Tetsuzo Yasunari

Monsoons: Processes, predictability, and the prospects for prediction

The Tropical Ocean-Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean-atmosphere system. The World Climate Research Programmes (WCRP) Global Ocean-Atmosphere-Land System (GOALS) program seeks to explore predictability of the global climate system through investigation of the major planetary heat sources and sinks, and interactions between them. The Asian-Australian monsoon system, which undergoes aperiodic and high amplitude variations on intraseasonal, annual, biennial and interannual timescales is a major focus of GOALS. Empirical seasonal forecasts of the monsoon have been made with moderate success for over 100 years. More recent modeling efforts have not been successful. Even simulation of the mean structure of the Asian monsoon has proven elusive and the observed ENSO-monsoon relationships has been difficult to replicate. Divergence in simulation skill occurs between integrations by different models or between members of ensembles of the same model. This degree of spread is surprising given the relative success of empirical forecast techniques. Two possible explanations are presented: difficulty in modeling the monsoon regions and nonlinear error growth due to regional hydrodynamical instabilities. It is argued that the reconciliation of these explanations is imperative for prediction of the monsoon to be improved. To this end, a thorough description of observed monsoon variability and the physical processes that are thought to be important is presented. Prospects of improving prediction and some strategies that may help achieve improvement are discussed.

Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident

The largest concern on the cesium-137 (137Cs) deposition and its soil contamination due to the emission from the Fukushima Daiichi Nuclear Power Plant (NPP) showed up after a massive quake on March 11, 2011. Cesium-137 (137Cs) with a half-life of 30.1 y causes the largest concerns because of its deleterious effect on agriculture and stock farming, and, thus, human life for decades. Removal of 137Cs contaminated soils or land use limitations in areas where removal is not possible is, therefore, an urgent issue. A challenge lies in the fact that estimates of 137Cs emissions from the Fukushima NPP are extremely uncertain, therefore, the distribution of 137Cs in the environment is poorly constrained. Here, we estimate total 137Cs deposition by integrating daily observations of 137Cs deposition in each prefecture in Japan with relative deposition distribution patterns from a Lagrangian particle dispersion model, FLEXPART. We show that 137Cs strongly contaminated the soils in large areas of eastern and northeastern Japan, whereas western Japan was sheltered by mountain ranges. The soils around Fukushima NPP and neighboring prefectures have been extensively contaminated with depositions of more than 100,000 and 10,000 MBq km-2, respectively. Total 137Cs depositions over two domains: (i) the Japan Islands and the surrounding ocean (130–150 °E and 30–46 °N) and, (ii) the Japan Islands, were estimated to be more than 5.6 and 1.0 PBq, respectively. We hope our 137Cs deposition maps will help to coordinate decontamination efforts and plan regulatory measures in Japan.

Impact of Indian Monsoon on the Coupled Atmosphere)Ocean System in the Tropical Pacific

SummaryThis study addresses the relationship between the Indian summer monsoon (ISM) and the coupled atmosphere/ocean system in the tropical Pacific on the interannual time scales. High positive correlations are found between ISM rainfall and both mixed layer sea water temperature (SWT) and sea surface temperature (SST) anomalies of the tropical western Pacific in the following winter. Negative correlations between ISM rainfall and SST in the central/eastern Pacific also appear to be most significant in the following winter. These parameters are correlated with each other mainly on a biennial time scale. Lag-correlations between the zonal wind and SST along the the equatorial Pacific show that the westerly (easterly) surface wind stress anomalies over the central/western Pacific are greatly responsible for the formation of negative (positive) SST/SWT anomalies in the western Pacific and positive (negative) SST/SWT anomalies in the central/eastern Pacific. Furthermore, it is evidenced that these lagcorrelations are physically based on the anomalies in the large-scale convection over the Asian monsoon region and the associated east-west circulation over the tropical Pacific, which first appear during the Indian summer monsoon season and evolve during the following autumn and winter. These results strongly suggest that the Asian summer monsoon may have an active, rather than a passive, role on the interannual variability, including the ENSO events, of the coupled atmosphere/ocean system over the tropical Pacific.

The Monsoon Year — A New Concept of the Climatic Year in the Tropics

The concept of “monsoon year” is proposed as a unit year of climatic anomalies (i.e., the climatic year) in the tropics. This monsoon year is defined as one year starting just before the northern summer monsoon season. It is also argued that this climatic year in the tropics is physically based upon the characteristic nature of the coupled ocean/land/atmosphere system over the Asian monsoon/Pacific Ocean sector.

Irregular droughts trigger mass flowering in aseasonal tropical forests in asia

General flowering is a community-wide masting phenomenon, which is thus far documented only in aseasonal tropical forests in Asia. Although the canopy and emergent layers of forests in this region are dominated by species of a single family, Dipterocarpaceae, general flowering involves various plant groups. Studying proximate factors and estimating the flowering patterns of the past and future may aid our understanding of the ecological significance and evolutionary factors behind this phenomenon. Here we show that this phenomenon is most likely triggered by irregular droughts based on 10 years of observations. In the aseasonal forests of SE Asia, droughts tend to occur during transition periods from La Niña to El Niño, which results in an irregular 6-7-yr cycle involving a dry period with several droughts and a wet period without droughts. The magnitude of a flowering event also depends on the timing of droughts associated with the El Niño southern oscillation (ENSO) cycle, with the largest events occurring after an interval of several years with no flowering. Because most plant species can only reproduce successfully during large flowering events, changes in the ENSO cycle resulting from global warming, may have serious ramifications for forest regeneration in this region.

Time–Space Characteristics of Diurnal Rainfall over Borneo and Surrounding Oceans as Observed by TRMM-PR

Abstract Five years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data were used to investigate the time and space characteristics of the diurnal cycle of rainfall over and around Borneo, an island in the Maritime Continent. The diurnal cycle shows a systematic modulation that is associated with intraseasonal variability in the large-scale circulation pattern, with regimes associated with low-level easterlies or westerlies over the island. The lower-tropospheric westerly (easterly) components correspond to periods of active (inactive) convection over the island that are associated with the passage of intraseasonal atmospheric disturbances related to the Madden–Julian oscillation. A striking feature is that rainfall activity propagates to the leeward side of the island between midnight and morning. The inferred phase speed of the propagation is about 3 m s−1 in the easterly regime and 7 m s−1 in the westerly regime. Propagation occurs over the entire island, causing a leeward enhan...

On the roles of the northeast cold surge, the Borneo vortex, the Madden-Julian Oscillation, and the Indian Ocean Dipole during the extreme 2006/2007 flood in southern Peninsular Malaysia

The mid-December 2006 to late January 2007 flood in southern Peninsular Malaysia was the worst flood in a century and was caused by three extreme precipitation episodes. These extreme precipitation events were mainly associated with strong northeasterly winds over the South China Sea. In all cases, the northeasterlies penetrated anomalously far south and followed almost a straight trajectory. The elevated terrain over Sumatra and southern Peninsular Malaysia caused low-level convergence. The strong easterly winds near Java associated with the Rossby wave-type response to Madden-Julian Oscillation (MJO) inhibited the counter-clockwise turning of the northeasterlies and the formation of the Borneo vortex, which, in turn, enhanced the low-level convergence over the region. The abrupt termination of the Indian Ocean Dipole (IOD) in December 2006 played a secondary role as warmer equatorial Indian Ocean helped in the MJO formation.

Changes in the Asian monsoon climate during 1700–1850 induced by preindustrial cultivation

Preindustrial changes in the Asian summer monsoon climate from the 1700s to the 1850s were estimated with an atmospheric general circulation model (AGCM) using historical global land cover/use change data reconstructed for the last 300 years. Extended cultivation resulted in a decrease in monsoon rainfall over the Indian subcontinent and southeastern China and an associated weakening of the Asian summer monsoon circulation. The precipitation decrease in India was marked and was consistent with the observational changes derived from examining the Himalayan ice cores for the concurrent period. Between the 1700s and the 1850s, the anthropogenic increases in greenhouse gases and aerosols were still minor; also, no long-term trends in natural climate variations, such as those caused by the ocean, solar activity, or volcanoes, were reported. Thus, we propose that the land cover/use change was the major source of disturbances to the climate during that period. This report will set forward quantitative examination of the actual impacts of land cover/use changes on Asian monsoons, relative to the impact of greenhouse gases and aerosols, viewed in the context of global warming on the interannual, decadal, and centennial time scales.

Western Disturbances: A review

Cyclonic storms associated with the midlatitude Subtropical Westerly Jet (SWJ), referred to as Western Disturbances (WDs), play a critical role in the meteorology of the Indian subcontinent. WDs embedded in the southward propagating SWJ produce extreme precipitation over northern India and are further enhanced over the Himalayas due to orographic land-atmosphere interactions. During December, January, and February, WD snowfall is the dominant precipitation input to establish and sustain regional snowpack, replenishing regional water resources. Spring melt is the major source of runoff to northern Indian rivers and can be linked to important hydrologic processes from aquifer recharge to flashfloods. Understanding the dynamical structure, evolution-decay, and interaction of WDs with the Himalayas is therefore necessary to improve knowledge which has wide ranging socioeconomic implications beyond short-term disaster response including cold season agricultural activities, management of water resources, and development of vulnerability-adaptive measures. In addition, WD wintertime precipitation provides critical mass input to existing glaciers and modulates the albedo characteristics of the Himalayas and Tibetan Plateau, affecting large-scale circulation and the onset of the succeeding Indian Summer Monsoon. Assessing the impacts of climate variability and change on the Indian subcontinent requires fundamental understanding of the dynamics of WDs. In particular, projected changes in the structure of the SWJ will influence evolution-decay processes of the WDs and impact Himalayan regional water availability. This review synthesizes past research on WDs with a perspective to provide a comprehensive assessment of the state of knowledge to assist both researchers and policymakers, and context for future research.

Agrometeorological conditions of grassland vegetation in central Mongolia and their impact for leaf area growth

[1] The long-term observation of surface heat and water budget and hydrometeorological elements has been carried out over a grassland site at Arvaikheer (46.23� N, 102.82� E) in central Mongolia as the framework of the GEWEX Asian Monsoon Experiment-Asian Automatic Weather Station Network (GAME-AAN). The purpose of this study is to clarify the relationship between vegetation and climate using long-term data (1982–2000) of satellite-derived leaf area index (LAI) and climatic data observed at Arvaikheer. Furthermore, we aimed to reveal physical process by comparing soil moisture and heat and water budgets in 1999 and 2000 as a case study of good and poor vegetation growth. Significant positive correlations with 99% confidence levels were found for July precipitation (P) and the LAI in July (0.538), August (0.826), and September (0.564). Composite analysis for five highest (H5) and lowest (L5) LAI years showed the significant positive anomalies of P in July and LAI in July and August for H5. In June and July 1999, soil moisture and P values were higher than values in 2000; this pattern was reversed in August and September. The mean LAI during the 1999 growing season (1.0) was about twice that of 2000 (0.6). In 1999 the ratio of evapotranspiration (ET) to P (ET/P) and change of stored soil moisture (DW )t o P( DW/P) were 0.79 and 0.15, respectively. In 2000, ET/P and DW/P were 0.94 and 0.0, respectively. These results suggest that the P and DW before July had the most influent on grass growth in central Mongolia. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1866 Hydrology: Soil moisture; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3322 Meteorology and Atmospheric Dynamics: Land/ atmosphere interactions; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Mongolia, grassland, leaf area index, soil moisture, rainfall, evapotranspiration