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Current climate models still have considerable biases in the simulation of the East Asian summer monsoon (EASM), which in turn reduces their reliability of monsoon projections under global warming. We hypothesize that a higher-resolution coupled climate model with atmospheric and oceanic components at horizontal resolutions of 0.25° and 0.1°, respectively, will better capture regional details and extremes of the EASM. Present-day (PD), 2 × CO2 and 4 × CO2 simulations are conducted with the Community Earth System Model (CESM1.2.2) to evaluate PD simulation performance and quantify future changes. Indeed, our PD simulation well reproduces the climatological seasonal mean and intra-seasonal northward advancement of the monsoon rainband, as well as climate extremes. Compared with the PD simulation, the perturbed CO2 experiments show an intensified EASM response to CO2-induced warming. We find that the precipitation increases of the Meiyu-Baiu-Changma band are caused by comparable contributions from the dynamical and thermodynamical components in 2 × CO2, while they are more driven by the thermodynamical component in 4 × CO2 due to stronger upper atmospheric stability. The regional changes in the probability distribution of the temperature show that extreme temperatures warm faster than the most often temperatures, increasing the skewness. Fitting extreme precipitation values with a generalized Pareto distribution model reveals that they increase significantly in 4 × CO2. Changes of temperature extremes scale with the CO2 concentrations over the monsoon domain but not for precipitation extreme changes. The 99th percentile of precipitation over the monsoon region increases at a super Clausius-Clapeyron rate, ~ 8% K–1, which is mainly caused by increased moisture transport through anomalous southerly winds.

Linear and nonlinear response functions (RF) are extracted for the climate system and the carbon cycle represented by the MPI-ESM and cGENIE models, respectively. Appropriately designed simulations are run for this purpose. Joining these RFs, we have a climate emulator with carbon emissions as the forcing and any desired observable quantity (provided the data is saved), such as the surface air temperature or precipitation, as the predictand. Like e.g. for atmospheric CO2 concentration, we also have RFs for the solar constant as a forcing — mimicking solar radiation management (SRM) geoengineering. We consider two application cases. 1. One is based on the Paris 2015 agreement, determining the necessary least amount of SRM geoengineering needed to keep the global mean surface air temperature below a certain threshold, e.g. 1.5 or 2 [oC], given a certain amount of carbon emission abatement (ABA) and carbon dioxide removal (CDR) geoengineering. 2. The other application considers the conservation of the Greenland ice sheet (GrIS). Using a zero-dimensional simplification of a complex ice sheet model, we determine (a) if we need SRM given some ABA and CDR, and, if possible, (b) the required least amount of SRM to avoid the collapse of the GrIS. Keeping temperatures below 2 [oC] even is hardly possible without sustained SRM (1.); however, the collapse of the GrIS can be avoided applying SRM even for moderate levels of CDR and ABA, an overshoot being affordable (2.).

In the past 5 years large efforts have been made to improve our understanding of scale-interactions in the Earth system, and to better resolve atmospheric and oceanic meso-scale processes and their response to greenhouse warming. Here, we provide an overview of the technical and scientific achievements of a new collaboration between the IBS Center for Climate Physics (South Korea) and the Alfred Wegener Institute for Polar and Marine Research (Germany) to simulate the climate system at km-scale resolution using the AWI Climate Model, version 3 (AWI-CM3). AWI-CM3 is based on the OpenIFS-FESOM2 coupled model and we conducted several control simulations and transient greenhouse warming runs in a medium-resolution (MR) configuration (31 km in the OpenIFS and 5~27 km in the FESOM2, ‘Tco319-DART’). These simulations will be used in future as initial conditions for shorter coupled storm-resolving (SR) simulations with target resolutions of 9 km and 4 km (Tco1279 and Tco2559). Our presentation focuses on the performance of the MR configuration (with a throughput of about 7 simulation years per day on 350 nodes) and its representation of the mean climate, climate variability such as the El Niño-Southern Oscillation, tropical cyclone statistics. We will also present preliminary estimates of the expected scaling behavior of the AWI-CM3 SR configuration on different multi-Petaflop supercomputing systems.

Tropical cyclones (TCs), the generic name for typhoons, are among the most destructive natural hazards. It is important to understand how climate change affects TC frequency, to minimize human and economic losses. Some studies have suggested that the number of TCs will decrease, and their formation will shift poleward. However, there is a major lack of fundamental understanding of the origin and development of the TCs from the initial precursory vortex, called a TC seed. The changes in the number of TC seeds and their survival rate (i.e., the proportion that successfully develops into TCs) will eventually control the future TC frequency. However, key challenges are mainly due to a lack of consensus in TC seed definition and a lack of computing resources for TC modeling.This study aims to meet the above challenges, through the following three tasks: (1) to identify the representation of TC seeds based on three different definitions from early-stage to matured stage; (2) to investigate the future changes in TC seeds and survival rate and their contribution to the poleward shift in TC genesis location; and (3) to unravel the physical mechanisms responsible for the change in response to climate change. We use a high-resolution fully-coupled Community Earth System Model (CESM) with an atmospheric resolution of 0.25° and an ocean resolution of 0.1° with present-day, doubling, and quadrupling CO2 concentrations. Our results show TC seeds defined by early-stage definition show more equatorward distribution with a strong connection to vertical velocity than those defined by matured stage. Interestingly, all three definitions exhibit a statistically significant reduction in the frequency of TC seeds while that in survival rate is not significant. Details of the methods and mechanisms will be further discussed during the presentation. The outcomes of this project will strengthen fundamental scientific knowledge of the TC seeds and their future change mechanisms, as well as provide a scientific basis for future risk assessment and precautionary strategies.

“Decadal influence” on the El Nin ̃o–Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection has been much studied but with plurality and ambiguity about the concept of influence. Here (I) we provide formal definitions of the apparent influence of a specific factor on the teleconnec- tion, as correlation coefficients, which enable us to test them as null-hypotheses. Using the recently released Community Earth System Model v2 Large Ensemble data, we show that a 50% chance for the detection of the apparent Indian Ocean influence under stationary conditions might take 2000 years of data. However, (II) we find that this influence is mostly apparent, causally not dominant, indeed, as it originates more dominantly from fluctuations of the decadal apparent moving window – as opposed to stationary climatological – ENSO variability, which causally influences an Indian Ocean Dipole-like apparent mean state. (III) We also show that unattributed so-called “decadal influence”, defined as a deviation from a linear regression model of the teleconnection as a null-hypothesis, cannot be detected in 20th c. observations in South Asia even regionally. However, as a new insight, we find our new simple influence concept as a significant correlation coefficient to be possibly consistent with such a null-hypothesis.

Annually, marine phytoplankton convert approximately 50 billion tons of dissolved inorganic carbon to particulate and dissolved organic carbon, a portion of which is exported to depth via the biological carbon pump. Despite its important roles in regulating atmospheric carbon dioxide via carbon sequestration and in sustaining marine ecosystems, model-projected future changes in marine net primary production are highly uncertain even in the sign of the change. Here, using an Earth system model, we show that frugal utilization of phosphorus by phytoplankton under phosphate-stressed conditions can overcompensate the previously projected 21st century declines due to ocean warming and enhanced stratification. Our results, which are supported by observations from the Hawaii Ocean Time-series program, suggest that nutrient uptake plasticity in the subtropical ocean plays a key role in sustaining phytoplankton productivity and carbon export production in a warmer world.

“Decadal influence” on the El Nin ̃o–Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection has been much studied but with plurality and ambiguity about the concept of influence. Here we provide formal definitions of the apparent influence of a specific factor on the teleconnection, as correlation coefficients, which enable us to test them as null-hypotheses. Using the recently released Community Earth System Model v2 Large Ensemble data, we show that a 50% chance for the detection of the apparent Indian Ocean influence under stationary conditions might take 2000 years of data. However, we find that this influence is mostly apparent indeed, as it originates from fluctuations of the decadal apparent moving window – as opposed to stationary climatological – ENSO variability, which causally influences an Indian Ocean Dipole-like apparent mean state. We also show that unattributed so-called “decadal influence”, defined as a deviation from a linear regression model of the teleconnection as a null-hypothesis, cannot be detected in 20th c. observations in South Asia even regionally. However, as a new insight, we find our new simple influence concept as a significant correlation coefficient to be possibly consistent with such a null-hypothesis.

The current generation of climate models does not properly resolve oceanic mesoscale processes in tropical oceans, such as tropical instability waves. The associated deficit in explicit vertical and lateral heat exchange can further contribute to large-scale equatorial temperature biases, which in turn impact the representation of the El Nino/Southern Oscillation (ENSO) and its sensitivity to greenhouse warming. Here, using a mesoscale-resolving global climate model with an improved representation of tropical climate, we show that a quadrupling of atmospheric CO2 causes a robust weakening of future simulated ENSO sea surface temperature variability. This sensitivity is caused mainly by stronger latent heat flux damping and weaker advective feedbacks. Stratification-induced weakening of tropical instability wave activity and the corresponding growth of ENSO instability partly offset the effect of other negative dynamical feedbacks. Our results demonstrate that previous lower-resolution greenhouse warming projections did not adequately simulate important ENSO-relevant ocean mesoscale processes. High-resolution climate models exhibit reduced tropical Pacific mean-state biases due to better representation of ocean mesoscale processes, like tropical instability waves. With climate warming, these improved dynamics project weaker El Nino/Southern Oscillation sea surface temperature variability.

The COVID-19 pandemic caused disruptions of public life and imposed lockdown measures in 2020 resulted in considerable reductions of anthropogenic aerosol emissions. It still remains unclear how the associated short-term changes in atmospheric chemistry influenced weather and climate on regional scales. To understand the underlying physical mechanisms, we conduct ensemble aerosol perturbation experiments with the Community Earth System Model, version 2. In the simulations reduced anthropogenic aerosol emissions in February generate anomalous surface warming and warm-moist air advection which promotes low-level cloud formation over China. Although the simulated response is weak, it is detectable in some areas, in qualitative agreement with the observations. The negative dynamical cloud feedback offsets the effect from reduced cloud condensation nuclei. Additional perturbation experiments with strongly amplified air pollution over China reveal a nonlinear sensitivity of regional atmospheric conditions to chemical/radiative perturbations. COVID-19-related changes in anthropogenic aerosol emissions provide an excellent testbed to elucidate the interaction between air pollution and climate.

“Decadal influence” on the El Nino–Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection has been much studied but with plurality and ambiguity about the concept of influence. Here we provide formal definitions of the apparent influence of a specific factor which enable us to test them as null-hypotheses. Using the recently released Community Earth System Model v2 (CESM2) Large Ensemble (LE) data, we show that a 50% chance for the detection of the apparent Indian Ocean (IO) influence under stationary conditions might take 2000 years of data. However, we find that this influence is mostly apparent indeed, as it originates from fluctuations of the decadal apparent – as opposed to climatological – ENSO variability, which causally influences an IO dipole-like apparent mean state. We also show that no unattributed so-called “decadal influence”, reflected in a deviation from a linear regression model of the teleconnection as a null-hypothesis, can be detected in 20th c. observations even regionally. However, as a new insight, we find our new influence concept to be possibly consistent with such a null-hypothesis.

Ocean eddies, which present different properties to their surroundings, play pivotal roles in transporting heat, salt, organic carbon, and nutrients around the ocean, ending up controlling regional and global climate. Eddy kinetic energy (EKE), defined as the kinetic energy of the time-varying component, is one of the most crucial indicators for observing the upwelling and downwelling induced by ocean eddies. We aim to understand the future changes in ocean eddy activities and find the possible cause of them using an ultra-high-resolution climate simulation of CESM 1.2.2, with about 25 km horizontal resolution and 30 vertical levels in the atmosphere, and about 10 km horizontal resolution and 62 levels in the ocean, under different levels of greenhouse gas conditions: Present-day run (PD, fixed CO2 concentration of 367 ppm), Doubling CO2 run (2xCO2, 734 ppm), Quadrupling CO2 run (4xCO2, 1468 ppm). Model simulation shows that compared to PD, the global EKE will increase about 6.7 % and 14.7 % in 2xCO2 and 4xCO2, respectively, but with the nonuniformed spatial distributions. The results show that the EKE increases about 12.5 % in 2xCO2 and decreases about 0.5 % in 4xCO2 in the Kuroshio Current region. In contrast, it decreases about 4.8 % (22.5 %) in 2xCO2 (4xCO2) in the Gulf Stream region. To find the underlying processes for the EKE change, we focus on identifying future changes in the energetics of eddy-mean flow interactions. Based on the energetics of eddy-mean flow interaction, the strengthened barotropic conversion will enhance the EKE in 2xCO2 over the Kuroshio Current region. Otherwise, the suppression of buoyancy flux will weaken the EKE in 2xCO2 and 4xCO2 over the Gulf Stream region.

A merged biomass burning aerosol (BBA) emission dataset of satellite observations with fire proxies and fire models has been used in the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. Although this utilizes best estimates of fire emissions based on available observations, it results in inconsistency in interannual variability of BBA forcing in CMIP6 between the period of satellite-based fire emissions (1997-2014) and the periods before and after. Using the Community Earth System Model version 2 Large Ensemble (CESM2-LE) simulations, we identify rectified multiyear mean climate responses to interannually varying BBA emissions. The comparison of 50 ensemble members forced by high BBA variability with 50 members by low BBA variability over a limited time domain provides a unique opportunity to identify a forced climate response to interannual fluctuations of fire emissions with high fidelity. While mean aerosol emissions are nearly conserved between the two sets of ensembles, there is detectable warming in northern high latitudes with regionally distinct seasonal changes in response to variable emissions. We find that the multiyear warming occurs in concert with a net loss of soil ice and moisture in addition to a loss of Arctic sea ice. Our results suggest that the magnitude of interannual variability of aerosol emissions can act as climate forcing over multiple years through nonlinear interactions with the cryosphere and soil processes.

The interpretation of East Asian monsoon speleothem δ18O records is heavily debated in the paleoclimate community. Besides developing new speleothem proxies, the use of isotope-enabled climate simulations is one of the key tools to enhance our understanding of speleothem δ18O records. Here we present results from novel climate simulations performed with the fully coupled isotope-enabled Community Earth System Model (iCESM1.2), which simulates global variations in water isotopes in the atmosphere, land, ocean, and sea ice. The model closely captures the major observed features of the isotopic compositions in precipitation over East Asia for the present-day conditions. To better understand the physical mechanisms causing interannual to orbital timescale variations in δ18O in East Asian speleothems, we ran a series of experiments with iCESM. We perturbed solar, orbital, bathymetry, ice-sheet, and greenhouse gas radiative forcings. The simulations supporting of observations/reconstructed records (GNIP/SISAL) from East Asia, help understand the controls on the isotope composition of East Asian monsoon rainfall and how speleothem δ18O records may be interpreted in terms of climate. The study provides new insights into the mechanisms of East Asian monsoon changes on different timescales.

“Decadal influence” on the El Nino--Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection have been much studied but with plurality and ambiguity about the concept of influence. We provide formal definitions of the apparent influence of a specific factor which enable us to test them as null-hypotheses. Using the recently released Community Earth System Model v2 (CESM2) Large Ensemble (LE) data, we show that a 50% chance for the detection of the apparent Indian Ocean (IO) influence under stationary conditions might take 2000 years of data. However, we find that this influence is mostly apparent indeed, as it originates from fluctuations of the decadal apparent -- as opposed to climatological -- ENSO variability, which causally influences an IOD-like apparent mean state. We also show that no unattributed so-called “decadal influence”, reflected in a deviation from a linear regression model of the teleconnection as a null-hypothesis, can be detected in 20th c. observations even regionally. Only the LE data is sizable enough to reveal this effect.

Rising temperatures and specific humidity are compounding influences that increase heat stress and reduce safe labor capacity. Here, we evaluate reductions in summertime labor capacity using Large Ensemble experiments from two Earth System Models (ESMs). Vulnerable regions, including the Indian subcontinent, Southeast Asia, and West Africa, are expected to begin experiencing 25% reductions as early as the 2040s. Internal climate variability can cloud the timing of such reductions, with differences in onset between ensemble members exceeding 40 years in high-variability locations. At regional scales, onset times are more certain, with differences between ensemble members typically less than 20 years. We demonstrate the benefits for maintaining human labor capacity associated with limiting the increase in global mean surface temperature (ΔGMST) to 1.5°C, consistent with the Paris Agreement. If ΔGMST exceeds 3.5°C, at least 15% of the global population is projected to experience 50% reductions in summertime labor capacity.

This study isolates the influence of sea ice mean state on pre-industrial climate and transient 1850-2100 climate change within a fully coupled global model: The Community Earth System Model versio...

Tropical cyclones (TCs) are extreme storm systems that form over warm tropical oceans. Along their track TCs can mix up cold water which can further impact their development. Due to the adoption of lower ocean model resolutions, previous modeling studies on the TC response to greenhouse warming underestimate such oceanic feedbacks. To address the robustness of TC projections in the presence of mesoscale air-sea interactions, we conduct century-long present-day, CO2 doubling and quadrupling experiments using the Community-Earth-System-Model 1.2.2 with ~25 km atmosphere and ~10 km ocean resolution. In these experiments an overall projected weakening of the rising branch of the Hadley Cells suppresses TC formation in the main genesis regions which weakens the TC-generated ocean cooling. Consistent with lower-resolution coupled modeling studies we find a reduction in global TC frequencies, a poleward shift of fast-moving extratropical TCs and an upsurge in precipitation rates and the intensity of landfalling events.

The COVID-19 pandemic has led to massive disruptions of public life on a global scale. To halt the spread of the disease, China temporarily shut down parts of the manufacturing and transportation sectors. Associated anthropogenic aerosol emissions in February 2020 plunged to record lows, causing a temporary improvement of air quality with uncertain effects on cloud formation, atmospheric radiation and climate. To determine the regional and remote climatic impacts of a wide-spread reduction of anthropogenic aerosol emissions in China in early 2020, we conduct a series of sensitivity experiments with a state-of-the art Earth System Model. By reducing February anthropogenic aerosol emissions by 65%, we find evidence for a substantial increase in cloud droplet size and a corresponding enhancement of low cloud cover over Eastern China which intensifies longwave downwelling radiation at the surface but reduces incoming shortwave fluxes, in accordance with observational data. Remote climatic effects of the improvement in air quality can be further identified over the Korean Peninsula and southern Japan. Even though regional climatic responses were only short-lived, the COVID-19-related human interference with earth’s radiation balance may hold important clues on how the climate system will respond to the implementation of future long-term air-pollution mitigation strategies.

To further improve our understanding of scale-interactions and key mechanisms leading to climate variability and extreme weather events, we conducted a series climate simulations using the ultrahigh-resolution Community Earth System Model (HR-CESM). The HR-CESM configuration uses a horizontal resolution of approximately 25 km for the atmospheric component and 10 km for the ocean. The simulations were carried out under present-day radiative forcing and for a variety of future greenhouse gas scenarios. This presentation highlights some key features of these simulations: the overall performance and important model biases, the representation of the Atlantic Meridional Overturning Circulation, tropical cyclone statistics, properties of sea-ice, the El Niño-Southern Oscillation, as well as their responses to future climate change. We will also present plans on how the data will be shared with the larger scientific community for further analysis and collaborative projects.

Atmospheric rivers (ARs) are narrow, elongated structures, transporting large amounts of water vapor from the tropics towards polar regions. These synoptic scale features play an important role in the global hydrological cycle and for extreme precipitation events. To study how ARs will change in response to greenhouse warming we use a series of century-long fully coupled ultra-high-resolution simulations conducted with CESM 1.2.2 with an approximate horizontal resolution of ~25 km in the atmosphere and 10 km in the ocean. The simulations were carried out for present-day, 2xCO2 and 4xCO2 conditions. In this high atmospheric resolution, we obtain a much more realistic representation of complex orographic features (such as the Rocky Mountains), which can greatly influence the extreme precipitation often associated with ARs. Results from the present-day simulation are compared with ERA-Interim data to validate the model's fidelity in representing ARs. Our analysis focuses on future greenhouse-warming induced changes in AR frequency, geometry, landfalling latitude and strength. We find a global increase in the frequency of ARs amounting to ~0.5% for 2xCO2 and 0.9% for 4xCO2 respectively. In subtropical areas, such as the southwestern part of the United States AR frequencies increase by up to 7%. The presentation will further document the underlying processes for this increase.

An intraseasonal genesis potential index (ISGPI) for Northern Hemisphere (NH) summer is proposed to quantify the anomalous tropical cyclone genesis (TCG) frequency induced by boreal summer intraseasonal oscillation (BSISO). The most important factor controlling NH summer TCG is found as 500-hPa vertical motion (ω500) caused by the prominent northward shift of large-scale circulation anomalies during BSISO evolution. The ω500 with two secondary factors (850-hPa relative vorticity weighted by the Coriolis parameter and vertical shear of zonal winds) played an effective role globally and for each individual basin in the northern oceans. The relative contributions of these factors to TCG have minor differences by basins except for the western North Atlantic (NAT), where low-level vorticity becomes the most significant contributor. In the eastern NAT, the BSISO has little control of TCG because weak convective BSISO and dominant 10–30-day circulation signal did not match the overall BSISO life cycle. The ISGPI is shown to reproduce realistic intraseasonal variability of TCG, but the performance is phase-dependent. The ISGPI shows the highest fidelity when BSISO convective anomalies have the largest amplitude in the western North Pacific and the lowest when they are located over the north Indian Ocean and eastern North Pacific. Along the NH major TCG zone, the TCG probability changes from a dry to a wet phase by a large factor ranging from 3 to 12 depending on the basins. The new ISGPI for NH summer can simulate more realistic impact of BSISO on TC genesis compared to canonical GPI derived by climatology.

Eastward propagation is an essential characteristic of the Madden–Julian oscillation (MJO). Yet, simulation of MJO propagation in general circulation models (GCMs) remains a major challenge and understanding the causes of propagation remains controversial. The present study explores why the GCMs have diverse performances in MJO simulation by diagnosis of 24 GCM simulations. An intrinsic linkage is found between MJO propagation and the zonal structural asymmetry with respect to the MJO convective center. The observed and realistically simulated MJO eastward propagations are characterized by stronger Kelvin easterly waves than Rossby westerly waves in the lower troposphere, which is opposite to the Gill pattern where Rossby westerly waves are 2 times stronger than Kelvin easterly waves. The GCMs simulating stronger Rossby westerly waves tend to show a stationary MJO. MJO propagation performances are robustly correlated with the quality of simulated zonal asymmetries in the 850-hPa equatorial zonal winds, 700-hPa diabatic heating, 1000–700-hPa equivalent potential temperature, and convective instability. The models simulating realistic MJO propagation are exemplified by an eastward propagation of boundary layer moisture convergence (BLMC) that leads precipitation propagation by about 5 days. The BLMC stimulates MJO eastward propagation by preconditioning and predestabilizing the atmosphere, and by generating lower-tropospheric heating and available potential energy to the east of precipitation center. The MJO structural asymmetry is generated by the three-way interaction among convective heating, moisture, and equatorial wave and boundary layer dynamics. In GCMs, differing convective heating representation could produce different MJO structural asymmetry, and thus different propagations. Diagnosis of structural asymmetry may help revealing the models’ deficiency in representing the complex three-way interaction processes, which involves various parameterized processes.