Examining the wave climate under climate change scenarios requires a concurrent analysis of both historical and predicted future wave data. This involves using historical wave data to understand seasonal fluctuations and long-term trends, while also utilizing future wave data to predict waves under diverse climate change scenarios. This information is pivotal for evaluating forthcoming risks and formulating strategies for climate change adaptation. This study employs historical wind field data, including ERA5 reanalysis data and CWB/WRF analysis field data, as well as wind field data from the CMIP6 dataset under the SSP5-8.5 extremely high emission scenario. These data are used to drive the WAVEWATCH III wave model for simulating waves. This study initially compared the simulated wave data from the WAVEWATCH III wave model with one year of observed wave data from met-ocean buoys. The results confirmed the high credibility of the simulated waves. Subsequently, extensive data simulations are conducted, encompassing historical wave data (1975-2022) and projected wave data for the future (2025-2100).

This study delves into the long-term temporal variations in wave height in Taiwanese waters and the differential regional trends in spatial changes. Regarding temporal changes, the wave heights are averaged year by year, and then linear regression is performed in units of years. The slope of the regression equation indicates the long-term linear trend of wave height in Taiwanese waters over the years, revealing an increasing trend from the past to the future. Regarding spatial changes, the average wave height at each grid point is calculated, and linear regression is applied to determine the long-term trends in wave height at each grid point from the past to the future. The findings unveil a positive growth trend in Taiwanese waters. Furthermore, Taiwanese watersexperience distinct weather patterns in each season, such as the influence of the northeasterly monsoon in winter and typhoons or southwestern winds in summer. This study further explores the differences and variations of wave during spring (March to May), summer (June to August), autumn (September to November), and winter (December to February of the following year). The analysis results indicate negative growth trends in spring and summer, and positive growth trends are observed in autumn and winter, indicating a noticeable increase in wave height in Taiwanese waters during autumn and winter under the influence of climate change.

How to cite: Fan, Y.-M.: Temporal and spatial varieties of future wave climate under the scenario of climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3018, https://doi.org/10.5194/egusphere-egu24-3018, 2024.

Coupled Model Intercomparison Project (CMIP) projections indicate a distinct future warming pattern in the tropical Pacific, with enhanced warming in the equatorial Pacific (resembling El Niño warming) and subdued warming in the southeast tropical Pacific. There is currently no consensus on the mechanisms shaping this pattern and its inter-model diversity.

Here, we employ the Sea Surface Temperature (SST) heat budget proposed by Zhang and Li (2014, ZL14), adapted to Relative SST (SST minus its tropical average), a proxy for atmospheric stability and circulation changes. This approach helps uncover the mechanisms that shape the tropical Pacific Multi-Model Mean (MMM) warming pattern and its diversity across historical and unmitigated scenario (RCP85 and SSP585) simulations from 53 CMIP5 and CMIP6 models.

We find that the MMM southeast Pacific relative cooling arises from locally intensified winds, leading to increased latent heat flux cooling. This process also explains the inter-model diversity in this region, alongside the diversity of cloud feedbacks.

Consistent with ZL14 conclusions, our results underscore that the MMM equatorial Pacific relative warming results from a less efficient evaporative cooling feedback over the climatologically cooler central and eastern Pacific. However, our study highlights a pivotal role of ocean dynamics in driving the equatorial Pacific relative warming inter-model diversity. In the eastern Pacific, this diversity is related to the cold tongue bias, with a stronger cold tongue bias leading to a more efficient thermostat mechanism that dampens the MMM relative warming. In the western Pacific, diversity is related to the intensity of the equatorial trade winds relaxation, with stronger westerly anomalies leading to enhanced warming, suggesting a strong role of the Bjerknes feedback.

These results advocate for more comprehensive studies using dynamical approaches to better understand the respective roles of the Bjerknes feedback and cold tongue bias in the equatorial Pacific warming pattern and, ultimately, in the Walker Circulation changes.

How to cite: Danielli, V., Lengaigne, M., Kwatra, S., Suresh, G., and Vialard, J.: Crucial role of ocean dynamics for the CMIP models equatorial Pacific warming pattern diversity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1969, https://doi.org/10.5194/egusphere-egu24-1969, 2024.

Here, we analyze projected tropical sea surface salinity (SSS) changes in 32 Coupled Model Intercomparison Projects phase 6 (CMIP6) global climate models historical simulations and representative concentration pathway 8.5 (SSP5-8.5) scenario. A robust “fresh gets fresher” pattern emerges by the end of the twenty-first century, with fresher tropical Indian and Pacific Oceans and saltier tropical Atlantic Ocean. We examine the inter-model diversity in this pattern using Empirical Orthogonal Function (EOF) analysis. The first two EOFs explain 45% of the total variance. EOF2 (22%) is a modulation of the multi-model mean SSS change, associated with the tropical-average warming intensity (r=0.61). Higher climate sensitivity leads to a more pronounced El Niño-like (positive IOD-like) warming pattern and stronger rainfall in the equatorial and north subtropical Pacific (west Indian) Ocean, leading to local freshening. In the equatorial Atlantic, an enhanced warming leads to more evaporation through the Clausius–Clapeyron relation, and a stronger SSS saltening. The “fresh gets fresher” SSS pattern inter-model diversity is thus more a response to the SST pattern diversity through the “warmer gets wetter” mechanism than an evidence of the “wet gets wetter” intensification of the hydrological cycle. EOF1 (25%) is characterized by saltening in the Indian Ocean and freshening in the Pacific Ocean, associated with changes in the inter-hemispheric relative SST gradient (r=0.55). Enhanced warming in the south hemisphere shifts the precipitation south, reducing total rainfall and saltening the Indian Ocean, while increasing rainfall and freshening the south Pacific Ocean. Overall, we find a strong influence of SST changes on the rainfall distribution, which influences SSS with some effects related to transport by the oceanic circulation.

How to cite: Pang, S., Vialard, J., Lengaigne, M., and Wang, X.: Understanding CMIP6 Inter-model Spread of Projected Change in Tropical Sea Surface Salinity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6654, https://doi.org/10.5194/egusphere-egu24-6654, 2024.

Future projections indicate the Atlantic Meridional Overturning Circulation (AMOC) will weaken and shoal in response to global warming, but models disagree widely over the amount of weakening. We analyse projected AMOC weakening in 34 CMIP6 climate models, in terms of changes in three return pathways of the AMOC. The branch of the AMOC that returns through diffusive upwelling in the Indo-Pacific, but does not later upwell in the Southern Ocean (SO), is particularly sensitive to warming, in part, because shallowing of the deep flow of the AMOC prevents it from entering the Indo-Pacific via the SO. In most models, this Indo-Pacific pathway declines to zero by 2100. Thus, the present-day strength of this pathway provides a strong constraint on the projected AMOC weakening. However, estimates of this pathway using four observationally based methods imply a wide range of AMOC weakening under the SSP5-8.5 scenario of 29%–61% by 2100. Our results suggest that improved observational constraints on this pathway would substantially reduce uncertainty in 21st century AMOC decline. We also present new findings that compare the AMOC response in realistic warming scenarios with those found under more extreme climate forcings, including quadrupled CO2 concentrations and large North Atlantic freshwater forcing.

How to cite: Baker, J., Bell, M., Jackson, L., Renshaw, R., Vallis, G., Watson, A., and Wood, R.: Overturning Pathways Control AMOC Weakening in CMIP6 Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3144, https://doi.org/10.5194/egusphere-egu24-3144, 2024.

Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO2 experiments conducted underthe 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, first from the Labrador Sea and then theNordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of theseparated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the currentweakens in response, reducing the inflow to the deepwater production regions. The AMOC cell weakens in tandem, firstnear the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perceptionthat freshwater caps the convection regions, stifling deepwater production; rather, it is the inflow to the subpolar gyre thatis suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local orremote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is theprecursor to AMOC weakening in these simulations.

How to cite: Madan, G., Gjermunsen, A., Iversen, S. C., and LaCasce, J. H.: The weakening AMOC under extreme climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4017, https://doi.org/10.5194/egusphere-egu24-4017, 2024.

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system, which keeps the North Atlantic relatively warm. It is predicted to weaken under climate change. The AMOC may have a threshold beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models.

We present results from the North Atlantic hosing model intercomparison project, where we applied an idealised forcing of a freshwater flux over the North Atlantic in 8 CMIP6 models to explore this threshold. The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in some models, and in others it stays in a weakened state. We will discuss mechanisms behind the different behaviour in the different models. 

How to cite: Jackson, L., Eduardo, A. D. A., Katinka, B., Gokhan, D., Helmuth, H., Aixue, H., Johann, J., Warren, L., Virna, M., Oleg, S., Andrew, S., and Didier, S.: AMOC thresholds in CMIP6 models: NAHosMIP, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3561, https://doi.org/10.5194/egusphere-egu24-3561, 2024.

Changes in surface freshwater fluxes are a main factor governing the response of the ocean circulation to future climate change. However, they are not well-represented in the most recent generation of Earth System Models (CMIP6), as most CMIP6 models do not include an interactive ice sheet component. Instead, most of them use a very idealized representation of ice sheets. While this approach may yield the correct order of magnitude for present-day meltwater runoff, it might not accurately extrapolate the increasing ice melt under future global warming.

Here, we address this deficiency by prescribing physically plausible meltwater fluxes from the Greenland ice sheet in a CMIP6 model, EC-Earth3, under a strong global warming scenario (SSP5-8.5) until the 23rd century. The meltwater fields were obtained from a CESM2-CISM simulation in which the Greenland ice sheet was fully coupled. The corresponding meltwater flux reaches about 0.4 Sv by the year 2300, comparable to what is often used in water hosing experiments. Using two EC-Earth ensembles of four members each (with and without Greenland meltwater flux), we compare the impact of this previously underestimated runoff on long-term projections of deep-water formation in the North Atlantic and on the evolution of the Atlantic Meridional Overturning Circulation. Our results allow us to quantify the importance of Greenland meltwater on AMOC weakening under strong global warming.

How to cite: Mehling, O., Bellomo, K., Fabiano, F., Devilliers, M., von Hardenberg, J., and Corti, S.: The impact of Greenland ice sheet melt on the future North Atlantic ocean circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11529, https://doi.org/10.5194/egusphere-egu24-11529, 2024.

We aim at reducing the uncertainties in the climate predictions of the Arctic region which is going under rapid changes with global repercussions. We analyse the spread in the Atlantic water core temperature across multi member CMIP6 historical simulations, focusing on different regions of the Arctic Ocean. While the redistribution of heat plays a critical role in the dynamics of the Arctic Ocean basins, it is usually not well represented in climate models, leading to divergent projections of future changes in the Arctic. To address this limitation, we compare CMIP6 model outputs with available reanalysis and observational products, in order to identify the biases within the model simulations and develop new metrics to constrain the model ensemble spread. Such metrics can be used to select the multi model ensemble members and construct a subsample with improved representation of the core temperature evolution over the historical period resulting in a reduced uncertainty in near-term future projections of the Arctic climate.

How to cite: Devilliers, M., Olsen, S. M., Yang, S., Langehaug, H. R., Tian, T., Guo, C., and Mahmood, R.: Constraining CMIP6 model ensemble spread to reduce uncertainty in the representation of the Atlantic water layer temperature in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11017, https://doi.org/10.5194/egusphere-egu24-11017, 2024.

Oceanic transports of heat, volume and salinity are an integral part of the Earth's energy and mass budgets and play a key role in regulating the Earth's climate. Changes in the ocean’s transport patterns may affect regional as well as global climates. Accurate monitoring is critical and there are several regional measuring lines like the RAPID 26N and OSNAP (Overturning in the Subpolar North Atlantic Program) array, as well as measuring lines across Arctic water straits, which are equipped with moorings and other advanced measuring systems. It is desirable to compare the transports calculated by these instruments with ocean reanalyses and climate models. However, this is challenging because the moorings are not aligned with the model grids, and the ocean model grids get complicated especially towards more northern latitudes.

To address this challenge, we introduce StraitFlux (https://pypi.org/project/straitflux/), a versatile tool enabling precise and mass-consistent calculation of volume, heat, and salinity transports across any oceanic section. We have used StraitFlux to calculate transports from reanalyses and climate models (CMIP6) in the Arctic region and to compare them to available observations. While we find some biases, especially in straits that are narrow and bathymetrically complicated, the results generally show that reanalyses capture the main current patterns quite well. Climate models on the other hand exhibit larger and often systematic deviations from the mooring and reanalysis output. The spread among climate models is 3-5 times larger than the spread between observation-based transports and reanalyses or among reanalyses, and it cannot be explained by natural variability. The large spread in flux quantities is related to mean-state biases in relevant state quantities. It helps to quantify and understand the strong connections between lateral OHT and the mean state as well as changes in the Arctic Ocean and sea ice.

Expanding on our methodology, we develop physically based metrics tailored to the Arctic, to detect outliers from the CMIP6 model ensemble and constrain model projections using a weighting approach incorporating the models’ performance and independence. This effectively reduces the spread of future projections of Arctic change. Further, using StraitFlux, we investigate constrained changes in Arctic volume, heat, and salinity transports for the main SSP scenarios. We examine cross-sections of the main Arctic gateways to assess future changes in the structures and strengths of the main currents and their effects on the Arctic system.

How to cite: Winkelbauer, S., Mayer, M., and Haimberger, L.: New Arctic quality metrics based on oceanic transports for CMIP6, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4310, https://doi.org/10.5194/egusphere-egu24-4310, 2024.