Possible changes in the tropical cyclones in a future, greenhouse-warmed climate are investigated using a 20 km-mesh, high-resolution, global atmospheric model of MRI/JMA, with the analyses focused on the evaluation of the frequency and wind intensity. Two types of 10-year climate experiments are conducted. One is a present-day climate experiment, and the other is a greenhouse-warmed climate experiment, with a forcing of higher sea surface temperature and increased greenhouse-gas concentration. A comparison of the experiments suggests that the tropical cyclone frequency in the warm-climate experiment is globally reduced by about 30% (but increased in the North Atlantic) compared to the presentday-climate experiment. Furthermore, the number of intense tropical cyclones increases. The maximum surface wind speed for the most intense tropical cyclone generally increases under the greenhousewarmed condition (by 7.3 m s À1 in the Northern Hemisphere and by 3.3 m s À1 in the Southern Hemisphere). On average, these findings suggest the possibility of higher risks of more devastating tropical cyclones across the globe in a future greenhouse-warmed climate.

New versions of the high-resolution 20- and 60-km-mesh Meteorological Research Institute (MRI) atmospheric general circulation models (MRI-AGCM version 3.2) have been developed and used to investigate potential future changes in tropical cyclone (TC) activity. Compared with the previous version (version 3.1), version 3.2 yields a more realistic simulation of the present-day (1979–2003) global distribution of TCs. Moreover, the 20-km-mesh model version 3.2 is able to simulate extremely intense TCs (categories 4 and 5), which is the first time a global climate model has been able to simulate such extremely intense TCs through a multidecadal simulation. Future (2075–99) projections under the Intergovernmental Panel on Climate Change (IPCC) A1B scenario are conducted using versions 3.1 and 3.2, showing consistent decreases in the number of TCs globally and in both hemispheres as climate warms. Although projected future changes in basin-scale TC numbers show some differences between the two versions, the projected frequency of TC occurrence shows a consistent decrease in the western part of the western North Pacific (WNP) and in the South Pacific Ocean (SPO), while it shows a marked increase in the central Pacific. Both versions project a future increase in the frequency of intense TCs globally; however, the degree of increase is smaller in version 3.2 than in version 3.1. This difference arises partly because version 3.2 projects a pronounced decrease in mean TC intensity in the SPO. The 20-km-mesh model version 3.2 projects a northward shift in the most intense TCs (category 5) in the WNP, indicating an increasing potential for future catastrophic damage due to TCs in this region.

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, MRI-AGCM3.1, 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).

A global atmospheric general circulation model, with the horizontal grid size of about 20 km, has been developed, making use of the Earth Simulator, the fastest computer available at present for meteorological applications. We examine the model's performance of simulating the present-day climate from small scale through global scale by time integrations of over 10 years, using a climatological sea surface temperature.Global distributions of the seasonal mean precipitation, surface air temperature, geopotential height, zonal-mean wind and zonal-mean temperature agree well with the observations, except for an excessive amount of global precipitation, and warm bias in the tropical upper troposphere. This model improves the representation of regional-scale phenomena and local climate, by increasing horizontal resolution due to better representation of topographical effects and physical processes, with keeping the quality of representation of global climate. The model thus enables us to study global characteristics of small-scale phenomena and extreme events in unprecedented detail.

[1] Future change in the frequency of atmospheric blocking is investigated through present-day and future simulations using 20-, 60-, 120-, and 180-km-mesh atmospheric general circulation models (AGCMs) under the Intergovernmental Panel on Climate Change Special Reports on Emission Scenarios A1B emission scenario, focusing on the Northern Hemisphere winter (December-February).The results of present-day climate simulations reveal that the AGCM with the highest horizontal resolution is required to accurately simulate Euro-Atlantic blocking, whereas the AGCM with the lowest horizontal resolution is in good agreement with reanalysis data regarding the frequency of Pacific blocking. While the lower-resolution models accurately reproduce long-lived Pacific blocking, they are unable to accurately simulate long-lived Euro-Atlantic blocking. This result suggests that the maintenance mechanism of Euro-Atlantic blocking is different from that of Pacific blocking. In the future climate simulations, both frequencies of Euro-Atlantic and Pacific blockings are predicted to show a significant decrease, mainly in the western part of each peak in present-day blocking frequency, where the westerly jet is predicted to increase in strength; no significant change is predicted in the eastern part of each peak. The number of Euro-Atlantic blocking events is predicted to decrease for almost all blocking durations, whereas the decrease in the number of Pacific blockings is remarkable for long-duration events. It is possible that long-lived (>25 days) Euro-Atlantic and Pacific blockings will disappear altogether in the future.Citation: Matsueda, M., R. Mizuta, and S. Kusunoki (2009), Future change in wintertime atmospheric blocking simulated using a 20-km-mesh atmospheric global circulation model,