Baiu-Changma-Meiyu is a rainy period in early summer over East Asia (Japan, Korea and China) and its variability and change is one of the major focus in climate change projections in these areas. We analyze the changes in intensity and duration of Baiu-Changma-Meiyu rain by global warming using daily precipitation data of fifteen coupled atmosphere-ocean general circulation model (AOGCM) simulations under the SRES AIB scenario at the end of the twenty-first century. It is revealed that a delay in early summer rain withdrawal over the region extending from Taiwan, Ryukyu Islands to the south of Japan is contrasted with an earlier withdrawal over the Yangtjze Basin, although the latter is not significant due to inconsistent sign of changes among the models. Higher mean sea-level pressure anomalies in the tropical western Pacific in the future may be related to these late withdrawals. Changes in onset dates are relatively less compared to those in withdrawal dates.
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.
We observed four dry intrusions that occurred over Sumatera Island (Sumatra) during the intensive rawinsonde observation periods in 1998-2004. The events were accompanied by westerly winds, and included the passage of a single organized synoptic-scale cloud system, with a structure similar to a squall line. The cloud systems had the properties of a Kelvin wave. Eastward propagation speed was around 13 m/s, and the horizontal scale was several thousand kilometers. Severe rain occurred as the cloud systems passed, and dry intrusion suppressed convections in the rear part.
Time-slice experiments are performed using a high-resolution North Pacific ocean general circulation model (NPOGCM) resolving the strong currents near Japan, such as the Kuroshio and the Oyashio, to investigate the effect of global warming on the North Pacific ocean circulation. The NPOGCM is forced by heat, momentum, and fresh-water fluxes obtained from a global warming projection using a global climate model (MRI-CGCM2.2). The annual mean sea-level pressure trend exhibits an annular pattern similar to the positive phase of the Arctic Oscillation in a global warming projection by MRI-CGCM2.2 based on the Intergovernmental Panel on Climate Change (IPCC) SRES A2 emission scenario. Associated with this trend, the anticyclonic atmospheric circulation is intensified over the mid-latitude North-Pacific, leading to a northward shift of the oceanic subtropical wind-driven gyre boundary, where extensions of the Kuroshio exist in MRI-CGCM2.2. Under these forcing changes, NPOGCM projects that in the future climate warm core eddies are more frequently pinched off from the Kuroshio off the eastern coast of Japan, leading to an annual mean SST rise over 5 K at its maximum, compared with the present climate. The projected annual mean sea-level rise ranges from 12 to 18 cm along the coasts of Japan, and about 40 cm over the ocean east of Japan.
Changes in the diurnal variation patterns of precipitation were analyzed using four-hourly data for 106 years (1898-2003) at 46 stations in Japan. On the annual and areal average, precipitation amount has a relative increasing trend in the early morning (02-06 JST) and a decreasing trend in the afternoon (14-18 JST), each with a rate of about 5% per century. These changes reflect those of intense precipitation (≥ 10 mm/4h) in spring and summer, while precipitation amount in autumn and winter shows different changes of diurnal variation patterns. Changes of diurnal cycles of weak precipitation (<3 mm/4h) are generally small, with a slight relative increase in 06-14 JST, and a decrease in 14-18 JST.
In this study, energy spectrum of blocking in the Northern Hemisphere is examined in the framework of the 3D normal mode decomposition using the NCEP/NCAR reanalysis for 51 years. Attention is concentrated to the barotropic component of the atmosphere, where the low-frequency variabilities dominate. The horizontal scale of disturbances is measured by the phase speed of a Rossby mode c instead of the horizontal wavenumber. According to the result of the energetics analysis in the phase speed domain, we find an accumulation of energy at the spherical Rhines speed cR when a blocking occurs. The energy level at cR exceeds the saturation spectrum E, denoted by E = ac2, which is derived theoretically from the criterion of Rossby wave breaking δq/δy <: 0, where a represents a constant proportional to mass of the atmosphere for the unit area, and q is the barotropic potential vorticity. The amplified Rossby wave persists longer at the same geographical location because the Rossby wave is stationary for the scale of cR. The wave cannot break down easily at the wave regime near cR. The result suggests that an atmospheric blocking occurs when the energy level at cR exceeds the Rossby wave saturation spectrum of E = ac2. The blocking structure is characterized by δq/δy < 0, which represents the saturation criterion of Rossby waves to derive the spectrum of E = ac2.
A new version of the Meteorological Research Institute (MRI) coupled general circulation model MRI-CGCM2 (MRI2.3) is developed and compared with the previous version (MRI2.0). The cloud scheme includes diagnostic function for cloud amount separately specified for convective and layer clouds, which is one of the major modifications contributing to the improved model performance. MRI2.3 exhibits better agreement with the observations in many aspects of present-day climate simulations, including the global energy budget, meridional distributions of shortwave and longwave radiation at the top of the atmosphere, and geographical distributions of surface air temperature and precipitation. The effective climate sensitivity of each version is evaluated based on an experiment with a transient (1%/year) increase of carbon dioxide concentration. The effective climate sensitivity of MRI2.3 (2.9 K) is about twice that of MRI2.0 (1.4 K). The change in the cloud-forcing response, particularly for shortwave cloud forcing, is essential for increasing climate sensitivity. A difference in tropical low-level clouds over the subsidence regions contributes significantly to the difference in cloud-forcing changes in response to a climate change. Analyses based on circulation regimes, defined by the vertical velocity at the mid-troposphere, suggest that the cloud-forcing response in the tropics is controlled more by thermodynamic characteristics, such as changes of the stability in the lower troposphere, rather than by large-scale circulation changes, such as a change in the subsidence strength.
Yamase is the name for local northeasterly winds that blow from the Pacific Ocean to the east coastsof Hokkaido, and the Tohoku District from May to August. They are cool and moist winds accompanied by low-level clouds. In the present study, we investigate the Yamase winds over the oceans around Japan, using ocean surface vector winds observed by a satellite-scatterometer. In order to define the Yamase Phenomena, we employed an Empirical Orthogonal Function (EOF) analysis of the AMeDAS surface air temperatures over the Tohoku District from May to August. Daily mean temperature anomaly (ΔT), which is a deviation from the climatological daily mean temperature, is calculated for the 167 AMeDAS stations. The spatial coeflicient function of the EOF first mode (EOF1), shows synchronous temperature changes in the whole Tohoku District, while that of the EOF second mode (EOF2), shows a seesaw pattern of temperature variation, between the Pacific and Japan Sea sides. The EOF-decomposed temperature variation, and ΔT at the Hachinohe station (a Yamase index), has high correlations; for EOF1, the correlation is 0.78, and for EOF1+2, it becomes 0.89. The Yamase Phenomena represented by the Hachinohe negative ΔT, is thus associated with two different patterns of the temperature variations in the Tohoku District. The composite wind map made through the sampling of days, with the Hachinohe ΔT < −2°C, shows northeasterly ocean winds blowing toward the Sanriku coast, an anti-cyclonic circulation center in the eastern Okhotsk Sea, and northeasterly surface winds along the Kuril Islands. When the winds approach the Tohoku District in the Pacific Ocean, they separate two flows around 40°N off Miyako. The surface wind fields associated with the cold phenomena of EOF1 and EOF2 corresponding to the Hachinohe negative ΔT are different. The former indicates the northeasterly wind blowing toward the Sanriku coast, and the latter that the northeasterly winds from the sea south of the Kuril Islands, change their direction around 145°E and the easterly winds blow toward the Sanriku coast. The Kitakami Mountains influence significantly on the Yamase winds, over the coastal Pacific Ocean.
Using GAME reanalysis upper-air data, we attempt to reveal the seasonal evolution of the heat source (Q1), and moisture sink (Q2) over and around the Indochina Peninsula (IP), from April to June 1998. Pre-monsoon rainfall occurs inland of IP prior to the large-scale monsoon onset in middle May. In this period, positive Q1, in excess of 3 K day−1, appears around the middle-lower Mekong River basin (MLMRB) centered around 500-600 hPa, which is accompanied by positive Q2, slightly below the peak level of Q1. This suggests that the pre-monsoon rainfall is associated with a cumulus-type convection. The horizontal distribution of vertically integrated Q1 (<Q1>) and Q2 (<Q2>) over MLMRB is similar, suggesting that the contribution of latent heating is nearly equivalent to sensible heat supply from the land surface. In contrast, negative <Q1>, less than −100 W m−2, is discernible over the Bay of Bengal (BoB), which is collocated with a strong downward motion. Once the large-scale monsoon commences, the value of <Q1> in MLMRB becomes roughly the same magnitude as those in the pre-monsoon season. The vertical profile of Q1 and Q2 are indicative of the existence of stratiform and cumulus-type clouds. On the other hand, the whole troposphere over BoB is abruptly occupied by deep heating, associated with convective activities as implied by positive <Q1> and<Q2>. These atmospheric heat and moisture budget analysis indicate the presence of a large regional difference between MLMRB and BoB, especially in the pre-monsoon season. The qualitative heating difference, which may be attributable to the structure of the cumulus-type convection, is recognizable even after the monsoon onset.
A ten-year integration of the Meteorological Research Institute Coupled atmosphere-ocean Regional Climate Model (MRI-CRCM) was conducted to evaluate the model’s performance in reproducing the present climate around Japan (CRCM run). MRI-CRCM couples a 20 km-mesh atmosphere Regional Climate Model (RCM20), and a North Pacific Ocean General Circulation Model, whose horizontal resolution is 1/4° (longitude) × 1/6° (latitude). Multi-nesting method is used for calculating the atmospheric part. A 60 km-mesh atmosphere Regional Climate Model (RCM60) is nested in MRI Coupled General Circulation Model, and RCM20 is nested in RCM60. To verify the effect of coupling, an atmospheric RCM run, that specified SST from a global coupled model run, was also conducted (ARCM run). Compared with observations, sea surface temperatures (SSTs) are overestimated in both runs for the winter over the Japan Sea. In the CRCM run, the SST simulation is somewhat improved and surface air temperature (1.5 m above the ground) is also lower than in the ARCM run. Japan is classified into seven regions according to climatic features. Compared with observational data, summer surface air temperatures in both runs are somewhat high in most of Japan, except for southern regions. This warm bias is reduced in the CRCM run as compared to the ARCM. The improvements in the CRCM run are especially large in northern Japan, where the differences between the runs exceed 1.5 K. The precipitation simulations are compared with radar-AMeDAS data. A typical winter pattern with much rainfall along the coast of the Japan Sea, compared to the coast of the Pacific Ocean, are well reproduced in both runs, but the simulated precipitation is overestimated on the Pacific coast in winter. Precipitation is generally overestimated in northern Japan, while underestimated in western Japan throughout the year. Precipitation is less in the CRCM than in the ARCM run, because of the reduced latent heat flux and lower SSTs. The MRI-CRCM is an effective means to reproduce the Japanese climate, but has still biases in temperature and precipitation. Further improvements are needed.
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−1, for experiments with an Arakawa-Schubert cumulus parameterization. Another threshold of 14 m s−1 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 Niño/La Niña SST conditions. A series of experiments is conducted with doubled CO2 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 Niño-like and La Niña-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 ≥ 25 m s−1) 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.