Key Earth System Processes to understand Arctic Climate Warming and Northern Latitude Hydrological Cycle Changes (KeyCLIM)

• Graff, Lise Seland; Landgren, Oskar Andreas; Parding, Kajsa; Levine, Xavier; Williams, Ryan S. & Mooney, Priscilla (2026). Selecting CMIP6 Models for Future Arctic Storylines Using a Novel Performance Score. Tellus A: Dynamic Meteorology and Oceanography. ISSN 0280-6495. 78(1), p. 1–21. doi: 10.16993/tellusa.4111. Full text in Research Archive Show summary

  Storylines are physically plausible scenarios of future climate change, statistically derived from an ensemble of climate model projections and organized according to the magnitude of projected changes in two or more remote drivers that strongly influence the spatial pattern of the climate response. Here, we provide novel insights into the Arctic storylines identified by Levine et al. (2024), where Barents-Kara Sea warming and lower-tropospheric Arctic warming during the extended summer season (May–October) were remote drivers, as we identify a set of models from the Coupled Model Intercomparison Project phase 6 to represent the storylines. We do this by first identifying models that are similar to these storylines in terms of each remote driver response and quantifying this similarity. Second, we evaluate the model’s performance in terms of a simple performance score based on the mean normalized root-mean-square error for multiple climate variables of importance for the storylines. The normalized values vary between 0 and 1 for all variables, allowing them to exert a comparable influence on the score. The advantage of the score is that it provides an easily implementable and interpretable way of identifying models that are characterized by large errors relative to the rest of the ensemble. Finally, we combine the similarity estimate and the score to select models to represent the storylines. We focus on the Arctic during the extended summer season for which the storylines were designed, but also consider other seasons and regions. Through this exercise, we also document the methodology, benefits, and limitations of the score.

• Goelzer, Heiko; Langebroek, Petra; Born, Andreas; Hofer, Stefan; Haubner, Konstanze & Petrini, Michele [Show all 9 contributors for this article] (2025). Interactive coupling of a Greenland ice sheet model in NorESM2. Geoscientific Model Development. ISSN 1991-959X. 18(20), p. 7867–7867. doi: 10.5194/gmd-18-7853-2025. Full text in Research Archive Show summary

  On the backdrop of observed accelerating ice sheet mass loss over the last few decades, there is growing interest in the role of ice sheet changes in global climate projections. In this regard, we have coupled the Norwegian Earth System Model (NorESM) with the Community Ice Sheet Model (CISM) and have produced an initial set of climate projections including an interactive coupling with a dynamic Greenland ice sheet. Our focus in this manuscript is the description of the coupling, the model setup and the initialisation procedure. To illustrate the effect of the coupling, we have further performed one chain of experiments under historical forcing and subsequently under high future greenhouse gas forcing (SSP5-8.5) until 2100 and extended until 2300. We find a limited impact of the dynamical ice sheet changes on the global response of the coupled model under the given forcing and experimental setup when comparing to a standard CMIP6 simulation of NorESM with a fixed ice sheet.

• Haugvaldstad, Ove Westermoen; Oliviè, Dirk Jan Leo; Storelvmo, Trude & Schulz, Michael (2025). Dust radiative forcing in CMIP6 Earth System models: insights from the AerChemMIP piClim-2xdust experiment. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 25, p. 13199–13219. doi: 10.5194/acp-25-13199-2025. Full text in Research Archive Show summary

  Mineral dust significantly affects the downwelling and upwelling shortwave (SW) and longwave (LW) radiative fluxes, and changes in dust can therefore alter the Earth's energy balance. This study analyses the dust effective radiative forcing (DuERF) in nine CMIP6 Earth System Models (ESMs) using the piClim-2xdust experiment from AerChemMIP. The piClim-2xdust experiment uses a global dust emission tuning factor to double the emission flux. The DuERF is decomposed into contributions from dust-radiation (direct DuERF) and dust-cloud (cloud DuERF) interactions. The net direct DuERF ranges from −0.56 to 0.05 W m−2. Models with lower (higher) dust absorption and a smaller (larger) fraction of coarse dust show the most negative (positive) direct DuERF. The cloud DuERF is positive in most models, ranging from −0.02 to 0.2 W m−2, however, they differ in their LW and SW flux contributions. Specifically, NorESM2-LM shows a positive LW cloud DuERF attributable to the effect of dust on cirrus clouds. The dust forcing efficiency varies tenfold among models, indicating that uncertainty in DuERF is likely underestimated in AerChemMIP. There is a consistent fast precipitation response associated with dust decreasing atmospheric radiative cooling (ARC). Models with strongly absorbing dust show reduced precipitation, explainable by decreased clear-sky ARC (up to 3.2 mm yr−1). In NorESM2-LM, this decrease is associated with a cloudy sky ARC due to an increase in cirrus clouds (up to 5.6 mm yr−1). Taken together, these findings suggest that the fast precipitation response induced by dust alone may be significant and comparable to that caused by anthropogenic black carbon.

• Graff, Lise Seland; Tjiputra, Jerry; Gjermundsen, Ada; Born, Andreas; Debernard, Jens Boldingh & Goelzer, Heiko [Show all 20 contributors for this article] (2025). Sensitivity of winter Arctic amplification in NorESM2. Earth System Dynamics (ESD). ISSN 2190-4979. 16(5), p. 1671–1698. doi: 10.5194/esd-16-1671-2025. Full text in Research Archive Show summary

  While Arctic amplification is a robust feature of both observed and projected climate change, projections of Arctic climate change are characterized by substantial uncertainty. To better understand the drivers of this uncertainty, we performed a coordinated set of fully coupled experiments with the second version of the Norwegian Earth System Model (NorESM2), in which selected processes of key importance for the Arctic climate have been modified. They include improved representation of (1) mixed-phase clouds, (2) eddy processes in the upper ocean, (3) Greenland ice sheet coupling, (4) snow on sea ice processes, and (5) ozone chemistry. For each modification, we carried out sensitivity experiments following the protocols for the historical simulation of the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and a future high-emissions scenario (ssp585). This results in an ensemble of modified historical and ssp585 experiments. The sensitivity experiments all demonstrate enhanced future Arctic warming compared to the unmodified historical and ssp585 experiments. Moreover, the amplitude of the additional warming varies considerably, with the difference between the experiment with the strongest and weakest Arctic mean warming reaching ∼ 9 K during the winter season by the end of the 21st century. The warming signal is dominated by a relatively uniform Arctic warming, which, according to the CMIP6 ssp585 long-term extension, starts to equilibrate during the 22nd century. Surface temperature decomposition shows that winter warming is primarily driven by an enhanced greenhouse effect due to increased cloud cover, near-surface humidity, and the resulting increase in downwelling longwave radiation. The temperature response is most pronounced in the sea ice retreat regions, with the greatest variability between experiments occurring on the Atlantic side. We also identify an emergent constraint, linking changes in Arctic surface temperatures to changes in ocean heat fluxes and sea ice area. This highlights the importance of correctly representing (contemporary) Northern Hemisphere (NH) sea ice when assessing future projected Arctic warming.

• Glaude, Q.; Noel, B.; Olesen, M.; Broeke, M Van den; Berg, W. J. van de & Mottram, R. [Show all 12 contributors for this article] (2024). A Factor Two Difference in 21st-Century Greenland Ice Sheet Surface Mass Balance Projections From Three Regional Climate Models Under a Strong Warming Scenario (SSP5-8.5). Geophysical Research Letters. ISSN 0094-8276. 51(22). doi: 10.1029/2024GL111902. Full text in Research Archive Show summary

  The Arctic is warming rapidly, significantly reducing the Greenland ice sheet (GrIS) surface mass balance (SMB) and raising its contribution to global sea-level rise. Since these trends are expected to continue, it is essential to explore the GrIS SMB response to projected climate warming. We compare projections from three polar regional climate models, RACMO, MAR, and HIRHAM, forced by the Community Earth System Model CESM2 under a high-end warming scenario (SSP5-8.5, 1970–2099). We reveal different modeled SMB by 2100, including a twofold larger annual surface mass loss in MAR (−1735 Gt/yr) and HIRHAM (−1698 Gt/yr) relative to RACMO (−964 Gt/yr). Discrepancies primarily stem from differences in projected runoff, triggering melt-albedo positive feedback and subsequent modeled ablation zone expansion. In addition, we find different responses of modeled meltwater production to similar atmospheric warming. Our analysis suggests clear avenues for model developments to further improve SMB projections and contribution to sea-level rise.

• Kim, Dongchul; Chin, Mian; Schuster, Greg; Yu, Hongbin; Takemura, Toshihiko & Tuccella, Paolo [Show all 14 contributors for this article] (2024). Where Dust Comes From: Global Assessment of Dust Source Attributions With AeroCom Models. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 129(16), p. 1–27. doi: 10.1029/2024JD041377. Full text in Research Archive
• Hofer, Stefan; Hahn, Lily; Shaw, Jonah; McGraw, Zachary; Bruno, Olimpia & Hellmuth, Franziska [Show all 11 contributors for this article] (2024). Realistic representation of mixed-phase clouds increases projected climate warming. Communications Earth & Environment. 5, p. 1–12. doi: 10.1038/s43247-024-01524-2. Full text in Research Archive Show summary

  Clouds are the main source of uncertainties when projecting climate change. Mixed-phase clouds (MPCs) that contain ice and supercooled-liquid particles are especially hard to constrain, and climate models neither agree on their phase nor their spatial extent. This is problematic, as models that underestimate contemporary supercooled-liquid in MPCs will underestimate future warming. Furthermore, it has recently been shown that supercooled-liquid water in MPCs is not homogeneously-mixed, neither vertically nor horizontally. However, while there have been attempts at observationally constraining MPCs to constrain uncertainties in future warming, all studies only use the phase of the interior of MPCs. Using novel satellite observations, and contrary to current knowledge, we show that MPCs are more liquid at the cloud top globally. We use these observations to constrain, for the first time, the cloud top phase in addition to the interior of MPCs in a global climate model, leading to +1C more 21st century warming i

• Hodnebrog, Øivind; Myhre, Gunnar; Jouan, Caroline; Andrews, Timothy; Forster, Piers M. & Jia, Hailing [Show all 12 contributors for this article] (2024). Recent reductions in aerosol emissions have increased Earth’s energy imbalance. Communications Earth & Environment. 5(1). doi: 10.1038/s43247-024-01324-8. Full text in Research Archive Show summary

  The Earth’s energy imbalance is the net radiative flux at the top-of-atmosphere. Climate model simulations suggest that the observed positive imbalance trend in the previous two decades is inconsistent with internal variability alone and caused by anthropogenic forcing and the resulting climate system response. Here, we investigate anthropogenic contributions to the imbalance trend using climate models forced with observed sea-surface temperatures. We find that the effective radiative forcing due to anthropogenic aerosol emission reductions has led to a 0.2 ± 0.1 W m−2 decade−1 strengthening of the 2001–2019 imbalance trend. The multi-model ensemble reproduces the observed imbalance trend of 0.47 ± 0.17 W m−2 decade−1 but with 10-40% underestimation. With most future scenarios showing further rapid reductions of aerosol emissions due to air quality legislation, such emission reductions may continue to strengthen Earth’s energy imbalance, on top of the greenhouse gas contribution. Consequently, we may expect an accelerated surface temperature warming in this decade.

• Siew, Peter Y.F.; Li, Camille; Sobolowski, Stefan Pieter; Dunn-Sigouin, Etienne & Ting, Mingfang (2024). Circulation responses to surface heating and implications for polar amplification. Weather and Climate Dynamics (WCD). 5(3), p. 985–996. doi: 10.5194/wcd-5-985-2024. Full text in Research Archive Show summary

  A seminal study by Hoskins and Karoly (1981) explored the atmospheric circulation response to tropospheric heating perturbations at low latitudes and midlatitudes. Here we revisit and extend their study by investigating the circulation and temperature response to low, middle, and high latitude surface heating using an idealised moist grey radiation model. Our results corroborate previous findings showing that heating perturbations at low latitudes and midlatitudes are balanced by different time-mean circulation responses – upward motion and horizontal-temperature advection, respectively. Transient eddy heat flux divergence plays an increasingly important role with latitude, becoming the main circulation response at high latitudes. However, this mechanism is less efficient at balancing heating perturbations than temperature advection, leading to greater reliance on an additional contribution from radiative cooling. These dynamical and radiative adjustments promote stronger lower-tropospheric warming in response to surface heating at high latitudes compared to lower latitudes. This elucidates the mechanisms by which sea ice loss contributes to polar amplification in a warming climate.

• Booge, Dennis; Tjiputra, Jerry; Oliviè, Dirk Jan Leo; Quack, Birgit & Krüger, Kirstin (2024). Natural marine bromoform emissions in the fully coupled ocean-atmosphere model NorESM2. Earth System Dynamics (ESD). ISSN 2190-4979. 15(3), p. 801–816. doi: 10.5194/esd-15-801-2024. Full text in Research Archive Show summary

  Oceanic bromoform (CHBr3) is an important precursor of atmospheric bromine. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, global CHBr3 production in the ocean and its emissions are still poorly constrained in observations and are mostly neglected in climate models. Here, we newly implement marine CHBr3 in the second version of the state-of-the-art Norwegian Earth System Model (NorESM2) with fully coupled interactions of ocean, sea ice, and atmosphere. Our results are validated using oceanic and atmospheric observations from the HalOcAt (Halocarbons in the Ocean and Atmosphere) database. The simulated mean oceanic concentrations (6.61 ± 3.43 pmol L−1) are in good agreement with observations from open-ocean regions (5.02 ± 4.50 pmol L−1), while the mean atmospheric mixing ratios (0.76 ± 0.39 ppt) are lower than observed but within the range of uncertainty (1.45 ± 1.11 ppt). The NorESM2 ocean emissions of CHBr3 (214 Gg yr−1) are within the range of or higher than previously published estimates from bottom-up approaches but lower than estimates from top-down approaches. Annual mean fluxes are mostly positive (sea-to-air fluxes); driven by oceanic concentrations, sea surface temperature, and wind speed; and dependent on season and location. During winter, model results imply that some oceanic regions in high latitudes act as sinks of atmospheric CHBr3 due to their elevated atmospheric mixing ratios. We further demonstrate that key drivers for oceanic and atmospheric CHBr3 variability are spatially heterogeneous. In the tropical West Pacific, which is a hot spot for oceanic bromine delivery to the stratosphere, wind speed is the main driver for CHBr3 fluxes on an annual basis. In the North Atlantic, as well as in the Southern Ocean region, atmospheric and oceanic CHBr3 variabilities interact during most of the seasons except for the winter months, when sea surface temperature is the main driver. Our study provides an improved process-based understanding of the biogeochemical cycling of CHBr3 and more reliable natural emission estimates, especially on seasonal and spatial scales, compared to previously published model estimates.

• Nummelin, Aleksi & Isachsen, Pål Erik (2024). Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography. Journal of Advances in Modeling Earth Systems. 16(3). doi: 10.1029/2023MS003806. Full text in Research Archive Show summary

  Most of the ocean's kinetic energy is contained within the mesoscale eddy field. Models that do not resolve these eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large-scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models rely on an assumption of a flat ocean floor even though observations and high-resolution modeling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. We show that buoyancy transport coefficient diagnosed from idealized eddy-resolving simulations is indeed reduced over both prograde and retrograde bottom slopes (topographic wave propagation along or against the mean flow, respectively) and that the reduction can be skilfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale. This modified “GM” parameterization enhances the strength of thermal wind currents over the slopes in coarse-resolution, non-eddying, simulations. We find that in realistic global coarse-resolution simulations the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean-flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean biases.

• Myhre, Gunnar; Byrom, Rachael; Andrews, Timothy; Forster, Piers M. & Smith, Christopher J. (2024). Efficacy of climate forcings in transient CMIP6 simulations. Frontiers in Climate. 6. doi: 10.3389/fclim.2024.1397358. Full text in Research Archive Show summary

  For effective radiative forcing (ERF) to be an ideal metric for comparing the strength of different climate drivers (such as CO2 and aerosols), the ratio of radiative forcing to global-mean temperature change must be the same for each driver. Typically, this ratio is divided by the same ratio for CO2 and termed efficacy. Previously it has been shown that efficacy is close to unity in abrupt perturbation experiments for a range of climate drivers, but efficacy with respect to CO2 has not been investigated in transient realistic simulations. Here, we analyse transient simulations from CMIP6 experiments and show comparable results between transient and abrupt perturbation experiments. We demonstrate that aerosol efficacy is not significantly different from unity, however inter-model differences in aerosol experiments are notably large.

• Sterl, Miriam F.; Lacasce, Joseph Henry; Groeskamp, Sjoerd; Nummelin, Aleksi Henrynpoika; Isachsen, Pål Erik & Baatsen, Michiel L. J. (2024). Suppression of Mesoscale Eddy Mixing by Topographic PV Gradients. Journal of Physical Oceanography. ISSN 0022-3670. 54(5), p. 1089–1103. doi: 10.1175/JPO-D-23-0142.1. Full text in Research Archive Show summary

  Oceanic mesoscale eddy mixing plays a crucial role in Earth’s climate system by redistributing heat, salt, and carbon. For many ocean and climate models, mesoscale eddies still need to be parameterized. This is often done via an eddy diffusivity K , which sets the strength of turbulent downgradient tracer fluxes. A well-known effect is the modulation of K in the presence of background potential vorticity (PV) gradients, which suppresses mixing in the direction of the PV gradients. Topographic slopes can induce such suppression through topographic PV gradients. However, this effect has received little attention, and topographic effects are often not included in parameterizations for K . In this study, we show that it is possible to describe the effect of topography on K analytically in a barotropic framework, using a simple stochastic representation of eddy–eddy interactions. We obtain an analytical expression for the depth-averaged K as a function of the bottom slope, which we validate against diagnos

• Edward, Hanna; Topal, Daniel; Box, Jason E.; Buzzard, Sammie; Christie, Frazer D. W. & Hvidberg, Christine [Show all 29 contributors for this article] (2024). Short- and long-term variability of the Antarctic and Greenland ice sheets. Nature Reviews Earth & Environment. 5, p. 193–210. doi: 10.1038/s43017-023-00509-7. Full text in Research Archive Show summary

  The variability of the Antarctic and Greenland ice sheets occurs on various timescales and is important for projections of sea level rise; however, there are substantial uncertainties concerning future ice-sheet mass changes. In this Review, we explore the degree to which short-term fluctuations and extreme glaciological events reflect the ice sheets’ long-term evolution and response to ongoing climate change. Short-term (decadal or shorter) variations in atmospheric or oceanic conditions can trigger amplifying feedbacks that increase the sensitivity of ice sheets to climate change. For example, variability in ocean-induced and atmosphere-induced melting can trigger ice thinning, retreat and/or collapse of ice shelves, grounding-line retreat, and ice flow acceleration. The Antarctic Ice Sheet is especially prone to increased melting and ice sheet collapse from warm ocean currents, which could be accentuated with increased climate variability. In Greenland both high and low melt anomalies have been observed since 2012, highlighting the influence of increased interannual climate variability on extreme glaciological events and ice sheet evolution. Failing to adequately account for such variability can result in biased projections of multi-decadal ice mass loss. Therefore, future research should aim to improve climate and ocean observations and models, and develop sophisticated ice sheet models that are directly constrained by observational records and can capture ice dynamical changes across various timescales.

• Heinze, Christoph; Michel, Clio; Torsvik, Tomas; Schwinger, Jörg & Tjiputra, Jerry (2024). More Frequent Abrupt Marine Environmental Changes Expected. Geophysical Research Letters. ISSN 0094-8276. 51(2), p. 1–10. doi: 10.1029/2023GL106192. Full text in Research Archive Show summary

  We quantify an elevated occurrence of abrupt changes in ocean environmental conditions under human-induced climate forcing using Earth system model output through a novel analysis method that compares the temporal evolution of the forcings applied with the development of local ocean state changes for temperature, oxygen concentration, and carbonate ion concentration. Through a multi-centennial Earth system model experiment, we show that such an increase is not fully reversible after excess greenhouse gas emissions go back to zero. The increase in occurrence of regional abrupt changes in marine environmental conditions has not yet been accounted for adequately in climate impact analyses that usually associate ecosystem shifts large-scale variability or extreme events. Estimates for remaining greenhouse gas emission targets need thus to be more conservative.

• Kalisoras, Alkiviadis; Georgoulias, Aristeidis K.; Akritidis, Dimitris; Allen, Robert J.; Naik, Vaishali & Kuo, Chaincy [Show all 16 contributors for this article] (2024). Decomposing the effective radiative forcing of anthropogenic aerosols based on CMIP6 Earth system models. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 24(13), p. 7837–7872. doi: 10.5194/acp-24-7837-2024. Full text in Research Archive
• Fiedler, Stephanie; Naik, Vaishali; O'Connor, Fiona M.; Smith, Christopher J.; Griffiths, Paul & Kramer, Ryan J. [Show all 21 contributors for this article] (2024). Interactions between atmospheric composition and climate change - progress in understanding and future opportunities from AerChemMIP, PDRMIP, and RFMIP. Geoscientific Model Development. ISSN 1991-959X. 17(6), p. 2387–2417. doi: 10.5194/gmd-17-2387-2024. Full text in Research Archive Show summary

  The climate science community aims to improve our understanding of climate change due to anthropogenic influences on atmospheric composition and the Earth's surface. Yet not all climate interactions are fully understood, and uncertainty in climate model results persists, as assessed in the latest Intergovernmental Panel on Climate Change (IPCC) assessment report. We synthesize current challenges and emphasize opportunities for advancing our understanding of the interactions between atmospheric composition, air quality, and climate change, as well as for quantifying model diversity. Our perspective is based on expert views from three multi-model intercomparison projects (MIPs) – the Precipitation Driver Response MIP (PDRMIP), the Aerosol Chemistry MIP (AerChemMIP), and the Radiative Forcing MIP (RFMIP). While there are many shared interests and specializations across the MIPs, they have their own scientific foci and specific approaches. The partial overlap between the MIPs proved useful for advancing the understanding of the perturbation–response paradigm through multi-model ensembles of Earth system models of varying complexity. We discuss the challenges of gaining insights from Earth system models that face computational and process representation limits and provide guidance from our lessons learned. Promising ideas to overcome some long-standing challenges in the near future are kilometer-scale experiments to better simulate circulation-dependent processes where it is possible and machine learning approaches where they are needed, e.g., for faster and better subgrid-scale parameterizations and pattern recognition in big data. New model constraints can arise from augmented observational products that leverage multiple datasets with machine learning approaches. Future MIPs can develop smart experiment protocols that strive towards an optimal trade-off between the resolution, complexity, and number of simulations and their length and, thereby, help to advance the understanding of climate change and its impacts.

• Lapere, Rémy; Thomas, Jennie L.; Marelle, Louis; Ekman, Annica M. L.; Frey, Markus M. & Lund, Marianne Tronstad [Show all 14 contributors for this article] (2023). The Representation of Sea Salt Aerosols and Their Role in Polar Climate Within CMIP6. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 128(6). doi: 10.1029/2022JD038235. Full text in Research Archive Show summary

  Natural aerosols and their interactions with clouds remain an important uncertainty within climate models, especially at the poles. Here, we study the behavior of sea salt aerosols (SSaer) in the Arctic and Antarctic within 12 climate models from CMIP6. We investigate the driving factors that control SSaer abundances and show large differences based on the choice of the source function, and the representation of aerosol processes in the atmosphere. Close to the poles, the CMIP6 models do not match observed seasonal cycles of surface concentrations, likely due to the absence of wintertime SSaer sources such as blowing snow. Further away from the poles, simulated concentrations have the correct seasonality, but have a positive mean bias of up to one order of magnitude. SSaer optical depth is derived from the MODIS data and compared to modeled values, revealing good agreement, except for winter months. Better agreement for aerosol optical depth than surface concentration may indicate a need for improving the vertical distribution, the size distribution and/or hygroscopicity of modeled polar SSaer. Source functions used in CMIP6 emit very different numbers of small SSaer, potentially exacerbating cloud-aerosol interaction uncertainties in these remote regions. For future climate scenarios SSP126 and SSP585, we show that SSaer concentrations increase at both poles at the end of the 21st century, with more than two times mid-20th century values in the Arctic. The pre-industrial climate CMIP6 experiments suggest there is a large uncertainty in the polar radiative budget due to SSaer

• Madan, Gaurav; Gjermundsen, Ada; Iversen, Silje Christine & Lacasce, Joseph Henry (2023). The weakening AMOC under extreme climate change. Climate Dynamics. ISSN 0930-7575. 62, p. 1291–1309. doi: 10.1007/s00382-023-06957-7. Full text in Research Archive Show summary

  Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO₂ experiments conducted under the sixth Coupled Model Intercomparison Project (CMIP6) are examined. Increased CO₂ triggers extensive Arctic warming, causing widespread melting of sea ice. The resulting freshwater spreads southward, frst from the Labrador Sea and then the Nordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of the separated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the current weakens in response, reducing the infow to the deepwater production regions. The AMOC cell weakens in tandem, frst near the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perception that freshwater caps the convection regions, stifing deepwater production; rather, it is the infow to the subpolar gyre that is suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local or remote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is the precursor to AMOC weakening in these simulations.

• Zhong, Qirui; Schutgens, Nick; Werf, Guido R. van der; Takemura, Toshihiko; Noije, Twan van & Mielonen, Tero [Show all 21 contributors for this article] (2023). Threefold reduction of modeled uncertainty in direct radiative effects over biomass burning regions by constraining absorbing aerosols. Science Advances. 9(48), p. 1–13. doi: 10.1126/sciadv.adi3568. Full text in Research Archive
• Iversen, Emilie Claussen; Hodnebrog, Øivind; Graff, Lise Seland; Nygaard, Bjørn Egil Kringlebotn & Iversen, Trond (2023). Future Winter Precipitation Decreases Associated With the North Atlantic Warming Hole and Reduced Convection. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 128(12). doi: 10.1029/2022JD038374. Full text in Research Archive Show summary

  Climate projections in the North Atlantic region suffer from great uncertainties, and projections of precipitation are given with a large spread. Some of this uncertainty is related to projections of the North Atlantic warming hole (NAWH). The Community Earth System Model version 2 (CESM2) projects a relatively strong and extensive NAWH, with future sea surface cooling extending to Northern Scandinavia. This study investigates the relatively large winter precipitation decrease projected by CESM2 in the northeastern North Atlantic region, reinforced in a regional model. Three future scenarios from CESM2 are dynamically downscaled with the Weather Research and Forecast model. A methodology to separate convective and orographic from stratiform precipitation is applied to explore the physical mechanisms. Changes in stratiform precipitation closely relate to storm-track changes, which varies between the scenarios. Convective precipitation decreases by up to 50% over the Norwegian Sea at the end of the century, which is robust across the scenarios. This is explained by the underlying reduced sea surface temperatures of the NAWH, leading to reduced evaporation and reduced convective activity and intensity. The orographic precipitation maximum over the Scandinavian mountains is shifted upstream, likely affected by increased static stability and flow blocking, which also relates to the NAWH. This shift is possibly also explained by more frequent rain versus snow, as well as reduced cross-barrier wind speeds. This study contributes to highlight the importance of focusing future research efforts on the NAWH, in order to constrain future climate projections in this region.

• Shu, Qi; Wang, Qiang; Guo, Chuncheng; Song, Zhenya; Wang, Shizhu & He, Yan [Show all 7 contributors for this article] (2023). Arctic Ocean simulations in the CMIP6 Ocean Model Intercomparison Project (OMIP). Geoscientific Model Development. ISSN 1991-959X. 16(9), p. 2539–2563. doi: 10.5194/gmd-16-2539-2023. Full text in Research Archive Show summary

  Arctic Ocean simulations in 19 global ocean–sea-ice models participating in the Ocean Model Intercomparison Project (OMIP) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are evaluated in this paper. Our findings show no significant improvements in Arctic Ocean simulations from the previous Coordinated Ocean-ice Reference Experiments phase II (CORE-II) to the current OMIP. Large model biases and inter-model spread exist in the simulated mean state of the halocline and Atlantic Water layer in the OMIP models. Most of the OMIP models suffer from a too thick and deep Atlantic Water layer, a too deep halocline base, and large fresh biases in the halocline. The OMIP models qualitatively agree on the variability and change of the Arctic Ocean freshwater content; sea surface height; stratification; and volume, heat, and freshwater transports through the Arctic Ocean gateways. They can reproduce the changes in the gateway transports observed in the early 21st century, with the exception of the Bering Strait. We also found that the OMIP models employing the NEMO ocean model simulate relatively larger volume and heat transports through the Barents Sea Opening. Overall, the performance of the Arctic Ocean simulations is similar between the CORE2-forced OMIP-1 and JRA55-do-forced OMIP-2 experiments.

• Ahsan, Hamza; Wang, Hailong; Wu, Jingbo; Wu, Mingxuan; Smith, Steven J. & Bauer, Susanne [Show all 20 contributors for this article] (2023). The Emissions Model Intercomparison Project (Emissions-MIP): Quantifying model sensitivity to emission characteristics. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 23(23), p. 14779–14799. doi: 10.5194/acp-23-14779-2023. Full text in Research Archive Show summary

  Anthropogenic emissions of aerosols and precursor compounds are known to significantly affect the energy balance of the Earth–atmosphere system, alter the formation of clouds and precipitation, and have a substantial impact on human health and the environment. Global models are an essential tool for examining the impacts of these emissions. In this study, we examine the sensitivity of model results to the assumed height of SO2 injection, seasonality of SO2 and black carbon (BC) particulate emissions, and the assumed fraction of SO2 emissions that is injected into the atmosphere as particulate phase sulfate (SO4) in 11 climate and chemistry models, including both chemical transport models and the atmospheric component of Earth system models. We find large variation in atmospheric lifetime across models for SO2, SO4, and BC, with a particularly large relative variation for SO2, which indicates that fundamental aspects of atmospheric sulfur chemistry remain uncertain. Of the perturbations examined in this study, the assumed height of SO2 injection had the largest overall impacts, particularly on global mean net radiative flux (maximum difference of −0.35 W m−2), SO2 lifetime over Northern Hemisphere land (maximum difference of 0.8 d), surface SO2 concentration (up to 59 % decrease), and surface sulfate concentration (up to 23 % increase). Emitting SO2 at height consistently increased SO2 and SO4 column burdens and shortwave cooling, with varying magnitudes, but had inconsistent effects across models on the sign of the change in implied cloud forcing. The assumed SO4 emission fraction also had a significant impact on net radiative flux and surface sulfate concentration. Because these properties are not standardized across models this is a source of inter-model diversity typically neglected in model intercomparisons. These results imply a need to ensure that anthropogenic emission injection height and SO4 emission fraction are accurately and consistently represented in global models.

• McGraw, Zachary; Storelvmo, Trude; Polvani, Lorenzo M.; Hofer, Stefan; Shaw, Jonah K. & Gettelman, Andrew (2023). On the Links Between Ice Nucleation, Cloud Phase, and Climate Sensitivity in CESM2. Geophysical Research Letters. ISSN 0094-8276. 50(17). doi: 10.1029/2023GL105053. Full text in Research Archive Show summary

  Abstract Ice nucleation in mixed‐phase clouds has been identified as a critical factor in projections of future climate. Here we explore how this process influences climate sensitivity using the Community Earth System Model 2 (CESM2). We find that ice nucleation affects simulated cloud feedbacks over most regions and levels of the troposphere, not just extratropical low clouds. However, with present‐day global mean cloud phase adjusted to replicate satellite retrievals, similar total cloud feedback is attained whether ice nucleation is simulated as aerosol‐sensitive, insensitive, or absent. These model experiments all result in a strongly positive total cloud feedback, as in the default CESM2. A microphysics update from CESM1 to CESM2 had substantially weakened ice nucleation, due partly to a model issue. Our findings indicate that this update reduced global cloud phase bias, with CESM2's high climate sensitivity reflecting improved mixed‐phase cloud representation.

• Tjiputra, Jerry; Negrel, Jean & Olsen, Are (2023). Early detection of anthropogenic climate change signals in the ocean interior. Scientific Reports. 13:3006, p. 1–14. doi: 10.1038/s41598-023-30159-0. Full text in Research Archive Show summary

  Robust detection of anthropogenic climate change is crucial to: (i) improve our understanding of Earth system responses to external forcing, (ii) reduce uncertainty in future climate projections, and (iii) develop efficient mitigation and adaptation plans. Here, we use Earth system model projections to establish the detection timescales of anthropogenic signals in the global ocean through analyzing temperature, salinity, oxygen, and pH evolution from surface to 2000 m depths. For most variables, anthropogenic changes emerge earlier in the interior ocean than at the surface, due to the lower background variability at depth. Acidification is detectable earliest, followed by warming and oxygen changes in the subsurface tropical Atlantic. Temperature and salinity changes in the subsurface tropical and subtropical North Atlantic are shown to be early indicators for a slowdown of the Atlantic Meridional Overturning Circulation. Even under mitigated scenarios, inner ocean anthropogenic signals are projected to emerge within the next few decades. This is because they originate from existing surface changes that are now propagating into the interior. In addition to the tropical Atlantic, our study calls for establishment of long-term interior monitoring systems in the Southern Ocean and North Atlantic in order to elucidate how spatially heterogeneous anthropogenic signals propagate into the interior and impact marine ecosystems and biogeochemistry.

• Stjern, Camilla Weum; Forster, Piers M.; Jia, Hailing; Jouan, Caroline; Kasoar, Matthew R. & Myhre, Gunnar [Show all 13 contributors for this article] (2023). The Time Scales of Climate Responses to Carbon Dioxide and Aerosols. Journal of Climate. ISSN 0894-8755. 36(11), p. 3537–3551. doi: 10.1175/JCLI-D-22-0513.1. Full text in Research Archive Show summary

  The climate system responds to changes in the amount of atmospheric greenhouse gases or aerosols through rapid processes, triggered within hours and days, and through slower processes, where the full response may only be seen after centuries. In this paper, we aim to elucidate the mechanisms operating on time scales of hours to years to better understand the response of key climate quantities such as energy fluxes, temperature, and precipitation after a sudden increase in either carbon dioxide (CO2), black carbon (BC), or sulfate (SO4) aerosols. The results are based on idealized simulations from six global climate models. We find that the effect of changing ocean temperatures kicks in after a couple of months. Rapid precipitation reductions start occurring instantly and are established after just a few days. For BC, they constitute most of the equilibrium response. For CO2 and SO4, the magnitude of the precipitation response gradually increases as surface warming/cooling evolves, and for CO2, the sign of the response changes from negative to positive after 2 years. Rapid cloud adjustments are typically established within the first 24 h, and while the magnitude of cloud feedbacks for CO2 and SO4 increases over time, the geographical pattern of the equilibrium cloud change is present already after the first year. While there are model differences, our work underscores the overall similarity of the major time-varying processes and responses simulated by current global models and hence the robustness of key features of simulated responses to historical and future anthropogenic forcing.

• Fiedler, Stephanie; Noije, Twan van; Smith, Christopher J.; Boucher, Olivier; Dufresne, Jean-Louis & Kirkevåg, Alf [Show all 11 contributors for this article] (2023). Historical Changes and Reasons for Model Differences in Anthropogenic Aerosol Forcing in CMIP6. Geophysical Research Letters. ISSN 0094-8276. 50(15). doi: 10.1029/2023GL104848. Full text in Research Archive
• Schmidt, Gavin A.; Andrews, Timothy; Bauer, Susanne E.; Durack, Paul J.; Loeb, Norman G. & Ramaswamy, V. [Show all 22 contributors for this article] (2023). CERESMIP: a climate modeling protocol to investigate recent trends in the Earth's Energy Imbalance. Frontiers in Climate. 5, p. 1–10. doi: 10.3389/fclim.2023.1202161. Full text in Research Archive
• Gomez, James; Allen, Robert J.; Turnock, Steven T.; Horowitz, Larry W.; Tsigaridis, Kostas & Bauer, Susanne E. [Show all 9 contributors for this article] (2023). The projected future degradation in air quality is caused by more abundant natural aerosols in a warmer world. Communications Earth & Environment. 4(1). doi: 10.1038/s43247-023-00688-7. Full text in Research Archive
• Axelsson, Josefine; Gao, Jing; Eckhardt, Sabine; Cassiani, Massimo; Cheng, Deliang & Zhang, Qiong (2023). A Precipitation Isotopic Response in 2014–2015 to Moisture Transport Changes in the Central Himalayas. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 128(13). doi: 10.1029/2023JD038568. Full text in Research Archive Show summary

  The impact of moisture transport and sources on precipitation stable isotopes (δ18O and d-excess) in the central Himalayas are crucial to understanding the climatic archives. However, this is still unclear due to the lack of in-situ observations. Here we present measurements of stable isotopes in precipitation at two stations (Yadong and Pali) in the central Himalayas during 2014–2015. Combined with simulations from the dispersion model FLEXPART, we investigate effects on precipitation stable isotopes related to changes in moisture sources and convections in the region, and possible influence by El Niño. Our results suggest that the moisture supplies related to evaporation over northeastern India and moisture losses related to convective activities over the Bay of Bengal (BoB) and Bangladesh region play important roles in changes in δ18O and d-excess in precipitation in the Yadong Valley. Outgoing longwave radiation and moisture flux divergence analysis further confirm that the contribution from continental evaporation dominates the moisture supply in the central Himalayas with a lesser contribution from convection over the BoB during the 2015 monsoon season compared with 2014. A change in the altitude effect is observed in 2015, which is more significant than the temperature and precipitation amount effect during the observation period. These findings provide valuable insights into climatic interpretations of paleo-isotopic archives with an isotopic response to changes in moisture transport to the central Himalayas.

• Zhong, Qirui; Schutgens, Nick; Werf, Guido R. van der; Noije, Twan van; Bauer, Susanne E. & Tsigaridis, Kostas [Show all 20 contributors for this article] (2022). Using modelled relationships and satellite observations to attribute modelled aerosol biases over biomass burning regions. Nature Communications. 13(1). doi: 10.1038/s41467-022-33680-4. Full text in Research Archive
• Lauritzen, P.H.; Kevlahan, N.K.-R.; Toniazzo, Thomas; Eldred, C.; Dubos, T. & Gassmann, A. [Show all 20 contributors for this article] (2022). Reconciling and Improving Formulations for Thermodynamics and Conservation Principles in Earth System Models (ESMs). Journal of Advances in Modeling Earth Systems. 14(9), p. 1–83. doi: 10.1029/2022MS003117. Full text in Research Archive Show summary

  Earth System Models (ESMs) have numerous total energy budget errors. This article establishes the governing total energy equations for large-scale ESMs and assesses the energy budget errors in real-world simulations in a widely used climate model. To move towards a closed energy budget in ESMs, further research on total energy conserving discretizations (in the dynamical core), unified thermodynamics (through thermodynamic potentials/conserved variables) and missing processes is paramount. This research is especially important since some of the energy budget errors will not improve with higher spatial resolution and may even get worse.

• Moseid, Kine Onsum; Schulz, Michael; Eichler, Anja; Schwikowski, Margit; McConnell, Joseph R. & Oliviè, Dirk Jan Leo [Show all 9 contributors for this article] (2022). Using Ice Cores to Evaluate CMIP6 Aerosol Concentrations Over the Historical Era. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 127(18). doi: 10.1029/2021JD036105. Full text in Research Archive Show summary

  The radiative effect of anthropogenic aerosols is one of the largest uncertainties in Earth's energy budget over the industrial period. This uncertainty is in part due to sparse observations of aerosol concentrations in the pre-satellite era. To address this lack of measurements, ice cores can be used, which contain the aerosol concentration record. To date, these observations have been under-utilized for comparison to aerosol concentrations found in state-of-the-art Earth system models (ESMs). Here we compare long term trends in concentrations of sulfate and black carbon (BC) between 15 ice cores and 11 ESMs over nine regions around the world during the period 1850–2000. We find that for sulfate concentration trends model results generally agree with ice core records, whereas for BC concentration the model trends differ from the records. Absolute concentrations of both investigated species are overestimated by the models, probably in part due to representation errors. However, we assume that biases in relative trends are not altered by these errors. Ice cores in the European Alps and Greenland record a maximum BC concentration before 1950, while most ESMs used in this study agree on a post-1950 maximum. We source this bias to an error in BC emission inventories in Europe. Emission perturbation experiments using NorESM2-LM support the observed finding that BC concentrations in Northern Greenland ice cores are recording European emissions. Errors in BC emission inventories have implications for all future and past studies where Coupled Model Intercomparison Project Phase 6 historical simulations are compared to observations relevant to aerosol forcing.

• Bourgeois, Timothée; Goris, Nadine; Schwinger, Jörg & Tjiputra, Jerry (2022). Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S. Nature Communications. 13. doi: 10.1038/s41467-022-27979-5. Full text in Research Archive Show summary

  The Southern Ocean between 30°S and 55°S is a major sink of excess heat and anthropogenic carbon, but model projections of these sinks remain highly uncertain. Reducing such uncertainties is required to effectively guide the development of climate mitigation policies for meeting the ambitious climate targets of the Paris Agreement. Here, we show that the large spread in the projections of future excess heat uptake efficiency and cumulative anthropogenic carbon uptake in this region are strongly linked to the models’ contemporary stratification. This relationship is robust across two generations of Earth system models and is used to reduce the uncertainty of future estimates of the cumulative anthropogenic carbon uptake by up to 53% and the excess heat uptake efficiency by 28%. Our results highlight that, for this region, an improved representation of stratification in Earth system models is key to constrain future carbon budgets and climate change projections.

• Shaw, Jonah; McGraw, Zachary; Bruno, O.; Storelvmo, Trude & Hofer, Stefan (2022). Using Satellite Observations to Evaluate Model Microphysical Representation of Arctic Mixed-Phase Clouds. Geophysical Research Letters. ISSN 0094-8276. 49(3). doi: 10.1029/2021GL096191. Full text in Research Archive Show summary

  Mixed-phase clouds play an important role in determining Arctic warming, but are parametrized in models and difficult to constrain with observations. We use two satellite-derived cloud phase metrics to investigate the vertical structure of Arctic clouds in two global climate models that use the Community Atmosphere Model version 6 (CAM6) atmospheric component. We report a model error limiting ice nucleation, produce a set of Arctic-constrained model runs by adjusting model microphysical variables to match the cloud phase metrics, and evaluate cloud feedbacks for all simulations. Models in this small ensemble uniformly overestimate total cloud fraction in the summer, but have variable representation of cloud fraction and phase in the winter and spring. By relating modeled cloud phase metrics and changes in low-level liquid cloud amount under warming to longwave cloud feedback, we show that mixed-phase processes mediate the Arctic climate by modifying how wintertime and springtime clouds respond to warming.

• Gangadharan, Nidheesh; Goosse, Hugues; Parkes, David C.; Goelzer, Heiko; Maussion, Fabien & Marzeion, Ben (2022). Process-based estimate of global-mean sea-level changes in the Common Era. Earth System Dynamics (ESD). ISSN 2190-4979. 13(4), p. 1417–1435. doi: 10.5194/esd-13-1417-2022. Full text in Research Archive Show summary

  Although the global-mean sea level (GMSL) rose over the twentieth century with a positive contribution from thermosteric and barystatic (ice sheets and glaciers) sources, the driving processes of GMSL changes during the pre-industrial Common Era (PCE; 1–1850 CE) are largely unknown. Here, the contributions of glacier and ice sheet mass variations and ocean thermal expansion to GMSL in the Common Era (1–2000 CE) are estimated based on simulations with different physical models. Although the twentieth century global-mean thermosteric sea level (GMTSL) is mainly associated with temperature variations in the upper 700 m (86 % in reconstruction and 74 ± 8 % in model), GMTSL in the PCE is equally controlled by temperature changes below 700 m. The GMTSL does not vary more than ±2 cm during the PCE. GMSL contributions from the Antarctic and Greenland ice sheets tend to cancel each other out during the PCE owing to the differing response of the two ice sheets to atmospheric conditions. The uncertainties of sea-level contribution from land-ice mass variations are large, especially over the first millennium. Despite underestimating the twentieth century model GMSL, there is a general agreement between the model and proxy-based GMSL reconstructions in the CE. Although the uncertainties remain large over the first millennium, model simulations point to glaciers as the dominant source of GMSL changes during the PCE.

• Wal, Roderik S.W. van de; Nicholls, Robert J.; Behar, David; McInnes, K.; Stammer, Detlef & Lowe, Jason A. [Show all 28 contributors for this article] (2022). A high-end estimate of sea-level rise for practitioners. Earth's Future. 10(11). doi: 10.1029/2022EF002751. Full text in Research Archive Show summary

  Sea level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate for the deep ocean and ice sheets. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientists and practitioners builds on a framework of discussing physical evidence to quantify high-end global SLR for practitioners. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2°C in 2100 (RCP2.6/SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for a (RCP8.5/SSP5-8.5), we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2°C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while here we emphasize the importance of the timing of ice shelf collapse around Antarctica. This is highly uncertain due to low understanding of the driving processes. Hence both process understanding and emission scenario control high-end SLR.

• Berends, Constantijn J.; Goelzer, Heiko; Reerink, Thomas J.; Stap, Lennert & Wal, Roderik S.W. van de (2022). Benchmarking the vertically integrated ice-sheet model IMAU-ICE (version 2.0). Geoscientific Model Development. ISSN 1991-959X. 15(14), p. 5667–5688. doi: 10.5194/gmd-15-5667-2022. Full text in Research Archive Show summary

  Ice-dynamical processes constitute a large uncertainty in future projections of sea-level rise caused by anthropogenic climate change. Improving our understanding of these processes requires ice-sheet models that perform well at simulating both past and future ice-sheet evolution. Here, we present version 2.0 of the ice-sheet model IMAU-ICE, which uses the depth-integrated viscosity approximation (DIVA) to solve the stress balance. We evaluate its performance in a range of benchmark experiments, including simple analytical solutions and both schematic and realistic model intercomparison exercises. IMAU-ICE has adopted recent developments in the numerical treatment of englacial stress and sub-shelf melt near the grounding line, which result in good performance in experiments concerning grounding-line migration (MISMIP, MISMIP+) and buttressing (ABUMIP). This makes it a model that is robust, versatile, and user-friendly, which will provide a firm basis for (palaeo-)glaciological research in the coming years.

• Michel, Clio; Sorteberg, Asgeir; Eckhardt, Sabine; Weijenborg, Christian; Stohl, Andreas & Cassiani, Massimo (2021). Characterization of the atmospheric environment during extreme precipitation events associated with atmospheric rivers in Norway - Seasonal and regional aspects. Weather and Climate Extremes. ISSN 2212-0947. 34. doi: 10.1016/j.wace.2021.100370. Full text in Research Archive Show summary

  Extreme precipitation events in Norway in all seasons are often linked to atmospheric rivers (AR). We show that during the period 1979–2018 78.5% of the daily extreme precipitation events in Southwestern Norway are linked to ARs, this percentage decreasing to 59% in the more northern coastal regions and ~40% in the inland regions. The association of extreme precipitation with AR occurs most often in fall for the coastal areas and in summer inland. All Norwegian regions experience stronger winds and 1–2°C increase of the temperature at 850 hPa during AR events compared to the climatology, the extreme precipitation largely contributing to the wet climatology (only considering rainy days) in Norway but also in Denmark and Sweden when the rest of Europe is dry. A cyclone is found nearby the AR landfall point in 70% of the cases. When the cyclone is located over the British Isles, as it is typically the case when ARs reach Southeastern Norway, it is associated with cyclonic Rossby wave breaking whereas when the ARs reach more northern regions, anticyclonic wave breaking occurs over Northern Europe. Cyclone-centered composites show that the mean sea level pressure is not significantly different between the eight Norwegian regions, that baroclinic interaction can still take place although the cyclone is close to its decay phase and that the maximum precipitation occurs ahead of the AR. Lagrangian air parcel tracking shows that moisture uptake mainly occurs over the North Atlantic for the coastal regions with an additional source over Europe for the more eastern and inland regions.

• Liddicoat, Spencer; Wiltshire, Andy; Jones, Chris D.; Arora, Vivek K; Brovkin, Victor & Cadule, Patricia [Show all 13 contributors for this article] (2021). Compatible fossil fuel CO2 emissions in the CMIP6 Earth system models’ historical and shared socioeconomic pathway experiments of the twenty-first century. Journal of Climate. ISSN 0894-8755. 34(8), p. 2853–2875. doi: 10.1175/JCLI-D-19-0991.1. Full text in Research Archive Show summary

  We present the compatible CO2 emissions from fossil fuel (FF) burning and industry, calculated from the historical and Shared Socioeconomic Pathway (SSP) experiments of nine Earth system models (ESMs) participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel mean FF emissions match the historical record well and are close to the data-based estimate of cumulative emissions (394 ± 59 GtC vs 400 ± 20 GtC, respectively). Only two models fall inside the observed uncertainty range; while two exceed the upper bound, five fall slightly below the lower bound, due primarily to the plateau in CO2 concentration in the 1940s. The ESMs’ diagnosed FF emission rates are consistent with those generated by the integrated assessment models (IAMs) from which the SSPs’ CO2 concentration pathways were constructed; the simpler IAMs’ emissions lie within the ESMs’ spread for seven of the eight SSP experiments, the other being only marginally lower, providing confidence in the relationship between the IAMs’ FF emission rates and concentration pathways. The ESMs require fossil fuel emissions to reduce to zero and subsequently become negative in SSP1-1.9, SSP1-2.6, SSP4-3.4, and SSP5-3.4over. We also present the ocean and land carbon cycle responses of the ESMs in the historical and SSP scenarios. The models’ ocean carbon cycle responses are in close agreement, but there is considerable spread in their land carbon cycle responses. Land-use and land-cover change emissions have a strong influence over the magnitude of diagnosed fossil fuel emissions, with the suggestion of an inverse relationship between the two.

• Brown, Hunter; Liu, Xiaohong; Pokhrel, Rudra; Murphy, Shane; Lu, Zheng & Saleh, Rawad [Show all 21 contributors for this article] (2021). Biomass burning aerosols in most climate models are too absorbing. Nature Communications. 12:277, p. 1–15. doi: 10.1038/s41467-020-20482-9. Full text in Research Archive Show summary

  Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation. Depending on the model, the top-of-the-atmosphere BB aerosol effect can range from cooling to warming. By relating aerosol absorption relative to extinction and carbonaceous aerosol composition from 12 observational datasets to nine state-of-the-art Earth system models/chemical transport models, we identify varying degrees of overestimation in BB aerosol absorptivity by these models. Modifications to BB aerosol refractive index, size, and mixing state improve the Community Atmosphere Model version 5 (CAM5) agreement with observations, leading to a global change in BB direct radiative effect of −0.07 W m−2, and regional changes of −2 W m−2 (Africa) and −0.5 W m−2 (South America/Temperate). Our findings suggest that current modeled BB contributes less to warming than previously thought, largely due to treatments of aerosol mixing state.

• Gjermundsen, Ada; Graff, Lise Seland; Bentsen, Mats; Breivik, Lars-Anders; Debernard, Jens Boldingh & Makkonen, Risto [Show all 10 contributors for this article] (2021). How representative is Svalbard for future Arctic climate evolution? An Earth system modelling perspective. SESS report. ISSN 2535-809X. 1(ISBN 978-82-691528-9-0), p. 38–79. doi: 10.5281/zenodo.4034104. Full text in Research Archive
• Berends, Constantijn J.; Goelzer, Heiko & Wal, Roderik S.W. van de (2021). The Utrecht Finite Volume Ice-Sheet Model: UFEMISM (version 1.0). Geoscientific Model Development. ISSN 1991-959X. 14, p. 2443–2470. doi: 10.5194/gmd-14-2443-2021. Full text in Research Archive Show summary

  Improving our confidence in future projections of sea-level rise requires models that can simulate ice-sheet evolution both in the future and in the geological past. A physically accurate treatment of large changes in ice-sheet geometry requires a proper treatment of processes near the margin, like grounding line dynamics, which in turn requires a high spatial resolution in that specific region, so that small-scale topographical features are resolved. This leads to a demand for computationally efficient models, where such a high resolution can be feasibly applied in simulations of 105–107 years in duration. Here, we present and evaluate a new ice-sheet model that solves the hybrid SIA–SSA approximation of the stress balance, including a heuristic rule for the grounding-line flux. This is done on a dynamic adaptive mesh which is adapted to the modelled ice-sheet geometry during a simulation. Mesh resolution can be configured to be fine only at specified areas, such as the calving front or the grounding line, as well as specified point locations such as ice-core drill sites. This strongly reduces the number of grid points where the equations need to be solved, increasing the computational efficiency. A high resolution allows the model to resolve small geometrical features, such as outlet glaciers and sub-shelf pinning points, which can significantly affect large-scale ice-sheet dynamics. We show that the model reproduces the analytical solutions or model intercomparison benchmarks for a number of schematic ice-sheet configurations, indicating that the numerical approach is valid. Because of the unstructured triangular mesh, the number of vertices increases less rapidly with resolution than in a square-grid model, greatly reducing the required computation time for high resolutions. A simulation of all four continental ice sheets during an entire 120 kyr glacial cycle, with a 4 km resolution near the grounding line, is expected to take 100–200 wall clock hours on a 16-core system (1600–3200 core hours), implying that this model can be feasibly used for high-resolution palaeo-ice-sheet simulations.

• Mulder, Thomas E.; Goelzer, Heiko; Wubs, Fred W. & Dijkstra, Henk A. (2021). Snowball Earth Bifurcations in a Fully-Implicit Earth System Model. International Journal of Bifurcation and Chaos in Applied Sciences and Engineering. ISSN 0218-1274. 31. doi: 10.1142/S0218127421300172. Full text in Research Archive Show summary

  There is now much geological evidence that the Earth was fully glaciated during several periods in the geological past (about 700Myr ago) and attained a so-called Snowball Earth (SBE) state. Additional support for this idea has come from climate models of varying complexity that show transitions to SBE states and undergo hysteresis under changes in solar radiation. In this paper, we apply large-scale bifurcation analyses to a novel, fully-implicit Earth System Model of Intermediate Complexity (I-EMIC) to study SBE transitions. The I-EMIC contains a primitive equation ocean model, a model for atmospheric heat and moisture transport, a sea ice component and formulations for the adjustment of albedo over snow and ice. With the I-EMIC, high-dimensional branches of the SBE bifurcation diagram are obtained through parameter continuation. We are able to identify stable and unstable equilibria and uncover an intricate bifurcation structure associated with the ice-albedo feedback. Moreover, large-scale linear stability analyses are performed near major bifurcations, revealing the spatial nature of destabilizing perturbations.

• Keen, Ann; Blockley, Ed; Bailey, David A.; Debernard, Jens Boldingh; Bushuk, Mitchell & Delhaye, Steve [Show all 18 contributors for this article] (2021). An inter-comparison of the mass budget of the Arctic sea ice in CMIP6 models. The Cryosphere. ISSN 1994-0416. 15, p. 951–982. doi: 10.5194/tc-15-951-2021. Full text in Research Archive
• Bock, Josué; Michou, Martine; Nabat, Pierre; Abe, Manabu; Mulcahy, Jane P. & Oliviè, Dirk Jan Leo [Show all 13 contributors for this article] (2021). Evaluation of ocean dimethylsulfide concentration and emission in CMIP6 models. Biogeosciences. ISSN 1726-4170. 18(12), p. 3823–3860. doi: 10.5194/bg-18-3823-2021. Full text in Research Archive Show summary

  Characteristics and trends of surface ocean dimethylsulfide (DMS) concentrations and fluxes into the atmosphere of four Earth system models (ESMs: CNRM-ESM2-1, MIROC-ES2L, NorESM2-LM, and UKESM1-0-LL) are analysed over the recent past (1980–2009) and into the future, using Coupled Model Intercomparison Project 6 (CMIP6) simulations. The DMS concentrations in historical simulations systematically underestimate the most widely used observed climatology but compare more favourably against two recent observation-based datasets. The models better reproduce observations in mid to high latitudes, as well as in polar and westerlies marine biomes. The resulting multi-model estimate of contemporary global ocean DMS emissions is 16–24 Tg S yr−1, which is narrower than the observational-derived range of 16 to 28 Tg S yr−1. The four models disagree on the sign of the trend of the global DMS flux from 1980 onwards, with two models showing an increase and two models a decrease. At the global scale, these trends are dominated by changes in surface DMS concentrations in all models, irrespective of the air–sea flux parameterisation used. In turn, three models consistently show that changes in DMS concentrations are correlated with changes in marine productivity; however, marine productivity is poorly constrained in the current generation of ESMs, thus limiting the predictive ability of this relationship. In contrast, a consensus is found among all models over polar latitudes where an increasing trend is predominantly driven by the retreating sea-ice extent. However, the magnitude of this trend between models differs by a factor of 3, from 2.9 to 9.2 Gg S decade−1 over the period 1980–2014, which is at the low end of a recent satellite-derived analysis. Similar increasing trends are found in climate projections over the 21st century.

• Meneghello, Gianluca; Marshall, John; Lique, Camille; Isachsen, Pål Erik; Doddridge, Edward & Campin, Jean-Michel [Show all 8 contributors for this article] (2021). Genesis and decay of mesoscale baroclinic eddies in the seasonally ice-covered Arctic Ocean. Journal of Physical Oceanography. ISSN 0022-3670. 51(1), p. 115–129. doi: 10.1175/JPO-D-20-0054.1. Full text in Research Archive Show summary

  Abstract Observations of ocean currents in the Arctic interior show a curious, and hitherto unexplained, vertical and temporal distribution of mesoscale activity. A marked seasonal cycle is found close to the surface: strong eddy activity during summer, observed from both satellites and moorings, is followed by very quiet winters. In contrast, subsurface eddies persist all year long within the deeper halocline and below. Informed by baroclinic instability analysis, we explore the origin and evolution of mesoscale eddies in the seasonally ice-covered interior Arctic Ocean. We find that the surface seasonal cycle is controlled by friction with sea ice, dissipating existing eddies and preventing the growth of new ones. In contrast, subsurface eddies, enabled by interior potential vorticity gradients and shielded by a strong stratification at a depth of approximately 50 m, can grow independently of the presence of sea ice. A high-resolution pan-Arctic ocean model confirms that the interior Arctic basin is baroclinically unstable all year long at depth. We address possible implications for the transport of water masses between the margins and the interior of the Arctic basin, and for climate models’ ability to capture the fundamental difference in mesoscale activity between ice-covered and ice-free regions.

• Thornhill, Gillian; Collins, William J.; Oliviè, Dirk Jan Leo; Skeie, Ragnhild Bieltvedt; Archibald, Alexander T. & Bauer, Susanne E. [Show all 26 contributors for this article] (2021). Climate-driven chemistry and aerosol feedbacks in CMIP6 Earth system models. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 21(2), p. 1105–1126. doi: 10.5194/acp-21-1105-2021. Full text in Research Archive Show summary

  Feedbacks play a fundamental role in determining the magnitude of the response of the climate system to external forcing, such as from anthropogenic emissions. The latest generation of Earth system models includes aerosol and chemistry components that interact with each other and with the biosphere. These interactions introduce a complex web of feedbacks that is important to understand and quantify. This paper addresses multiple pathways for aerosol and chemical feedbacks in Earth system models. These focus on changes in natural emissions (dust, sea salt, dimethyl sulfide, biogenic volatile organic compounds (BVOCs) and lightning) and changes in reaction rates for methane and ozone chemistry. The feedback terms are then given by the sensitivity of a pathway to climate change multiplied by the radiative effect of the change. We find that the overall climate feedback through chemistry and aerosols is negative in the sixth Coupled Model Intercomparison Project (CMIP6) Earth system models due to increased negative forcing from aerosols in a climate with warmer surface temperatures following a quadrupling of CO2 concentrations. This is principally due to increased emissions of sea salt and BVOCs which are sensitive to climate change and cause strong negative radiative forcings. Increased chemical loss of ozone and methane also contributes to a negative feedback. However, overall methane lifetime is expected to increase in a warmer climate due to increased BVOCs. Increased emissions of methane from wetlands would also offset some of the negative feedbacks. The CMIP6 experimental design did not allow the methane lifetime or methane emission changes to affect climate, so we found a robust negative contribution from interactive aerosols and chemistry to climate sensitivity in CMIP6 Earth system models.

• Gliss, Jonas; Mortier, Augustin; Schulz, Michael; Andrews, Elisabeth; Balkanski, Yves & Bauer, Susanne E. [Show all 31 contributors for this article] (2021). AeroCom phase III multi-model evaluation of the aerosol life cycle and optical properties using ground- and space-based remote sensing as well as surface in situ observations. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 21, p. 87–128. doi: 10.5194/acp-21-87-2021. Full text in Research Archive Show summary

  Within the framework of the AeroCom (Aerosol Comparisons between Observations and Models) initiative, the state-of-the-art modelling of aerosol optical properties is assessed from 14 global models participating in the phase III control experiment (AP3). The models are similar to CMIP6/AerChemMIP Earth System Models (ESMs) and provide a robust multi-model ensemble. Inter-model spread of aerosol species lifetimes and emissions appears to be similar to that of mass extinction coefficients (MECs), suggesting that aerosol optical depth (AOD) uncertainties are associated with a broad spectrum of parameterised aerosol processes. Total AOD is approximately the same as in AeroCom phase I (AP1) simulations. However, we find a 50 % decrease in the optical depth (OD) of black carbon (BC), attributable to a combination of decreased emissions and lifetimes. Relative contributions from sea salt (SS) and dust (DU) have shifted from being approximately equal in AP1 to SS contributing about 2∕3 of the natural AOD in AP3. This shift is linked with a decrease in DU mass burden, a lower DU MEC, and a slight decrease in DU lifetime, suggesting coarser DU particle sizes in AP3 compared to AP1. Relative to observations, the AP3 ensemble median and most of the participating models underestimate all aerosol optical properties investigated, that is, total AOD as well as fine and coarse AOD (AODf, AODc), Ångström exponent (AE), dry surface scattering (SCdry), and absorption (ACdry) coefficients. Compared to AERONET, the models underestimate total AOD by ca. 21 % ± 20 % (as inferred from the ensemble median and interquartile range). Against satellite data, the ensemble AOD biases range from −37 % (MODIS-Terra) to −16 % (MERGED-FMI, a multi-satellite AOD product), which we explain by differences between individual satellites and AERONET measurements themselves. Correlation coefficients (R) between model and observation AOD records are generally high (R>0.75), suggesting that the models are capable of capturing spatio-temporal variations in AOD. We find a much larger underestimate in coarse AODc (∼ −45 % ± 25 %) than in fine AODf (∼ −15 % ± 25 %) with slightly increased inter-model spread compared to total AOD. These results indicate problems in the modelling of DU and SS. The AODc bias is likely due to missing DU over continental land masses (particularly over the United States, SE Asia, and S. America), while marine AERONET sites and the AATSR SU satellite data suggest more moderate oceanic biases in AODc. Column AEs are underestimated by about 10 % ± 16 %. For situations in which measurements show AE > 2, models underestimate AERONET AE by ca. 35 %. In contrast, all models (but one) exhibit large overestimates in AE when coarse aerosol dominates (bias ca. +140 % if observed AE < 0.5). Simulated AE does not span the observed AE variability. These results indicate that models overestimate particle size (or underestimate the fine-mode fraction) for fine-dominated aerosol and underestimate size (or overestimate the fine-mode fraction) for coarse-dominated aerosol. This must have implications for lifetime, water uptake, scattering enhancement, and the aerosol radiative effect, which we can not quantify at this moment. Comparison against Global Atmosphere Watch (GAW) in situ data results in mean bias and inter-model variations of −35 % ± 25 % and −20 % ± 18 % for SCdry and ACdry, respectively. The larger underestimate of SCdry than ACdry suggests the models will simulate an aerosol single scattering albedo that is too low. The larger underestimate of SCdry than ambient air AOD is consistent with recent findings that models overestimate scattering enhancement due to hygroscopic growth. The broadly consistent negative bias in AOD and surface scattering suggests an underestimate of aerosol radiative effects in current global aerosol models. Considerable ...

• Gjermundsen, Ada; Nummelin, Aleksi; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2021). Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Nature Geoscience. ISSN 1752-0894. 14, p. 724–731. doi: 10.1038/s41561-021-00825-x. Full text in Research Archive Show summary

  The effective climate sensitivity estimates the equilibrium response of near-surface temperature to doubling atmospheric carbon dioxide concentration and is a widely used metric to characterize potential global warming. Earth system models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) exhibit considerable spread in effective climate sensitivity estimates. Cloud feedbacks are thought to be the cause of this, with marine boundary layer clouds over the Southern Ocean playing an important role. Here, we show that Southern Ocean deep convection is a major contributor to the CMIP6 intermodel spread in effective climate sensitivity. By comparing two Earth system models with very different sensitivities, we find that greater storage of heat at depth can delay the Southern Ocean surface warming and associated cloud response, thereby delaying global surface warming by centuries. The link between Southern Ocean convection and effective climate sensitivity is seen across 41 CMIP6 models, with low-sensitivity models exhibiting substantial deep ocean warming. Our results reveal the influence of Southern Ocean convection on potential long-term climate warming.

• Jones, Chris D.; Hickman, Jonathan; Rumbold, Steven T; Walton, Jeremy; Lamboll, Robin & Skeie, Ragnhild Bieltvedt [Show all 49 contributors for this article] (2021). The Climate Response to Emissions Reductions Due to COVID-19: Initial Results From CovidMIP. Geophysical Research Letters. ISSN 0094-8276. 48(8). doi: 10.1029/2020GL091883. Full text in Research Archive Show summary

  Many nations responded to the corona virus disease-2019 (COVID-19) pandemic by restricting travel and other activities during 2020, resulting in temporarily reduced emissions of CO2, other greenhouse gases and ozone and aerosol precursors. We present the initial results from a coordinated Intercomparison, CovidMIP, of Earth system model simulations which assess the impact on climate of these emissions reductions. 12 models performed multiple initial-condition ensembles to produce over 300 simulations spanning both initial condition and model structural uncertainty. We find model consensus on reduced aerosol amounts (particularly over southern and eastern Asia) and associated increases in surface shortwave radiation levels. However, any impact on near-surface temperature or rainfall during 2020–2024 is extremely small and is not detectable in this initial analysis. Regional analyses on a finer scale, and closer attention to extremes (especially linked to changes in atmospheric composition and air quality) are required to test the impact of COVID-19-related emission reductions on near-term climate.

• Payne, Antony; Nowicki, Sophie; Abe-Ouchi, Ayako; Agosta, Cécilie; Alexander, Patrick & Albrecht, Torsten [Show all 63 contributors for this article] (2021). Future sea level change under coupled model intercomparison project phase 5 and phase 6 scenarios from the Greenland and Antarctic ice sheets. Geophysical Research Letters. ISSN 0094-8276. 48(16). doi: 10.1029/2020GL091741. Full text in Research Archive Show summary

  Projections of the sea level contribution from the Greenland and Antarctic ice sheets (GrIS and AIS) rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous Coupled Model Intercomparison Project phase 5 (CMIP5) effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

• Lambert, Erwin; Bars, Dewi Le; Goelzer, Heiko & Wal, Roderik S.W. van de (2021). Correlations Between Sea‐Level Components Are Driven by Regional Climate Change. Earth's Future. 9(2), p. 1–17. doi: 10.1029/2020EF001825. Full text in Research Archive Show summary

  The accurate quantification of uncertainties in regional sea-level projections is essential for guiding policy makers. As climate models do not currently simulate total sea level, these uncertainties must be quantified through summation of uncertainties in individual sea-level components. This summation depends on the correlation between the components, which has previously been prescribed or derived from each individual component's dependence on global mean surface temperature. In this study, we quantify, for the first time, regional correlations between sea-level components based on regional climate change projections. We compute regional sea-level projections consistent with climate projections from an ensemble of 14 Earth System Models. From the multi-model spread, we estimate the uncertainty in the regional climate's response to greenhouse forcing. To quantify the total uncertainty, we add the uncertainty in the response of sea-level components to this regional climate change. This approach reveals how regional climate processes impose correlations between sea-level components, affecting the total uncertainty. One example is an anti-correlation between North Atlantic sterodynamic change and Antarctic dynamic mass loss, suggesting a teleconnection established by the large-scale ocean circulation. We find that prescribed correlations, applied in the fifth assessment report of the Intergovernmental Panel on Climate Change, lead to a global overestimation in the uncertainty in regional sea-level projections on the order of 20%. Regionally, this overestimation exceeds 100%. We conclude that accurate uncertainty estimates of regional sea-level change must be based on projections of regional climate change and cannot be derived from global indicators such as global mean surface temperature.

• Thornhill, Gillian D.; Collins, William J.; Kramer, Ryan J.; Oliviè, Dirk Jan Leo; Skeie, Ragnhild Bieltvedt & O'Connor, Fiona M. [Show all 30 contributors for this article] (2021). Effective radiative forcing from emissions of reactive gases and aerosols-A multi-model comparison. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 21(2), p. 853–874. doi: 10.5194/acp-21-853-2021. Full text in Research Archive Show summary

  This paper quantifies the pre-industrial (1850) to present-day (2014) effective radiative forcing (ERF) of anthropogenic emissions of NOX, volatile organic compounds (VOCs; including CO), SO2, NH3, black carbon, organic carbon, and concentrations of methane, N2O and ozone-depleting halocarbons, using CMIP6 models. Concentration and emission changes of reactive species can cause multiple changes in the composition of radiatively active species: tropospheric ozone, stratospheric ozone, stratospheric water vapour, secondary inorganic and organic aerosol, and methane. Where possible we break down the ERFs from each emitted species into the contributions from the composition changes. The ERFs are calculated for each of the models that participated in the AerChemMIP experiments as part of the CMIP6 project, where the relevant model output was available. The 1850 to 2014 multi-model mean ERFs (± standard deviations) are −1.03 ± 0.37 W m−2 for SO2 emissions, −0.25 ± 0.09 W m−2 for organic carbon (OC), 0.15 ± 0.17 W m−2 for black carbon (BC) and −0.07 ± 0.01 W m−2 for NH3. For the combined aerosols (in the piClim-aer experiment) it is −1.01 ± 0.25 W m−2. The multi-model means for the reactive well-mixed greenhouse gases (including any effects on ozone and aerosol chemistry) are 0.67 ± 0.17 W m−2 for methane (CH4), 0.26 ± 0.07 W m−2 for nitrous oxide (N2O) and 0.12 ± 0.2 W m−2 for ozone-depleting halocarbons (HC). Emissions of the ozone precursors nitrogen oxides (NOx), volatile organic compounds and both together (O3) lead to ERFs of 0.14 ± 0.13, 0.09 ± 0.14 and 0.20 ± 0.07 W m−2 respectively. The differences in ERFs calculated for the different models reflect differences in the complexity of their aerosol and chemistry schemes, especially in the case of methane where tropospheric chemistry captures increased forcing from ozone production.

• Brenna, Hans; Kutterolf, Steffen; Mills, Michael; Niemeier, Ulrike; Timmreck, Claudia & Krüger, Kirstin (2021). Decadal Disruption of the QBO by Tropical Volcanic Supereruptions. Geophysical Research Letters. ISSN 0094-8276. 48(5), p. 1–13. doi: 10.1029/2020GL089687. Full text in Research Archive Show summary

  The Los Chocoyos (14.6°N, 91.2°W) supereruption happened ∼75,000 years ago in Guatemala and was one of the largest eruptions of the past 100,000 years. It emitted enormous amounts of sulfur, chlorine, and bromine, with multi‐decadal consequences for the global climate and environment. Here, we simulate the impact of a Los Chocoyos‐like eruption on the quasi‐biennial oscillation (QBO), an oscillation of zonal winds in the tropical stratosphere, with a comprehensive aerosol chemistry Earth System Model. We find a ∼10‐year disruption of the QBO starting 4 months post eruption, with anomalous easterly winds lasting ∼5 years, followed by westerlies, before returning to QBO conditions with a slightly prolonged periodicity. Volcanic aerosol heating and ozone depletion cooling leads to the QBO disruption and anomalous wind regimes through radiative changes and wave‐mean flow interactions. Different model ensembles, volcanic forcing scenarios and results of a second model back up the robustness of our results.

• Hagen, Jenny Sjåstad; Leblois, Etienne; Lawrence, Deborah; Solomatine, Dimitri & Sorteberg, Asgeir (2021). Identifying major drivers of daily streamflow from large-scale atmospheric circulation with machine learning. Journal of Hydrology. ISSN 0022-1694. 596, p. 126086–126086. doi: 10.1016/j.jhydrol.2021.126086. Full text in Research Archive Show summary

  Previous studies linking large-scale atmospheric circulation and river flow with traditional machine learning techniques have predominantly explored monthly, seasonal or annual streamflow modelling for applications in direct downscaling or hydrological climate-impact studies. This paper identifies major drivers of daily streamflow from large-scale atmospheric circulation using two reanalysis datasets for six catchments in Norway representing various Köppen-Geiger climate types and flood-generating processes. A nested loop of roughly pruned random forests is used for feature extraction, demonstrating the potential for automated retrieval of physically consistent and interpretable input variables. Random forest (RF), support vector machine (SVM) for regression and multilayer perceptron (MLP) neural networks are compared to multiple-linear regression to assess the role of model complexity in utilizing the identified major drivers to reconstruct streamflow. The machine learning models were trained on 31 years of aggregated atmospheric data with distinct moving windows for each catchment, reflecting catchment-specific forcing-response relationships between the atmosphere and the rivers. The results show that accuracy improves to some extent with model complexity. In all but the smallest, rainfall-driven catchment, the most complex model, MLP, gives a Nash-Sutcliffe Efficiency (NSE) ranging from 0.71 to 0.81 on testing data spanning five years. The poorer performance by all models in the smallest catchment is discussed in relation to catchment characteristics, sub-grid topography and local variability. The intra-model differences are also viewed in relation to the consistency between the automatically retrieved feature selections from the two reanalysis datasets. This study provides a benchmark for future development of deep learning models for direct downscaling from large-scale atmospheric variables to daily streamflow in Norway.

• Moseid, Kine Onsum; Schulz, Michael; Storelvmo, Trude; Julsrud, Ingeborg Rian; Oliviè, Dirk Jan Leo & Nabat, Pierre [Show all 12 contributors for this article] (2020). Bias in CMIP6 models as compared to observed regional dimming and brightening. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 20(24), p. 16023–16040. doi: 10.5194/acp-20-16023-2020. Full text in Research Archive Show summary

  Abstract. Anthropogenic aerosol emissions have increased considerably over the last century, but climate effects and quantification of the emissions are highly uncertain as one goes back in time. This uncertainty is partly due to a lack of observations in the pre-satellite era, making the observations we do have before 1990 additionally valuable. Aerosols suspended in the atmosphere scatter and absorb incoming solar radiation and thereby alter the Earth's surface energy balance. Previous studies show that Earth system models (ESMs) do not adequately represent surface energy fluxes over the historical era. We investigated global and regional aerosol effects over the time period 1961–2014 by looking at surface downwelling shortwave radiation (SDSR). We used observations from ground stations as well as multiple experiments from eight ESMs participating in the Coupled Model Intercomparison Project Version 6 (CMIP6). Our results show that this subset of models reproduces the observed transient SDSR well in Europe but poorly in China. We suggest that this may be attributed to missing emissions of sulfur dioxide in China, sulfur dioxide being a precursor to sulfate, which is a highly reflective aerosol and responsible for more reflective clouds. The emissions of sulfur dioxide used in the models do not show a temporal pattern that could explain observed SDSR evolution over China. The results from various aerosol emission perturbation experiments from DAMIP, RFMIP and AerChemMIP show that only simulations containing anthropogenic aerosol emissions show dimming, even if the dimming is underestimated. Simulated clear-sky and all-sky SDSR do not differ greatly, suggesting that cloud cover changes are not a dominant cause of the biased SDSR evolution in the simulations. Therefore we suggest that the discrepancy between modeled and observed SDSR evolution is partly caused by erroneous aerosol and aerosol precursor emission inventories. This is an important finding as it may help interpret whether ESMs reproduce the historical climate evolution for the right or wrong reason.

• Seland, Øyvind; Bentsen, Mats; Oliviè, Dirk Jan Leo; Toniazzo, Thomas; Gjermundsen, Ada & Graff, Lise Seland [Show all 30 contributors for this article] (2020). Overview of the Norwegian Earth System Model (NorESM2) and key climate response of CMIP6 DECK, historical, and scenario simulations. Geoscientific Model Development. ISSN 1991-959X. 13(12), p. 6165–6200. doi: 10.5194/gmd-13-6165-2020. Full text in Research Archive Show summary

  The second version of the coupled Norwegian Earth System Model (NorESM2) is presented and evaluated. NorESM2 is based on the second version of the Community Earth System Model (CESM2) and shares with CESM2 the computer code infrastructure and many Earth system model components. However, NorESM2 employs entirely different ocean and ocean biogeochemistry models. The atmosphere component of NorESM2 (CAM-Nor) includes a different module for aerosol physics and chemistry, including interactions with cloud and radiation; additionally, CAM-Nor includes improvements in the formulation of local dry and moist energy conservation, in local and global angular momentum conservation, and in the computations for deep convection and air–sea fluxes. The surface components of NorESM2 have minor changes in the albedo calculations and to land and sea-ice models. We present results from simulations with NorESM2 that were carried out for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Two versions of the model are used: one with lower (∼ 2∘) atmosphere–land resolution and one with medium (∼ 1∘) atmosphere–land resolution. The stability of the pre-industrial climate and the sensitivity of the model to abrupt and gradual quadrupling of CO2 are assessed, along with the ability of the model to simulate the historical climate under the CMIP6 forcings. Compared to observations and reanalyses, NorESM2 represents an improvement over previous versions of NorESM in most aspects. NorESM2 appears less sensitive to greenhouse gas forcing than its predecessors, with an estimated equilibrium climate sensitivity of 2.5 K in both resolutions on a 150-year time frame; however, this estimate increases with the time window and the climate sensitivity at equilibration is much higher. We also consider the model response to future scenarios as defined by selected Shared Socioeconomic Pathways (SSPs) from the Scenario Model Intercomparison Project defined under CMIP6. Under the four scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5), the warming in the period 2090–2099 compared to 1850–1879 reaches 1.3, 2.2, 3.0, and 3.9 K in NorESM2-LM, and 1.3, 2.1, 3.1, and 3.9 K in NorESM-MM, robustly similar in both resolutions. NorESM2-LM shows a rather satisfactory evolution of recent sea-ice area. In NorESM2-LM, an ice-free Arctic Ocean is only avoided in the SSP1-2.6 scenario.

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• Bourgeois, Timothée; Goris, Nadine; Schwinger, Jörg & Tjiputra, Jerry (2024). Emergent constraint on future anthropogenic carbon and excess heat uptake in the Southern Ocean. Full text in Research Archive Show summary

  The Southern Ocean is a major sink of anthropogenic carbon and excess heat. In this region, the Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread. This contributes significantly to the large uncertainties in the overall climate sensitivity and remaining carbon budgets for ambitious climate targets. Hence, a reduction in the uncertainty of the future Southern Ocean carbon and heat sinks is urgently needed. Globally, Bronselaer and Zanna (2020) identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emissions scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. Here, we use this approach for the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake. We found that, in this region, the variations in the models’ contemporary water-column stability over the first 2000 m is highly correlated to both their future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data of water-column stability, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Independent studies have found similar constraints in the Southern Ocean and globally, strengthening our findings (Liu et al., 2023; Newsom et al., 2023; Terhaar et al., 2021, 2022), and pinpointing that a better representation of water-column stratification in Earth system models is essential to improve future anthropogenic climate change projections. Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J. F.: Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S, Nat Commun, 13, 340, https://doi.org/10.1038/s41467-022-27979-5, 2022. Bronselaer, B. and Zanna, L.: Heat and carbon coupling reveals ocean warming due to circulation changes, Nature, 584, 227–233, https://doi.org/10.1038/s41586-020-2573-5, 2020. Liu, M., Soden, B. J., Vecchi, G. A., and Wang, C.: The Spread of Ocean Heat Uptake Efficiency Traced to Ocean Salinity, Geophys. Res. Lett., 50, e2022GL100171, https://doi.org/10.1029/2022GL100171, 2023. Newsom, E., Zanna, L., and Gregory, J.: Background Pycnocline Depth Constrains Future Ocean Heat Uptake Efficiency, Geophys. Res. Lett., 50, e2023GL105673, https://doi.org/10.1029/2023GL105673, 2023. Terhaar, J., Frölicher, T. L., and Joos, F.: Southern Ocean anthropogenic carbon sink constrained by sea surface salinity, Sci. Adv., 7, eabd5964, https://doi.org/10.1126/sciadv.abd5964, 2021. Terhaar, J., Frölicher, T. L., and Joos, F.: Observation-constrained estimates of the global ocean carbon sink from Earth system models, Biogeosciences, 19, 4431–4457, https://doi.org/10.5194/bg-19-4431-2022, 2022.

• Booge, Dennis; Tjiputra, Jerry; Olivie, Dirk Jan Leo; Quack, Birgit & Krüger, Kirstin (2024). Contrasting trends of marine bromoform emissions in a future climate. Full text in Research Archive
• Bentsen, Mats & Ilicak, Mehmet (2024). Hybrid vertical coordinate in the Bergen Layered Ocean Model (BLOM). Full text in Research Archive
• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2023). Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Full text in Research Archive
• He, Yanchun; Torsvik, Tomas & Gupta, Alok Kumar (2023). ESMValTool Workshop 2023. Full text in Research Archive
• Graff, Lise Seland; Parding, Kajsa & Landgren, Oskar Andreas (2023). Evaluating CMIP6 models with a simple climatology score. Full text in Research Archive
• Oliviè, Dirk Jan Leo; Seland, Øyvind; Krüger, Kirstin & Schulz, Michael (2023). Analysis of NorESM2 full-chemistry simulation of the historical period (1850–2014). Full text in Research Archive
• Gjermundsen, Ada; Bentsen, Mats; Oliviè, Dirk Jan Leo; Nummelin, Aleksi Henrynpoika; Seland, Øyvind & Schulz, Michael (2023). The Norwegian Earth System Model. Full text in Research Archive
• Tjiputra, Jerry; Goris, Nadine; Schwinger, Jörg; Bourgeois, Timothée; Ayar, Pradeebane Vaittinada & Johannsen, Klaus (2023). Constraining ocean carbon sink projections in CMIP6 models. Full text in Research Archive
• Tjiputra, Jerry (2023). Detection timescales of environmental stressors in the interior ocean. Full text in Research Archive
• Eckhardt, Sabine; Svendby, Tove Marit; Steensen, Birthe Marie Rødssæteren; Myhre, Gunnar; Gjermundsen, Ada & Oliviè, Dirk Jan Leo (2023). Moisture transport into the Arctic in a past and future climate. Full text in Research Archive
• Booge, Dennis; Tjiputra, Jerry; Olivie, Dirk Jan Leo; Birgit, Quack & Krüger, Kirstin (2023). Global and regional marine bromoform emissions in a fully coupled ocean-atmosphere-model. Full text in Research Archive Show summary

  Bromoform (CHBr3) is one of the most important precursors of atmospheric reactive bromine with an atmospheric lifetime of ~20 days. Natural production, being the main source of oceanic CHBr3, is high at the coasts and in open ocean upwelling regions due to production by macroalgae and phytoplankton. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, the global bromoform production in the ocean and their emissions are still poorly constrained in observations and are mostly neglected in Earth System Model (ESM) climate projections. Here, we show first model results of fully coupled ocean-atmosphere bromoform interactions in the Norwegian ESM (NorESM) with the ocean model BLOM and the ocean biogeochemistry component iHAMOCC for the CMIP6 historical period from 1850 to 2014. Our results are validated using oceanic and atmospheric measurements listed in the HalOcAt (Halocarbons in the Ocean and Atmosphere) data base and show an overall good agreement with those observations in open ocean regions. The NorESM open ocean emissions of CHBr3 are higher than previously published emission estimates from bottom-up approaches. Moreover, the emissions are mainly positive (sea-to-air fluxes) driven by the oceanic production, sea surface temperature and wind speed, dependent on season and location. However, during low-productive winter seasons, model results also show local negative fluxes (air-to-sea fluxes) in high latitudes, suggesting some oceanic regions to be a sink of atmospheric bromoform. Driving factors will be shown for different case studies, e.g. the tropical West Pacific, which is a hot spot for oceanic bromine delivery to the stratosphere. How to cite: Booge, D., Tjiputra, J., Olivié, D., Quack, B., Schulz, M., and Krüger, K.: Global and regional marine bromoform emissions in a fully coupled ocean-atmosphere-model, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12225, https://doi.org/10.5194/egusphere-egu23-12225, 2023.

• Goelzer, Heiko; Langebroek, Petra Margaretha; Born, Andreas; Haubner, Konstanze; Hofer, Stefan & Storelvmo, Trude (2023). Coupled Greenland ice sheet-climate simulations with the Norwegian Earth System Model (NorESM2). Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra Margaretha; Born, Andreas; Haubner, Konstanze; Hofer, Stefan & Storelvmo, Trude (2023). Coupled Greenland ice sheet-climate simulations with the Norwegian Earth System Model (NorESM2). Full text in Research Archive
• Goelzer, Heiko (2023). Ice sheets, sea-level and climate change. Full text in Research Archive
• Goelzer, Heiko (2023). Lecture on ice sheets. Full text in Research Archive
• Tjiputra, Jerry; Negrel, Jean & Olsen, Are (2023). Detecting anthropogenic climate change signals of marine stressors in the interior. Full text in Research Archive
• Oliviè, Dirk Jan Leo; Seland, Øyvind; Tjiputra, Jerry & Schwinger, Jörg (2023). Lessons from COVID-19 emission reduction simulations with NorESM2. Full text in Research Archive
• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2022). Assessing the oceanic control on ECS in CMIP6. Full text in Research Archive
• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2022). NorESM2 and Climate Sensitivity. Full text in Research Archive
• Krüger, Kirstin; Brenna, Hans; Fuglestvedt, Herman; Zhuo, Zhihong; Mills, Mike & Niemeier, Ulrike [Show all 7 contributors for this article] (2022). How large has a volcanic eruption to be to disrupt the QBO? Full text in Research Archive Show summary

  The dominating circulation phenomenon in the tropical stratosphere is the quasi-biennial oscillation (QBO), an approximately 28 months oscillation of alternating easterly or westerly winds centered at the equator. Volcanic aerosols may have an impact on the QBO phase as was observed for the Pinatubo eruption, which showed a “remarkably” prolonged Easterly wind regime. In contrast, geo-engineering studies model a prolonged Westerly phase or a shut-down of the QBO due to the artificial injection of sulfate aerosols. Extremely large volcanic eruptions which inject several 10s to 100s Tg SO2 might therefore have the potential to disturb the QBO. In addition, recent observations revealed two anomalous disruptions of the QBO in the 2010s with anomalous westerly winds probably due to Rossby wave propagating from the extratropics. We use an Earth System Model taking volcanic aerosol chemistry climate interactions into account to study the QBO response to violent volcanic eruptions. Simulating a tropical supereruption with 1000 Tg SO2 and halogens injection into the stratosphere reveals a disruption of the QBO for up to 10 years first with anomalous Easterlies followed by anomalous Westerlies before returning to a QBO regime with slightly longer period. Volcanic aerosol heating and ozone depletion cooling lead to the QBO disruption and anomalous wind regimes through radiation and wave-mean flow interactions. Here, we present a new set of sulfur- and halogen-rich volcanic eruptions experiments with 17 and 200 Tg SO2 injections into the tropical and Northern Hemisphere extratropics during January and July season to answer the above raised question.

• Krüger, Kirstin; Booge, Dennis; Quack, Birgit; Tjiputra, Jerry & Olivie, Dirk (2022). Future stratospheric halogen loading and ozone cycle: Modelling biogenic halocarbons in the ocean and their impact on the atmosphere. Full text in Research Archive Show summary

  Halogenated very short-lived substances (VSLSs) contribute to the tropospheric and stratospheric halogen burden, take part in ozone depletion, and, thus, impact climate. The contribution of oceanic VSLSs is estimated to be 10–40 % of the current total stratospheric bromine. Among them, is bromoform (CHBr3) one the most important precursor of atmospheric reactive bromine with a lifetime of ~20 days in the atmosphere. Oceanic sources of bromoform are highest at the coasts and in the open ocean in upwelling regions due to macroalgae and phytoplankton production. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, the global bromoform production in the ocean and their emissions are still poorly constrained in observations and are mostly neglected in chemistry climate and Earth System Models (ESMs). Here, we show first model results following Stemmler et al (2015) but adding the full coupling of the oceanic bromoform production with the atmosphere through air-sea gas exchange and atmospheric chemistry in the Norwegian ESM (NorESM) with the ocean model BLOM and the biogeochemistry component iHAMOCC. We run NorESM for the CMIP6 historic scenario from 1850 to 2014. Our results reveal higher global bromoform emissions to the atmosphere than previously reported from bottom up approaches. Comparing our modelled bromoform results with HalOcAt observations reveal an overall good agreement for the open ocean and the seasonal cycle. Case studies for the tropical Indian Ocean and Pacific will be shown, which are hot spots in the ocean and for the stratospheric bromine delivery.

• Heinze, Christoph (2022). Ocean tipping points - an overview. Full text in Research Archive
• Gjermundsen, Ada; Buraas, Marte Sofie; Oliviè, Dirk Jan Leo; Debernard, Jens Boldingh; Graff, Lise Seland & Nummelin, Aleksi Henrynpoika [Show all 9 contributors for this article] (2022). Assessing Arctic climate change with the Norwegian Earth System Model. Full text in Research Archive
• Guo, Chuncheng; Bentsen, Mats; Ilicak, Mehmet & Nummelin, Aleksi Henrynpoika (2022). CMIP6/OMIP simulations of the Arctic Ocean and Resolution effects. Full text in Research Archive
• Krüger, Kirstin & et, al (2022). Future stratospheric halogen loading and ozone cycle: Modelling biogenic halocarbons in the ocean and their impact on the atmosphere. Full text in Research Archive Show summary

  Halogenated very short-lived substances (VSLSs) contribute to the tropospheric and stratospheric halogen burden, take part in ozone depletion, and, thus, impact climate. The contribution of oceanic VSLSs is estimated to be 10–40 % of the current total stratospheric bromine. Among them, is bromoform (CHBr3) one the most important precursor of atmospheric reactive bromine with a lifetime of ~20 days in the atmosphere. Oceanic sources of bromoform are highest at the coasts and in the open ocean in upwelling regions due to macroalgae and phytoplankton production. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, the global bromoform production in the ocean and their emissions are still poorly constrained in observations and are mostly neglected in chemistry climate and Earth System Models (ESMs). Here, we show first model results following Stemmler et al (2015) but adding the full coupling of the oceanic bromoform production with the atmosphere through air-sea gas exchange and atmospheric chemistry in the Norwegian ESM (NorESM) with the ocean model BLOM and the biogeochemistry component iHAMOCC. We run NorESM for the CMIP6 historic scenario from 1850 to 2014. Our results reveal higher global bromoform emissions to the atmosphere than previously reported from bottom up approaches. Comparing our modelled bromoform results with HalOcAt observations reveal an overall good agreement for the open ocean and the seasonal cycle. Case studies for the tropical Indian Ocean and Pacific will be shown, which are hot spots in the ocean and for the stratospheric bromine delivery.

• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2022). Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Full text in Research Archive
• Oliviè, Dirk Jan Leo (2022). The ESM2025 project and interactive methane modelling. Full text in Research Archive
• Tjiputra, Jerry; Booge, Dennis; Oliviè, Dirk Jan Leo; Quack, Birgit & Krüger, Kirstin (2022). Modeling bromoform in Norwegian Earth system model. Full text in Research Archive
• Heinze, Christoph (2022). Triple threats to our ocean - the triple blow and how one can try to address it:ocean warming, acidification, and deoxygenation. Full text in Research Archive
• Krüger, Kirstin; Brenna, Hans & et, al (2022). How large has a volcanic eruption to be to disrupt the QBO? Full text in Research Archive Show summary

  The dominating circulation phenomenon in the tropical stratosphere is the quasi-biennial oscillation (QBO), an approximately 28 months oscillation of alternating easterly or westerly winds centered at the equator. Volcanic aerosols may have an impact on the QBO phase as was observed for the Pinatubo eruption, which showed a “remarkably” prolonged Easterly wind regime. In contrast, geo-engineering studies model a prolonged Westerly phase or a shut-down of the QBO due to the artificial injection of sulfate aerosols. Extremely large volcanic eruptions which inject several 10s to 100s Tg SO2 might therefore have the potential to disturb the QBO. In addition, recent observations revealed two anomalous disruptions of the QBO in the 2010s with anomalous westerly winds probably due to Rossby wave propagating from the extratropics. We use an Earth System Model taking volcanic aerosol chemistry climate interactions into account to study the QBO response to violent volcanic eruptions. Simulating a tropical supereruption with 1000 Tg SO2 and halogens injection into the stratosphere reveals a disruption of the QBO for up to 10 years first with anomalous Easterlies followed by anomalous Westerlies before returning to a QBO regime with slightly longer period. Volcanic aerosol heating and ozone depletion cooling lead to the QBO disruption and anomalous wind regimes through radiation and wave-mean flow interactions. Here, we present a new set of sulfur- and halogen-rich volcanic eruptions experiments with 17 and 200 Tg SO2 injections into the tropical and Northern Hemisphere extratropics during January and July season to answer the above raised question.

• Oliviè, Dirk Jan Leo; Balkanski, Yves; Bergman, Tommi; Carslaw, Ken; Checa-Garcia, Ramiro & O'Connor, Fiona M [Show all 17 contributors for this article] (2022). Impact of natural aerosol uncertainty on anthropogenic aerosol forcing. Full text in Research Archive
• Oliviè, Dirk Jan Leo; Seland, Øyvind; Krüger, Kirstin & Schulz, Michael (2022). Implementation of gas-phase chemistry in NorESM2. Full text in Research Archive
• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2021). Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Full text in Research Archive
• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2021). Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Full text in Research Archive
• Gjermundsen, Ada; Graff, Lise Seland; Bentsen, Mats; Breivik, Lars-Anders; Debernard, Jens Boldingh & Makkonen, Risto [Show all 10 contributors for this article] (2021). How representative is Svalbard for future Arctic climate evolution? An Earth system modelling perspective. Full text in Research Archive
• Heinze, Christoph (2021). EU H2020 Project COMFORT. Full text in Research Archive
• Gjermundsen, Ada; Graff, Lise Seland; Bentsen, Mats; Breivik, Lars-Anders; Debernard, Jens Boldingh & Makkonen, Risto [Show all 10 contributors for this article] (2021). How representative is Svalbard for future Arctic climate evolution? An Earth system modelling perspective. Full text in Research Archive
• Gjermundsen, Ada; Nummelin, Aleksi Henrynpoika; Oliviè, Dirk Jan Leo; Bentsen, Mats; Seland, Øyvind & Schulz, Michael (2021). Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Full text in Research Archive
• Oliviè, Dirk Jan Leo; Gjermundsen, Ada; Kirkevåg, Alf; Moseid, Kine Onsum; Schulz, Michael & Seland, Øyvind (2021). Comparing the climate impact of CMIP5 versus CMIP6 aerosol (precursor) emissions in NorESM2. Full text in Research Archive
• Oliviè, Dirk Jan Leo; Fagerli, Hilde & Schulz, Michael (2021). METs work on methane in ESM2025 (Earth System Modelling) and for DG-ENV (air quality). Full text in Research Archive
• Auganæs, Sigrid Marie & Krüger, Kirstin (2023). Ozone Above the Tropical Atlantic Ocean. Universitetet i Oslo. Full text in Research Archive Show summary

  Ozon spelar ei avgjerande rolle i atmosfæren både i stratosfæren og troposfæren. Denne avhandlinga undersøker troposfærisk ozon over det tropiske Atlanterhavet ved hjelp av observasjonar, trajektoriar og den norske jordsystemmodellen (NorESM). Forskingsspørsmåla som blir svara på inkluderar tilstadeværet av eit minimum av troposfærisk ozon over det tropiske Atlanterhavet, samanlikning av ozonprofilar mellom skipskampanjar og landbaserte stasjonar, variasjonen i ozon over Atlanterhavet samanlikna med andre havbasseng, og kor godt NorESM modellen representerer observerte ozoneigenskapar. Observasjonar frå SO287 CONNECT skipskampanjen som kryssa det tropiske Atlanterhavet, avslører ingen ozonminimum lik det som førre er observert i det tropiske vestlige Stillehavet. Ozonkonsentrasjonar over det tropiske Atlanterhavet varierte frå 25 til 80 delar per milliard (ppb), med eit gjennomsnitt på 45 ppb gjennom troposfæren. Dei analyserte luftmassane viste karakteristiske eigenskapar frå ulike regionar, som indikerer skogbrannar og stratosfæriske påverknader. Samanlikningar med nærliggande SHADOZ stasjonar bekrefta vellykka prøvetaking av dei same luftmassane av skipskampanjen og Paramaribo SHADOZ stasjonen. Vidare er ozonkonsentrasjonar frå skipskampanjar i med ulike tropiske havbasseng samanklikna, og avdekkjer høgare ozonkonsentrasjonar over det tropiske Atlanterhavet samanlikna med andre tropiske havbasseng. NorESM, inkludert MOZART-TS1 kjemisystemet, fanger opp dei forventa mønstera av overflateozon og wave-one distribusjonar, med nokre avvik. Likevel simulerer modellen den brå økinga av ozon på lågare høgde enn det som er observert, sannsynlegvis på grunn av grov oppløysing i øvre troposfære og stratosfære.

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