Abstract. The sixth version of the Model for Interdisciplinary Research on Climate (MIROC), called MIROC6, was cooperatively developed by a Japanese modeling community. In the present paper, simulated mean climate, internal climate variability, and climate sensitivity in MIROC6 are evaluated and briefly summarized in comparison with the previous version of our climate model (MIROC5) and observations. The results show that the overall reproducibility of mean climate and internal climate variability in MIROC6 is better than that in MIROC5. The tropical climate systems (e.g., summertime precipitation in the western Pacific and the eastward-propagating Madden–Julian oscillation) and the midlatitude atmospheric circulation (e.g., the westerlies, the polar night jet, and troposphere–stratosphere interactions) are significantly improved in MIROC6. These improvements can be attributed to the newly implemented parameterization for shallow convective processes and to the inclusion of the stratosphere. While there are significant differences in climates and variabilities between the two models, the effective climate sensitivity of 2.6 K remains the same because the differences in radiative forcing and climate feedback tend to offset each other. With an aim towards contributing to the sixth phase of the Coupled Model Intercomparison Project, designated simulations tackling a wide range of climate science issues, as well as seasonal to decadal climate predictions and future climate projections, are currently ongoing using MIROC6.

Abstract. This article describes the new Earth system model (ESM), the Model for Interdisciplinary Research on Climate, Earth System version 2 for Long-term simulations (MIROC-ES2L), using a state-of-the-art climate model as the physical core. This model embeds a terrestrial biogeochemical component with explicit carbon–nitrogen interaction to account for soil nutrient control on plant growth and the land carbon sink. The model's ocean biogeochemical component is largely updated to simulate the biogeochemical cycles of carbon, nitrogen, phosphorus, iron, and oxygen such that oceanic primary productivity can be controlled by multiple nutrient limitations. The ocean nitrogen cycle is coupled with the land component via river discharge processes, and external inputs of iron from pyrogenic and lithogenic sources are considered. Comparison of a historical simulation with observation studies showed that the model could reproduce the transient global climate change and carbon cycle as well as the observed large-scale spatial patterns of the land carbon cycle and upper-ocean biogeochemistry. The model demonstrated historical human perturbation of the nitrogen cycle through land use and agriculture and simulated the resultant impact on the terrestrial carbon cycle. Sensitivity analyses under preindustrial conditions revealed that the simulated ocean biogeochemistry could be altered regionally (and substantially) by nutrient input from the atmosphere and rivers. Based on an idealized experiment in which CO2 was prescribed to increase at a rate of 1 % yr−1, the transient climate response (TCR) is estimated to be 1.5 K, i.e., approximately 70 % of that from our previous ESM used in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The cumulative airborne fraction (AF) is also reduced by 15 % because of the intensified land carbon sink, which results in an airborne fraction close to the multimodel mean of the CMIP5 ESMs. The transient climate response to cumulative carbon emissions (TCRE) is 1.3 K EgC−1, i.e., slightly smaller than the average of the CMIP5 ESMs, which suggests that “optimistic” future climate projections will be made by the model. This model and the simulation results contribute to CMIP6. The MIROC-ES2L could further improve our understanding of climate–biogeochemical interaction mechanisms, projections of future environmental changes, and exploration of our future options regarding sustainable development by evolving the processes of climate, biogeochemistry, and human activities in a holistic and interactive manner.

A new high-resolution atmosphere-ocean coupled general circulation model named MIROC4h has been developed, and its performance in a 120-year control experiment (including a 50-year spin-up) Compared with MIROC3h and MIROC3m, many improvements have been achieved; for example, errors in the surface air temperature and sea surface temperature are smaller, there is less drift of the ocean water temperature in the subsurface-deep ocean, and the frequency of heavy rain is comparable to observations. The fine horizontal resolution in the atmosphere makes orographic wind and its effects on the ocean more realistic than those of the former models, and the treatment of coastal upwelling motion in the ocean has been improved. Phenomena in the atmosphere and ocean related to the El Niño and southern oscillation are now closer to observations than was obtained by MIROC3h and MIROC3m. The effective climate sensitivity for CO 2 doubling is calculated to be about 5.7 K, which is much larger than the value obtained using the IPCC AR4 models, and is mainly due to a decrease in the low-level clouds at low latitudes.

Abstract. This study presents the results from the Tropospheric Chemistry Reanalysis version 2 (TCR-2) for the period 2005–2018 at 1.1∘ horizontal resolution obtained from the assimilation of multiple updated satellite measurements of ozone, CO, NO2, HNO3, and SO2 from the OMI, SCIAMACHY, GOME-2, TES, MLS, and MOPITT satellite instruments. The reanalysis calculation was conducted using a global chemical transport model MIROC-CHASER and an ensemble Kalman filter technique that optimizes both chemical concentrations of various species and emissions of several precursors, which was efficient for the correction of the entire tropospheric profile of various species and its year-to-year variations. Comparisons against independent aircraft, satellite, and ozonesonde observations demonstrate the quality of the reanalysis fields for numerous key species on regional and global scales, as well as for seasonal, yearly, and decadal scales, from the surface to the lower stratosphere. The multi-constituent data assimilation brought the model vertical profiles and interhemispheric gradient of OH closer to observational estimates, which was important in improving the description of the oxidation capacity of the atmosphere and thus vertical profiles of various species. The evaluation results demonstrate the capability of the chemical reanalysis to improve understanding of the processes controlling variations in atmospheric composition, including long-term changes in near-surface air quality and emissions. The estimated emissions can be employed for the elucidation of detailed distributions of the anthropogenic and biomass burning emissions of co-emitted species (NOx, CO, SO2) in all major regions, as well as their seasonal and decadal variabilities. The data sets are available at https://doi.org/10.25966/9qgv-fe81 (Miyazaki et al., 2019a).

Global multiconstituent concentration and emission fields obtained from the assimilation of the satellite retrievals of ozone, CO, NO 2 , HNO 3 , and SO 2 from the Ozone Monitoring Instrument (OMI), Global Ozone Monitoring Experiment 2, Measurements of Pollution in the Troposphere, Microwave Limb Sounder, and Atmospheric Infrared Sounder (AIRS)/OMI are used to understand the processes controlling air pollution during the Korea‐United States Air Quality (KORUS‐AQ) campaign. Estimated emissions in South Korea were 0.42 Tg N for NO x and 1.1 Tg CO for CO, which were 40% and 83% higher, respectively, than the a priori bottom‐up inventories, and increased mean ozone concentration by up to 7.5 ± 1.6 ppbv. The observed boundary layer ozone exceeded 90 ppbv over Seoul under stagnant phases, whereas it was approximately 60 ppbv during dynamical conditions given equivalent emissions. Chemical reanalysis showed that mean ozone concentration was persistently higher over Seoul (75.10 ± 7.6 ppbv) than the broader KORUS‐AQ domain (70.5 ± 9.2 ppbv) at 700 hPa. Large bias reductions (>75%) in the free tropospheric OH show that multiple‐species assimilation is critical for balanced tropospheric chemistry analysis and emissions. The assimilation performance was dependent on the particular phase. While the evaluation of data assimilation fields shows an improved agreement with aircraft measurements in ozone (to less than 5 ppbv biases), CO, NO 2 , SO 2 , PAN, and OH profiles, lower tropospheric ozone analysis error was largest at stagnant conditions, whereas the model errors were mostly removed by data assimilation under dynamic weather conditions. Assimilation of new AIRS/OMI ozone profiles allowed for additional error reductions, especially under dynamic weather conditions. Our results show the important balance of dynamics and emissions both on pollution and the chemical assimilation system performance.

Abstract. We evaluate global tropospheric nitrogen dioxide (NO 2 ) simulations using the CHASER V4.0 global chemical transport model (CTM) at horizontal resolutions of 0.56, 1.1, and 2.8 • . Model evaluation was conducted using satellite tropospheric NO 2 retrievals from the Ozone Monitoring Instrument (OMI) and the Global Ozone Monitoring Experiment-2 (GOME-2) and aircraft observations from the 2014 Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ). Agreement against satellite retrievals improved greatly at 1.1 and 0.56 • resolutions (compared to 2.8 • resolution) over polluted and biomass burning regions. The 1.1 • simulation generally captured the regional distribution of the tropospheric NO 2 column well, whereas 0.56 • resolution was necessary to improve the model performance over areas with strong local sources, with mean bias reductions of 67 % over Beijing and 73 % over San Francisco in summer. Validation using aircraft observations indicated that high-resolution simulations reduced negative NO 2 biases below 700 hPa over the Denver metropolitan area. These improvements in high-resolution simulations were attributable to (1) closer spatial representativeness between simulations and observations and (2) better representation of large-scale concentration fields (i.e., at 2.8 • ) through the consideration of small-scale processes. Model evaluations conducted at 0.5 and 2.8 • bin grids indicated that the contributions of both these processes were comparable over most polluted regions, whereas the latter effect (2) made a larger contribution over eastern China and biomass burning areas. The evaluations presented in this paper demonstrate the potential of using a high-resolution global CTM for studying megacity-scale air pollutants across the entire globe, potentially also contributing to global satellite retrievals and chemical data assimilation.

This paper documents the procedure of ocean data assimilation that initializes the climate models MIROC3m, MIROC4h, and MIROC5 for decadal climate predictions following the CMIP5 protocol, and summarizes the performance of the climate models using this data assimilation. Only anomalies of observed ocean hydrographic data are assimilated using the incremental analysis update method in order to prevent model climate drifts during predictions. In the case of MIROC4h, which has an eddy-permitting ocean model, a spatial smoother is used in calculating analysis increments so that oceanic mesoscale eddies cannot be damped by observational constraints and that they are generated and decay physically in response to the assimilated background state. Globally, the decadal-scale variations of ocean temperatures in the assimilation runs are highly correlated with the observations. Variations of surface air temperature over oceans are also consistent with the observations, but this is not the case in some regions over continents. Atmospheric responses to the SST variations corresponding to the Pacific Decadal Oscillations and the Atlantic Multi-decadal Oscillation are better represented in MIROC4h and MIROC5 than in MIROC3m. The high resolution of MIROC4h and new cloud parameterizations in MIROC5 may contribute to this improvement. Root-mean-squared amplitudes of sea surface height variations associated with oceanic eddies (hereafter, eddy activity) are not suppressed undesirably in the MIROC4h assimilation run and these are comparable with those in the uninitialized runs. In the Kuroshio-Oyashio confluence zone, eddy activity is modulated on a decadal timescale. This modulation is reasonably represented in the assimilation run compared with the observations. In the hindcast experiments, significant decadal prediction skills are found for the North Atlantic, the subtropical North Pacific, and the Indian Ocean. The decadal climate predictions are expected to contribute to the IPCC AR5 and political decision-making for the coming decades.

Abstract. This study developed a new Model for Interdisciplinary Research on Climate, Earth System version2 for Long-term simulations (MIROC-ES2L) Earth system model (ESM) using a state-of-the-art climate model as the physical core. This model embeds a terrestrial biogeochemical component with explicit carbon–nitrogen interaction to account for soil nutrient control on plant growth and the land carbon sink. The model’s ocean biogeochemical component is largely updated to simulate biogeochemical cycles of carbon, nitrogen, phosphorus, iron, and oxygen such that oceanic primary productivity can be controlled by multiple nutrient limitations. The ocean nitrogen cycle is coupled with the land component via river discharge processes, and external inputs of iron from pyrogenic and lithogenic sources are considered. Comparison of a historical simulation with observation studies showed the model could reproduce reasonable historical changes in climate, the carbon cycle, and other biogeochemical variables together with reasonable spatial patterns of distribution of the present-day condition. The model demonstrated historical human perturbation of the nitrogen cycle through land use and agriculture, and it simulated the resultant impact on the terrestrial carbon cycle. Sensitivity analyses in preindustrial conditions revealed modeled ocean biogeochemistry could be changed regionally (but substantially) by nutrient inputs from the atmosphere and rivers. Through an idealized experiment of a 1 %CO2 increase scenario, we found the transient climate response (TCR) in the model is 1.5 K, i.e., approximately 70 % that of our previous model. The cumulative airborne fraction (AF) is also reduced by 15 % because of the intensified land carbon sink, resulting in an AF close to the multimodel mean of the Coupled Model Intercomparison Project Phase 5 (CMIP5) ESMs. The transient climate response to cumulative carbon emission (TCRE) is 1.3 K EgC−1, i.e., slightly smaller than the average of the CMIP5 ESMs, suggesting optimistic model performance in future climate projections. This model and the simulation results are contributing to the Coupled Model Intercomparison Project Phase 6 (CMIP6). The ESM could help further understanding of climate–biogeochemical interaction mechanisms, projections of future environmental changes, and exploration of our future options regarding sustainable development by evolving the processes of climate, biogeochemistry, and human activities in a holistic and interactive manner.