Journal of the Meteorological Society of Japan. Ser. II
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Volume 70 , Issue 1B
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  • T. N. Krishnamurti, M. C. Sinha, Ruby Krishnamurti, D. Oosterhof, J. C ...
    Volume 70 (1992) Issue 1B Pages 131-166
    Released: March 31, 2009
    JOURNALS FREE ACCESS
    In this paper some global aspects of the intraseasonal oscillations on the time scale of 30 to 50 days are explored. Noting that the variability of zonal flow of the monsoon, the atmospheric angular momentum and the length of day are strongly correlated on this time scale, we have made an effort to examine the global variability using the length of day as a point of reference. The scenario of this cycle is presented starting from a super cloud cluster at the near equatorial latitudes. This seems to be accompanied with an acceleration of zonal flows, an increase of the atmospheric angular momentum and an increase in the length of day. The transfer of westerly angular momentum from the earth to the atmosphere occurs over regions of the surface easterlies to the east of the super cloud clusters resulting in an increase in the length of day. During this transition from a mean length of day to a maximum length of day, an active phase of the Indian summer monsoon is noted. The interesting aspect of the length of day transition occurs on its return cycle when the near equatorial cloud cover eases or moves away from the equator with a decrease in the monsoonal zonal flows and a reduction of this component of atmospheric angular momentum. The length of day does not simply go back to an equilibrium value, but the long term data from the laser ranger shows an overshooting beyond that to a minimum value. This transition is characterized in general by monsoon break-like conditions, counter monsoon flows in the low levels and by a transition from high index to low index conditions in the upper troposphere of the middle latitudes. Phenomenologically, some blocking situations have been noted over the higher middle latitudes during this transition. The reduction of the angular momentum is attributed to the transfer of the westerly angular momentum from the atmosphere to the earth via frictional and mountain torques. These torques exhibit a clear relationship to the changes in the atmospheric angular momentum on this time scale. The behavior of the middle latitude low frequency variability is also in part explained by the meridional wave energy flux. That problem is examined in this context with the full non-linear equations in the frequency domain. It is shown that unlike the linear problems where such fluxes are inhibited beyond the critical latitude, the nonlinear problem permits the temporal oscillations of zonal flows on this time scale. As a consequence, a significant tropical-middle latitude coupling is noted by this process.
    A simple mathematical model of the oscillation is also presented. It is a local theory in which ocean and atmosphere interact. Initially, the atmosphere is stably stratified with weak winds at the sea surface and stronger winds aloft; the ocean has a surface mixed layer of temperature Ts lying over deep cold water. Solar heating gradually increases Ts which leads to atmospheric convection with associated transport of horizontal momentum and increased winds at the sea surface. Increased winds lead to deepening of the mixed layer and a drop in Ts because of mixing of deep cold water with surface waters. Convection ceases, winds decay, and the cycle repeats only after solar heating has once more increased Ts. The period of this oscillation is shown to be on the order of 30 days.
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  • Toshio Yamagata, Yukio Masumoto
    Volume 70 (1992) Issue 1B Pages 167-175
    Released: March 31, 2009
    JOURNALS FREE ACCESS
    Long-term hydrographic observations repeated by the Japan Meteorological Agency in the western Pacific have revealed not only the oceanic thermal variability associated with the ENSO events but also another interdecadal variability seemingly related to the global warming trend from the late 1970s. Since the background SST is high in the tropical western Pacific, even weak SST anomalies may affect strongly the atmospheric circulation including the Aleutian Low and the Asian winter monsoon. The extratropical atmospheric response to the interdecadal SST anomaly is global and looks quite different from that for the ENSO time scale. We note that the response is even reversed in the Asian monsoon region.
    To the west of the date line the ocean behaves like a dynamical slave to the winter Asian monsoon as demonstrated using the ocean general circulation model. In particular, the winter monsoon and related easterly wind variations are responsible for maturity or immaturity of the cold Mindanao Dome off the Phillipine coast. However, the active (inactive) summer monsoon followed by the anomalous easterlies (westerlies) intensified over the tropical western Pacific from summer through winter appears to be responsible for the positive (negative) SST anomalies in the same area at least for the ENSO time scale. This suggests that an interesting positive feedback mechanism responsible for natural climate variabilities ranging from several years to decades may exist in the coupled ocean-atmosphere-land system in the western tropical Pacific.
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  • Tetsuzo Yasunari, Yuji Seki
    Volume 70 (1992) Issue 1B Pages 177-189
    Released: March 31, 2009
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    The role of the Asian summer monsoon on the interannual variability of the global climate system particularly relevant to the ENSO time scales is discussed, by examining the statistical and dynamical links between the Asian summer monsoon, the atmosphere/ocean system in the tropics and the westerly flow régimes in the extratropics.
    The Asian monsoon, the ocean and the atmosphere in the tropical Pacific are tightly linked together as one climate system, named here as the MAOS (Monsoon and the Coupled Atmosphere/Ocean System). The MAOS prominently shows the biennial oscillatory nature which tends to have anomalous states starting in the northern summer monsoon season and persisting for about one year (Yasunari, 1990a: 1991).
    The anomalous state of the MAOS produces the anomalous atmospheric circulation over the subtropics and the extratropics of the north Pacific during summer through the early winter, through the modulation of the subtropical high and the stationary Rossby wave propagation mechanism. In the mid winter, this anomalous circulation over the north Pacific is evolved to the hemispheric winter anomalous circulation with wavenumber-one and/or-two structure.
    The anomalous circulation over Eurasia associated with this hemispheric anomalous flow régime seems to provide a favorable condition for the extensive (or diminished) snow cover area over central Asia, which in turn is responsible for the reversed anomalous state of the next Asian summer monsoon and the MAOS. That is, the biennial nature of the climate system in the northern hemisphere may be due, at least partly, to this two-way interactions between the tropics and the extratropics. In these processes, the Asian monsoon plays a key role as a transmitter of climate signals between the tropics and the extratropics through the land/atmosphere/ocean interaction in the seasonal cycle.
    In addition, it is strongly suggested that the North Atlantic Oscillation (NAO), in reality, plays a crucial role in the timing of the occurrence of the ENSO event, by stochastically amplifying or damping the biennial oscillation of this coupled climate system. That is, the more or less irregular ENSO cycle may result from this interaction between the MAOS and the NAO, where the former seems to have the nature of an almost-intransitive climate system, while the latter seems to represent the more chaotic nature of the westerly flow régime.
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  • Takio Murakami, Bin Wang, Steven W. Lyons
    Volume 70 (1992) Issue 1B Pages 191-210
    Released: March 31, 2009
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    Structure of the July mean local Hadley circulation is investigated by utilizing wind, surface pressure, sea surface temperature (SST), and outgoing longwave radiation (OLR) data along 90°E and 110°W. The objective is to compare two types of monsoon, one is totally driven by land-sea contrast (Asiatic summer monsoon) and the other primarily induced by SST contrast over the ocean (eastern North Pacific summer monsoon).
    In July, there exists a large-scale Hadley circulation covering both hemispheres along 90°E. Superimposed upon this are distinctly separate regional-scale Hadley cells, one each over either hemisphere. The northern hemisphere Hadley cell is associated with southerly surface winds accelerating from the equator toward the monsoon trough as a low-level branch, vigorous summer monsoon rains near the head Bay of Bengal representing an updraft leg, and 200mb northerly winds as an upper-level return flow. Finally, the northern cell is completed by an ill-defined downdraft leg due to divergent southerly surface winds over the equatorial Bay of Bengal. Continentality with high mountains is the major contributor to enhancement of the summer monsoon over the Bay of Bengal, while the contribution due to SST is minimal. The southern hemisphere Hadley cell is supported by low-level southerly surges bursting out of the midlatitude anticyclone and subjected to considerable warming and air mass modification due to strong SST gradients between 20°S and 10°S. Modified air converges into the near-equatorial trough and causes winter-time rains near 2°-8°S, where an updraft leg of the southern hemisphere Hadley cell is located. The southern Hadley cell interacts with the northern counterpart via low-level cross-equatorial southerlies that are generated as a response to an overall heat contrast between the winter and summer hemispheres.
    Continentality appears to be insignificant along 110°W. The eastern Pacific is characterized by a zonally oriented cold SST tongue along the equator. This exceptionally strong south-to-north SST gradient causes a prominent poleward surface pressure gradient, which tends to accelerate southerly surface winds to the north of the equator. A large amount of energy is likely to be furnished to these southerlies due to a rapid increase of SST between the equator and about 10°N. Convection becomes strongest near 12°N, two to three degrees south of the thermal equator where SST is maximum (28.3°C). No convection occurs over the tropical South Pacific because of cold SST (less than 25°C). Throughout the year, the inter-tropical convergence zone (ITCZ) remains in the tropical North Pacific. The summer monsoon occurs when the ITCZ (updraft leg of the Hadley circulation) reaches it's northern most latitude of about 14°N in July. Over the equatorial eastern Pacific, the annual variation of surface meridional winds (low-level Hadley circulation) is much more pronounced than the annual variation of surface zonal winds (low-level Walker circulation). This favors the development of the summer monsoon over the eastern North Pacific.
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  • K. -M. Lau
    Volume 70 (1992) Issue 1B Pages 211-242
    Released: March 31, 2009
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    In this paper, recent progress in the study of the East Asian summer Monsoon (EAM) and its impact on global climate fluctuations are reviewed. The review is focused on the climatology and variability of the EAM rainfall and its relationship with regional and global scale circulation systems. Climatologically, the EAM rainfall is dominated by convective activities associated with the northward advance of the Mei-Yu trough from southern China during April-May to central China during mid-June. After staying in the same position for one to two weeks, the Mei-Yu trough disappears abruptly and a new rainfall zone is developed over northern China. This is followed by a quasi-20 days oscillatory rainfall regime which develops over central China. Subsequently, the maximum rainfall zone returns to the coastal region of south and southeast China. Regional features unique to the EAM include the extraordinary length of the extended monsoon season (April to late August), the extent of the northward penetration of the major precipitation, the multiple onset and interspersed propagation and stationary nature of the rainfall. Planetary scale features that directly influence the EAM include the western Pacific Subtropical High, the Tibetan High, the local Hadley and the equatorial Walker circulations.
    It is stressed that the EAM rainfall is only a small part of global scale precipitation system which migrates northward from the equatorial Indian Ocean and the Western Pacific to the EAM region and Indian subcontinent during the boreal summer. The EAM possesses a wide range of spatial and temporal scales of variabilities including the seasonal cycle, intraseasonal oscillations, subseasonal scale inter-monsoon interactions, sub-synoptic scale variability and supercluster organization in the western Pacific. These variabilities are in turn linked to interannual variations associated with the biennial oscillation and the El Nino/Southern oscillation. Also discussed is evidence showing the presence of an atmospheric teleconnection pattern connecting eastern Asia and North America (ANA) via the North Pacific. The ANA has profound impact on the climate of eastern Asia including Japan. Dynamically, it may be associated with a marginally unstable barotropic mode in the mean northern hemisphere summertime circulation. This mode is also related to latent heating in the western Pacific near Philippines as well as the Indian Ocean region. While there are some successes in the general circulation model (GCM) simulation of the planetary scale features of the EAM, most GCMs still have problems obtaining realistic regional rainfall over East Asia and India. The intraseasonal and interannual variability of the EAM are generally not very well-simulated in GCMs. Much work is needed to improve modeling of the variability of the EAM.
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  • Ronghui Huang, Fengying Sun
    Volume 70 (1992) Issue 1B Pages 243-256
    Released: March 31, 2009
    JOURNALS FREE ACCESS
    In this paper, the impacts of the convective activities in the western Pacific warm pool on the interannual and intraseasonal variations of the summer monsoon in East Asia are analyzed by using the observed data for 12 summers from 1978 to 1989. The analyzed results show that both interannual and intraseasonal variabilities of the East Asian summer monsoon are greatly influenced by the convective activities in the warm pool. Generally, the monsoon rainfall is below normal in East Asia and the abrupt change of the monsoon circulation is obvious in the summer of strong convective activities around the Philippines.
    The impacts of the convective activities in the warm pool on the summer monsoon in East Asia and the East Asia/Pacific teleconnection pattern of summer circulation anomalies due to the convection are discussed by using the theory of planetary wave propagation and the numerical modelling by the IAP-GCM, respectively.
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  • Jun Matsumoto
    Volume 70 (1992) Issue 1B Pages 257-273
    Released: March 31, 2009
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    The seasonal variations of the wind fields mainly at 850mb and the outgoing long wave radiation (OLR) over Asian and Australian monsoon regions are examined using 5-day mean data.
    Some abrupt changes of the circulation systems and the OLR distribution are recognized in transitional seasons almost simultaneously over a wide region. They occur in early March, late March, mid-April, mid-May, mid-June, late July, early September, late September, late October, mid-November and the end of the year. According to these distinct periods, eleven natural seasons are established. A close relationship is shown between these natural seasons and the advance and retreat of each monsoon system. The seasonal composite maps of low level (850mb) wind and the OLR fields are constructed and the features of each season are described.
    The main factors influencing these abrupt seasonal changes are discussed by utilizing difference maps of seasonal mean 500mb height and 300mb temperature between two consecutive seasons. They are the deepening and shallowing, and the displacement of the quasi-stationary long wave trough at the east coast of the Eurasian Continent, the warming and cooling over the Tibetan Plateau and the wave-like interaction between tropical convection and midlatitude westerlies, part of which may be related to the behavior of the long-wave trough mentioned above.
    Lastly, the regional divisions are performed based on the mean wind of two extreme seasons at both 850mb and 1000mb. Three main regions with seven sub-regions are distinguished.
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  • M. J. Manton, J. L. McBride
    Volume 70 (1992) Issue 1B Pages 275-285
    Released: March 31, 2009
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    Owing to the data flowing from a number of observational programs, there has been over the last few years a sustained research effort on improving our understanding of the Australian monsoon. This paper discusses the findings of that research, including results on the large-scale structure of the monsoon, interannual variability, onset, intraseasonal variability, and mesoscale structure. Although there has been significant progress, much work remains to be done on relating regional aspects of the monsoon to the general tropical circulation.
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  • C. -P. Chang, Jeng-Ming Chen
    Volume 70 (1992) Issue 1B Pages 287-302
    Released: March 31, 2009
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    A 14-winter (1974/75-1987/88, November-March) tropical global band data set is used to study the relationship between the occurrence of cold surges over the South China Sea and the variation of the 200mb and surface tropical divergence from the western Pacific to the eastern Indian Ocean. The data are truncated by 12 empirical orthogonal function modes. A canonical correlation analysis (CCA) between the surface meridional winds over the South China sea between the surge onset and 48h later is used to define a basic cold surge mode. This mode represents the spatial- and time-coherent variations of cold surge events and is constructed from 8 CCA modes which contain approximately half of the total variance of the surface northerly winds. Lag correlations between this mode and the surface meridional velocity, 200mb velocity potential and surface velocity potential, respectively, are computed to examine the effect of the surges.
    The results show that, within 72h of the surge onset, the anomalous surface northerlies reach 5m/s centered in the northern South China Sea, and the anomalous southerlies reach 2m/s over Burma. They represent about 70% and 30% of the total variance of the local surface meridional winds in the two areas, respectively. The cold surge-related 200mb tropical divergence variations have two centers. The primary one is located in the equatorial South China Sea between the Malay Peninsula, Sumatra and Borneo, which is about 50° to the west of the long-term winter mean divergence center in the equatorial western Pacific around 155°E. The secondary center is over the Bay of Bengal. Divergence around both centers increases after the surge onset, with the primary one accounting for >25% of the local variance after 5 days, and increasing to 40-50% if the surge lasts 7-8 days. The magnitude of the increased divergence in the equatorial South China Sea is positively correlated with the surge intensity only during the first three days, while that in the Bay of Bengal is correlated with the surge intensity throughout the surge period. At the surface, anomalous tropical convergence increases during a surge event and develops into two centers. The first one is over northwestern Borneo near the 200mb anomalous divergence center. It probably reflects the importance of the diurnal Borneo convection system. The other surface center is in the eastern Indian Ocean south of the Bay of Bengal. At both centers the surge-related fractional variances are approximately 40% two days after the onset, and become >50% five days after the onset. There is no indication of cold surge influences on the long-term mean divergence center in the equatorial western Pacific.
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  • R. P. Pearce
    Volume 70 (1992) Issue 1B Pages 303-318
    Released: March 31, 2009
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    The main features of the monsoon are first described emphasising particularly its essentially interhemispheric character and showing how the latent heat release associated with the rainfall over India and its neighbours and radiative cooling over the Indian Ocean determine the monsoon flow at lower and upper levels.
    An area is then selected between 22.5°N and 32.5°S, and 37.5°E and 117.5°E for estimating surface transfers of latent and sensible heat and net tropospheric heating for the six half-monthly periods of June-August 1990 using ECMWF operational analyses. The results show that less than a quarter of the surface energy input is exported from the region; the rest is offset by radiative cooling, mainly over the S. Indian Ocean.
    It is inferred that much of the observed variability associated with the location and intensity of monsoon rainfall, at least on time scales up to about a month, are determined primarily by feed-back mechanisms within this region. It is proposed that such studies, carried out routinely and linked to limited area numerical experiments, could contribute substantially to our understanding of these processes. At the same time analyses of the much smaller lateral boundary fluxes could enable the nature of the interaction of the monsoon circulation with larger scale phenomena such as ENSO to be separately identified and quantified.
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  • Michio Yanai, Chengfeng Li, Zhengshan Song
    Volume 70 (1992) Issue 1B Pages 319-351
    Released: March 31, 2009
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    Using the objectively analyzed FGGE II-b upper-air data, the large-scale circulation, heat sources and moisture sinks over the Tibetan Plateau and surrounding areas are examined for a 9-month period from December 1978 to August 1979. In addition to the FGGE data, special soundings obtained during the Chinese Qinghai-Xizang (Tibet) Plateau Meteorological Experiment (QXPMEX) from May to August 1979 are also used in the objective analyses.
    The evolution of the large-scale flow patterns, temperature, outgoing longwave radiation (OLR) and vertical circulation is described in order to identify the distinct seasonal changes from winter to summer that lead to the onset of the Asian summer monsoon. The Tibetan Plateau maintains a large-scale thermally driven vertical circulation which is originally separated from the planetary-scale monsoon system. The rising motion exists only on the western Plateau in winter and then spreads to the whole Plateau as the season progresses. The monsoon onset over Asia is an interaction process between the Plateau-induced circulation and the circulation associated with the principal rainbelt migrating northward.
    During winter the Plateau is a heat sink, but it is surrounded by regions of more intense cooling. In spring the Plateau becomes a heat source, but the cooling in the surrounding areas continues. The sensible heat flux from the surface provides the major source of heating on the Plateau. However, additional contribution from condensation heating is observed in the western Plateau during all seasons and, more significantly, in the eastern Plateau during summer. The sensible heating of the elevated Plateau surface and the radiative cooling in the environment maintain the horizontal temperature contrast that drives the thermally direct vertical circulation.
    The detailed examination of the warming process of the upper troposphere during two transition periods, i. e., the onset of the Southeast Asian monsoon in May and that of the Indian monsoon in June, reveals that the temperature increase over the eastern Plateau during the first onset was mainly the result of diabatic heating, whereas that over the Iran-Afghanistan-western Plateau region leading to the second onset was caused by intense subsidence.
    There are large diurnal variations in the boundary layer and vertical circulation over the Plateau. As a result of diurnal heating of the surface, a deep mixed layer of nearly uniform potential temperature exists over the Plateau in the evening (1200 UTC), suggesting the role of thermal convection in the upward transport of heat. However, moisture is not well mixed vertically and there is a large horizontal temperature gradient in the boundary layer. From late spring to summer the boundary layer becomes more stable for dry convection. On the other hand, the vertical distributions of equivalent potential temperature in late spring and afterwards show a conditionally unstable stratification for moist convection with the increase of moisture of surface air.
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  • Richard H. Johnson
    Volume 70 (1992) Issue 1B Pages 353-372
    Released: March 31, 2009
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    The structure and properties of, heat and moisture sources and sinks of the Asian monsoon are reviewed. Results from the First GARP Global Experiment (FGGE) have yielded important information on these sources, ranging from the planetary scale down to the scale of individual convective systems. The emerging picture is one of a complex spatial and temporal distribution of heat sources over the enormous area covered by the Asian monsoon, with the detailed structure of this distribution determined in large part by a wide variety of types of precipitating systems.
    Several recent experiments, the 1987 Australian Monsoon Experiment (AMEX), the 1987 Equatorial Mesoscale Experiment (EMEX), the Taiwan Area Mesoscale Experiment (TAMEX), and the 1988-1990 Down Under Doppler and Electricity Experiment (DUNDEE), have provided new knowledge concerning the nature of mesoscale convective systems within the monsoon and their contributions to monsoon heat and moisture sources and sinks. Some of the findings of these experiments confirm previous conceptual models of precipitating systems, but also provide new insight into convective processes in the Asian monsoon.
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  • Yihui Ding
    Volume 70 (1992) Issue 1B Pages 373-396
    Released: March 31, 2009
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    This paper presents a comprehensive review of summer monsoon rainfalls in China, with a special emphasis on the effects of the monsoonal airflow and large-scale terrain features in East Asia. The following aspects have been addressed: climatology of summer monsoon rainfalls in China, regional patterns of the summer monsoon rainfall, including the pre-summer rainfalls over South China, the Meiyu season and the rainy season in North China, the low-level jet associated with summer monsoon rainfalls, meso-scale features of precipitating systems during the summer monsoon season, atmospheric heat sources and their effect on the change in the general circulation in East Asia, and numerical experiments and operational prediction of the summer monsoon precipitation in China.
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  • Longxun Chen, Min Dong, Yongning Shao
    Volume 70 (1992) Issue 1B Pages 397-421
    Released: March 31, 2009
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    By utilizing the meteorological observational data over the mainland of China and surrounding oceans from 1951 to 1990, the characteristics of the climatic trend of East Asian Monsoon and the interannual variation of monsoon overlapped on the general trend have been studied. The results show that during the past 40 years, especially in the 1980s, China has generally been drying and the areas south of 35°N in China are getting cooler. This may be attributed to the weakening of the winter and summer monsoon in China. Mei-Yu in middle and lower reaches of Yangtze River and the precipitation in other areas of China are affected by the climatic change trend.
    It has long been believed that the Indian Monsoon is correlative to the summer precipitation of China. But in our studies, it can be concluded that there is a significant correlation only between the summer precipitation over India and that over the western part of North China, and for other areas of China the correlation is not obvious.
    The interannual anomaly of the summer monsoon in China is mainly attributed to the variation of each member of the circulation system of the East Asian summer monsoon. The interactions of air-sea and air-land have also been discussed. Although a significant correlation can not be found between the SST of equatorial eastern Pacific/Polar ice and the winter/summer monsoon of China, they all have similar significant interannual oscillations and the correlation between these interannual oscillation components of the same frequency is very high.
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  • Pao K. Wang, De'er Zhang
    Volume 70 (1992) Issue 1B Pages 423-446
    Released: March 31, 2009
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    Recent work on the reconstruction of past climate of the part of China that is influenced by the East Asian monsoon based on Chinese historical literature are reviewed. General climate information that can be extracted from historical documents are described and some examples are given. The sources of available documents useful for more quantitative reconstruction purposes are given and assessed. Reconstruction of winter and summer temperature series and dryness/wetness series are then reviewed and discussed. The reconstruction of temperature series were generally based on climatological interpretation of phenological records such as the dates of lake or river freezing, bird migration, or the beginning and ending of snow. Reconstructed temperature series for various parts of China are given. Next, the reconstruction of humidity series are reviewed. The data sources for this purpose are mainly flood, drought, and rain records. Some high quality data sources, such as the daily weather reports, the Clear and Rain Records of Qing Dynasty, are also discusses.
    The reconstructed series are then used to summarize the general evolution of the monsoon climate of China from 300 to 1900 AD. A brief conclusion is made.
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  • Tsuyoshi Nitta, Takanori Mizuno, Kiyotoshi Takahashi
    Volume 70 (1992) Issue 1B Pages 447-466
    Released: March 31, 2009
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    Temporal and spatial characteristics of tropical convective systems and their associated atmospheric circulations such as intraseasonal variations (ISV), super cloud clusters (SCC), twin cyclones and westerly wind bursts (WWB) developed during the initial phase of the 1986/87 El Niño are investigated.
    Most ISVs originated in the Indian Ocean and propagated eastward through the western Pacific and consisted of SCCs, twin cyclones and WWBs. Time evolution and horizontal-vertical structure of major ISVs in 1986, which appeared to play an important role in the El Niño development, such as (1) twin cyclone in May, (2) severely enhanced ITCZ in August and (3) SCC in November which brought about a major El Niño development, are analyzed in detail.
    It is suggested that the WWBs associated with the SCCs propagating eastward along the equator from the Indian Ocean toward the western Pacific region are blocked by the surface topography over the “maritime continent”, especially over Sumatra.
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  • Kozo Ninomiya, Takako Akiyama
    Volume 70 (1992) Issue 1B Pages 467-495
    Released: March 31, 2009
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    This review paper summarizes the authors' recent studies on the Baiu, the summer monsoon, over the Japan Islands and the adjacent areas.
    The most important feature of the Baiu is the formation of the Baiu front, which is a long quasistationary precipitation belt extending from the southeastern foot of the Tibetan Plateau to Japan and further to the northwestern Pacific. The cold Baiu trough, the polar front and the Pacific subtropical anticyclone are the major large-scale circulation systems which have influence on the Baiu front. The frontogenesis and the generation of convective instability in the southwesterly flows along the westnorthwest periphery of the Pacific subtropical anticyclone are the primary factors for the formation of the Baiu front.
    The structure of the Baiu front and the associated Baiu low-level jet stream, and the water vapor budget during the peak Baiu period are studied. The generation and release of the convective instability in the intense Baiu precipitation area are also analyzed.
    The features of the meso-α-scale disturbances in the Baiu frontal zone are studied by the spectral analysis of the relative vorticity field and the cloud amount. The meso-β- and meso-γ-scale fine structures within the meso-α-scale disturbances are demonstrated.
    The most outstanding feature of the Baiu is that the motions of many scales (planetary, synoptic, meso-α-, meso-β- and meso-γ-scales) are interacting with each other. The multi-scale aspects of the Baiu are stressed in this review paper.
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  • George Tai-Jen Chen
    Volume 70 (1992) Issue 1B Pages 497-516
    Released: March 31, 2009
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    Mei-Yu (Baiu in Japan) is a weather and climate phenomenon in the area of Japan, Taiwan, and subtropical China where the seasonal rainfall distribution reaches a peak in late spring and early summer due to the repeated occurrence of the Mei-Yu front. From the satellite pictures, the Mei-Yu front is usually accompanied by a nearly continuous cloud band with organized mesoscale convective systems (MCSs). To the south of the Mei-Yu front, a low-level jet (LLJ) is often observed and is closely related to the formation of MCSs and heavy rainfall events. As the Mei-Yu front approaches Taiwan, the front and the accompanying MCSs and LLJ tend to be affected by the mesoscale topography of Taiwan, the Central Mountain Range (CMR). Besides, the land-sea contrast coupled with island topography produces the land-sea breeze, the mesolow, and the island circulations which are important in modulating the local precipitation.
    In this paper, an overview of the current understanding of the structure and dynamics of the mesoscale features observed in the Taiwan Mei-Yu season is presented. Research results in the pre-TAMEX era as well as those derived from TAMEX program are discussed for the Mei-Yu front, the LLJ, the MCSs, the mesolow, the land-sea breeze, and the island circulations.
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  • Sulochana Gadgil, G. Asha
    Volume 70 (1992) Issue 1B Pages 517-527
    Released: March 31, 2009
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    Within the summer monsoon, the circulation and rainfall over the Indian region exhibit large variations over the synoptic scale of 3-7 days and the supersynoptic scales of 10 days and longer. In this paper we discuss some facets of intraseasonal variation on the supersynoptic scale on the basis of existing observational studies and some new analysis. The major variation of the summer monsoon rainfall on this scale is the active-break cycle. The deep convection over the Indian region on a typical day in the active phase is organized over thousands of kilometers in the zonal direction and is associated with a tropical convergence zone (TCZ). The intraseasonal variations on the supersynoptic scale are also coherent on these scales and are related to the space-time variation of the large-scale TCZ. The latitudinal distribution of the occurrence of the TCZ is bimodal with the primary mode over the heated continent and a secondary mode over the ocean. The variation of the continental TCZ is generally out of phase with that of the oceanic TCZ. During the active spells, the TCZ persists over the continent in the monsoon zone. The revival from breaks occurs either by northward propagation of the TCZ over the equatorial Indian Ocean or by genesis of a disturbance in the monsoon zone (often as a result of westward propagations from W. Pacific). The mechanisms governing the fluctuation between active spells and breaks, the interphase transition and the complex interactions of the TCZ over the Indian subcontinent with the TCZ over the equatorial Indian Ocean and the W. Pacific, have yet to be completely understood.
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  • Ravi S. Nanjundiah, J. Srinivasan, Sulochana Gadgil
    Volume 70 (1992) Issue 1B Pages 529-550
    Released: March 31, 2009
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    A class of simple zonally symmetric models of increasing complexity viz. Webster and Chou (1980a), Gadgil and Srinivasan (1990) and Srinivasan et al. (1991) have yielded increasingly realistic simulation of the intraseasonal variation of the tropical convergence zone (TCZ) over the Indian region in the summer. A new model in this class, which incorporates the space-time variation of surface pressure is discussed in this paper. As in the earlier models, the prominent feature of intraseasonal variation in the Indian region viz. poleward propagations of the TCZ is simulated. The rate of propagations in this model is more realistic and the period between propagations is also more realistic being about thirty five days. A new feature simulated by this model is the active phase of the monsoon with the TCZ persisting over the continental trough for about 20 days. The underlying mechanisms, unravelled by analysis of this and earlier models of this class are discussed in this paper.
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  • R. R. Kelkar, A. V. R. K. Rao
    Volume 70 (1992) Issue 1B Pages 551-561
    Released: March 31, 2009
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    The paper presents a review of techniques being used for the retrieval of quantitative meteorological information from INSAT VHRR (Very High Resolution Radiometer) data. It describes how the constraints imposed by the spacecraft design and the ground segment capabilities led to the development of innovative and unconventional techniques of product retrievals from INSAT.
    The utility of the INSAT derived products, viz. Cloud Motion Vectors, Sea Surface Temperature, Outgoing Longwave Radiation and Quantitative Precipitation Estimates is discussed. The variation of these parameters on different space and time scales is presented.
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  • Akio Kitoh
    Volume 70 (1992) Issue 1B Pages 563-583
    Released: March 31, 2009
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    Interannual variations of the South Asian and the Australian summer monsoons in a 20-year simulation by an atmospheric general circulation model (GCM) with the observed sea surface temperature (SST) for the period 1970-1989 are investigated. Emphasis is laid on moisture flux variations, as the moisture flux convergence plays a more dominant role in the rainfall variations than the local evaporation, although their relative roles are comparable for the 20-year mean moisture budget.
    The anomalous precipitation in the Bay of Bengal in boreal summer is significantly correlated with the SST anomalies in the western Pacific, but not with the in situ SST anomalies. The positive SST anomalies in the western Pacific are favorable to the intensified South Asian summer monsoon circulation with strong surface westerlies over the north Indian Ocean. A contrast in anomalous precipitation in the model between the South Arabian Sea and the India subcontinent is also discussed. The anomalous precipitation over the oceans north of Australia in austral summer is explained mainly by the anomalous moisture flux converging into this region from the west. This is accompanied by an intensified cyclonic circulation northwest of Australia, but cross-equatorial moisture flux from the winter hemisphere is found to be important. Some comparisons with observations are also made and discrepancies between the model and these observation are discussed.
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  • Shao-Fen Tian, Tetsuzo Yasunari
    Volume 70 (1992) Issue 1B Pages 585-596
    Released: March 31, 2009
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    The time and space structure of interannual fluctuations of summer rainfall (May to September) for the period from 1951 to 1990 over China is described. First, a harmonic analysis is applied to the summer rainfall series. The variation with periods of two to four years (hereafter, refered to 2-4 year component) accounts for more than 40 percent of the total variance for all stations, and more than 70 percent of the total variance for 61 percent of stations. The three-year period seems most prevailent among this period band.
    An EOF (Empirical Orthogonal Function) technique is applied to the 2-4 year period series. The first two EOF modes account for 12.8 and 10.7 percent of the total variance, respectively. EOF mode 1 reveals the seesaw between the Yangtze River valley and the Northern part of China. The spatial pattern of EOF mode 2 is more complicated, but it may be characterized by the oscillation between the Mei-yu region including the Yangtze River valley, and the rest of the country. Although the contribution proportion of the two principle modes is not high, composite maps show that they present the situations (before applying the EOF analysis) well. These two EOF modes exhibit large amplitude modulations. The amplitudes or the squares of the time coefficients tend to become large or small alternatively.
    Correlation and composite analyses show that EOF mode 1 correlates with the Indian summer monsoon and the SOI (Southern Oscillation Index). This result agrees with that of preceding studies and suggests that the summer rainfall over China is associated with the ENSO (El Nino and Southern Oscillation) events on the time-scale of two to four years. EOF mode 1 seems to precede the anomaly of the SOI. This result supports the previous proposals that the ENSO signals in the eastern equatorial Pacific may originate in the central Asia or the Indian Ocean region. EOF mode 2 seems not to be related to the Indian monsoon and the ENSO.
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  • Tetsuo Nakazawa
    Volume 70 (1992) Issue 1B Pages 597-611
    Released: March 31, 2009
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    By using the climatological mean daily value from the 10-year objective analysis dataset and the 12-year OLR data, the enhancement of the Asian summer monsoon activity, which occurs intermittently with a period of 30-60 days, is examined. In particular, this work describes the seasonal phase lock of the intraseasonal variation (ISV). In the northern hemisphere summer, the enhancement of the connective activity and the low-level monsoon westerlies is evident along 10°N around early June and middle July over the Indian Ocean and around middle June and late July to early August and early-middle September over the western Pacific. The enhancement is also found around late May and middle July over the equatorial Indian Ocean. The first enhancement in late May and early June over the Indian Ocean corresponds to the climatological monsoon onset, dated June 1 in the South India and in southern Japan. The second enhancement in late July over the western Pacific is accompanied by a rather abrupt change. The monsoon westerlies in the lower latitude suddenly extend to the western Pacific up to 150°E. The penetration of the westerlies to the subtropical western Pacific is associated with the eastward propagation of the ISV from the Arabian Sea and the equatorial Indian Ocean in middle July. The westerlies over the subtropical western Pacific result in cyclonic circulation over the western Pacific and anti-cyclonic circulation over Japan, which brings the withdrawal of the Baiu and the summer season in Japan.
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  • Minoru Tanaka
    Volume 70 (1992) Issue 1B Pages 613-629
    Released: March 31, 2009
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    The seasonal cycle and the onset and retreat dates in the summer monsoon in East Asia, Southeast Asia and the Western Pacific region are analyzed using 12-year (April 1978-December 1989) means of the 5-day mean 1-degree latitude-longitude gridded GMS high-cloud-amount data. An analysis of these data showed the detailed seasonal cycles of two convective zones (ITCZ and the Baiu front) of the summer monsoon cloud defined by the regions with more than 30 (25 for Baiu front) percent of the mean high-cloud amount. The clouds associated with the ITCZ were observed to increase in Southeast Asia during May and subsequently spread to the South China Sea and the Western Pacific region in a series of sudden expansions during June and July.
    The Baiu front branch is preceded by an increase in the high-cloud amount along the polar front located near 30°N just south of Japan around 26 April. During the middle of May, this cloud band rapidly moved south to 20-25°N. This period coincides with the onset of the Baiu season in Okinawa. Subsequently, this cloud band moves north to mainland Japan and the Yellow Sea with a series of sudden northward advances during June and July. These abrupt changes are associated with the phase-locking between the intraseasonal oscillation and the seasonal cycle of the monsoon clouds.
    The onset and retreat dates derived from this study fill the major gaps in these dates over Southeast Asia and the adjacent Western Pacific region where the large area of the ocean prevented analysis of the onset (retreat) dates based on rainfall data.
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  • Kuranoshin Kato, Yasumasa Kodama
    Volume 70 (1992) Issue 1B Pages 631-647
    Released: March 31, 2009
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    The formation of the “quasi-stationary” Baiu front at the beginning of May 1979 and its relation to the seasonal transition of the large-scale baroclinicity in East Asia was examined by using the observational data.
    The frontal zone to the south of the Japan Islands, corresponding to the southern branch of the middle-level westerly jet around the Tibetan Plaeau, is characterized by the alternative passages of synoptic-scale extratropical cyclones and anticyclones in April. On the other hand, the cloud belt associated with the stationary front is sustained at -25°N in May, even after the passage of the synoptic-scale (or larger meso-α-scale) cyclone at -30°N. In other words, the frontal zone there becomes quasi-stationary at the beginning of May (the formation of the “quasi-stationary Baiu front” around Japan). The analysis of the location of the front on twice-daily surface weather maps in 1985, 1986, 1987 and 1988, as well as 1979, shows that the change into the quasi-stationary frontal zone occurred in early May of those other years.
    The two baroclinic zones, corresponding to the southern and the northern branches of the westerly jet, respectively, are separate from each other around the Japan Islands in May. This results in the weakening of baroclinicity in May just to the north of the Baiu front as well as across the Baiu front. Thus the development of a migratory anticyclone is suppressed, which provides a favorable condition for sustaining the “quasi-stationary” Baiu front in May.
    It is noted that the change into the “quasi-stationary” frontal zone at the beginning of May 1979 mentioned above is a different event from the abrupt disappearance of the temperature gradient across the Baiu front in China in late May pointed out by Kato (1985a, 1987).
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  • Masashi Nagata
    Volume 70 (1992) Issue 1B Pages 649-671
    Released: March 31, 2009
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    A prediction experiment is performed for a typical case of cold-air outbreak with large temporal variation over the Japan Sea in the Asian winter monsoon situation with a very-fine-mesh, primitive-equation model. The purposes are to elucidate the typical evolution of the Japan-Sea convergent cloud band (CCB) that responds to the large-scale environment varying according to the passage of a short-wave trough accompanied by a marked cold vortex aloft, and to diagnose the physical processes responsible for the formation of a paired middle-level jet and weak-wind zone analyzed along the CCB.
    The model simulates well the overall orientation and intensity of the CCB. The CCB reaches its peak intensity under the short-wave trough, where the layer of low stability in the troposphere is deepest and the largest air-mass transformation occurs. The CCB weakens as the stability of the lower troposphere increases. Net air-mass transformation over the sea decreases behind the trough and eventually it dies away, even though air-mass transformation in cold advection persists. Although the orientation of the CCB tends to be parallel with the large-scale flow, this is not always the case. This is because each part of it migrates, following its own local flow field which has both temporal and spatial variations according to the phase of travelling short waves. The variation in thermal structure may be primarily attributable to the location and orientation of the CCB which govern the relative influences of each lower-boundary forcing.
    A paired middle-level jet and weak-wind zone develop along the CCB when the CCB is in the mature stage. A sensitivity experiment suggests that the pair of wind anomalies are closely linked with the CCB. Analysis of the ageostrophic wind reveals that the jet is rather geostrophic and nearly balanced with the mesoscale temperature field featuring the CCB, while the weak-wind zone is highly sub-geostrophic. The former develops in a mass-momentum adjustment process due to concentrated diabatic heating along the CCB, while the latter is produced directly by rapid upward motion with diabatic heating in a baroclinic environment. The diabatic vertical displacement in cold advection explains the wind-speed minimum in the vertical observed along the ascending zone.
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