Sea surface temperature (SST) predictability in the Pacific on decadal timescales is examined in hindcast experiments using the coupled atmosphere-ocean model MIROC with low, medium, and high resolutions. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity while prescribing natural and anthropogenic forcing based on the IPCC concentration scenarios. Our hindcast experiments show the predictability of SST in the western subtropical Pacific, the Indian Ocean, and the tropics to the North Atlantic. Previous studies have examined the SST predictability in the Indian Ocean and the Atlantic, but SST predictability in the western subtropical Pacific has not been evaluated. In the western Pacific, the observed SST anomalies in the subtropics of both hemispheres increased rapidly from the early 1990s to the early 2000s. While this SST warming in the western subtropical Pacific is partly explained by global warming signals, the predictions of our model initialized in 1995 or 1996 tend to simulate the pattern of the SST increase and the associated precipitation changes. This large climate change around the late 1990s may be related to phenomena such as the recent increase in the typhoon frequency in Taiwan and the weakened East Asian monsoon reported by recent studies.
A new global climate model, MRI-CGCM3, has been developed at the Meteorological Research Institute (MRI). This model is an overall upgrade of MRI’s former climate model MRI-CGCM2 series. MRI-CGCM3 is composed of atmosphere-land, aerosol, and ocean-ice models, and is a subset of the MRI’s earth system model MRI-ESM1. Atmospheric component MRI-AGCM3 is interactively coupled with aerosol model to represent direct and indirect effects of aerosols with a new cloud microphysics scheme. Basic experiments for pre-industrial control, historical and climate sensitivity are performed with MRI-CGCM3. In the pre-industrial control experiment, the model exhibits very stable behavior without climatic drifts, at least in the radiation budget, the temperature near the surface and the major indices of ocean circulations. The sea surface temperature (SST) drift is sufficiently small, while there is a 1 W m-2 heating imbalance at the surface. The model’s climate sensitivity is estimated to be 2.11 K with Gregory’s method. The transient climate response (TCR) to 1 % yr-1 increase of carbon dioxide (CO2) concentration is 1.6 K with doubling of CO2 concentration and 4.1 K with quadrupling of CO2 concentration. The simulated present-day mean climate in the historical experiment is evaluated by comparison with observations, including reanalysis. The model reproduces the overall mean climate, including seasonal variation in various aspects in the atmosphere and the oceans. Variability in the simulated climate is also evaluated and is found to be realistic, including El Niño and Southern Oscillation and the Arctic and Antarctic oscillations. However, some important issues are identified. The simulated SST indicates generally cold bias in the Northern Hemisphere (NH) and warm bias in the Southern Hemisphere (SH), and the simulated sea ice expands excessively in the North Atlantic in winter. A double ITCZ also appears in the tropical Pacific, particularly in the austral summer.
Future changes in precipitation and the vertical structure of the frontal zone around Japan during the Baiu season are investigated using regional climate experiments with a 5-km-mesh non-hydrostatic model, driven for the present-day (1979-2003) and future (2075-2099) climates by output from global warming experiments using a 20-km-mesh atmospheric global circulation model under the SRES-A1B scenario. Significant increases are projected for the future climate relative to the present-day in daily precipitation amounts around western Japan during the late Baiu season. The percentage of precipitation occurring in intense precipitation (over 100 mm day-1) increases with statistical confidence above the 98% confidence level. In the present-day climate, 9% of the 25-year-mean precipitation amount during early July in the region within 30°N-35°N and 127°E-137°E is estimated to be dominated by intense precipitation; this value rises to 15% in the future climate. Changes in the Baiu front and in environmental conditions around western Japan are investigated. In the future climate, a delay in the northward march of the Baiu front is projected, and the mean location of the front during the late Baiu season is 33.0°N, compared with 33.5°N in the present-day. The mean amount of water vapor on the southern side of the front at a 500 m height increases to 19.7 g kg-1 in the future climate, an increase of 2.8 g kg-1 relative to the present-day. Mean vertical cross-sections of the front reveal tall structures of intense vertical vorticity (> 1.0 × 10-5 s-1) along an intensified frontal zone with intense mean updrafts and large amounts of rainwater in the future climate. Two of the characteristic jets associated with the Baiu front, located at a 700 hPa level on the southern side of the front and at a 200 hPa level on the northern side, are also intensified. The increases in the supply of water vapor and the intensified convective activity along the Baiu front could contribute to the projected increase in the occurrence of intense daily precipitation.
Numerous efforts have been made for evaluating the performance of global climate models with such expectation that those models with higher reproducibility of the current climate should provide more reliable projections of climate changes into the future. Attempts have been made to define a single general metric through which the overall performance of a global climate model can be assessed. On the basis of general metrics defined through several techniques of multivariate analysis, the present study compares global climate models from a viewpoint of their reproducibility of climatological-mean fields of multiple variables. The analyses indicate that a reproducibility of a particular variable is not necessarily independent of that of others, which may bring redundant information into a general metric. The model reproducibility in upper and mid-tropospheric temperature and lower-tropospheric humidity, for example, tends to be anti-correlated with that in upper and mid-tropospheric humidity. It is argued that attention has to be paid to this kind of trade-off relationships among some variables and resultant redundancy in synthesizing multiple metrics. A possibility is suggested that an arbitrary selection of variables can yield some redundant information of variables. The redundancy is, however, found to exert no serious influence on the quality of a general metric as long as it is based on the sufficient number of variables. In our attempt to evaluate the climate models by introducing general performance metrics with reduced redundancy of variables, the overall model ranking is found rather insensitive to the specific definition of the metric.
Future changes in summertime temperature extremes over Japan are projected by a well-developed regional climate model (RCM) with a spatial resolution of 5 km. The performance of the RCM with respect to temperature in the present climate is evaluated first based on observations. Although the differences in the biases (from observations) of daily temperatures between the RCM and the driving atmospheric general circulation model (AGCM) simulations are not statistically significant, root-mean-square errors (from observations) in daily mean and minimum temperatures reproduced by the RCM are smaller than those by the AGCM with a coarser spatial resolution. These results indicate that RCM with a higher spatial resolution has better performance than the driving AGCM in simulating temperature variability. A test of whether the RCM can capture temperature extremes reveals a good quantitative agreement between simulated and observed values in appearance frequency of extreme high temperature for the daily minimum temperature during July and August. The spatial pattern of the upper tail of the frequency distribution for the daily maximum temperature is also in good agreement with observations, although the model values are underestimated. Projected changes for extremely high daily minimum temperature are relatively large in July on the lee side of the Hidaka Mountains, southeast of Hokkaido. This finding can be explained by the foehn phenomenon: changes in the windward Froude number indicate that atmospheric conditions in the future climate are more favorable for the intensification of foehn. In terms of extremely high daily maximum temperature, the difference between the present-day and future climates during August is relatively large over the Tama area, west of Tokyo. Although the foehn phenomenon is not solely responsible for the projected changes, the phenomenon can explain the projected changes when westerly winds prevail over the Tama area, located on the lee side of the Kanto Mountains.
Northeasterly winds blowing from high latitudes of the North Pacific in boreal summer, including the Sea of Okhotsk and the Bering Sea, are called the “Yamase” in Japanese. The Yamase brings unusually cold and cloudy summers over northeastern Japan, in contrast to climatological southeasterly winds, having great impact on agriculture and life in the region. Therefore, future changes of the Yamase, which may be caused by global warming, are a major concern. This study is the first attempt to investigate future changes of Yamase frequency, which are defined using 10day mean surface winds, by analyzing eighteen coupled atmosphere-ocean general circulation model (AOGCM) experiments for May to August in the CMIP3 archives. We first assess present-day climate experiments (1981-2000, 20C3M) and then examine the changes in future climate (2081-2100, SRES A1B scenario). In the present-day climate, an eighteen multi-model ensemble (MME18) mean modestly reproduces seasonal variation of the Yamase frequency, although each model generally underestimates the Yamase frequency compared to the reanalysis data and large differences are seen among the models. In the future climate, most models project increases of the Yamase frequency in August in contrast to decreases of the frequency in May, whereas projected frequency changes in June, July and May to August (MJJA) are inconsistent among the MME18. Inter-model comparison suggests that weakening of mean tropical circulation, including the Walker circulation, may contribute to the increased Yamase frequency in August. A projection employing only nine of the models with higher skill (MME9hi), based on a defined metric, is also tried. Negative anomalies in June over the Sea of Okhotsk and eastern Siberia in the mean sea level pressure field are contrasted with positive anomalies in July, which are unclear in the MME18 projections. In August, almost all the MME9hi project increase of the Yamase frequency, consistent with the MME18 projections.
The impact of climate change on river flow in the Chao Phraya River basin in Thailand is analyzed by feeding future runoff projection data into a distributed flow routing model. The projection data used consists of daily runoff generation, which is downscaled into hourly data, by assuming the temporal pattern is proportional to GCM generated hourly precipitation. The GCM dataset used is a 20 km spatial resolution general circulation model (MRI-AGCM3.1S) developed by the Meteorological Research Institute, Japan Meteorological Agency, for the present climate experiment (1979-2003), the near future climate experiment (2015-2039), and the future climate experiment (2075-2099). The main findings of the river discharge projections are as follows: 1) clear changes in hourly flood peak discharge, daily drought discharge, and monthly discharge were detected; 2) for each discharge, the degree of change differed by location; 3) the changes appeared in the near future climate experiment and became clearer in the future climate experiment; and 4) a significant decrease in discharge was detected at the Pasak River basin in October.
Possible future changes in the elevation dependency of summertime precipitation over the Tibetan Plateau and the surrounding regions are investigated in time-slice ensemble experiments using a 60-km-mesh atmospheric general circulation model (AGCM). Four different lower boundary conditions in future climate simulations are synthesized based on World Climate Research Program/Coupled Model Intercomparison Project Phase-3 (WCRP/CMIP3) multi-model datasets. Simulated summer-time precipitation pattern in the present-day climate is compared with the observation based on rain-gauge-based gridded precipitation datasets. Precipitation is projected to increase in future climate simulations at high (above 4000 m) and low (1500 m or less) altitudes. These projected increases become more prominent with warmer prescribed global mean sea surface temperatures. Analysis of the elevation dependency of the atmospheric water budget indicates that projected future precipitation increases at high altitudes are caused by increases of evaporation from the surface of the Tibetan Plateau. These increases in surface evaporation are accompanied by increases in surface air temperature and snow-free area at high altitudes, suggesting that the snow/ice albedo feedback is a key component of future climate change over the Tibetan Plateau.
This study extends a comprehensive modeling study by Ito and Feng to investigate the effect of iron mobilization in size-segregated particles on soluble iron deposition to the open ocean using a global aerosol chemistry transport model. The iron dissolution from relatively insoluble iron in mineral aerosols due to chemical reactions with acidic species is calculated from the online simulation of our aerosol chemistry model. In addition, the iron from combustion sources such as biomass and fossil fuels burning is readily released into solutions in aerosols assuming constant iron solubility (i.e., the mass fraction of dissolved to total iron). The simulation results indicate that fine particles play a major role in soluble iron supply to the Pacific and Atlantic oceans in the Northern Hemisphere (70-100%) due to acid mobilization. In significant portions of the Southern Ocean, in contrast, the amount of acidic trace gases is not high enough to promote the iron dissolution in the fine aerosols. Consequently, coarse particles are important source of soluble iron in the South Atlantic Ocean (40-60%) downwind of regions from Patagonian desert in South America. The model response of soluble iron deposition to perturbation by iron solubility suggests that large fires may supply a potentially important source of soluble iron to the open ocean, compared to dust.
Understanding and forecasting of summertime afternoon precipitation due to rapidly developing cumulonimbus clouds without any significant synoptic-scale influences are important to prevent and mitigate the induced disasters. Future changes in the behavior of such precipitation events have recently gained scientific and societal interests. This study investigates the environmental stability for afternoon precipitation that develops under synoptically undisturbed conditions in summer by using the outputs of 20-km-mesh, super-high-resolution atmospheric general circulation model (GCM) simulations for a present, a near-future, and a future climate under global warming with the Intergovernmental Panel on Climate Change A1B emission scenario. The Kanto Plain was chosen as the analysis area. After verifying the usefulness of the GCM present-climate outputs with observations and gridded mesoscale analyses, we examine the future changes in the environmental stability for the afternoon precipitation by conducting statistical analyses. In the future climates, temperature lapse rate decreased in the lower troposphere, while water vapor mixing ratio increased throughout the deep troposphere. The changes in the temperature and moisture profiles resulted in the increase in both precipitable water vapor and convective available potential energy. These projected changes will be enhanced with the future period. Furthermore, the statistical analyses for the differences of the stability parameters between no-rain and rain days under the synoptically undisturbed condition in each simulated climate period indicated that the representations of the stability parameters that distinguish no-rain and rain events are basically unchanged between the present and the future climates. This result suggests that the environmental characteristics favorable for afternoon precipitation in the synoptically undisturbed environments will not change under global warming.
Sea ice has a large impact on climatic system and its variability. A good reproducibility of the past state of the sea ice in global climate models will reduce uncertainty in future projection. Here, we present sea-ice simulations for new versions of atmosphere-ocean coupled general circulation models, the Model for Interdisciplinary Research on Climate version 4h (MIROC4h) and version 5 (MIROC5), and assess the reproducibility of the sea ice prior to the future projection. The horizontal resolution of MIROC4h is significantly high for a coupled climate model, although its sea-ice component is based on the previous version. MIROC5 employs some improved schemes including subgrid-scale ice thickness distribution. Hindcast simulations of twentieth-century climate by the new models are compared with observations and with the results of previous versions of MIROC. For the Northern Hemisphere, Arctic sea-ice simulations are improved in both MIROC4h and MIROC5 compared with previous models. MIROC5 generally agrees well with observational data, whereas in MIROC4h, the Arctic sea ice is smaller in summer extent and in thickness. Employment of the ice thickness distribution, which allows large heat exchange through subgrid-scale thin ice regardless of the grid-averaged thickness, and relatively high albedo parameters contribute to reproduce more realistic ice thickness in MIROC5 compared with that in MIROC4h. For the Southern Hemisphere, MIROC4h well reproduces the observed ice edge, especially in winter, while MIROC5 underestimates sea-ice extent. Both models indicate decreasing trends in Arctic sea ice in the late twentieth century. A heat budget analysis of the MIROC5 Arctic Ocean suggests that intensification of ice-albedo feedback accelerates the rate of Arctic ice decline.
A new version of the atmospheric general circulation model of the Meteorological Research Institute (MRI), with a horizontal grid size of about 20 km, has been developed. The previous version of the 20-km model, MRIAGCM3.1, which was developed from an operational numerical weather-prediction model, provided information on possible climate change induced by global warming, including future changes in tropical cyclones, the East Asian monsoon, extreme events, and blockings. For the new version, MRI-AGCM3.2, we have introduced various new parameterization schemes that improve the model climate. Using the new model, we performed a present-day climate experiment using observed sea surface temperature. The model shows improvements in simulating heavy monthly-mean precipitation around the tropical Western Pacific, the global distribution of tropical cyclones, the seasonal march of East Asian summer monsoon, and blockings in the Pacific. Improvements in the model climatologies were confirmed numerically using skill scores (e.g., Taylor’s skill score).
In the climate-carbon cycle system, the terrestrial ecosystem feedback is significant. In studies on feedback analysis, the ecosystem feedback is divided into the sensitivity of carbon storage to atmospheric CO2 concentration (βL), and temperature change (γL). Although ecosystems include many nonlinear processes, the scenario-and time-dependency of βL and βL have not been explicitly discussed. To check the validity of this simplification and its robustness, we carried out two, 1% per year (p.a.) increase and RCP4.5 scenario, experiments, using a process-based terrestrial ecosystem model forced by an existing GCM output, combined with an energy moisture balance model for the latter experiment, with 300 ensemble members perturbing twelve important and a priori unconstrained parameters. In the 1% p.a. experiment, βL peaked around 500 ppm and then gradually decreased with increasing CO2 level, while βL decreased with some inter-annual variability as temperature increases. The time-dependency of βL was small (at least for CO2 level > 550 ppm), but that of βL was significant, and the effect of this was larger than that of the nonlinear term (i.e., combined effect of CO2 and temperature change). The scenario-dependency is also significant, but the effect in the estimated carbon uptake was smaller than that by the time-dependency. By investigating the background of this effect, we found that in the 1% p.a. CO2 increase scenario, the maximum photosynthesis rate and specific leaf area (SLA, leaf area per unit dry mass) had the most significant contribution to both of βL and βL, and the contributions are dependent on the climate state (i.e., temperature and atmospheric CO2 level). For carbon uptake in both experiments, SLA and coefficient of plant respiration were most significant. We also attempted to constrain the ensemble members, and found that for net primary production, soil carbon and soil respiration, the default parameter set was already well-tuned, while observations of leaf area index (LAI) strongly constrains βL, βL, airborne fraction and ecosystem carbon balance as the default model overestimated the LAI.
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.
Direct simulations of the surface all-sky UV-B radiation in an Earth system model (MIROC-ESM-CHEM) are validated against the operational ground-based measurements of spectral UV radiation at Sapporo, Tateno, Kagoshima, Naha, and Chengkung. The model reasonably reproduces the mean seasonal evolution of the surface UV-B radiation at Sapporo and Naha, but substantially overestimates it at Tateno and Kagoshima during the summer. Underestimation of the cloud effect at these two sites contributes to the overestimation. A projection of the seasonal evolution of the monthly mean UV-B radiation into the future shows a general increase in the 2090s compared to the 2000s. While a slight reduction of the column ozone contributes to the UV-B increase, the effects of the reduction in cloud forcing and, especially, of the reduction in aerosols contribute more significantly. The month for which the largest increase in the UV-B radiation is projected varies site-by-site, reflecting local changes in forcing due to aerosols and clouds.
The projected changes in annual mean precipitation and large-scale circulations between the two periods 1981-2000 and 2081-2100 obtained from the Coupled Model Intercomparison Project phase 3 (CMIP3) and version 5 of the Model for Interdisciplinary Research on Climate (MIROC5), which is a newly developed model and not included in CMIP3, are investigated over tropical oceans (30°S-30°N). We first evaluate skill scores for precipitation reproducibility, based on which five highest and lowest score models (HSMs and LSMs) are selected from 24 CMIP3 models. The score of MIROC5 is higher than that of any CMIP3 model without flux adjustment. While the HSMs, the LSMs, and MIROC5 commonly show precipitation increase over the equatorial central to eastern Pacific region, and weakening of the Walker circulation in future projections; the magnitudes of these changes are significantly larger in the HSMs than in the LSMs. This difference in the magnitudes of the changes between in HSMs and in LSMs is consistent with the different sensitivities of deep convection to the environmental humidity in the mid-lower troposphere. The vertical structure of the vertical velocity changes indicates that the precipitation increase around 160°E-150°W in the HSMs and LSMs is associated with deep convection, whereas that in MIROC5 is associated with middle-level convection.
Features of year-to-year variations in the Meiyu frontal rain zone (MFZ, in 110°E-125°E) and the Baiu frontal rain zone (BFZ, in 125°E-140°E) in June over 20 years (1980-1999), obtained by 22 climate models from the 20th Century Climate in Coupled Models experiment (20C3M) of the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3), were compared with the year-to-year variations in two observed precipitation datasets. Year-to-year variations in the latitudes of and precipitation in the MFZ and BFZ produced by the models were examined. The MFZ and BFZ were clearly defined in every year in the observed precipitation data, but not in every simulation case. Medians of the latitude and precipitation in the modeled zones also did not always coincide with observations. Whereas several models showed large variation ranges in latitude and precipitation, a few other models had unrealistically small variation ranges. In addition, the mutual relations between year-to-year variations in latitude and precipitation in the modeled MFZ and BFZ were compared with the relations in the observational data. Some models showed unrealistically high correlations between latitude and precipitation, whereas several other models showed unrealistically low correlation or opposite correlation compared to the observations. Many of the models did not reproduce realistic year-to-year variations in MFZ and BFZ consistent with the observations.
This study examined features of intense rainfalls over southwestern Japan in the Baiu season (June and July) in the 20th Century Climate in Coupled Models (20C3M) and 21st Century projection (Special Report on Emission Scenarios [SRES] A1B) used by 18 climate models that contributed to the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3). Characteristics of daily precipitation over an area of 5° x 5° within 30°N-35°N and 130°E-135°E obtained by 20th century climate simulation were compared with observed precipitation data for a 3-year period (1997-1999). The studied area includes four meshed subareas of 2.5° x 2.5°. The four-area averaged daily precipitation and the maximum precipitation in the four areas were analyzed. The order statistics of the maximum precipitation indicated a very wide difference from model to model. To measure the temporal and spatial concentration of precipitation, “concentration ratio,” defined as (the average of the first-to sixth-largest maximum precipitation)/(time-area averaged precipitation) was examined. Although a few models reproduced a reasonable concentration ratio comparable to observations, many other models did not reproduce a reasonable ratio. Models of medium and high horizontal resolution with the Arakawa-Schubert cumulus scheme tended to reproduce a reasonable concentration ratio. Models that simulate a realistic Baiu frontal precipitation zone were able to simulate a proper concentration ratio. Features of the intense rainfalls in the 21st century climate projection by the models were examined for June and July over a 3-year period (2097-2099). The characteristics of the models in regard to the intense rainfalls in the 20th century simulation were commonly found in the 21st century projections. Because the difference among the models was too large with respect to the intense rainfalls, the change in intense rainfalls under the climate changes in the 21st century cannot be definitely determined by CMIP3 models.
A 5-km-mesh nonhydrostatic cloud-system-resolving regional climate model (NHM-5km) has been developed at the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) by improving upon the JMA operational mesoscale model (MSM). Three major changes have been made to MSM: the Kain-Frisch convective parameterization scheme has been improved to reduce the incidence of false predictions of rainfall areas along coastlines during the warm season, a spectral nudging method has been introduced to avoid phase-gap between the inner model (NHM-5km) and the outer model, and a Simple Biosphere model has been applied for sophisticated representation of land surface processes. This article presents details of the first two of these modifications. A present-day climate simulation is performed using NHM-5km by nesting within the results of a 20-kmmesh atmospheric global climate model (MRI-AGCM3.2S). Taylor’s skill score is used to compare the performances of NHM-5km and MRI-AGCM3.2S in terms of reproducing the spatial pattern of precipitation-based extreme indices over the Japanese Islands. The comparison shows that NHM-5km yields a significant improvement in reproducing the present-day climatology (e.g., the maximum number of consecutive dry days and the simple daily precipitation intensity index), suggesting that NHM-5km is a reliable tool for accurately predicting future changes in extreme weather at a fine spatial resolution.
The sensitivity of the equatorial quasi-biennial oscillation (QBO) to increasing greenhouse gas (GHG) concentrations over time is evaluated using a high-top (85 km) earth system model, which generates the QBO using a non-orographic gravity wave drag parameterization. Based on a pre-industrial control run (1850), a 1/2 CO2 run and a 4 x CO2 run are performed as sensitivity experiments. In addition, long-term transient behaviors of the QBO in a historical climate run (1850-2005) and a future projection run (2006-2100) are investigated. The period of the simulated QBO lengthens in a GHG-rich warmer climate, changing from a quasi-regular 24 months in 1850-1980 to a variable 24-31 months in 1980-2050, although never exceeding 31 months even in the extremely warm 4 x CO2 run. Such elongation of the QBO period is mostly caused by strengthening of the mean tropical upwelling of the Brewer-Dobson circulation, which counteracts the wave forcing that drives the QBO. Simultaneous reductions in the easterly wind maximum of the QBO are also caused by this mechanism.
The reproducibility of the extratropical teleconnection in East Asia associated with the Madden-Julian Oscillation (MJO) during the boreal winter is evaluated in the 20th century experiment (20C3M) outputs from 16 global climate models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). It is revealed that five of the models (BEST models) realistically simulate the MJOextratropical convection teleconnection. These models successfully represent the MJO convective signals better than the other models. Through analysis of the wave activity flux and Rossby wave source (RWS), the upper-level extratropical wave train induced by MJO convection is demonstrated to propagate northeastward along the Asian jet with its reinforcement, which eventually affects the convective variability in East Asia. The BEST models pronouncedly reproduce the extratropical wave train and RWS. However, the other models produce unrealistic or no extratropical wave trains due to unrealistically produced RWSs, although all models realistically exhibit the climatological absolute vorticity over the Asian jet as a waveguide. Furthermore, only the BEST models reproduce the low-level moisture transport with southerlies from the tropics into East Asia, which are associated with realistically reproduced anticyclonic circulation over the North Pacific, that is largely formed as a Rossbywave response to the cooling anomaly with meaningful suppressed convection over tropical western and central Pacific. In summary, correctly simulating the intensity of the MJO convection and its eastward propagation into the Pacific is necessary in order to predict the wintertime climate variability over East Asia.
In line with the experimental design for near-term climate prediction toward the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR5) and the Coupled Model Intercomparison Project Phase 5 (CMIP5), we perform ensembles of initialized decadal hindcast experiments using two recent versions of the Model for Interdisciplinary Research On Climate (MIROC): MIROC4h (T213L56 AGCM and 1/6-1/4 deg. 48 level OGCM) and MIROC5 (T85L40 AGCM and 0.56-1.4 deg. 50 level OGCM). We analyze sets of 10-yearlong 9-ensemble hindcasts (3 members by MIROC4h and 6 members by MIROC5) with initialization every five years after 1961 and explore the predictability of decadal climate changes. The most predictable variation on decadal timescales is the global warming signal due to the favorable response of the models to external forcing. The results of these initialized hindcast experiments using MIROC5 validate our ability to enhance decadal predictability primarily through the initialization, particularly of the Pacific Decadal Oscillation (PDO) for a few years and of the Atlantic Multidecadal Oscillation (AMO) for almost a decade. The initialization has large impacts on the upper ocean temperature hindcasts over the mid-and high latitudes of the North Pacific and the high latitudes of the North Atlantic, where the PDO and AMO signals are observed to be strongest. In contribution to process and assessment studies in IPCC-AR5 and CMIP5, further analysis of our hindcast data (and near-term prediction data) using MIROC4h and MIROC5 is worthwhile. We note that the initialized hindcasts using MIROC4h have predictive skill inferior to the MIROC5 results and that at this stage, fully significant discussions may not be possible due to the small number of ensembles with limited computational resources.
Regional patterns of wintertime sea level pressure (SLP) trends over the North Pacific and their uncertainty were investigated based on the phase 3 of the Coupled Model Intercomparison Project (CMIP3) multi-model projections under the Special Report on Emissions Scenarios (SRES) A1B emission scenario for the 21st century (2000-2099). While the 24-model ensemble mean of the 100-yr SLP trend over the North Pacific shows a northward shift of the Aleutian low (AL), regional patterns of the SLP change vary among the models. Projected changes deepen the AL in several models but it shifts northward in some others. The different response of the AL results in a large inter-model spread over the North Pacific, which is largest of the Northern Hemisphere and comparable in magnitude to the ensemble mean in the same region. This large spread means a high degree of uncertainty in the 100-yr SLP trend over the North Pacific. For the total uncertainty in the SLP trends over the North Pacific, we examined the relative importance of the internal climate variability and model uncertainty due to different treatments of physical processes and computational scheme. To evaluate each of contributions, a single-realization ensemble using a subset of 10 CMIP3 models is compared to a multi-realization ensemble for the same models in the A1B projections. Additionally the control simulations under preindustrial conditions are examined to evaluate the background internal variability in each of the CMIP3 models. Our analysis shows that both the model uncertainty and internal climate variability contribute to the total uncertainty in the 100-yr SLP trends during the 21st century, while the internal climate variability largely explains the total uncertainty in the 50-yr SLP trends during the first half of the 21st century. The changes in surface heat flux and North Pacific subtropical gyre in association with the different response of the AL affect regional patterns of the sea surface temperature trends among models.
In order to explore the hypothesized mechanisms for the reduction of global tropical cyclone (TC) frequency due to greenhouse warming, an experiment has been conducted using a most recent version of MRI-AGCM with a new convection scheme. In addition to a present climate run (HPA run) and a future climate run (HFA run), two more runs are conducted. In CO2F run, future values of CO2 and other greenhouse gas (GHG) concentrations are used with present value of sea surface temperature (SST), while in the SSTF run, future value of SST is used with present values of CO2 and other GHG concentrations. The reductions of global TC frequency in HFA run, CO2F run and SSTF run from HPA run are 25%, 9% and 18%, respectively. These results are basically consistent with previous studies. Based on the results of the experiment, we examined three key relations in our hypothesized mechanism for the reduction of TC frequency. First, the relation between changes in atmospheric radiative cooling and precipitation is confirmed to be valid in the experiment, in which not only CO2 but other GHG is increased. It is also confirmed that the effect of increasing CO2 is decreasing precipitation, while the effect of increasing other GHG is increasing precipitation. Second, the relation between changes in precipitation and upward mass flux is clarified by using a simple approximate thermodynamic equation. Third, regarding the relation between changes in upward mass flux and TC genesis frequency, we examined the changes in four parameters (precipitation, upward mass flux, vertical wind shear and mid-troposphere saturation deficit) which are closely related to deep convective activities in the tropics and may affect TC genesis frequency. The results of our experiment support the idea suggested by the previous studies that the reduction of TC frequency is closely related to a reduction of upward mass flux, although the chain of causality linking the two remains unclear. In addition, our experiment suggests a possibility that the changes in mid-troposphere saturation deficit may also contribute to the changes in TC genesis frequency.