Tomoo Ogura

Improved Climate Simulation by MIROC5: Mean States, Variability, and Climate Sensitivity

Abstract A new version of the atmosphere–ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1° ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5. A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Nino–Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between th...

The impact of parametrized convection on cloud feedback.

We investigate the sensitivity of cloud feedbacks to the use of convective parametrizations by repeating the CMIP5/CFMIP-2 AMIP/AMIP + 4K uniform sea surface temperature perturbation experiments with 10 climate models which have had their convective parametrizations turned off. Previous studies have suggested that differences between parametrized convection schemes are a leading source of inter-model spread in cloud feedbacks. We find however that ‘ConvOff’ models with convection switched off have a similar overall range of cloud feedbacks compared with the standard configurations. Furthermore, applying a simple bias correction method to allow for differences in present-day global cloud radiative effects substantially reduces the differences between the cloud feedbacks with and without parametrized convection in the individual models. We conclude that, while parametrized convection influences the strength of the cloud feedbacks substantially in some models, other processes must also contribute substantially to the overall inter-model spread. The positive shortwave cloud feedbacks seen in the models in subtropical regimes associated with shallow clouds are still present in the ConvOff experiments. Inter-model spread in shortwave cloud feedback increases slightly in regimes associated with trade cumulus in the ConvOff experiments but is quite similar in the most stable subtropical regimes associated with stratocumulus clouds. Inter-model spread in longwave cloud feedbacks in strongly precipitating regions of the tropics is substantially reduced in the ConvOff experiments however, indicating a considerable local contribution from differences in the details of convective parametrizations. In both standard and ConvOff experiments, models with less mid-level cloud and less moist static energy near the top of the boundary layer tend to have more positive tropical cloud feedbacks. The role of non-convective processes in contributing to inter-model spread in cloud feedback is discussed.

Emission Scenario Dependency of Precipitation on Global Warming in the MIROC3.2 Model

Abstract The precipitation sensitivity per 1 K of global warming in twenty-first-century climate projections is smaller in an emission scenario with larger greenhouse gas concentrations and aerosol emissions, according to the Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2) coupled atmosphere–ocean general circulation model. The authors examined the reasons for the precipitation sensitivity to emission scenarios by performing separated individual forcing runs under high and low emission scenarios. It was found that the dependency on emission scenario is mainly caused by differences in black and organic carbon aerosol forcing (the sum of which is cooling forcing) between the emission scenarios and that the precipitation is more sensitive to carbon aerosols than well-mixed greenhouse gases. They also investigated the reason for the larger precipitation sensitivity (larger magnitude of precipitation decrease per 1 K cooling of temperature) in the carbon aerosol runs. Surface dimming due to the ...

Rapid Adjustments of Cloud and Hydrological Cycle to Increasing CO2: a Review

Rapid cloud response to instantaneous radiative perturbation in the troposphere due to change in CO2 concentration is called cloud adjustment. Cloud adjustment develops on a short timescale because it is separated from surface temperature-mediated changes in cloud. Adjustments in cloud and tropospheric properties including the hydrological cycle have attracted considerable attention because of their importance in the interpretation of mechanisms of climate change and the identification of sources of uncertainty in climate sensitivity. Modeling studies have clearly revealed that major aspects of the tropospheric adjustment including the warming and drying of the troposphere, associated reduction of low cloud and increasing shortwave cloud radiative forcing, downward shift of the low-cloud layer, and slowdown of the global hydrological cycle, are common among many climate model simulations. Combinations of model simulations with realistic and idealized aqua-planet settings have helped demonstrate the roles of land and robust aspects of the tropospheric adjustment.

Using a Multiphysics Ensemble for Exploring Diversity in Cloud–Shortwave Feedback in GCMs

AbstractThis study proposes a systematic approach to investigate cloud-radiative feedbacks to climate change induced by an increase of CO2 concentrations in global climate models (GCMs). Based on two versions of the Model for Interdisciplinary Research on Climate (MIROC), which have opposite signs for cloud–shortwave feedback (ΔSWcld) and hence different equilibrium climate sensitivities (ECSs), hybrid models are constructed by replacing one or more parameterization schemes for cumulus convection, cloud, and turbulence between them. An ensemble of climate change simulations using a suite of eight models, called a multiphysics ensemble (MPE), is generated. The MPE provides a range of ECS as wide as the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel ensemble and reveals a different magnitude and sign of ΔSWcld over the tropics, which is crucial for determining ECS.It is found that no single process controls ΔSWcld, but that the coupling of two processes does. Namely, changing the cloud and...

Comparison of equilibrium and transient responses to CO2 increase in eight state-of-the-art climate models

We compared the climate response of doubled CO2 equilibrium experiments (2 × CO2) by atmosphere—slab ocean coupled general circulation models (ASGCMs) and that of 1% per year CO2 increase experiments (1%CO2 by atmosphere—ocean coupled general circulation models (AOGCMs) using eight state-of-the-art climate models. Climate feedback processes in 2 × CO2 are different from those in 1%CO2, and the equilibrium climate sensitivity (T2×) in 2 × CO2 is different from the effective climate sensitivity (T2×, eff) in 1%CO2. The difference between T2× and T2×, eff is from −1.3 to 1.6 K, a large part of which can be explained by the difference in the ice-albedo and cloud feedback. The largest contribution is cloud SW feedback, and the difference in cloud SW feedback for 2 ×CO2 and 1%CO2 could be determined by the distribution of the SAT anomaly which causes differences in the atmospheric thermal structure. An important factor which determines the difference in ice-albedo feedback is the initial sea ice distribution at the Southern Ocean, which is generally overestimated in 2 ×CO2 as compared to 1%CO2 and observation. Through the comparison of climate feedback processes in 2 ×CO2 and 1%CO2, the possible behaviour of the time evolution of T2×, eff is discussed.

Relative contribution of feedback processes to Arctic amplification of temperature change in MIROC GCM

The finding that surface warming over the Arctic exceeds that over the rest of the world under global warming is a robust feature among general circulation models (GCMs). While various mechanisms have been proposed, quantifying their relative contributions is an important task in order to understand model behavior. Here we apply a recently proposed feedback analysis technique to an atmosphere–ocean GCM under two and four times CO2 concentrations which approximately lead to seasonally and annually sea ice-free climates. The contribution of feedbacks to Arctic temperature change is investigated. The surface warming in the Arctic is contributed by albedo, water vapour and large-scale condensation feedbacks and reduced by the evaporative cooling feedback. The surface warming contrast between the Arctic and the global averages (AA) is maintained by albedo and evaporative cooling feedbacks. The latter contributes to AA predominantly by cooling the low latitudes more than the Arctic. Latent heat transport into the Arctic increases and hence evaporative cooling plus large-scale condensation feedback contributes positively to AA. On the other hand, dry-static energy transport into the Arctic decreases and hence dynamical heating feedback contributes negatively to AA. An important contribution is thus made via changes in hydrological cycle and not via the ‘dry’ heat transport process. A larger response near the surface than aloft in the Arctic is maintained by the albedo, water vapour, and dynamical heating feedbacks, in which the albedo and water vapour feedbacks contribute through warming the surface more than aloft, and the dynamical heating feedback contributes by cooling aloft more than the surface. In our experiments, ocean and sea ice dynamics play a secondary role. It is shown that a different level of CO2 increase introduces a latitudinal and seasonal difference into the feedbacks.

Importance of instantaneous radiative forcing for rapid tropospheric adjustment

To better understand CFMIP/CMIP inter-model differences in rapid low cloud responses to CO2 increases and their associated effective radiative forcings, we examined the tropospheric adjustment of the lower tropospheric stability (LTS) in three general circulation models (GCMs): HadGEM2-A, MIROC3.2 medres, and MIROC5. MIROC3.2 medres showed a reduction in LTS over the sub-tropical ocean, in contrast to the other two models. This reduction was consistent with a temperature decrease in the mid-troposphere. The temperature decrease was mainly driven by instantaneous radiative forcing (RF) caused by an increase in CO2. Reductions in radiative and latent heating, due to clouds, and in adiabatic and advective heating, also contribute to the temperature decrease. The instantaneous RF in the mid-troposphere in MIROC3.2 medres is inconsistent with the results of line-by-line (LBL) calculations, and thus it is considered questionable. These results illustrate the importance of evaluating the vertical profile of instantaneous RF with LBL calculations; improved future model performance in this regard should help to increase our confidence in the tropospheric adjustment in GCMs.

Influence of the Antarctic Ice Sheet on southern high latitude climate during the Cenozoic: Albedo vs topography effect

Sensitivity of high latitude temperature distribution to the variation of the Antarctic ice sheet is investigated using an atmospheric general circulation model coupled with a mixed layer ocean. Results suggest that cooling effect of the ice sheet is not negligible compared to the effect of atmospheric CO2 decrease such as during the Cenozoic. Cooling effect of high surface albedo of the ice sheet is enhanced over the ice sheet and slightly suppressed over the circumpolar ocean by the high topography of the ice sheet. High surface albedo and high topography of the ice sheet each causes increase and decrease of southward atmospheric heat transport respectively, whose contribution to maintaining the heat balance anomaly of atmosphere-ocean system is comparable to that of local adjustment of radiation balance.

Lower-Tropospheric Mixing as a Constraint on Cloud Feedback in a Multiparameter Multiphysics Ensemble

AbstractFactors and possible constraints to extremely large spread of effective climate sensitivity (ECS) ranging about 2.1–10.4 K are examined by using a large-member ensemble of quadrupling CO2 experiments with an atmospheric general circulation model (AGCM). The ensemble, called the multiparameter multiphysics ensemble (MPMPE), consists of both parametric and structural uncertainties in parameterizations of cloud, cumulus convection, and turbulence based on two different versions of AGCM. The sum of the low- and middle-cloud shortwave feedback explains most of the ECS spread among the MPMPE members. For about half of the perturbed physics ensembles (PPEs) in the MPMPE, variation in lower-tropospheric mixing intensity (LTMI) corresponds well with the ECS variation, whereas it does not for the other half. In the latter PPEs, large spread in optically thick middle-cloud feedback over the equatorial ocean substantially affects the ECS, disrupting the LTMI–ECS relationship. Although observed LTMI can constr...