Journal of the Meteorological Society of Japan. Ser. II, 1999
The mechanism of the tropospheric biennial oscillation (TBO) of the ENSO-monsoon system is invest... more The mechanism of the tropospheric biennial oscillation (TBO) of the ENSO-monsoon system is investigated by an MRI coupled atmosphere-ocean general circulation model. In this mechanism, biennial variability of the South Asian monsoon affects the global scale climate variability through interactions with the air-sea coupled system over the Pacific and/or the extratropical circulation. In the strong phase of the TBO, the area of relatively strong monsoon convective maximum over South Asia in the spring to summer season moves southeastward to Indonesia in the autumn to winter season. This movement superimposes on its climatological seasonal cycle. It suggests that the northern winter monsoon convection tends to be strong around Indonesia to northern Australia when the summer Asian monsoon is strong. The anomalous state of the air-sea coupled system in the Pacific sector which forms in the summer season, seems to dissipate from its eastern edge. This occurs by a local atmosphere-ocean coupling process through a large scale Walker circulation. The convection anomalies persist during the entire monsoon season over Indonesia and northern Australia. As a response to this equatorial monsoon convection anomaly, a Matsuno-Gill type stationary Rossby wave is established over the South Asian region. The appearance of upper level anticyclonic circulation and lower level cyclonic circulation anomaly in a strong monsoon year is a favorable condition for bringing the cold air advection over the Eurasian continent. Cold air advection after the strong monsoon persists through the whole winter to spring season to form the cold tropospheric temperature around Central to South Asia. Then reduced land-sea, or north-south temperature contrasts sets up the following weak South Asian summer monsoon. The simulated TBO of the South Asian monsoon is tightly phase locked with a seasonal cycle. The phase of the TBO changes in northern spring, which suggests that the extratropicaltropical interaction be realized mainly during winter to spring through the onset of South Asian monsoon. Our results imply that the TBO is an inherent feature in the land-monsoon-ocean coupled system, and emphasize a more active role of monsoon-extratropical interaction in the Indian sector in winter to spring season for regulating the TBO cycle.
Journal of the Meteorological Society of Japan. Ser. II, 1988
The impact of the sea surface temperature (SST) anomalies and cumulus parameterizations is invest... more The impact of the sea surface temperature (SST) anomalies and cumulus parameterizations is investigated for the case of the 1984 northern summer with the use of the MRI general circulation model. Integrations are performed either with the observed SST for 1984 or with the chmatological SST, either with the original Arakawa-Schubert cumulus parameterization (the A-model) or with a modified scheme (the M-model) which imposes an additional constraint between the minimum entrainment rate and the depth of the predicted planetary boundary layer. Intraseasonal oscillations in the low latitudes are better simulated with the M than with the A. However, the forecast skill with the M depends sensitively on the initial conditions. In one case, the observed eastward moving wave in the velocity potential field at 200mb over the equator is forecasted well, both in its phase and amplitude up to 20 days. Using different initial conditions, the forecast skill for the transient fields was poor. The M-model also has many advantages over the A-model in its climate simulation. It has simulated the Pacific subtropical high in the proper position and succeeded in simulating a Baiu-like rain band. The M-model has a stronger monsoon activity and a stronger sensitivity to the boundary forcing (here the SST anomalies) than the A-model, due to more unstable stratification. The impact of the SST anomaly is better simulated with the M than with the A. Monthly mean forecast skill in the low latitudes varies from month to month, but overall useful skill is found in the three-month mean forecast with the M over the tropics except at the very beginning of the forecast. The SST anomaly impact is not clearly determined, mainly because of the limited forecast samples and partially because of the modest SST anomalies in 1984.
Journal of the Meteorological Society of Japan. Ser. II, 1999
Climatological features and interannual variability of the Asian summer monsoon and its relations... more Climatological features and interannual variability of the Asian summer monsoon and its relationship with equatorial Pacific sea surface temperature (SST) anomalies simulated by a global coupled atmosphereocean GCM (CGCM) are investigated. The coupled model results are compared with the observation as well as the simulations by an atmospheric general circulation model (ALCM). Overall features of the model climatology and variability of the Asian summer monsoon in the CGCM are as close to the observed one as in the ALCM. The simulated SST and its variability in the CGCM shows some bias compared to the observation. The monsoon region in the models, as defined by the seasonal change of wind direction and convection proxy, agrees with the observed. The models are less successful in simulating the summertime wind system in the western Pacific region. The CGCM reasonably well reproduces the observed ENSO-related interannual variability of the tropical circulation system. Its main deficit is associated with a westward displacement of simulated SST variability. There is an underestimation of precipitation around the Philippines. Differences are found between the CGCM and the AGCM in the variability over the Indian monsoon region. The AGCM responds well to the prescribed SST anomaly in the Pacific. It behaves erroneously over the Indian Ocean. This may be related to the fact that the AGCM is only responding to the prescribed SST fields, while the CGCM includes two-way atmosphere-ocean interactions. The CGCM results show that the simulated Indian summer monsoon rainfall anomalies are negatively correlated with the equatorial Pacific SST anomalies. It is consistent with the observed one, i, e., good monsoon is associated with La Nina. As a precursory signal, ground temperature is significantly warmer in spring in central Asia preceding a good monsoon. It is noted that the snow cover anomalies are negative in the above region, but its significance is marginal.
Journal of the Meteorological Society of Japan. Ser. II, 1997
Possible changes in the Asian summer monsoon due to increased atmospheric CO2 are investigated by... more Possible changes in the Asian summer monsoon due to increased atmospheric CO2 are investigated by an MRI global coupled atmosphere-ocean general circulation model. The summer (June-August) monsoon rainfall in India increases significantly with global warming. On the other hand, the monsoon wind shear index, defined as the difference between 850 hPa and 200 hPa zonal winds over the northern Indian Ocean, decreases. At 850 hPa, the westerly wind shifts northward and intensifies from the Sahel to northwest of India, but the monsoon westerly over the Arabian Sea weakens. It is found that increased moisture content in the warmer air leads to larger moisture flux convergence, contributing to the increased rainfall. Therefore, the monsoon wind shear index is not a good indicator for identifying any change of monsoon accompanying global warming. In contrast to the increased rainfall in India, change in rainfall is little over China where soil moisture becomes drier at times of increased CO2. It is also noted that the northern Eurasian continent becomes wetter in the increased-CO2 climate. The magnitude of the interannual variability of the Asian summer monsoon rainfall becomes larger in the CO2 experiment than in the control experiment, particularly in the later stage of the experiment after CO2 doubling. However it should be noted that the interdecadal variation of this interannual variability is also large both in the control and the CO2 experiments.
The performance of the July simulation conducted with a five-level atmospheric general circulatio... more The performance of the July simulation conducted with a five-level atmospheric general circulation model of the Meteorological Research Institute(MRI・GCM-1)is described．The model is the same as was described in Tokiokaα召よ(1985)．
Papers in Meteorology an(l Geophyslcs VoL45，No。4，pp．121-148，May1995 AMIP Simulations of the MRl G... more Papers in Meteorology an(l Geophyslcs VoL45，No。4，pp．121-148，May1995 AMIP Simulations of the MRl GCM by Akio Kitoh，Akira No-a，Yoshinobu Nika藍dou，Tomoaki Ose and Tatsushi Tokioka＊ ハ48オ607rolo9∫(⊃αl Rε56αr6h ln5孟露μオε，丑配ゑ祝翫z，／6αアrαゑぎ305血〆〉αn
Journal of the Meteorological Society of Japan. Ser. II, 2005
This study investigates the impact of anthropogenic climate change on the Indian summer monsoon, ... more This study investigates the impact of anthropogenic climate change on the Indian summer monsoon, and the ENSO-monsoon teleconnection, using the transient climate change simulations of the MRI coupled model (MRI-CGCM2.2). In the present simulations atmospheric greenhouse gas (GHG) concentrations and aerosols are varied to represent observed changes during 1850-2000, and from 2001-2100, at the rate prescribed by the SRES-A2 and B2 scenarios to study the response of the monsoon climate during 2071-2100. All India annual temperature shows warming by 2.35 C (1.64 C), while all-India monsoon rainfall (JJAS) indicates an increase by 9% (6%) in SRES-A2 (B2) scenario relative to the present climate . Increase in the monsoon precipitation is evident over parts of south India, parts of the Bay of Bengal and northwest India. The simulations feature stronger warming over South Asia relative to the Indian Ocean, leading to enhanced land-sea temperature contrast. Enhanced moisture in a warmer scenario, coupled with enhanced monsoon circulation, leads to enhanced moisture transport over the Indian region from both the Bay of Bengal and Arabian Sea leading to increased monsoon precipitation. Decomposition of the moisture transport into divergent and non-divergent components shows that the enhanced monsoon rainfall is mainly due to intensification in the non-divergent component of the moisture transport. While strong and continued increase in monsoon rainfall suggests a change towards wetter mean state, warming of summer (JJA) Nin ˜o-3 SSTs suggests a change towards warmer El Nin ˜o like mean state in the east Pacific. The variability in the Nin ˜o-3 SST shows an increasing trend, as well as fluctuations about the trend. The correlation between the Indian monsoon rainfall and Nin ˜o-3 SST decrease particularly after 2050. This weakening of ENSO-monsoon relationship is also seen in the form of change in the impact of ENSO events on the intra-seasonal monsoon rainfall over India.
Modulation of El Niño–Southern Oscillation at the mid-Holocene [6000 yr before present (6 ka)] is... more Modulation of El Niño–Southern Oscillation at the mid-Holocene [6000 yr before present (6 ka)] is investigated with a coupled ocean–atmosphere general circulation model. The model is integrated for 300 yr with 6-ka and present (0 ka) insolation both with and without flux adjustment, and the effect of flux adjustment on the simulation of El Niño is investigated. The response in the equatorial Pacific Ocean in 6 ka is in favor of weaker El Niño variability resulting from lowered sea surface temperature (SST) and a more diffuse thermocline. Atmospheric sensitivity in 6 ka is larger than that in 0 ka because of increased trade winds, while oceanic sensitivity in 6 ka is weaker than that in 0 ka, resulting from destabilization of the upper ocean, both in the flux- and non-flux-adjusted experiments. However, the use of flux adjustment causes a difference in the total response. El Niño variability in 6 ka does not change much from that in 0 ka with the flux-adjusted case, while the 6-ka El...
Geological Society, London, Special Publications, 2010
Impacts of mountain uplift on the Asian monsoon and adjacent seas are investigated by climate mod... more Impacts of mountain uplift on the Asian monsoon and adjacent seas are investigated by climate model sensitivity studies. Two sets of general circulation model (GCM) experiments are performed. Using an atmosphere-ocean coupled GCM, a progressive mountain uplift experiment is performed. During boreal summer, monsoon precipitation is confined in the deep tropics around 10°N in the no-mountain case, but as mountains become higher, heavy rain areas move inland from the East Asian coast with stronger upward winds and increased rainfall over the southeastern Tibetan Plateau region. An increase of freshwater discharge from the Asian rivers results in a significant decrease of sea surface salinities over the Bay of Bengal, the South China, East China and Yellow Seas. A high-resolution atmospheric GCM experiment, which shows improvement in reproducing the present-day model climatology, gives more precise information on precipitation and the circulation changes caused by mountain uplift.
Journal of the Meteorological Society of Japan. Ser. II, 1991
An atmospheric general circulation model is integrated, forced by the observed near-global (40*S-... more An atmospheric general circulation model is integrated, forced by the observed near-global (40*S-60*N) sea surface temperatures (SST) from September 1969 to February 1990. Recurrent patterns of the rnorithly mean Northern Hemisphere atmosphere in the simulated 20 winters are identified and compared with the observations. The simulated first mode, in the zonal wind at 200 mb, geopotential height at 500 mb, and zonal-mean zonal wind, fluctuates with a time-scale of about 10 years. This decade-scale variation seems to result from the specified SST for this period particularly over the tropical western Pacific northeast of New Guinea. A meridional shift of the East Asian subtropical jet and associated circulation changes contribute to this variation. Surface air temperature variations from around Japan to the extreme northern Pacific are mostly explained by this mode. This mode resembles the observed first and fourth modes of thee geopotential height at 500 mb. The model's systematic error of emphasizing variability over the North Pacific more than that over the North Atlantic results in extracting this mode associated with the meridional shift of the East Asian jet as the leading mode. Additionally, a trend of the equatorward shift of the Southern Hemisphere subtropical jet is also simulated related to the trend in the SST.
Journal of the Meteorological Society of Japan, 1998
Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) during the ... more Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) during the Indian summer monsoon, one over the continent and the other over the equatorial Indian Ocean. An active spell of one TCZ coincides with a weak spell of the other TCZ. Observations also show the presence of positive sea surface temperature (SST) anomalies south of the equator over the Indian Ocean during the weak Indian summer monsoon years. The impact of such SST anomalies on the Indian summer monsoon is investigated through general circulation model ensemble experiments. The results indicate significant response over the Indian region and this response is manifested as a decrease in the monsoon precipitation and the weakening of the mean monsoon circulation. A series of identical experiments with a negative SST anomaly prescribed over the same region with the same magnitude confirm these findings.
Impact of the sea surface temperature anomalies (SST As) on atmospheric circulations are studied ... more Impact of the sea surface temperature anomalies (SST As) on atmospheric circulations are studied with emphasis on the winter climate in Japan with the use of an atmospheric general circulation model. The empirical orthogonal function analyses are performed for precipitation, geopotential height at 500 mb and surface air temperature. It is shown that leading eigenvectors of precipitation are zonally elongated in the tropics and that the distribution of positive and negative precipitation anomalies is dependent on the SST and precipitation field in the control run. Surface air temperature in East Asia is mostly governed by the temperature contrast between Japan-East China region and the Sea of Okhotsk. This is associated with high pressure anomalies in the North Pacific Ocean, which weakens the cold surge from Siberia. Thil circulation is found in the first eigenvector in the present experiment EI(Z500) and is dominant in the run which uses the composited January SSTA observed during warm winter in Japan. The simulated anomalies in mid-latitude circulation correspond well with the observations. An additional run with the SSTA over the equator east of the dateline gives the largest anticyclonic circulation response over the North Pacific Ocean and the warmest surface air temperature anomalies in East Asia. The pattern relevant to warm winter in Japan is not a simple atmospheric response to tropical heating. It is conceivable that inherently there is a dominant circulation mode like EI(Z500) and that this mode can be activated either by a direct and/or an indirect orographic effect or by anomalous heating induced by the SST A. This pattern can be understood as a response to subtropical mass source/sink distribution unaer the framework of a linear theory.
Journal of the Meteorological Society of Japan. Ser. II, 1991
The effect of excessive snow mass over the Eurasian continent on the spring and summer climate is... more The effect of excessive snow mass over the Eurasian continent on the spring and summer climate is investigated by using the MRI * GCM. The ensemble mean of the four runs (SNOW runs) with the excessive snow mass of 5 cm (water equivalent) at the beginning of March over the snow cover area of the continent is compared with that of the control runs, to deduce the effect of the snow mass on the climatic parameters in the later seasons. The main results are summarized as follows: (1) In spring, the albedo effect is dominated in the lower latitudes particularly over the Tibetan Plateau. The reduced net radiation by the anomalous snow cover balances the reduced surface sensible and latent heat fluxes, which account for the significant decrease of surface temperature, cloudiness and total diabatic heating over there in the SNOW runs. (2) In summer, in contrast, the snow-hydrological effect is significant, particularly in the mid-latitudes. The increase of ground wetness in the SNOW runs causes anomalous cooling and higher pressure near the surface. A moderate signal of the weakened Asian summer monsoon is also obtained. However, the increase of evaporation activates cumulus convection, which partly compensates for the decrease of total diabatic heating by the cooling near the surface. This evaporation/convection feedback seems to work, on the other hand, to sustain the increased ground wetness throughout the summer. (3) The atmospheric teleconnection patterns induced by the anomalous snow cover over the Tibetan Plateau and east Asia significantly appear over the north Pacific and the North American continent in spring through late summer. These anomalous circulations cause the considerable decrease of surface temperature over the northeastern part of North America. (4) The implication of these results for the Ice Age issue is also briefly discussed.
Journal of the Meteorological Society of Japan. Ser. II, 1998
Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) during the ... more Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) during the Indian summer monsoon, one over the continent and the other over the equatorial Indian Ocean. An active spell of one TCZ coincides with a weak spell of the other TCZ. Observations also show the presence of positive sea surface temperature (SST) anomalies south of the equator over the Indian Ocean during the weak Indian summer monsoon years. The impact of such SST anomalies on the Indian summer monsoon is investigated through general circulation model ensemble experiments. The results indicate significant response over the Indian region and this response is manifested as a decrease in the monsoon precipitation and the weakening of the mean monsoon circulation. A series of identical experiments with a negative SST anomaly prescribed over the same region with the same magnitude confirm these findings.
Simulations under present (end of the 20th Century) and future conditions (end of the 21st Centur... more Simulations under present (end of the 20th Century) and future conditions (end of the 21st Century with SRES A1B scenario) by using a 20 km-mesh atmospheric general circulation model (AGCM) over 10 years are conducted and the changes in snow due to global warming are investigated. The seasonal march of the snow cover in the present simulation is comparable to that of satellite-based observational data. Distributions of the simulated snow cover and snow water equivalent (SWE) reflect the detailed geographical features. Due to global warming, the beginning of the snowaccumulating season (the end of the snow-melting season) will occur later (earlier) in most snow regions, and the snow cover will decrease except for very few exceptions. SWE will also decrease in wide areas, but over the cold regions (Siberia and the northern parts of North America), SWE will increase due to increases of snowfall in the coldest season. In both the change and the percentage change of the SWE, we can find that the detailed geographical features effect on them. In Japan, the SWE will decrease over the heavy snow areas. However, the percentage changes are relatively smaller over the colder areas. Recently, the Japan Meteorological Agency and the Meteorological Research Institute succeeded in simulating climate for a period in excess of 10 years using a 20 km-mesh global AGCM for the present (end of the 20 th Century) and future (end of the 21st Century) conditions. The results of the present simulation show, for the most part, good agreement with the observational data in the distributions of precipitation, snow cover and others. This manuscript describes changes in snow cover and SWE due to global warming.
The influence of planetary boundary layer parameterization on large-scale conditions was investig... more The influence of planetary boundary layer parameterization on large-scale conditions was investigated using the improved Mellor and Yamada (MY) scheme, containing modified closure constants based on large-eddy simulation data and a modified mixing length, in which it varies with stability of the surface layer. The analyses focused on diurnal variations in vertical mixing of the planetary boundary layer and their effect on large-scale conditions in a climate model. The modified MY scheme produces enhanced vertical mixing due to increased mixing length under unstable conditions in the lower atmosphere and increased stability function under stable conditions in the upper atmosphere, especially over land in the summer hemisphere. Greater mixing during the daytime leads to enhanced maximum values of heat and moisture and diurnal amplitude, and resulting in increased mean value. During nighttime, mechanical mixing affects the minimum values of heat and moisture, mitigating the abrupt decr...
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