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Understanding the systematic characteristics of tropical cyclones (TCs) represented in global climate models (GCMs) is important for reliable climate change impact assessments. The atmospheric GCM (AGCM) and ocean wave models were coupled by incorporating the wave-dependent momentum flux. Systematic impacts of wave-dependent momentum flux on TC characteristics were estimated by analyzing 100 historical TCs that occurred in the western North Pacific Ocean. Wave-dependent momentum flux parameterization considering wind and wave direction misalignment was used for assessing the wave–atmosphere interaction. The larger the wave age and misalignment are, the larger the drag coefficient is. The drag coefficient at the left-hand side of the TC was enhanced by the wave condition. It was found that the wave-dependent momentum flux did not have any impact on peak TC intensity. On the other hand, the wave-dependent momentum flux showed a significant impact on TC development during the early development stage. Although systematic differences in TC intensity at most developed stages were not detected, systematic differences in TC tracks between experiments were observed. The TC tracks of the wave-coupled AGCM tend to pass in a relatively eastward direction in comparison with those from the uncoupled AGCM. This is because the wave-dependent momentum flux in the coupled AGCM altered the environmental steering flow and the smaller beta effect of smaller TC at the early developing stage. Systematic differences in TC tracks have significant impacts on climate change assessments, such as extreme sea level changes in coastal regions due to climate change.

Slow-moving tropical cyclones (TCs) can cause heavy rain because of their duration of influence. Combined with expected increase in rain rates associated with TCs in a warmer climate, there is growing interest in TC translation speed in the past and future. Here we present that a slowdown trend of the translation speed is not simulated for the period 1951–2011 based on historical model simulations. We also find that the annual-mean translation speed could increase under global warming. Although previous studies show large uncertainties in the future projections of TC characteristics, our model simulations show that the average TC translation speed at higher latitudes becomes smaller in a warmer climate, but the relative frequency of TCs at higher latitudes increases. Since the translation speed is much larger in the extratropics, the increase in the relative frequency of TCs at higher latitudes compensates the reduction of the translation speed there, leading to a global mean increase in TC translation speed., How the translation speed of tropical cyclones has changed in recent decades and will change in the future has been the subject of debate. Model results show that on average, they have not slowed down in the past, but despite a slowing of tropical cyclones at higher latitudes, a poleward shift in their mean track location causes a general speed up under high greenhouse gas emissions.

There is growing evidence that the atmospheric dynamics of the Euro‐Atlantic sector during winter is driven in part by the presence of quasi‐persistent regimes. However, general circulation models typically struggle to simulate these with, for example, an overly weakly persistent blocking regime. Previous studies have showed that increased horizontal resolution can improve the regime structure of a model but have so far only considered a single model with only one ensemble member at each resolution, leaving open the possibility that this may be either coincidental or model dependent. We show that the improvement in regime structure due to increased resolution is robust across multiple models with multiple ensemble members. However, while the high‐resolution models have notably more tightly clustered data, other aspects of the regimes may not necessarily improve and are also subject to a large amount of sampling variability that typically requires at least three ensemble members to surmount., Key Points Climate models have difficulty representing Euro‐Atlantic regime structure correctlyIncreasing horizontal resolution improves the significance of regime clustering across multiple modelsSpatial patterns and persistence levels of regimes do not necessarily improve with increased resolution

Future changes in tropical cyclone properties are an important component of climate change impacts and risk for many tropical and midlatitude countries. In this study we assess the performance of a multimodel ensemble of climate models, at resolutions ranging from 250 to 25 km. We use a common experimental design including both atmosphere‐only and coupled simulations run over the period 1950–2050, with two tracking algorithms applied uniformly across the models. There are overall improvements in tropical cyclone frequency, spatial distribution, and intensity in models at 25 km resolution, with several of them able to represent very intense storms. Projected tropical cyclone activity by 2050 generally declines in the South Indian Ocean, while changes in other ocean basins are more uncertain and sensitive to both tracking algorithm and imposed forcings. Coupled models with smaller biases suggest a slight increase in average TC 10 m wind speeds by 2050., Key Points Biases in tropical cyclone distribution, frequency, and intensity are generally reduced in models at 25 km resolutionNorthern Hemisphere basins show mixed responses to future forcing, while Southern Indian Ocean activity projected to declineFuture changes in 10 m wind speed in coupled models are mixed, and models with lower bias suggest small increases

The predictability of a downward-propagating event of stratospheric planetary waves observed in early March 2007 is examined by conducting ensemble forecasts using an AGCM. It is determined that the predictable period of this event is about 7 days. Regression analysis using all members of an ensemble forecast also reveals that the downward propagation is significantly related to an amplifying quasi-stationary planetary-scale anomaly with barotropic structure in polar regions of the upper stratosphere. Moreover, the anomaly is 90° out of phase with the ensemble-mean field. Hence, the upper-stratospheric anomaly determines the subsequent vertical-propagating direction of incoming planetary waves from the troposphere by changing their vertical phase tilt, which depends on its polarity. Furthermore, the regressed anomaly is found to have similar horizontal structure to the pattern of greatest spread among members of the predicted upper-stratospheric height field, and the spread growth rate reaches a maximum prior to the occurrence of the downward propagation. Hence, the authors propose a working hypothesis that the regressed anomaly emerges as a result of the barotropic instability inherent to the upper-stratospheric circulation. In fact, the stability analysis for basic states constituting the ensemble-mean forecasted upper-stratospheric streamfunction field using a nondivergent barotropic vorticity equation on a sphere supports this hypothesis. Thus, the barotropic instability inherent to the distorted polar vortex in the upper stratosphere forced by incoming planetary waves from the troposphere determines whether the planetary waves are eventually absorbed or emitted downward in the stratosphere.

An unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the assessment of probabilistic change in localized severe events that have large uncertainty from internal variability. The simulation outputs are open to the public as a database called “Database for Policy Decision Making for Future Climate Change” (d4PDF), which is intended to be utilized for impact assessment studies and adaptation planning for global warming.

In this study, we compare the resolution sensitivity of the Asian Summer Monsoon (ASM) in two Atmospheric General Circulation Models (AGCMs): the MRI-AGCM and the MetUM. We analyze the MetUM at three different resolutions, N96 (approximately 200-km mesh on the equator), N216 (90-km mesh) and N512 (40-km mesh), and the MRI-AGCM at TL95 (approximately 180-km mesh on the equator), TL319 (60-km mesh), and TL959 (20-km mesh). The MRI-AGCM and the MetUM both show decreasing precipitation over the western Pacific with increasing resolution, but their precipitation responses differ over the Indian Ocean. In MRI-AGCM, a large precipitation increase appears off the equator (5–20°N). In MetUM, this off-equatorial precipitation increase is less significant and precipitation decreases over the equator. Moisture budget analysis demonstrates that a changing in moisture flux convergence at higher resolution is related to the precipitation response. Orographic effects, intra-seasonal variability and the representation of the meridional thermal gradient are explored as possible causes of the resolution sensitivity. Both high-resolution AGCMs (TL959 and N512) can represent steep topography, which anchors the rainfall pattern over south Asia and the Maritime Continent. In MRI-AGCM, representation of low pressure systems in TL959 also contributes to the rainfall pattern. Furthermore, the seasonal evolution of the meridional thermal gradient appears to be more accurate at higher resolution, particularly in the MRI-AGCM. These findings emphasize that the impact of resolution is only robust across the two AGCMs for some features of the ASM, and highlights the importance of multi-model studies of GCM resolution sensitivity.

Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951–2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO–AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS.

This study investigates future changes in the accumulated and daily heavy winter snowfall in central Japan and the surrounding regions. We analyze outputs of the 48-member ensemble regional climate simulations in the historical and future climates. In the historical climate simulations, each ensemble member has a 61-year simulation from September 1950 to August 2011. For the future climate simulations, we also conduct 61-year simulations assuming the climate at the end of the twenty-first century (2080–2099) when the global mean surface air temperature is about 4 °C warmer than the pre-industrial climate (1861–1880) as projected under the Representative Concentration Pathway (RCP) 8.5 scenario. Our simulations show that the heavy snowfall occurring at a frequency of every 10 years is enhanced in the inland areas of the central part of the Japanese archipelago (central Japan) where the total winter snowfall amount decreases significantly. Heavy snowfall is also intensified in the northern part of the Asian continent where the surface air temperature is much colder than over central Japan. A composite analysis of heavy snowfall events in central Japan indicates that such events occur when the Japan Sea polar air mass convergence zone (JPCZ) appears during the East Asian winter monsoon season. In the future climate projections, the JPCZ is intensified since the warm ocean supplies more moisture due to warming. An upward wind anomaly is also found over the windward side of mountains where the upward flow is prevalent climatologically. The intensification of both the JPCZ and the upward wind over the mountain ranges result in the enhancement of heavy snowfall in inland areas where the surface air temperature is still below 0 °C.

The first multimodel study to estimate the predictability of a boreal sudden stratospheric warming (SSW) is performed using five NWP systems. During the 2012/13 boreal winter, anomalous upward propagating planetary wave activity was observed toward the end of December, which was followed by a rapid deceleration of the westerly circulation around 2 January 2013, and on 7 January 2013 the zonal-mean zonal wind at 60°N and 10 hPa reversed to easterly. This stratospheric dynamical activity was followed by an equatorward shift of the tropospheric jet stream and by a high pressure anomaly over the North Atlantic, which resulted in severe cold conditions in the United Kingdom and northern Europe. In most of the five models, the SSW event was predicted 10 days in advance. However, only some ensemble members in most of the models predicted weakening of westerly wind when the models were initialized 15 days in advance of the SSW. Further dynamical analysis of the SSW shows that this event was characterized by the anomalous planetary wavenumber-1 amplification followed by the anomalous wavenumber-2 amplification in the stratosphere, which resulted in a split vortex occurring between 6 and 8 January 2013. The models have some success in reproducing wavenumber-1 activity when initialized 15 days in advance, but they generally failed to produce the wavenumber-2 activity during the final days of the event. Detailed analysis shows that models have reasonably good skill in forecasting tropospheric blocking features that stimulate wavenumber-2 amplification in the troposphere, but they have limited skill in reproducing wavenumber-2 amplification in the stratosphere.

A series of ensemble reforecast experiments is conducted to investigate the predictability and the occurrence mechanism of a stratospheric sudden warming occurred in late January 2009, which is a typical polar vortex splitting event. To fully examine the rapid vortex splitting evolution and predictability variation, ensemble forecasts are carried out every day during January 2009. The vortex splitting event is reliably predicted by forecasts initialized after 6 days prior to the vortex breakup. It is also found that the propagating property of planetary waves within the stratosphere is a key to the successful prediction for the vortex splitting event. Planetary waves incoming from the troposphere are reflected back into the troposphere for failed forecasts, whereas they are absorbed within the stratosphere for succeeded forecasts. Composite analysis reveals the following reflection process of planetary waves for the failed forecast: Upward propagation of planetary wave activity from a tropospheric blocking over Alaska is weaker during initial prediction periods; then, the deceleration of the zonal wind in the upper stratosphere becomes weaker over Europe, which produces a preferable condition for the wave reflection; hence, subsequently incoming wave activity from the troposphere over Europe is reflected back over the Siberia inducing the eastward phase tilt of planetary waves, which shuts down the further upward propagation of planetary waves leading to the vortex splitting. Thus, this study shows that the stratospheric condition would be another important control factor for the occurrence of the vortex splitting event, besides anomalous tropospheric circulations enforcing upward propagation of planetary waves.

The authors have developed a new spectral cumulus parameterization scheme that explicitly considers an ensemble of multiple convective updrafts by interpolating in-cloud variables between two convective updrafts with large and small entrainment rates. This cumulus scheme has the advantages that the variables in entraining and detraining convective updrafts are calculated in detail layer by layer as in the Tiedtke scheme, and that a spectrum of convective updrafts with different heights due to the difference in entrainment rates is explicitly represented, as in the Arakawa–Schubert scheme. A conservative and monotonic semi-Lagrangian scheme is used for calculation of transport by convection-induced compensatory subsidence. Use of the semi-Lagrangian scheme relaxes the mass-flux limit due to the Courant–Friedrichs–Lewy (CFL) condition, and moreover ensures nonnegative natural material transport. A global atmospheric model using this cumulus scheme gives an atmospheric simulation that agrees well with the observational climatology.

[1] We conducted three-member ensemble simulations using a global atmospheric model with a high horizontal resolution of a 60 km grid size for the period 1872–2099 (228 years). Between 1872 and 2005, the model was forced with observed historical sea surface temperatures (SST), while between 2006 and 2099, the boundary SST data were estimated using the multimodel ensemble of the Coupled Model Intercomparison Project Phase 3 models and assuming A1B emission scenario. Annual mean precipitation (PAVE), the Simple Daily Precipitation Intensity Index (SDII), and the maximum 5 day precipitation total (R5d) averaged over East Asia increase almost monotonically through the 21st century. The statistically significant area of precipitation intensity increase is larger for 2080–2099 than for 2046–2065. In particular, intense rainfall will increase over northern and southern China during 2080–2099. The conversion rate from water vapor to precipitation per 1°C rise in surface air temperature for SDII and R5D is much larger than that for PAVE during the 21st century. This suggests that extreme rainfall events will occur more frequently than moderate rainfall events even if the amount of temperature rise is same. Future changes in the horizontal transport of water vapor also lead to more intense precipitation over East Asia. In particular, the increase in clockwise water vapor transport due to intensification of the subtropical high contributes to increased intense precipitation over southern China.

Future changes in Northern Hemisphere wintertime storm activity as a consequence of global warming are investigated using the AGCM of Meteorological Research Institute (MRI-AGCM) with horizontal grid sizes of 60 and 20 km. A future (2075–99) climate experiment, in which the change in sea surface temperature (SST) derived from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel ensemble mean is added to observed SST, is compared with a present-day (1979–2003) climate experiment. Results of three-member simulations using the 60-km model are presented. A single simulation using the 20-km model is also presented, showing that similar results are obtained. In the future climate experiment, the number of intense cyclones (sea level pressure below 980 hPa) shows a significant increase whereas the number of total cyclones shows a significant decrease, similar to the results obtained from the CMIP3 models themselves. The increase in intense cyclones is seen on the polar side and downstream side of Atlantic and Pacific storm tracks. At the same time, the growth rate of the cyclones increases in areas upstream of these regions. For the regions with the increasing growth rate, a high correlation is seen between the growth rate of the surface cyclones and upper-tropospheric zonal wind at a monthly-mean time scale. Months of high cyclone growth rate with strong zonal wind in these regions become more frequent, and months of low cyclone growth rate with weak zonal wind become less frequent. One of the possibilities that can explain this relationship is changes in the wave-breaking pattern, that is, a decrease in wave breakings in areas of cyclonic shear and an increase in wave breakings in areas of anticyclonic shear. Associated with these changes, rapid cyclone developments are more commonly seen, and weak, long-lived cyclones become less frequent.

Author(s): Shiogama, H; Imada, Y; Mori, M; Mizuta, R; Stone, D; Yoshida, K; Arakawa, O; Ikeda, M; Takahashi, C; Arai, M; Ishii, M; Watanabe, M; Kimoto, M | Abstract: © 2016, the Meteorological Society of Japan. We describe two unprecedented large (100-member), longterm (61-year) ensembles based on MRI-AGCM3.2, which were driven by historical and non-warming climate forcing. These ensembles comprise the "Database for Policy Decision making for Future climate change (d4PDF)". We compare these ensembles to large ensembles based on another climate model, as well as to observed data, to investigate the influence of anthropogenic activities on historical changes in the numbers of record-breaking events, including: the annual coldest daily minimum temperature (TNn), the annual warmest daily maximum temperature (TXx) and the annual most intense daily precipitation event (Rx1day). These two climate model ensembles indicate that human activity has already had statistically significant impacts on the number of record-breaking extreme events worldwide mainly in the Northern Hemisphere land. Specifically, human activities have altered the likelihood that a wider area globally would suffer record-breaking TNn, TXx and Rx1day events than that observed over the 2001- 2010 period by a factor of at least 0.6, 5.4 and 1.3, respectively. However, we also find that the estimated spatial patterns and amplitudes of anthropogenic impacts on the probabilities of record-breaking events are sensitive to the climate model and/or natural-world boundary conditions used in the attribution studies.

In this study, the authors have investigated the likely future changes in the summer monsoon over the Western Ghats (WG) orographic region of India in response to global warming, using time-slice simulations of an ultra high-resolution global climate model and climate datasets of recent past. The model with approximately 20-km mesh horizontal resolution resolves orographic features on finer spatial scales leading to a quasi-realistic simulation of the spatial distribution of the present-day summer monsoon rainfall over India and trends in monsoon rainfall over the west coast of India. As a result, a higher degree of confidence appears to emerge in many aspects of the 20-km model simulation, and therefore, we can have better confidence in the validity of the model prediction of future changes in the climate over WG mountains. Our analysis suggests that the summer mean rainfall and the vertical velocities over the orographic regions of Western Ghats have significantly weakened during the recent past and the model simulates these features realistically in the present-day climate simulation. Under future climate scenario, by the end of the twenty-first century, the model projects reduced orographic precipitation over the narrow Western Ghats south of 16°N that is found to be associated with drastic reduction in the southwesterly winds and moisture transport into the region, weakening of the summer mean meridional circulation and diminished vertical velocities. We show that this is due to larger upper tropospheric warming relative to the surface and lower levels, which decreases the lapse rate causing an increase in vertical moist static stability (which in turn inhibits vertical ascent) in response to global warming. Increased stability that weakens vertical velocities leads to reduction in large-scale precipitation which is found to be the major contributor to summer mean rainfall over WG orographic region. This is further corroborated by a significant decrease in the frequency of moderate-to-heavy rainfall days over WG which is a typical manifestation of the decrease in large-scale precipitation over this region. Thus, the drastic reduction of vertical ascent and weakening of circulation due to ‘upper tropospheric warming effect’ predominates over the ‘moisture build-up effect’ in reducing the rainfall over this narrow orographic region. This analysis illustrates that monsoon rainfall over mountainous regions is strongly controlled by processes and parameterized physics which need to be resolved with adequately high resolution for accurate assessment of local and regional-scale climate change.

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.

Global warming projection experiments were conducted using a 20-km mesh global atmospheric model, focusing on the change in the rain band of East Asian summer monsoon. To assess the uncertainty of climate change projections, we performed ensemble simulations with the 60-km resolution model combining four different SSTs and three atmospheric initial conditions. In the present-day climate simulations, the 20-km model reproduces the rain band of East Asian summer monsoon better than lower resolution models in terms of geographical distribution and seasonal march. In the future climate simulation by the 20-km model, precipitation increases over the Yangtze River valley in May through July, Korean peninsula in May, and Japan in July. The termination of rainy season over Japan tends to be delayed until August. Ensemble simulations by the 60-km model show that precipitation in the future climate for July increases over the Yangtze River valley, the East China Sea and Japan. These changes in precipitation are partly consistent with those projected by the 20-km model. Simulations by the 20-km and 60-km models consistently show that in the future climate the termination of rainy season over Japan tends to be delayed until August. The changes in the vertically integrated water vapor flux show the intensification of clockwise moisture transport over the western Pacific subtropical high. Most precipitation changes over the East Asia can be interpreted as the moisture convergence resulting from change in the horizontal transport of water vapor.

Tropical mean convection and its organization on different spatiotemporal scales in a simulation using the Meteorological Research Institute/Japan Meteorological Agency (MRI/JMA) global atmospheric general circulation model (AGCM) at 20-km resolution (TL959L60) has been investigated. Comparison with two lower resolution simulations of 120 km (TL159L40) and 180 km (TL95L40) shows that convection, climatological fields, and moist stability over the Indian Ocean and South Pacific convergence zone are better represented in TL959L60 than in lower resolution simulations. However, the simulated three-dimensional structure of the Walker and Hadley cells, and the vertical structure of convective heating, do not show marked improvement relative to lower resolution simulations. The amplitude and phase speed of convectively coupled equatorial waves show that, although the Kelvin waves, the observed hierarchical structure of cloud clusters associated with the Madden–Julian oscillation (MJO) convection, and the wave interrelationship among MJO, Kelvin, and equatorial Rossby waves are marginally improved in TL959L60, increased resolution does not impact the simulation of the MJO itself. In the model, the MJO appears as a standing oscillation with much weaker variance compared to observations. Power spectra and a composite MJO life cycle constructed based on extended empirical orthogonal functions reveal that the amplitude, structure, and propagation characteristics of the MJO remain deficient at all resolutions. Thus, while the very high resolution yields improvements in some aspects of tropical mean convection, these appear to be less important compared to the basic deficiency of the parameterized convection in capturing tropical convection and its spatiotemporal variability.

Global warming projection experiments were conducted using a 20-km mesh global atmospheric model (the 20-km model), focusing on the change in the rain band of the East Asian summer monsoon (the Baiu rain band in Japan). To quantify the dependence of the projected change on the sea surface temperature (SST) prescribed to the 20-km model, we have taken different SSTs given by the two Atmospheric-Ocean General Circulation Model (AOGCM)s of MRI-CGCM2.3.2 and MIROC(hires). In the future climate simulations, the Intergovermental Panel on Climate Change (IPCC) A1B emission scenario was assumed. The future climate simulations show that precipitation and its intensity increase over the Yangtze River valley of China and Western Japan. The termination of the Baiu season tends to be delayed until August. These changes were consistently found in the simulations regardless of different SSTs.

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.

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 present-day-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 greenhouse-warmed condition (by 7.3 m s - 1 in the Northern Hemisphere and by 3.3 m s - 1 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.

Changes in temperature-based extremes over land due to global warming estimated by a global 20-km-mesh atmospheric model are analyzed using Frich’s five extremes indices. At the end of the 21st Century, under the Intergovernmental Panel on Climate Change (IPCC) SRES A1B scenario, the model projects that the total number of frost days (Fd) decreases by more than 20 days per year and the length of the growing season (GSL) increases by about 14-34 days in northern mid- and high latitudes. The heat wave duration index (HWDI) and the percentage of time with a daily minimum temperature above the present-day 90th percentile (Tn90) increase worldwide. The intra-annual extreme temperature range (ETR) decreases in northern high latitudes, east Asia, and eastern North America by 1.3-3.9°C, but it increases by 1.0°C in the Amazon. The high-resolution simulation reveals that changes in these indices are influenced by regional properties, such as the altitude and distance from the coast.

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 Nino 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.

Since the late 1990s, land surface temperatures over Japan have increased during the summer and autumn, while global mean temperatures have not risen in this duration (i.e., the global warming hiatus). In contrast, winter and spring temperatures in Japan have decreased. To assess the impact of both global warming and global-scale decadal variability on this enhanced seasonal temperature contrast, we analyzed the outputs of 100 ensemble simulations of historical and counterfactual non-warming climate simulations conducted using a high-resolution atmospheric general circulation model (AGCM). Our simulations showed that atmospheric fields impacted by the La Nina-like conditions associated with Interdecadal Pacific Oscillation (IPO) have predominantly contributed to the seasonal temperature contrast over Japan. Compared with the impact of negative IPO, the influence of global warming on seasonal temperature contrasts in Japan was small. In addition, atmospheric variability has also had a large impact on temperatures in Japan over a decadal timescale. The results of this study suggest a future increase in heatwave risk during the summer and autumn when La Nina-like decadal phenomena and atmospheric perturbations coincide over a background of global warming.

Chaotic mixing processes and transport barriers around the wintertime stratospheric polar vortex are investigated with an idealized barotropic model, previously used by Ishioka and Yoden. A barotropically unstable jet is forced in order to obtain a fluctuating polar vortex. A flow with quasiperiodic time dependence and an aperiodic flow with similar behavior are investigated using several Lagrangian methods. A typical chaotic mixing process is observed in the quasiperiodic flow, resulting in effective mixing inside and outside of the polar vortex. The mixing regions are on the critical latitudes of several planetary waves that grow through barotropic instability. Poincare sections give accurate locations of chaotic mixing regions, and transport barriers are identified as the edges of invariant torus regimes. In addition to the transport barriers associated with strong potential vorticity gradients, another type of transport barrier exists, which is not related to the steep potential vorticity gra...

[1] The transport and exchange process across the extratropical tropopause is investigated using an atmospheric global climate model with a horizontal grid size of about 20 km, which can depict filamental structures near the tropopause. The advection of an idealized passive tracer, which is initialized to have a nonzero value only above the tropopause, is calculated in January and July in the Northern Hemisphere. A version of the model with coarse resolution is used to examine the horizontal resolution dependence. The exchange between the stratosphere and the troposphere has its maximum amounts at 300 hPa in January and at 200-250 hPa in July. Above 400 hPa, the amount of exchange in the 20-km model is about one-half of that of the 180-km model, in accordance with the existence of a sharp tropopause. In contrast, the amount in the 20-km model below 400 hPa is more than that in the 180-km model, partly due to the inability of simulating filamental structures along with the storms in the 180-km model. However, the net transport across the tropopause does not have large dependence on the horizontal resolution. The net transport below 400 hPa is from the stratosphere to the troposphere and is large over the Pacific and Atlantic storm track in January. The net troposphere to stratosphere transport is seen above 300 hPa and is large near the subtropical jet over the Eurasian continent. A large part of this transport is attributed to the temporal change of the potential vorticity estimated from a vertical difference in the longwave radiation.

Future change in the frequency of atmospheric blocking is investigated through present-day (1979-2003) and future (2075-2099) 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 AlB 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. Copyright 2009 by the American Geophysical Union.

[1] Interannual variations of the flow field and large-scale horizontal transport and mixing inside the wintertime polar vortex of the Southern Hemisphere upper stratosphere are investigated using isentropic winds obtained from the U.K. Met Office assimilated data for nine years of 1992–2000. We focused on the midwinter, July, when the polar vortex is not much distorted, although an eastward propagating wave called the 4-day wave is observed in some years in the polar region. Finite-time Lyapunov exponents are computed, and contour advections are done to examine stirring and mixing in the polar region. When the 4-day wave has a large amplitude, effective mixing through a stretching and folding process is seen inside the polar vortex. Finite-time Lyapunov exponents are sometimes as large as the midlatitudes, and the material contours of small areas grow exponentially in time on the poleward side of 70°S. Such mixing properties are not uniform inside the vortex. When the wave is not clearly seen, on the other hand, wind fields are close to a solid body rotation around the pole, and mixing is very small; Lyapunov exponents are small, and the material contours grow linearly in time by the stretching due to the meridional shear of the polar night jet. Such interannual variability of the strength of the mixing is correlated with the variability of the perturbation amplitude of potential vorticity in the polar region.

[1] This study focuses on projecting future changes in mean and extreme precipitation in Asia, and discusses their uncertainties. Time-slice experiments using a 20-km-mesh atmospheric general circulation (AGCM) were performed both in the present-day (1979–2003) and the future (2075–2099). To assess the uncertainty of the projections, 12 ensemble projections (i.e., combination of 3 different cumulus schemes and 4 different sea surface temperature (SST) change patterns) were conducted using 60-km-mesh AGCMs. For the present-day simulations, the models successfully reproduced the pattern and amount of mean and extreme precipitation, although the model with the Arakawa–Schubert (AS) cumulus scheme underestimated the amount of extreme precipitation. For the future climate simulations, in South Asia and Southeast Asia, mean and extreme precipitation generally increase, but their changes show marked differences among the projections, suggesting some uncertainty in their changes over these regions. In East Asia, northwestern China and Bangladesh, in contrast, mean and extreme precipitation show consistent increases among the projections, suggesting their increases are reliable for this model framework. Further investigation by analysis of variance (ANOVA) revealed that the uncertainty in the precipitation changes in South Asia and Southeast Asia are derived mainly from differences in the cumulus schemes, with an exception in the Maritime Continent where the uncertainty originates mainly from the differences in the SST pattern.