New global warming projections in CMIP6 contain extensions beyond year 2100 up to 2300/2500. The Atlantic Meridional Overturning Circulation (AMOC) in these runs essentially ceases in all models forced by a high emissions (SSP585) scenario and sometimes also in models forced by intermediate (SSP245) or low (SSP126) scenarios. These extremely weak overturning states merely maintain a wind-driven shallow overturning at depths less than 200 m. Northward mass transport below this maximum is either completely absent or less than 2 Sverdrup. In all cases, this AMOC cessation is preceded by a mid-21st century collapse of deep convection in Labrador, Irminger and Nordic Seas, which likely represents the tipping point triggering the terminal AMOC decline.
How to cite: Rahmstorf, S., Drijfhout, S., Angevaare, J., and Mecking, J.: The Atlantic overturning circulation (AMOC) ceases in many CMIP6 projections after 2100, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3604, https://doi.org/10.5194/egusphere-egu25-3604, 2025.
The Atlantic Meridional Overturning Circulation (AMOC), crucial for transporting heat northward across the Atlantic Ocean, is expected to weaken due to global warming, with implications for global climate. It is uncertain, however, how much it will weaken or if it may even completely collapse this century. Hence, we assess the AMOC's ability to withstand extreme increases in greenhouse gases and freshwater forcing in the North Atlantic by examining the upwelling pathways that return deep waters of the AMOC to the surface in 34 CMIP6 climate models. Our findings show that upwelling in the Southern Ocean, maintained by persistent overlying westerly winds, prevents a total AMOC collapse and impacts its future strength. This Southern Ocean upwelling must be balanced by downwelling in the Atlantic or Pacific oceans, so only the development of a strong Pacific Meridional Overturning Circulation (PMOC) could enable an AMOC collapse. While a PMOC does appear in nearly all models, it is insufficient to offset the upwelling in the Southern Ocean, suggesting an AMOC collapse this century is unlikely. We uncover novel stabilising mechanisms that enhance the resilience of the AMOC, with implications for its past and future change. Our findings highlight the critical need for improved observation-based estimates of the Indo-Pacific and Southern Ocean circulations to reduce uncertainty in AMOC projections.
How to cite: Baker, J., Bell, M., Jackson, L., Vallis, G., Watson, A., and Wood, R.: Continued Atlantic overturning circulation even under climate extremes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4450, https://doi.org/10.5194/egusphere-egu25-4450, 2025.
Past trends in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are still debated, and its fate under global warming is uncertain. Observational studies suggest that there has been persistent weakening since the mid-twentieth century, whereas CMIP models systematically simulate a stable circulation. Here, using Earth System and eddy-permitting coupled ocean–sea-ice models, we show that a freshening of the subarctic Atlantic Ocean and weakening of the overturning circulation increase the temperature and salinity of the South Atlantic on a decadal timescale through the propagation of Kelvin and Rossby waves. Applying this finding to observations provides novel evidence on the recent AMOC slow-down. We also show that accounting for upper-end meltwater input in CMIP6 historical simulations significantly improves the data–model agreement on past changes in the Atlantic Meridional Overturning Circulation, indicating the AMOC has weakened by 13% (9 to15%) under a 1ºC global warming, reached in 2017. Including estimates of subarctic meltwater input for the coming century suggests that this circulation could weaken by 33% under a 2ºC global warming, which can be attained over the coming decade, being twice the AMOC weakening projected by radiative forcing only (16%).
How to cite: Pontes, G. M. and Menviel, L.: Past and future of the Atlantic Meridional Overturning Circulation under radiative and meltwater forcings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9669, https://doi.org/10.5194/egusphere-egu25-9669, 2025.
Future climate projections in CMIP indicate that sea ice will continue to decline under all emission scenarios, although there is uncertainty regarding the timing of a summer ice-free Arctic. Additionally, while models agree that the Atlantic Meridional Overturning Circulation (AMOC) will weaken, they diverge on the rate of weakening and the underlying processes. Results from the HighResMIP initiative have highlighted the benefits of increased model resolution in improving the representation of key climate processes in the North Atlantic and Arctic oceans.
Building on this, we use the high-resolution global climate model EC-Earth3-HR, a higher-resolution version of the EC-Earth3 configuration developed for CMIP6, to investigate future changes in the Arctic and North Atlantic Oceans. EC-Earth3-HR consists of the IFS atmospheric model (T511, ~40 km), the NEMO ocean model (0.25°), and the LIM3 sea ice model. The model has undergone a tuning process, a long spin-up, and 350 years of pre-industrial control simulations, followed by full historical (1850–2014) and future (2015–2100, SSP2-4.5) simulations.
We will highlight the impact of increased horizontal resolution, compared to the lower-resolution version, on simulating the historical climate, with a focus on ocean and sea ice conditions in the North Atlantic and Arctic and their projected changes under SSP2-4.5. Our results show that EC-Earth-HR effectively captures sea ice trend and rapid sea ice loss events, and projecting a summer ice-free Arctic by 2050. It also shows a 40% weakening of the AMOC by the end of the century. Furthermore, we present a novel method for estimating deep water formation rates and examining the processes contributing to the weakening of the AMOC.
How to cite: Karami, M. P., Koenigk, T., Wang, S., Kruschke, T., Carreric, A., Ortega, P., Wyser, K., Fuentes Franco, R., Navarro Labastida, R., de Boer, A., Sicard, M., and Aldama Campino, A.: Historical Climate and Future Projection in the Arctic and North Atlantic: Insights from High-Resolution EC-Earth3 Simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16442, https://doi.org/10.5194/egusphere-egu25-16442, 2025.
Records of Atlantic sea surface temperature (SST) and sea surface salinity (SSS) integrated over different areas have been proposed as fingerprints of the AMOC over the period of observations. Boers (2021) analyses eight different time series of fingerprints as AMOC indices and finds for all of them early warming signals of an approach to a tipping point. However, if two different AMOC indices are not strongly correlated, they can obviously not both be trustworthy representations of the AMOC. Thus, if significant EWSs are found in both of such records, at least one is spurious. In that case, it is questionable if EWSs for a forthcoming collapse can be trusted at all, since it is observed for unrelated reasons in (at least) one record. Here we propose a consistent fingerprint based on a combination of all the fingerprints.
How to cite: Ditlevsen, P. and Ditlevsen, S.: Obtaining optimal fingerprints of the AMOC from sea surface observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4309, https://doi.org/10.5194/egusphere-egu25-4309, 2025.
A realistic representation of ocean salinity is essential for understanding large-scale ocean circulation, water mass transformation, and the global hydrological cycle. This study thoroughly assesses CMIP6 historical simulations by comparing ocean salinity outputs from climate models with EN4 datasets, investigating horizontal and vertical spatial patterns as well as temporal variability. Our analysis shows that while CMIP6 models display a strong spatial correlation with observations across multiple ocean basins, significant discrepancies arise in the temporal trends of salinity variations. Specifically, the models often hard to capture the magnitude and persistence of long-term salinity trends observed in the EN4 dataset. The evaluation also emphasizes vertical salinity profiles, where maximum and minimum salinity values serve as simplified indicators for structural assessment. To ensure consistent comparisons, model-simulated salinity extrema are standardized by aligning their values with the corresponding depths in the observational data. These results highlight both the strengths and limitations of CMIP6 models in representing key oceanographic features, such as the depth and magnitude of salinity extrema, mixed layer depth, and regional salinity gradients. By identifying areas of agreement and divergence between models and observations, this study provides valuable insights for improving the physical realism and predictive accuracy of historical climate simulations, ultimately guiding future model development.
How to cite: Lyu, Y., Bindoff, N., Mohanpatra, S., Rathore, S., and Phillips, H.: A Multi-Dimensional Evaluation of Global Ocean salinity in CMIP6 Historical simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9412, https://doi.org/10.5194/egusphere-egu25-9412, 2025.
We employ the Adriatic Sea and Coast (AdriSC) fine-resolution atmosphere-ocean model, operating at the kilometer scale, to assess the effects of a far-future extreme warming scenario on the formation, transport, and accumulation of the Adriatic dense and deep water. It is well-established that North Adriatic Dense Water (NAddW) is spreading across the entire basin and accumulates within the Jabuka Pit, while Adriatic Deep Water (AdDW) is only located within the Southern Adriatic Pit (SAP). However, unlike earlier studies that relied on coarser-resolution Mediterranean climate models, our approach incorporates updated thresholds for defining dense and deep water, reflecting far-future background density shifts due to increased sea surface temperatures.
Our analysis reveals a 15% increase in surface buoyancy losses at NAddW generation sites under extreme warming, driven primarily by evaporation, despite a 25% decline in both the intensity and spatial extent of the winter windstorms responsible for the surface cooling. Consequently, far-future NAddW formation is projected to remain comparable to present-day conditions. However, heightened stratification in the far-future scenario is expected to reduce the volume of dense water retained in the Jabuka Pit. Furthermore, the transport of dense water between the Jabuka Pit and the SAP's deepest regions is likely to cease, as future NAddW will be less dense than the AdDW.
Regarding the exchanges between the Adriatic and Ionian Seas, we find that the Bimodal Oscillation System's influence on Adriatic salinity variability will persist under extreme warming. Nonetheless, future AdDW dynamics will primarily be driven by density changes in the northern Ionian Sea.
These findings underscore the intricate nature of climate change impacts on Adriatic atmosphere-ocean interactions and highlight the necessity of higher resolution models for producing more reliable far-future projections at the coastal scale.
How to cite: Denamiel, C., Vrdoljak, I., and Pranić, P.: A New Vision of the Adriatic Dense Water Future under Extreme Warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7835, https://doi.org/10.5194/egusphere-egu25-7835, 2025.
Taiwan’s coastal areas are highly vulnerable to typhoon-induced waves and storm surges, posing severe threats to coastal defenses, port operations, and maritime safety. Over the past decade, Taiwan has experienced an average of 4.5 typhoons annually, resulting in wave heights typically exceeding 10 meters, with maximums reaching 23 meters, and storm surges ranging from 20 to 40 cm. Understanding these extreme events under climate change scenarios is essential for effective disaster prevention and coastal planning. This study utilizes EC-Earth3 wind field data from the SSP5-8.5 high-emission scenario to simulate historical (1961–2020) and projected future (2021–2100) typhoon wave and storm surge data, followed by an analysis of their characteristics. Although the EC-Earth3 outputs have a spatial resolution of 100 km and a temporal resolution of 3 hours, this study refined them using an artificial intelligence-based downscaling model. A convolutional recurrent neural network (CRNN) enhanced the resolution to 3 km and 1 hour, enabling more detailed and accurate data for analyzing typhoon waves and storm surges.
Historical analysis reveals distinct regional variations in wave changes around Taiwan. The eastern coast exhibits the largest wave change due to its exposure to the Pacific Ocean, while the Taiwan Strait remains the most stable, predominantly influenced by monsoonal conditions. The southern and northern waters show intermediate wave change levels, with the latter affected by shifting typhoon tracks. Future projections suggest moderate increases in wave heights around Taiwan, particularly in the northern and northeastern waters, driven by the northeast monsoon and typhoon activity. These findings underscore the increasing risks to Taiwan’s coastal regions under changing climate conditions. This study further examines the regional characteristics of design wave heights, revealing the differential impacts of climate change on extreme wave conditions. Wave conditions in the Taiwan Strait are predominantly influenced by monsoons, with minimal climate change effects and relatively stable future design wave heights, ranging from approximately 3.63 to 3.68 meters for projected scenarios. In contrast, the eastern coastal waters, affected by typhoons and the Kuroshio Current, display moderate variability, with historical wave heights ranging from 4.42 to 4.92 meters and projected heights from 4.08 to 4.22 meters. The northern coastal waters show the most significant increases, with future wave heights reaching up to 5.65 meters for a 200-year return period. Meanwhile, the southern coastal waters exhibit limited changes, with future wave heights remaining stable between 5.21 and 5.26 meters, reflecting distinct regional response patterns to extreme wave conditions.
Storm surge simulations reveal additional risks. Historical records indicate that storm surges typically range from 10 to 30 cm, with peaks exceeding 40 cm during extreme typhoon events. Future scenarios indicate an increased frequency of extreme surges surpassing 40 cm, with localized peaks exceeding 50 cm, compounding risks when combined with large waves. These findings provide a scientific foundation for developing a joint probability distribution model for wave heights and storm surges, leading to the establishment of a Coastal Impact Index (CII) that quantifies the combined effects of waves and storm surges during typhoons.
How to cite: Fan, Y.-M.: Typhoon-Induced Waves and Storm Surges along Taiwan’s Coast: Historical Trends and Future Projections, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2104, https://doi.org/10.5194/egusphere-egu25-2104, 2025.
Projections of regional sea-level change based on CMIP simulations are essential for coastal management, adaptation, and mitigation of impacts, as they serve as the foundation for producing policy-relevant reports like those from the IPCC (Intergovernmental Panel on Climate Change). Although evaluating future estimates of sea-level change against observations is important for asserting confidence in projected values, this has only been done for global-mean sea-level change and multiple tide gauge locations. In this study, we evaluate projections of regional sea-level change for the entire range of satellite altimetry measurements. Projections can only be evaluated against observations when a significant length of data has been collected. For instance, the latest IPCC assessment report (AR6) provides projections from 2020, which leaves only five years of data for comparison with recent observations up to 2025. Therefore, we turn to the previous IPCC report (AR5), based on CMIP5, as this allows for a longer overlapping period (2007–2022). We evaluate the IPCC AR5 projections of total sea-level change and its two main components—mass changes due to melting land ice and sterodynamic changes—against satellite altimetry, gravity measurements, and ocean reanalysis products, respectively. Over short periods and on regional scales, the large internal climate variability in observed sea levels obscures the underlying trend, hindering model comparisons. To address this, we use three methods to reduce internal climate variability in the observations before comparing them to the projections: low-frequency component analysis, multivariate regression analysis, and self-organizing maps. Error metrics indicate that low-frequency component analysis performs best at reducing internal climate variability, and we proceed with this method. The projected and observed total sea-level trends agree well for the overlapping period: in 98% of the ocean area, satellite observations fall within one standard deviation of AR5 projections for the middle-of-the-road emissions scenario (RCP4.5). We find that, in general, the AR5 projections underestimate total and mass-driven sea-level change, while they overestimate the sterodynamic component. The largest discrepancies occur in regions with strong ocean currents, such as the Kuroshio Current and the Southern Ocean, where models appear to inadequately capture sea-level trends. Overall, our results provide confidence in future sea-level trends as estimated in AR5, particularly over longer time periods and broad regions.
How to cite: Malagón-Santos, V., Camargo, C., Scheen, J., Oerlemans, B., and Slangen, A.: Comparing projected regional sea-level change from CMIP5 against observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12966, https://doi.org/10.5194/egusphere-egu25-12966, 2025.
To address shortcomings of global climate models (CMIP6) and project the impacts of climate change in the Atlantic Ocean with focus on regional scales, we downscale globally a 4-member ensemble of future climate projections (2 emission scenarios and 2 global climate models) with a NEMO-ERSEM coupled hydrodynamic-ecosystem ocean model. In our global ocean downscaling of climate projections, the resolution for the ocean is increased to ¼ of a degree and the river runoffs are represented more realistically in an attempt to better resolve the shelf break and the exchange of water between the shelf break and the deep ocean, as well as the influence of this exchange on both large and regional scales. Here, using the projections from our global ocean downscaling we investigate the cross shelf-open ocean volume, heat and salt transports along the Atlantic Ocean margins and how they may change in the near future (up to year 2070). We also present preliminary analysis for the role of changes in the large-scale circulation patterns versus changes in the regional water content/properties (e.g., heat and salt) on driving changes in the shelf-open ocean exchange, with a particular interest in the North Atlantic western boundary region and the Gulf Stream.
How to cite: Katavouta, A., Holt, J., Artioli, Y., and de Mora, L.: Investigating climate-induced changes in shelf – open ocean exchange in the Atlantic using global ocean downscaling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13538, https://doi.org/10.5194/egusphere-egu25-13538, 2025.
Sterodynamic sea level (SdynSL) is an essential component of sea level changes that climate models can simulate directly. Here we untangle the impact of intermodel uncertainty, internal variability, and scenario uncertainty on SdynSL projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) models. At global scale, intermodel (scenario) uncertainty reigns before (after) ~2070, but internal variability is negligible. At regional scale, intermodel uncertainty is the largest contributor (50~100%), internal variability is secondary (20~50%) in the Indian Ocean and tropical Pacific in the near term and midterm. Scenario uncertainty is negligible until emerging in the long term. The anthropogenic signal of global mean SdynSL emerges from the beginning of this century relative to the 1971-2000 climatology. However, only 70% of the ocean can detect anthropogenic SdynSL signals until the long term. Assuming that model differences are eliminated, anthropogenic SdynSL signals will emerge 38 almost globally after the midterm.
How to cite: Jin, C., Liu, H., Lin, P., Lyu, K., and Li, Y.: Uncertainties in the projection of sterodynamic sea level in CMIP6 models , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6379, https://doi.org/10.5194/egusphere-egu25-6379, 2025.
How much heat is pumped into the interior of the oceans directly affects projected future warmings of the atmosphere and surface climate, with both global and regional implications. Kuroshio-Oyashio Extension region (KOE) influences the local marine ecosystems and is part of the North Pacific decadal variation systems, it also tele-connects with the North American weather and is a key projection indicator for the marine heatwaves. For a more reliable understanding and future projection of the future climate and extreme events in the North Pacific, it is important to predict potential future spatial and temporal warming patterns in KOE more accurately. The future KOE warming pattern and warming mechanisms are analysed, with future scenario simulation by skillful high-resolution coupled model FGOALS-f3-H, compared with low-resolution model FGOALS-f3-L. Results show that high-resolution models simulate a future deep, strong warming reaching 600 m in the Kuroshio-Oyashio region, while the warming in low-resolution models is only above 300 m. Deep warming includes two spatial parts, one in the south of Kuroshio, which is contributed by heaving, and one in the north around Oyashio which is contributed by the northward movement of the subtropical gyre. The skill of high-resolution models to simulate future deep warming is co-contribute by the improvements in the ability to realistically capture the Kuroshio Extension current, with its meridional position, sharp front as well as large horizontal current speed, and mixed layer depth with mesoscale eddies effects.
How to cite: An, B., Yu, Y., Hewitt, H., Wu, P., and Furtado, K.: Contrasting deeper ocean responses around the Kuroshio-Oyashio Extension in high and low-resolution coupled climate models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7841, https://doi.org/10.5194/egusphere-egu25-7841, 2025.
In the context of global warming, understanding the effects of rising ocean surface temperatures on ocean stratification and mixing is essential. Recent research challenges the traditional notion of a linear relationship between surface warming and increased stratification, raising critical questions about the ocean's interior response to climate forcing and future climate change.
This study addresses these questions by analyzing upper-ocean stratification using historical simulations and two different warming scenarios from a representative set of CMIP6 models. We characterize the ocean's vertical structure by applying the sharp homogenization/diffusive retreat (SHDR) algorithm to fit vertical density profiles with an analytical one-dimensional model. This approach integrates various stratification metrics, including mixed layer depth, pycnocline amplitude, and slope, providing a comprehensive representation of the upper ocean.
Our analysis reveals critical gaps in the representation of ocean stratification in historical simulations, including an asymmetric bias in the main pycnocline slope between hemispheres. Building on these findings, we examine how stratification responds and evolves under SSP2-4.5 and SSP5-8.5 scenarios, addressing these discrepancies within the context of a warming climate. These insights contribute to our understanding of ocean stratification dynamics and their implications for future climate projections.
How to cite: Vallès Casanova, I., Somavilla, R., Naveira Garabato, A., González Pola, C., and Fernández Diaz, J.: Exploring ocean stratification in CMIP6 models: biases and evolution in a warming world, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13527, https://doi.org/10.5194/egusphere-egu25-13527, 2025.
Due to rising global air and sea temperatures, the Antarctic ice shelves and sheet is expected to melt more rapidly in the future, resulting in an increasing freshwater input into the Southern Ocean. The additional freshwater input could affect the MOC (Meridional Overturing Circulation), which extends over the oceans and distributes heat, carbon, and biogeochemical components globally. The majority of the global coupled climate models have fixed ice sheets, so the missing ice dynamics of the Antarctic ice sheet represent a key uncertainty in their future projections. In this study we investigate how additional freshwater input from the Antarctic ice sheet affects the upper cell of the global MOC.
For this purpose, we analyze output from the Southern Ocean Freshwater Input from Antarctica (SOFIA) initiative to investigate the change in the upper cell of the MOC. This analysis is carried out with regard to different freshwater inputs and different models based on the calculation of the stream function, intending to gain a better understanding of the changing of the upper cell MOC in response to Antarctic meltwater.
How to cite: Wagner, J., Brown, R., and Haumann, A.: The influence of freshwater input in the SOFIA simulations on the upper cell of the global MOC, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13960, https://doi.org/10.5194/egusphere-egu25-13960, 2025.
Marine Protected Areas (MPAs) are vital for preserving marine biodiversity and supporting sustainable resource management. However, the intensifying impacts of future temperature extremes, such as marine heatwaves (MHWs), pose significant risks to marine ecosystems. This study utilizes high-resolution climate projections under the SSP585 scenario (AWI-CM3) to examine 21st century changes in MHWs, defined as extreme deviations from the seasonally varying long-term temperature trend, within global MPAs, focusing on the surface to a depth of 300 meters. Our findings reveal that, while the frequency of MHWs is expected to decline, their intensity and duration are projected to increase, exacerbating accumulative impacts. On average, accumulative intensity is anticipated to rise by approximately 30 %, surpassing the global average increase of 25%. Notably, substantial increases are identified in mid- and high-latitude MPAs below the surface, while in the surface layer, such increases are observed in tropical MPAs. These results highlight the urgent need to understand how marine organisms and habitats will adapt to escalating heat stresses. Such insights are critical for ensuring the resilience and sustainability of marine ecosystems in the face of future climate challenges.
How to cite: Cho, E. B., Kwon, E. Y., Timmermann, A., Jung, T., Semmler, T., Hegewald, J., and Lee, S.-S.: Intensifying Heat Stresses in Marine Protected Areas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14882, https://doi.org/10.5194/egusphere-egu25-14882, 2025.
The possibility of large and potentially irreversible changes in the AMOC, with potentially significant impacts on global and regional climate, are currently being debated. Whilst many studies have been conducted investigating temperature and salinity changes associated with AMOC variability, feedbacks related to the in-situ density/pressure gradient have not thus far been studied in detail. We identify and examine large changes in the AMOC on timescales up to centennial in the CMIP6 multimodel climate model ensemble (preindustrial, historical, and future climate) and historical observations. Our specific focus is on constructing budgets of in-situ density at the continental boundaries in order to understand the mechanisms leading to permanent reduction of the transbasin zonal density/pressure gradient required to maintain the AMOC. We further investigate whether and how reduction in the AMOC impacts boundary densities, and whether positive feedback loops resulting in rapid change can occur.
How to cite: Sinha, B. and Grist, J.: Dynamics of large changes in the Atlantic Meridional Overturning Circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20518, https://doi.org/10.5194/egusphere-egu25-20518, 2025.
