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.

A global warming projection experiment was conducted on the Earth Simulator using a very high horizontal resolution atmospheric general circulation model, with 20-km grid size (the 20-km model). Such high horizontal resolution in a global climate model is unprecedented for a global warming projection. Experiments using the 20-km model were conducted by adopting the time-slice method, in which future changes in sea surface temperature (SST) were predicted by an atmosphere-ocean general circulation model (AOGCM) called MRI-CGCM2.3. The A1B emission scenario, proposed by the Intergovernmental Panel on Climate Change (IPCC), was assumed in the experiment.The model reproduces a realistic Baiu rain band under the present-day climate conditions in terms of geographical distribution and northward seasonal march. Experiments of the dependency of the horizontal resolution on the reproducibility of the Baiu rain band have revealed that the 20-km model generally exhibits higher performance than a model with a lower horizontal resolution. The future climate simulation shows that precipitation, and its intensity increases over the Yangtze River valley of China, the East China Sea, Western Japan, and the ocean to the south of the Japan archipelago. Conversely, precipitation and its intensity decrease over the Korean peninsula and Northern Japan. The termination of the Baiu season tends to be delayed until August.The future precipitation change is mainly attributable to the change in the horizontal transport of water vapor flux and its convergence associated with the intensification of a subtropical high. This can be interpreted as an atmospheric response to the El Niñ o condition of the ocean. The change in the wind field mainly contributes to the change in the water vapor flux in the case of the Baiu rain band.

Results from a 20-yr simulation of a high-resolution AGCM forced with climatological SST, along with simplified model experiments and supplementary data diagnostics, are used to investigate internal feedbacks arising from monsoon-midlatitude interactions during droughts in the Indian summer monsoon. The AGCM simulation not only shows a fairly realistic mean monsoon rainfall distribution and large-scale circulation features but also exhibits remarkable interannual variations of precipitation over the subcontinent, with the 20-yr run showing incidence of four ''monsoon droughts.''The present findings indicate that the internally forced droughts in the AGCM emanate largely from prolonged ''monsoon breaks'' that occur on subseasonal time scales and involve dynamical feedbacks between monsoon convection and extratropical circulation anomalies. In this feedback, the suppressed monsoon convection is shown to induce Rossby wave dispersion in the summertime subtropical westerlies and to set up an anomalous quasi-stationary circulation pattern extending across continental Eurasia in the middle and upper troposphere. This pattern is composed of a cyclonic anomaly over west central Asia and the IndoPakistan region, a meridionally deep anticyclonic anomaly over East Asia (;1008E), and a cyclonic anomaly over the Far East. The results suggest that the anchoring of the west central Asia cyclonic anomaly by the stagnant ridge located downstream over East Asia induces anomalous cooling in the middle and upper troposphere through cold-air advection, which reduces the meridional thermal contrast over the subcontinent. Additionally, the intrusion of the dry extratropical winds into northwest India can decrease the convective instability, so that the suppressed convection can in turn weaken the monsoon flow. The sustenance of monsoon breaks through such monsoon-midlatitude feedbacks can generate droughtlike conditions over India.

Influences of sea surface temperature (SST) spatial patterns and cumulus parameterizations on tropical cyclone (TC) frequency, in the context of global warming impacts, are investigated using an atmospheric general circulation model at T106 horizontal resolution. Simulated TCs in this high-resolution model are categorized into tropical storms (TSs) and tropical depressions (TDs). Model TSs are defined as TCs with maximum surface wind speed more than, or equal to 16 m s À1 , for experiments with an Arakawa-Schubert cumulus parameterization. Another threshold of 14 m s À1 is used for those with a Kuo cumulus parameterization. Model TDs are defined as weaker TCs. Although the maximum wind speed, and the minimum central pressures of intense TCs are not realistically simulated in the model, geographical patterns of TS formation seem to be realistically simulated, with climatological and El Niñ o/La Niñ a SST conditions.A series of experiments is conducted with doubled CO 2 and with increased SSTs. A spatial pattern of SST, made by uniform 2 K warming, is used for experiments with both of the cumulus parameterizations. El Niñ o-like and La Niñ a-like warming patterns of SSTs, are used with the Arakawa-Schubert scheme. In these global warming experiments, frequency of TS formation decreases by 9.0-18.4% globally, and some of these changes are statistically significant. While no coherent changes in global frequency of relatively intense TCs (e.g., maximum surface wind f 25 m s À1 ) are found in the warm-climate experiments, significant reduction in the total frequency of TSs and TDs resulted from all of these experiments. The results suggest that global frequency of relatively weak TCs may decrease in the future warm climate, but frequency of intense storms may either decrease or increase. Mean precipitation near TC centers is significantly heavier in the warming experiments than in the present-day experiments, as compared for TCs with the same maximum wind speed.

In this report, we present the results from our recent experiments using 20 km-mesh and 60 km-mesh atmospheric general circulation models with prescribed sea surface temperatures (SST). The results of the experiments consistently show a reduction in the global tropical cyclone frequency due to global warming. By the experiments with the models of different resolution and with different SST changes, we find that the reduction in the global tropical cyclone frequency due to global warming is a very robust feature. In contrast, the regional tropical cyclone frequency change varies a lot among the experiments with different SST change distribution. We find that the regional tropical cyclone frequency change is sensitive to relative SST change distribution. This suggests that the regional change is strongly affected by the change in tropical circulation and convective activity which is dominated by relative SST distribution patterns, and therefore, for a reliable projection of the regional change, a reliable projection of the pattern of SST change is vitally important. IntroductionIn the Summary for Policy Makers (SPM) of the Intergovernmental Panel on Climate Change (IPCC) (2007), the conclusion regarding the future tropical cyclone frequency change is written as "There is less confidence in projections of a global decrease in numbers of tropical cyclones". This conclusion is based on the discussion in Chapter 10 of the IPCC (2007). In the chapter, the discussion on the global tropical cyclone frequency change is based on six references (Sugi et al. 2002;Yoshimura et al. 2006;McDonald et al. 2005;Bengtsson et al. 2006;Tsutsui 2002;Oouchi et al. 2006). Among these six references, results of the medium resolution (grid size of about 100 km) models (Sugi et al. 2002;Yoshimura et al. 2006;McDonald et al. 2005) and a result of high resolution (20 km mesh) model (Oouchi et al. 2006) consistently indicate a significant decrease in global numbers of tropical cyclones, while results of lower resolution (grid size of 180 km or larger) models (Bengtsson et al. 2006;Tsutsui 2002) show insignificant increase or decrease. In the discussion of IPCC (2007), the medium resolution models are regarded to fall into first category models, which are not able to simulate tropical cyclones reasonably and are not reliable.Generally speaking, higher resolution models are able to simulate tropical cyclones more realistically, particularly the intense mature stage tropical cyclones, and the results of higher resolution models are considered to be more reliable. Regarding projections of the change in maximum intensity of tropical cyclones, the results from the medium resolution (grid size of about 100 km) models are different from those of high resolution models (grid size of about 20 km), indicating that the medium resolution is not sufficient to reasonably simulate intense tropical cyclones. On the other hand, regarding the projection of the change in global frequency in tropical cyclones, the results of the medium resolution models a...