poorly quantified. Here, we use a novel technique called low-frequency component analysis to identify the dominant patterns of winter and summer decadal Arctic sea-ice variability in the satellite record. The identified patterns account for most of the observed regional sea-ice variability and trends, and they thus help to disentangle the role of forced and internal sea-ice changes over the satellite record. In particular, we identify a mode of decadal ocean–atmosphere–sea-ice variability, characterized by an anomalous atmospheric circulation over the central Arctic, that accounts for approximately 30 % of the accelerated decline in pan-Arctic summer sea-ice area between 2000 and 2012 but accounts for at most 10 % of the decline since 1979. For winter sea ice, we find that internal variability has dominated decadal trends in the Bering Sea but has contributed less to trends in the Barents and Kara seas. These results, which detail the first purely observation-based estimate of the contribution of internal variability to Arctic sea-ice trends, suggest a lower estimate of the contribution from internal variability than most model-based assessments.

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

Surface-Forced Variability in the Nordic Seas Overturning Circulation and Overflows

Årthun, M. 2023: Surface-Forced Variability in the Nordic Seas Overturning Circulation and Overflows. Geophys Res Lett. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GL104158

Summary: Water mass transformation in the Nordic Seas and the associated overflow of dense waters across the Greenland-Scotland Ridge (GSR) acts to maintain the lower limb of the Atlantic meridional overturning circulation. Here, we use ocean and atmospheric reanalysis to assess the temporal variability in the Nordic Seas overturning circulation between 1950 and 2020 and its relation to surface buoyancy forcing. We find that variable surface-forced transformation of Atlantic waters in the eastern Nordic Seas can explain variations in overflow transport across the GSR. The production of dense water masses in the Greenland and Iceland Seas is of minor importance to overflow variability. The Nordic Seas overturning circulation shows pronounced multidecadal variability that is in phase with the Atlantic Multidecadal Variability (AMV) index, but no long-term trend. As the AMV is currently transitioning into its negative phase, the next decades could see a decreased overflow from the Nordic Seas.

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

Phytoplankton abundance in the Barents Sea is predictable up to five years in advance

Fransner, F., Olsen, A., Årthun, M., Counillon, F., Tjiputra, J., Samuelsen, A., Keenlyside, N. 2023: Phytoplankton abundance in the Barents Sea is predictable up to five years in advance. Commun Earth Environ. https://doi.org/10.1038/s43247-023-00791-9

Summary: The Barents Sea is a highly biologically productive Arctic shelf sea with several commercially important fish stocks. Interannual-to-decadal predictions of its ecosystem would therefore be valuable for marine resource management. Here, we demonstrate that the abundance of phytoplankton, the base of the marine food web, can be predicted up to five years in advance in the Barents Sea with the Norwegian Climate Prediction Model. We identify two different mechanisms giving rise to this predictability; 1) in the southern ice-free Atlantic Domain, skillful prediction is a result of the advection of waters with anomalous nitrate concentrations from the Subpolar North Atlantic; 2) in the northern Polar Domain, phytoplankton predictability is a result of the skillful prediction of the summer ice concentration, which influences the light availability. The skillful prediction of the phytoplankton abundance is an important step forward in the development of numerical ecosystem predictions of the Barents Sea.

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

Future strengthening of the Nordic Seas overturning circulation

Årthun, M., Asbjørnsen, H., Chafik, L., Johnson, H.L., Våge, K. 2023: Future strengthening of the Nordic Seas overturning circulation. Nat Commun. https://www.nature.com/articles/s41467-023-37846-6

Summary: The overturning circulation in the Nordic Seas involves the transformation of warm Atlantic waters into cold, dense overflows. These overflow waters return to the North Atlantic and form the headwaters to the deep limb of the Atlantic meridional overturning circulation (AMOC). The Nordic Seas are thus a key component of the AMOC. However, little is known about the response of the overturning circulation in the Nordic Seas to future climate change. Here we show using global climate models that, in contrast to the North Atlantic, the simulated density-space overturning circulation in the Nordic Seas increases throughout most of the 21st century as a result of enhanced horizontal circulation and a strengthened zonal density gradient. The increased Nordic Seas overturning is furthermore manifested in the overturning circulation in the eastern subpolar North Atlantic. A strengthened Nordic Seas overturning circulation could therefore be a stabilizing factor in the future AMOC.

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

Rapid sea ice changes in the future Barents Sea

Rieke, O., Årthun, M., Dörr, J.S. 2023: Rapid sea ice changes in the future Barents Sea. The Cryosphere. https://doi.org/10.5194/tc-17-1445-2023

Summary: Observed and future winter Arctic sea ice loss is strongest in the Barents Sea. However, the anthropogenic signal of the sea ice decline is superimposed by pronounced internal variability that represents a large source of uncertainty in future climate projections. A notable manifestation of internal variability is rapid ice change events (RICEs) that greatly exceed the anthropogenic trend. These RICEs are associated with large displacements of the sea ice edge which could potentially have both local and remote impacts on the climate system. In this study we present the first investigation of the frequency and drivers of RICEs in the future Barents Sea, using multi-member ensemble simulations from CMIP5 and CMIP6. A majority of RICEs are triggered by trends in ocean heat transport or surface heat fluxes. Ice loss events are associated with increasing trends in ocean heat transport and decreasing trends in surface heat loss. RICEs are a common feature of the future Barents Sea until the region becomes close to ice-free. As their evolution over time is closely tied to the average sea ice conditions, rapid ice changes in the Barents Sea may serve as a precursor for future changes in adjacent seas.

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

Impact of initialization methods on the predictive skill in NorCPM: an Arctic–Atlantic case study

Passos, L., Langehaug, HR., Årthun, M., Eldevik, T., Bethke, I., Kimmritz, M. 2022: Impact of initialization methods on the predictive skill in NorCPM: an Arctic–Atlantic case study. Clim Dyn. https://doi.org/10.1007/s00382-022-06437-4

Summary: The skilful prediction of climatic conditions on a forecast horizon of months to decades into the future remains a main scientific challenge of large societal benefit. Here we assess the hindcast skill of the Norwegian Climate Prediction Model (NorCPM) for sea surface temperature (SST) and sea surface salinity (SSS) in the Arctic–Atlantic region focusing on the impact of different initialization methods. We find the skill to be distinctly larger for the Subpolar North Atlantic than for the Norwegian Sea, and generally for all lead years analyzed. For the Subpolar North Atlantic, there is furthermore consistent benefit in increasing the amount of data assimilated, and also in updating the sea ice based on SST with strongly coupled data assimilation. The predictive skill is furthermore significant for at least two model versions up to 8–10 lead years with the exception for SSS at the longer lead years. For the Norwegian Sea, significant predictive skill is more rare; there is relatively higher skill with respect to SSS than for SST. A systematic benefit from more complex data assimilation approach can not be identified for this region. Somewhat surprisingly, skill deteriorates quite consistently for both the Subpolar North Atlantic and the Norwegian Sea when going from CMIP5 to corresponding CMIP6 versions. We find this to relate to change in the regional performance of the underlying physical model that dominates the benefit from initialization.

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

Prediction of the Northeast Arctic cod biomass in the Barents Sea

Mariko Koseki joined BCPU as an intern in autumn 2021. Mariko has a Masters in Environmental Science which she obtained at Hokkaido University in Japan.

“I, Mariko Koseki, am an intern within BCPU, and I have been working with Dr. Anne Britt Sandø at the Institute of Marine Research since autumn of 2021.
During the internship, we have focused on the Northeast Arctic cod (NEA cod/Gadus Morhua) biomass in the Barents Sea and developed regression models to predict variations in cod biomass in the future.
The NEA cod is one of the most important species in the Barents Sea for both the ecosystem and as a commercial stock. Several earlier studies reported that the recent warming condition in the Barents Sea has led to high cod biomass.
To construct regression models for total stock biomass of the NEA cod, we used hydrographic and biological variables, such as temperature, salinity, sea ice fraction, primary- and secondary production as explanatory variables. These variables were obtained from hindcast simulations with regional ocean and ecosystem models. Finally, we used the same regression models with variables from downscaled climate scenarios to project future variations in the NEA cod.
We found that several of the regression models have high prediction skills and captured the variations in total stock biomass in the Barents Sea well. Moreover, based on downscaled climate projections, we made maps of spatial distributions of cod biomass in the future. However, errors between observations and predictions of cod biomass necessitate further improvement of the regression models. Now we are preparing to publish this study as a scientific article.
I would like to thank everyone who has supported my internship, and I hope to make use of my experience in my next steps.” – Mariko

On the trail of the disappearing polar sea ice

Jakob Dörr is a PhD student at the University of Bergen, working with Marius Årthun in the BCPU research area on “Understanding mechanisms for climate predictability”. In the spring of 2022, he travelled to California, to visit Dave Bonan. For four weeks, Jakob had the opportunity to work with Dave and his working group at the California Institute of Technology (Caltech) in Pasadena. Dave is also a PhD student, and also interested in understanding present and future changes in the Earth’s sea ice cover, and which processes these are caused by.

In the Arctic, the sea ice cover has strongly declined in all seasons over the last 40 years, and this is mostly due to the Earth’s warming, driven by anthropogenic greenhouse gas emissions. However, because the sea ice interacts with the ocean and the atmosphere and is thus part of the chaotic climate system, it is affected by random fluctuations and internal variability which is independent from the long-term warming trend. These variability modes can affect the sea ice for periods of up to several decades. It is therefore not entirely clear exactly how much of the sea ice loss we have observed over the last decades was due to global warming, and how much was because of internal variability. Dave and Jakob are working to detect and separate those variability modes that affect the sea ice over long periods (decades and longer) in the observational record of sea ice. They use a novel technique developed by Robb Wills at the University of Washington.

Jakob and Dave are hoping to determine for different regions of the Arctic, which modes of variability affect the sea ice cover, and how much their influence compares to the long-term sea ice loss due to global warming. This will help to understand and attribute past sea ice changes and enhance our ability to predict the future regional sea ice loss. While Jakob focuses on the Arctic, Dave applies the same technique to the Antarctic, where a steady increase in sea ice cover over the last decades, followed by a strong decline since 2016, has been observed. Their analysis might shed some light on the mechanisms behind this puzzling evolution, and how much of it was caused by changes due to global warming. The goal of the BCPU-supported visit was to prepare work for two separate publications on Arctic and Antarctic sea ice, respectively, and to discuss how experience from observations can be applied to analyse climate model simulations of future sea ice change.


“During the visit, we exchanged our experience and discussed new ideas for our analysis. I also got to meet scientists in both the Oceanography (Andrew Thompson) and the Climate Dynamics (Tapio Schneider) group at Caltech. I was also lucky to come at a time where Caltech was opening fully again, with many international scientists visiting the institute. Furthermore, I got invited to be part of a sea ice reading course where we had intense discussions about sea ice models, trends and mechanisms with Dave and other members of the Oceanography group. On top of that, I had the chance to visit some friends from the Scripps Institute of Oceanography in San Diego. I had a lot of interactions during the visit and learned a lot about how science is conducted at Caltech and other US institutions. I hope that I can continue the collaboration between Caltech and the Bjerknes Center beyond our work on sea ice observations. The visit showed me how important it is to physically meet people to exchange ideas and develop collaborations across the globe. There are plans that Dave visits us in Bergen next spring, and I hope to return to Pasadena after that.” – Jakob Dörr