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Dynamics on variabilities of global/regional climate and related water cycle processes We are studying interannual variability and long-term trends of global/regional-scale hydro-climate and atmospheric/surface water balance, based on long-term observational data and GCM experiments, to assess both natural variabilities and changes due to the effect of greenhouse gas increase. |
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Asian Monsoon variabilities and related energy and related water cycle
processes The Asian monsoon plays an important role in the global climate system with its energy and water cycle processes. We are studying dynamics of the Asian monsoon variabilities (with diurnal, intraseasonal, seasonal to interannual time scales), based on diagnostic analyses of observational data and numerical experiments by climate models. |
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The role and function of biosphere in the climate system The biosphere (terrestrial ecosystem) plays an essential role in the climate system by controling continental to regional scale energy and water cycle processes. We are investigating these processes based on in-situ observational data obatained through some field campaigns and projects (e.g., in the boreal forest in Siberia, grassland in Mongolia and tropical rainforest in Sarawak), satellite data and numerical experiments by climate models. |
Now in progressing specific theme
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This study investigated the climatological pentad mean annual cycle of
rainfall in Thailand and the associated atmospheric circulation fields.
The data used included two different data of rainfall: rain gauge data
for Thailand from the Thai Meteorological Department and satellite-derived
rainfall data from the Climate Prediction Center (CPC) Merged Analysis
of Precipitation (CMAP). Climatological mean pentad values of rainfall taken over 50 yr clearly show a distinct climatologicalmonsoon break (CMB) occurring over Thailand in late June. The occurrence of the CMB coincides with adrastic change of large-scale monsoon circulation in the seasonal march. The CMB is a significant singularityin the seasonal march of the Southeast Asia monsoon, which divides the rainy season into the earlymonsoon and the later monsoon over the Indochina Peninsula. A quasi-stationary ridge dynamically induced by the north–south-oriented mountain range in Indochina is likely to cause the CMB. The formation of the strong ridge over the mountain ranges of Indochina is preceded by a sudden enhancement (northward expansion) of the upstream monsoon westerlies along a latitudinal band between 15° and 20°N in late June. The CMB also has an impact downstream. The orographically induced stationary Rossby waves enhance the cyclonic circulation to the lee of Indochina, and over the South China Sea. The enhancement of cyclonic circulation may be responsible for the summer monsoon rains peaking in late June over the South China Sea and the western North Pacific, and in the baiu front. (Takahashi, H. G., and T. Yasunari 2006: A climatological monsoon break in rainfall over Indochina in the summer and its influence on the seasonal march of the Asian monsoon circulation. Journal of Climate, 19, 1545-1556.) |
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This study investigated diurnal cycles in convection and precipitation over the complex mountain-valley terrain of the southern Tibetan Plateau (TP) during the mature phase of the summer monsoon. Cloud-cover frequency (CCF) for high cloud increased after 13 LST (07 UTC) over the mountain ranges along 28.5°N and 30.2°N, reaching a maximum near 18 LST (12 UTC). Areas of high CCF subsequently moved towards the valley area along 29.3°N; relatively high CCF persisted there until early morning. Tropical Rainfall Measuring Mission (TRMM) PR data show a nearly identical variation in rainfall frequency. Formation and development of convective-type clouds and phase differences in the diurnal cycle were strongly affected by TP topography. Possible mechanisms for convective enhancement over the southern TP are also discussed. (Fujinami, H., S. Nomura, and T. Yasunari 2005: Characteristics of diurnal variations in convection and precipitation over south Tibeatan Plateau during summer. SOLA, 1, 49-52.) |
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Convective variability at submonthly timescales (7-20 days) over the Tibetan
Plateau and the associated large-scale atmospheric circulation and convection
were examined over regions affected by the Asian Monsoon. The mature phase of
the Asian summer monsoon (July-August) was analyzed for those years (1986, 93,
98) in which convective variability on timescales of 14 days was notable over
the Tibetan Plateau. Composite analyses of OLR, based on the filtered Tbb time series over the southern Tibetan Plateau, show that significant convective signals rotate clockwise around 28°N, 90°E, affecting the Tibetan Plateau, Indochina, the Bay of Bengal, and India. Significant signals also appear around the Philippines and the South China Sea. A well-developed wave train extending from North Africa to far-east Asia along the Asian subtropical jet is associated with convective fluctuations over the plateau. The waves are quasi-stationary and have a Rossby wave-like downward wave train with wavenumber 7. The waves control convective fluctuations over the plateau. During the transition to active (inactive) convection, an upper-level trough (ridge) develops west of the plateau. Simultaneously, cyclonic anomalies strengthen over India between the lower and middle troposphere. The development of the two troughs induces a southerly flow of moist air toward the plateau. Moistening of the lower atmosphere creates favorable conditions for subsequent active moist convection. Possible processes for forming the wave train over the subtropical jet and a link for convective signals between midlatitudes and the Asian monsoon are discussed. (Fujinami, H. and T. Yasunari, 2004: Submonthly Variability of Convection and Circulation over and around the Tibetan Plateau during the Boreal Summer. J. Meteor. Soc. Japan, 82, 1545-1564. ) |
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We investigated the effects of large-scale orography on the tropical coupled
atmosphere-ocean system over the Indian and Pacific Oceans in northern summer,
using the Meteorological Research Institute coupled atmosphere-ocean General
Circulation Model (GCM). Six different experiments were conducted with mountain
heights of 100%, 80%, 60%, 40%, 20%, and 0% of the standard mountain height. The
results show that a pool of warm sea surface temperatures (SSTs) appears in the
western Pacific as orography increases, although SST in the tropical Pacific
decreases as a whole. In addition, easterly winds at low levels over the
equatorial Pacific strengthen as mountains rise. The enhanced easterlies alter
surface heat flux and ocean dynamics, changing the water temperature field in
the upper Pacific Ocean. Water temperatures between the surface and 300 m in the
western Pacific increase as upwelling is suppressed and the thermocline deepens.
Water temperatures in the eastern Pacific decrease and the thermocline rises.
Therefore, the east-west gradient of water temperature in the Pacific is
enhanced for cases with mountain heights of 80% and 100% of the standard
mountain height. In the equatorial Indian Ocean, the east-west gradient of ocean
heat content weakens as mountain heights increase, in connection with the
evolution of the Asian summer monsoon. An increase in diabatic heating over
South Asia as mountain heights increase causes sea level pressure (SLP) to
decline over the Indian Ocean, and enhances upper atmospheric divergence over
the eastern hemisphere. Consequently, the east-west circulation over the Indian
and Pacific Oceans strengthens as mountains become taller. The east-west
circulation may also be enhanced by changes in convective activity associated
with SST changes. The coupled general circu- lation model (GCM) results show
that uplift of large-scale orography, particularly the Tibetan Plateau,
significantly affects the tropical atmospheric and oceanic climate, by changing
the east-west circulation and altering the evolution of the Asian summer
monsoon. (Abe, M., T. Yasunari and A. Kitoh, 2004: Effects of Large-scale Orography on the Coupled Atmosphere-Ocean System in the Tropical Indian and Pacific Oceans in Boreal Summer. J. Meteor. Soc. Japan, 82, 745-759. ) |
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Using the MRI global atmosphere-ocean coupled general circulation model, we had
six simulations with different mountain heights, i.e., 0% (M0), 20% (M2), 40%
(M4), 60% (M6), 80% (M8), and 100% (M, control run) of the present global
orography, respectively, to study climate changes due to progressive mountain
uplift. The changes of the Asian summer monsoon, with progressive mountain
uplift is studied in this paper. An active convection region extends with mountain uplift to form a moist climate in South and East Asia. Monsoon circulation such as low-level westerly, and upper-level anticyclonic circulation, is also enhanced with mountain uplift. The increase in precipitation, and the enhancement of southwesterly, in the later stages of the mountain uplift, appear only over India and the south and southeastern slope of the Tibetan Plateau. Over the coastal region of Southeast and East Asia, where the maximum precipitation appears in M0, precipitation decreases gradually with mountain uplift, and the southwesterly in the later stages becomes weaker. In the connection with these changes, surface heat flux changes remarkably over moist Asia in the earlier stages of mountain uplift, compared with that in the later stages. The intensity of the Indian, Southeast Asian, and East Asian monsoon was investigated with indices which are defined by area mean precipitation. The Indian monsoon becomes strong gradually with mountain uplift; particularly, in the later stages, the remarkable enhancement is found. The intensity of the South Asian monsoon is the strongest in M4. Thus, in the later stages of mountain uplift, that becomes weaker in association with the northwestward migration of the convective activity. Although the East Asian monsoon is enhanced gradually with mountain uplift, the enhancement in the earlier stages is larger than that in the later stages. In the equatorial Indian Ocean, SST also increases with mountain uplift, resulting in the increase in precipitation. The increase in SST results from the change of the ocean surface dynamics due to the enhanced monsoon circulation. This result could not be obtained if CGCM was not used in this study. (Abe, M., A. Kitoh and T. Yasunari, 2003: An Evolution of the Asian Summer Monsoon Associated with Mountain Uplift -Simulation with the MRI Atmosphere-Ocean Coupled GCM-. J. Meteor. Soc. Japan, 81, 909-933. ) |
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The onset processes of the zonally asymmetric anomalies of convection and sea
surface temperature(SST) over the tropical Indian Ocean are investigated with
considering seasonal evolution, and interannualvariability, of the large-scale
convection anomalies in the Asian summer monsoon, using outgoinglongwave
radiation (OLR), SST, and NCEP/NCAR reanalysis data. This asymmetric pattern
ofthe convection anomalies is particularly dominated in boreal autumn. Some
recent studies have notedthat these anomalies, based on the atmosphere-ocean
coupling phenomenon, can be developed and maintainedby itself. The time evolution shows that the eastern part of the zonally asymmetric anomalies over the IndianOcean lead the western part of those. In July, the negative SST anomalies and positive OLR anomaliesfirst appeared off the Sumatra coast, and southeasterly wind anomaly accelerated the climatologicalsoutheasterly wind along the west coast of Sumatra. This southeasterly wind acceleration provide a SSTcooling over the southeastern Indian Ocean, and play a role in triggering of the zonally asymmetricanomalies in the following autumn. It is suggested that this southeasterly wind acceleration over thesoutheastern Indian Ocean is closely linked to the meridionally asymmetric anomalies of convection,between the maritime continent and the South China Sea/ Philippine Sea (SCS/PS). That is, the intensificationof the local Hadley circulation over the western Pacific associated with the enhanced convectionover the SCS/PS, and suppressed convection over the maritime continent, is found to be a clearprecursory signal of the zonally asymmetric anomalies over the Indian Ocean. It has also been notedthat the convection anomalies over the southern and northern parts of the meridionally asymmetricanomalies over the western Pacific are not always the opposite sign, and seem to have different interannualvariability respectively. It is likely that the former might be strongly influenced by the ENSO,through the Walker circulation anomalies and the latter might be affected by the modulation of the intraseasonalvariation of the Asian summer monsoon. The seasonality of the zonally asymmetric anomaliesis also suggested from the occurrence of the intensification of the local Hadley circulation in borealsummer. (Kajikawa, Y., T. Yasunari and R. Kawamura, 2003: The Role of the Local Hadley Circulation over the Western Pacific on the Zonally Asymmetric Anomalies over the Indian Ocean. J. Meteor. Soc. Japan, 81-2, 259-276) |