International Workshop on the Climate System of Monsoon Asia (IWCSMA)

3-7 December 1996, Kyoto, Japan

edited by Nobuo Yamazaki
Typhoon Research Department, Meteorological Research Institute, Japan


1. Introduction

by Tomio Asai (Chairperson of the Organizing Committee of IWCSMA)

The Monsoon Asia is the area where more than one-half of the world population carry out their socio-economic activities. This is also the area which receives the influence of monsoon, one of the most vigorous circulation systems in existence over the globe. In addition to its dominance in the local climate over the Asian continent and its surrounding oceans, there is increasing evidence that shows the important role of the Asian monsoon in the global climate systems such as El Niño. The growing concern on this importance has led to the planning of GEWEX Asian Monsoon Experiment (GAME) which aims at investigating the atmosphere/land processes over Asia.

The International Workshop on the Climate System of Monsoon Asia (IWCSMA) was held in Kyoto, Japan, in December 3-7, 1997. The members of the International Programme Committee which are responsible for the overall organizations of the workshops were;

Tomio Asai (Chairperson: Chiba University, Japan)
Ding Yihui (Co-chairperson: National Climate Center, China)
Akimasa Sumi (University of Tokyo, Japan)
Tsuyoshi Nitta (University of Tokyo, Japan)
Toshio Yamagata(University of Tokyo, Japan)
Tetsuzo Yasunari(University of Tsukuba, Japan)
Masato Murakami(Meteorological Research Institute, Japan)
Chen Longxun (Chinese Academy of Meteorological Sciences, China)
Huang Ronghui (Institute of Atmospheric Physics, China)
B.K. Cheang (Malaysian Meteorological Service, Malaysia)
Michael Manton (Bureau of Meteorology Research Center, Aurtrailia)
Greg Holland (Bureau of Meteorology Research Center, Aurtrailia)
Michio Yanai (University of California, Los Angeles, USA)
K.M. Lau (Goddard Space Flight Center/NASA, USA)
C.P. Chang (Naval Postgraduate School, USA)
Roger Lukas (University of Hawaii, USA).

The IWCSMA was dedicated to discuss the scientific challenges regarding the climate system in Monsoon Asia and its surrounding oceans. The scientific fields discussed in the workshop included meteorology, hydrology and oceanography. The impact of land surface conditions to the climate system and the remote sensing technologies applicable to Monsoon Asia was also in the scope of the workshop. These scientific discussions are summarized in section 2.

The workshop participants summarized the major scientific issues discussed during the preceding sessions and adopted the recommendations for clarifying the scientific challenge toward the coming 21st century and enhancing the international/interdisciplinary cooperation for the future research project. The recommendations adopted in this workshop are in section 3.

If you have any comments on the recommendations, please send your comments to the local secretariat:

Dr. Nobuo Yamazaki
Typhoon Research Department
Meteorological Research Institute
1-1 Nagamine, Tsukuba-shi
Ibaraki-ken 305, Japan
Tel: +81-298-53-8668 Fax: +81-298-55-2683
E-mail: nyamazak@mri-jma.g o.jp

2. Summary


Reports on International Programs and Projects
Reports on Monsoon Onset and Intraseasonal Variation
Reports on Asian Monsoon and Climate Variability - Part 1
Reports on Asian Monsoon and Climate Variability - Part 2
Reports on Simulation and Prediction of the Asian Monsoon
Reports on Land Surface Processes
Reports on Asian Monsoon and ENSO
Reports on Oceanography and Air-Sea Interaction
Reports on Radar and Satellite Observation over Monsoon Asia

Reports on International Programs and Projects
by Greg Holland (Bureau of Meteorology Research Centre, Australia)

The opening session of the Workshop contained presentations on a number of the international programs and projects related to Monsoon research and experimentation. Hartmut Grassl (WCRP) presented an overview of WCRP and its sub-components, with emphasis on the monsoon-related activities within CLIVAR, GEWEX and WIGNE. GEWEX is emphasizing activities that exploit and investigate the regional aspects of soil moisture memory and the impacts on short climatic scales and Dr. Grassl described a number of continental-scale programs that have been developed in this regard. Of particular interest was the highly active GAME program. CLIVAR is primarily involved in the climate variability on seasonal to longer time scales. A major focus of this program is the monsoon circulations in West Africa, Asia, Australia and the Americas.

Peter Webster elaborated on the CLIVAR GOALS program. He emphasized the difficulty in partitioning time and spatial scales due to the wide range of, often non-linear, interactions involved. Three major focus regions were identified: the Asian-Australian Monsoon, the Equatorial Pacific, and the Pan American Climate System. Within these major questions remain on the predictability of the monsoon and the monsoon-ENSO interactions and on the processes contributing to the highly variable tropospheric moisture content. Dr. Webster recommended a long-term monitoring program in the Indian Ocean/Southern Asian region to build up data on these issues and on the overall ocean/land/atmosphere interactions that are occurring.

Tetsuzo Yasunari described the GAME activities in Asia. He noted that major discrepancies between original and reanalyzed ECMWF analyses of water vapor fluxes and the difficulties that the remaining errors posed for adequate answering of short and long term variability in the atmosphere. The importance of heating on the Tibetan Plateau also was emphasized, particularly the deep mixed layer and the high diurnal variability. Professor Yasunari outlined the main programs under way in Asia to address these and related issues. These include: the South China Sea Monsoon Experiment, the Korean Monsoon Experiment, and the Tibetan Plateau Experiment. Also described were the GAME monitoring sites in China and Thailand.

Ding Yihui and Jai-Ho Oh presented details of the South China Sea Monsoon Experiment and the Korean Monsoon Experiment, respectively. Both of these experiments are occurring in the "meeting ground" of the major air flows from east to west in the northern hemisphere and from Asia to Australia. The SCSMEX, which is organized under the WMO CAS Tropical Meteorology Research Program and the Pacific Science Association, is aimed at a detailed investigation of the May onset by providing an integrated range of observations from the regional scale, to a mesoscale focus in the onset region. The KOMEX is smaller scale and seeks to address important questions of the monsoon, particularly the water cycle, which is of great importance to South Korea, where the available supplies are marginal and under stress.

Huang Ronhui presented a detailed account of the climate research programs in the Chinese Academy of Sciences. These include: severe climate variations and their effect on water and agriculture, climate dynamics and prediction theory, integrated field experiments, including those described above, and research into predicting climate variability and anomalies.

Reports on Monsoon Onset and Intraseasonal Variation
by Peter J. Webster (University of Colorado) and Bing Wang (University of Hawaii)

This session dealt with the subseasonal time scales of the Asian summer monsoon. This time scale is important for a number of points of view.

  1. The intraseasonal variability determines the time of the onset of the rainy season and also its cessation.
  2. Intraseasonal variability determines the active and break sequence of the monsoon and, then, the periods of wet and dry weather, and,
  3. intraseasonal variability plays a key role in interannual variability of the monsoon.

Thus, intraseasonal variability is an important component of the monsoon climate system. Furthermore, the predictability of the monsoon is clearly associated with intraseasonal variability on time scale of weeks to years.

The session contained over 20 papers ranging from modeling studies to diagnostic studies. There were a number of interesting points made in the presentation :

  1. Intraseasonal variability is a chaostic element of the monsoon system with an "oscillation" between one or more physical reliable solution. (e.g., convection near the equator but dry South Asia (break), the reverse (active), convection in East Asia, dry in South, the reverse). ENSO nudges the system to be more in one phase than another thus providing a probabilistic bias toward drought or heavy rainfall monsoon season in different part of the monsoon system.
  2. A number of papers compared the climatological progression of the monsoon onset with detailed region studies of the monsoon. The climatological view has the monsoon arriving first in East Asia and propagating northward along the Asian coast from the South. The monsoon arrives in South Asia at a later time. However, it appears that the monsoon may arrive first inland in SE Asia. Other studies strongly suggest that there is no propagation in East Asia but rather a jump from near the equator to further north. In northern South China Sea, total cloudiness surge in early May leads onset of rainy season in late May by 20 days.
  3. Deciphering the timing of the onset is difficult because of the influence of altitude and strong diurnal variability.
  4. There was some discussion about how to define a strong or a weak monsoon. Two monsoon indices were considered: the monsoon shear index and the South Asia rainfall index (specifically the all-India rainfall index). These two indices provide different strong and weak monsoon and depend upon events occurring at different times of the monsoon season. Also the rainfall index depends on local South Asia rainfall but late in the season: the shear index appears to be more a function of remote western Pacific Ocean influences. Each, though, tells different things about the monsoon.
  5. A number of papers discussed the Tibetan Plateau and related anomalous condition of the monsoon with the shear of the monsoon. Eg. in the Tibet, if winter is warm, East Asia monsoon is strong and subtropical rain bands tend to be north of normal position. Also there were some diagnostic studies that questioned earlier results. over eastern Tibet less than western Tibet (compared to Yanai). Also eastward moving disturbances over Plateau linked to Baiu activity.
  6. August Korean rainfall is better correlated with western North Pacific SST. This means that August rainfall were predictable than Changima (July rainfall).
  7. New technological innovations were discussed. The UAV aerosonde, a small automated aircraft, were discussed. Applications to MCTEX(95) future probabilities for GAME experiments and general monitoring of climate, hurricanes etc. were considered.

Reports on Asian Monsoon and Climate Variability - Part 1
by Cheang Buon Khean (Malaysian Meteorological Service)

In the first paper presented by Professor B. Wang, "On Northern Hemisphere Summer Monsoon Singularities", by removing the first four harmonics (basic annual cycle), Professor Wang and Xu obtained the climatological intraseasonal oscillation, CISO, which displays oscillatory behavior with considerable amplitude from May to November. The intraseasonal oscillation in the monsoon exhibits year to year variations, but it tends to phase lock to annual cycle which was found by Professor Wang and Xu to have resulted in statistically significant CISO. The extreme phases were found by them to be responsible for the summer monsoon singularities after applying these rigid statistical test of significance. CISO was found by them to exhibit dynamical coherent structure between convection represented by regressed climatological pentad mean outgoing long-wave radiation (OLR) and circulation represented by 850- and 200- hPa wind anomalies. Wang and Xu also showed preferred seasonal-dependent movements paths such as northward phase propagation along the longitudinal band 122.5-132.5E and pronounced westward propagation primarily over the western Pacific between 110-160E. They also found the timing of monsoon singularities varied with locations, and the relationship in subseasonal variability between Indian summer monsoon and western North Pacific summer monsoon varied with seasonal march. They concluded that understanding the dynamics of CISO is important to understanding the temporal structure of monsoon and its year to year variability and it bears profound implications to seasonal climate predictions.

In the second paper of Professor Ronghui Huang and Zhenzhou Zhang, the interannual and intraseasonal variabilities of the transport of water vapor by the Asian summer monsoon circulation and their impact on the drought and flood in East Asia were analyzed by using the ECMWF global data for 12 summers from 1978 to 1989. Professor Hunag and Zhang showed from the analyzed and numerical simulated results that the interannual and intraseasonal variabilities of water vapor are large in East Asia and are closely associated with the thermal state of the tropical Pacific especially the thermal state of the western Pacific warm pool. When the SST in the warm pool is above normal, they found that the transport of water vapor by the Asian monsoon was strong from the Indo-China Peninsula to the east to the Philippines, and the convective activities were intensified in this region. However, in this case, the transport is weak in the Yangze River basin and the Huaihe River basin. On the contrary, they found the reverse when the SST in the warm pool is below normal. Huang and Zhang also found coherent anomalous circulation pattern associated with the afore-mentioned results regarding the transport of water vapor. The anomaly distributions of water vapor transport flux according to Huang and Zhang appeared to have a teleconnection pattern from East Asia to North America through the North Pacific.

In the third paper, "Climate Trend In Bias-Corrected OLR Time Series Over the Tropical Indian And Western Pacific Oceans" of Dr. P.S. Chu and J.B. Wang, presented by Dr. Chu, they found that the original OLR in the 1980s and 1990s were consistently lower than those in the 1970s in the latitudinal band of 22.5N and 22.5S. To examine the climate trend over the tropical Indian and western Pacific oceans, they corrected the systematic bias in daily averaged OLR using the method suggested by Gadgil et. al. Chu and Wang used to statistical approaches to detect climate trends in the area mentioned earlier. Their results showed during the 223 months from 1974to 1992 there was an increasing trend in convection and possibly rainfall over the Indian Ocean and the South China Sea and the central/ western Pacific. This result is consistent with that of Nitta and Kach is of 1994 although the latter removed the bias in the OLR series. They also found reduced convection and rainfall over northwestern India, southern Madagascar, southern New Guinea and tropical Australia. The increase in convection showed a preference to occur in the summer hemisphere. During the Austral summer, enhanced convection was found over the South and Central Indian Ocean and the equatorial South-Central Pacific. A decrease in convection was observed over Australia and a weaker summer. Australian summer monsoon was implied by Chu and Wang.

In the fourth paper "The South Asian Monsoon and the Tropospheric Biennial Oscillation (TBO)" presented by Dr. Gerald A. Meehl, a mechanism involving the South Asian monsoon as an active part of the tropospheric biennial oscillation (TBO) was described. This mechanism depends on coupled interactions between (1) land-atmosphere-ocean in the Indian sector; (2) large-scale atmospheric east-west circulations in the tropics; (3) convective heating anomalies over Africa and Pacific; and (4) tropical-midlatitude interactions in the Northern Hemisphere. Important results in Meehl's study is that soil moisture anomalies contribute to land surface temperature anomalies for only one season after the summer monsoon. An experiment using a global atmospheric GCM running in perpetual January mode with observed SSTs with specified convective heating anomalies demonstrated that convective heating anomalies elsewhere in the tropics associated with the coupled ocean-atmosphere biennial mechanism can contribute to altering seasonal midlatitude circulation. These changes in the midlatitude longwave pattern, forced by a combination of tropical convective heating over East Asia, South-East Asia and the western Pacific was found by Meehl to be able to maintain the temperature anomalies over Asia via advection through winter and spring to set up the land-sea meridional tropospheric temperature contrast for the subsequent monsoon. The role of the Indian Ocean was found to provide a moisture source and a low amplitude coupled response component for meridional contrast to drive the South Asian monsoon. The role of the Pacific is to produce shifts in regionally coupled convection SST-anomalies. These regions were found to tie together and mutually interact via the atmosphere and contribute to altering midlatitude circulation.

In the fifth paper, "Statistical Study on Kelvin Wave With Large Amplitude Near the Equatorial Tropopause" presented by Dr. Nishi Noriyuki, the characteristics of the distribution near the equatorial tropopause were examined. He found that most of the disturbances with period longer than several days have maximum amplitude near the tropopause statistical amplitude near the tropopause was found to be at most two time larger than 200hPa level. Noriyuki explained this feature by 1) density decrease with height 2) weak dissipation in the upper troposphere 3) vertical wind shear near the tropopause and 4) N increase with height. In about ten years (1983-1993) he found abrupt remarkable amplification of Kelvin wave in specific year/season (mainly in northern winter and spring) which can not be explained by the above features but suggested that longitudinal variability in the basic zonal wind could bring about such amplification where the basic easterly was stronger in the eastern part.

In the sixth paper, "Rossby Wave Propagation in Asian Jet" by Dr. Hiroaki Naoe and Dr. Yoshihisa Matsuda and presented by Dr. Naoe, the main aim of their study is to examine the behavior of Rossby waves in the jet exit region when the Rossby waves propagate through the Asian jet stream. They carried out nonlinear experiments of a barotropic model with the basic state of 300hPa monthly mean flow in a winter. Their results showed that when an anticyclonic Rossby wave propagates in the jet exit region, a large isolated wave was growing and on the contrary, when a Rossby wave propagates in the jet exit region, it soon disappear. Naoe and Matsuda concluded that nonlinear effects in the jet exit region play an important role in growing the isolated wave. In the last paper, "Summer Climate Variability And In Association With 500hPa Height And Tropical Convection" of Professor Tsuyoshi Nitta and Dr. ZhengZhen Hu, presented by Professor Nitta, the authors focused on the interannual and interdecadal variabilities and long-term trend of summer rainfall and temperature patterns and China and their association with atmospheric circulations and tropical convection. The singular value decomposition analysis was employed to identify the dominant coupled modes between the summer rainfall and temperature variations. Their results show that there was correlation pattern with dominant amplitudes of rainfall and temperature along the Yangtze River with opposite signs. Time coefficients of the first SVD showed decreasing trends, indicating that the summer temperature in China was decreasing and the summer rainfall was increasing during the 1951-1994 period and the QBO signal could be seen after mid 70S. Association with 500hPa height showed by Nitta and Hu that positive height anomalies generally corresponded to below normal precipitation and above normal temperative over the above-mentioned region and vice versa. The authors also found that there existed large positive correlation between summer climate over China and the Philippine Sea probably due to atmospheric teleconnection pattern generated by conventional leating.

Reports on Asian Monsoon and Climate Variability - Part 2
by Jai-Ho Oh (Meteorological Research Institute, KMA)

The second part of the Asian monsoon and climate variability session is basically focused on the regional rainfall variations. The first topic was the relationship between monsoon rainfall in China and snow cover over Qinghai-Xizangt Plateau by Dr. L. Chen. Through the anomaly analysis of the snow cover data of 60 stations for the period of 1958 to 1992 and the monthly (May to August) rainfall data of 140 stations in the eastern China for the period of 1959 to 1990, the biennial and 6-7 year interannual oscillations have been found in the snow cover over the Qinghai-Xizang Plateau. Also it has been found the interdecadal jump during 1970's. The QBO of summer monsoon rainfall in the southern China may be caused from the biennial oscillation in the snow cover over the plateau. In snow-rich year, the northward advance of the main rainfall belt has been delayed and then, it may cause the negative anomaly rainfall in the northern China and the positive anomaly in the southern China.

The second topic is the variation of d 18O/dT in precipitation at the Qinghai-Xizang Plateau by Dr. Zhang. A significant positive correlation between 18O in precipitation and sampling temperature of three stations in the Qingzang Plateau has been found. Also, the distinguished difference between synoptic and climatic timescale has been analyzed, which might be corresponding to the vapor transportation to the Qingzang Plateau.

Dr. Lu and Prof. Huang presented the interannual variation of the blocking Highs over the northeastern Asia during summer season. They showed the blocking highs over the northeastern Asia have obvious interannual variation and it might be related to the variation of the SSTA in the tropical western Pacific through the wave train of EAP pattern.

The following four papers are focused on the regional precipitation characteristics. Dr. Patipat presented the monsoon phenomena over Thailand. He pointed out two cases of severe summer monsoon and winter monsoon. During summer, the highlights of phenomena are the presence of tropical cyclones in the South China Sea induced the intensification of the southwest monsoon and orientation over the region. During winter, the severe northeast monsoon was caused mainly by the prolonged existence of cyclonic disturbances in most area in southern Thailand. He also demonstrated the baroclinic model yielding forecast results possible to be used for quantitative monsoon rain precipitation in the tropics.

The seasonal migration of ITCZ over the Indian Ocean has been investigated by Dr. Kelkar through the analysis of large-scale feature in OLR and precipitation data. The ITCZ migration depends on the elevation of heat source over the Tibetan Plateau and the warming of the Indian Ocean. Dr. Hu and Prof. Nitta presented the time scale regimes in the summer rainfall variation over the North China and India with the wavelet transform analysis. They demonstrated that there is a strong localization and nonstationary evolution in the interannual and decadal-scale variations of the rainfall data. A strong interaction has been found between the Indian summer monsoon and ENSO cycle, while the less significant association is found in the North China rainfall data. They concluded that similarity in the variation of Indian summer rainfall and north China rainfall record may be caused by similar associations between the rainfall variation and ENSO cycle and the general circulation anomalies in the middle and high latitudes over Eurasian continent, while the difference may related to the different character in these associations.

The last topic was the climatic trend and periodicities of annual rainfall over India by Prof. Rao with the use of rainfall time series for the period of 124 years (1871-1994). Significant decreasing trends are found during 1890-1920 and 1960-1980 and increasing trend during 1920-1960. The trends over southeast peninsula are out of phase with rest of the country. Also, significant periodicities in the ranges 2-3 years, 6-8 years and 13-15 years are observed which are attributed to QBO, ENSO and inter decadal variability, respectively. From the results, it needs further investigation to understand why the trend is significant over isolated areas with opposite direction.

In summary, there is many timescale variations in the Asian summer monsoon rainfall. Many papers indicate that the interannual variability of the Asian monsoon rainfall based on the air-sea interaction associated with variation of tropical SST, while biennial oscillation or QBO might be based on the air-land interaction including variation of snow cover over Tibetan Plateau. Also, there is intraseasonal variation in monsoonal rainfall associated with synoptic patter changes. However, some of variability of monsoon rainfall is nor clearly explained. We may still need a significant effort for the better understanding of Asian monsoon and climate variability. For the long-term variability study in addition to the future international field experiments, such as GAME, in particular, an international collaboration work is highly desirable to exchange the long period climatological data of each country to understand whether it is regional climate change with a significant trend or just a part of natural variation of climate.

Reports on Simulation and Prediction of the Asian Monsoon
by Akio Kitoh (Meteorological Research Institute, Japan)

Thirteen papers were presented in the session of "Simulation and Prediction of the Asian Monsoon". AGCM, CGCM, and RegCM are used for these studies. Topics covered are interannual variability of simulated monsoons with respect to SST and land surface conditions, impact of SST anomaly, land surface processes, cloud formation, and cumulus parameterizations. Also shown are topics about prediction of heavy rainfall and monsoon changes due to global warming.

From the excellent presentations made in the session, we can tentatively see that the followings are important for clarifying monsoon mechanism using numerical models.

  1. Only considering a monsoon-land interaction or an ENSO-monsoon interaction may not be enough. We should understand the monsoon variability under the coupled processes of ocean-atmosphere-land system as a whole. Processes include water cycle and cloud, snow and ice, biosphere and atmospheric chemistry.
  2. The results of the climate simulations are generally good in their broad-scale features, but the performance of regional scale precipitation is not so good, particularly the precipitation pattern in East Asia. Reasons for poor simulation may include resolution problem, design of orography, convection parameterization, cloud parameterization and land surface processes.
  3. Different interpretations are made of the possible impact of the South China Sea SST on the monsoon. Reason for this may be that in a coupled system the SST is determined through various interactions. However in AGCM sensitivity experiments, the model only responds to given SST anomalies.

Action:

  1. To study interactions among ENSO, monsoon and land surface conditions by using various types of models including a coupled atmosphere-ocean GCM.
  2. To intercompare East Asian climate simulated by many model by participating an international comparison project.

Reports on Land Surface Processes
by Tetsuzo Yasunari (University of Tsukuba)

  1. Seasonal change of surface water balance shows large spatial variability , depending on land-surface conditions and climate. (e.g. P, E, P-E) - Tao, Ohata
  2. Vegetation control of surface energy budget seems to be important. - Kondo
  3. Remarkable seasonal and spatial changes of surface radiation and energy budget characteristics (SH, LE, CD etc.) are noted over the Tibetan Plateau. - Endoh, Haginoya, Li, Li
  4. The role on snowcover, permafrost and frozen ground with soil moisture on water and energy budget and climate seems to be very essential. - Endoh, Ohata, Sato, Bamzai
  5. Stable isotope ($B&D(J18O,$B&D(JD) budget study is important for water cycle study, particularly over the Tibetan Plateau. - Nakawo, (Yao)
  6. Glaciers in High-Asia are generally retreating due to warming (?) and precipitation change? - Han

Recommendations for Land Surface Processes:

  1. Process studies and monitoring effort as in situ AWS and satellites need to be further intensified particularly to SNOW COVER, PERMAFROST and SOIL MOISTURE over the Eurasian continent.
  2. Modeling studies based on these process studies and monitoring data are very essential task for Asian monsoon modeling activity.
  3. Stable isotope studies should be involved in water cycle and hydrological processes of the monsoon climate system.

Reports on Asian Monsoon and ENSO
by Huang Ronhui (Institute of Atmospheric Physics, Chinese Academy of Sciences)

Asian monsoon and ENSO play significant roles in climate variability in East Asia. There are six presentations concerning the Asian monsoon and ENSO in the session. The main results are as follows:

  1. The impact of the East Asian winter monsoon on ENSO was investigated. It is pointed out that, on the one hand, the intraseasonal oscillation coming from the Indian Ocean to the western equatorial Pacific can be greatly modulated by the East Asian winter monsoon, which can influence significantly the evolution of the ENSO cycle (Dr. Sui's presentation). On the other hand, when the strong East Asian winter monsoon flow arrived at the equatorial western Pacific, the strong convective activities on the intraseasonal time-scale can be stimulated, which can be act as a triggering mechanism of ENSO event through the effect on the dynamical processes in the atmosphere and the ocean (Prof. Li's presentation).
  2. Through the diagnostic studies of the impact of El Niño on the monsoon precipitation in China, it is pointed that in northern winter, spring and autumn, the positive precipitation anomalies with statistical significance appeared in the southern part of China during the El Niño mature phase. In summer, significant negative precipitation anomalies appeared in the northern part of China. The possible physical process through which the El Niño affects the precipitation are proposed in term of the anomalous features of the circulation during the El Niño mature phase. (Prof. Zhang's presentation)
  3. It is revealed that the multi-scale perturbations exist in the tropical Pacific air-sea coupled system and they strongly interacted each other. The interactions of the perturbations of different time-scales can greatly affect the duration and amplitude of the ENSO cycle (Prof. He's presentation). In particular, the interaction between the perturbation of intraseasonal time-scale and that of the interannual time-scale can be important in the tropical air-sea coupled system (Dr. Li's presentation)

The interaction between the Asian monsoon and the El Niño is an important research topic and should be studied further.

Reports on Oceanography and Air-Sea Interaction
by T. Yamagata (University of Tokyo) and R. Lukas (University of Hawaii)

Asian Monsoon is one of the most remarkable phenomena of the ocean-atmosphere-land system in response to a seasonal march of solar insolation with complex feedback mechanisms. The effects including human dimensions manifest themselves in variety ways both regionally and globally as shown by Sribimawati. The ocean comes into play of this drama as one of major players via heat and moisture fluxes through its interface. Those fluxes are dependent of both atmospheric and oceanic variables such as sea surface temperature (SST), winds, etc. In particular, the sea surface temperature is one of the most important variables in the prediction of the monsoon system since the monsoon is driven by land-sea thermal contrast. he sea surface temperature is also involved in moisture fluxes which amplify the monsoon circulation by diabatic in the atmosphere.

Lukas showed that the seasonal and interannual variability of SST in the western Pacific are related, with some interannual variability being associated with amplitude and phase variations of the annual cycle of SST anomaly modes exhibit the classical ENSO east-west dipole pattern, the north-south see-saw related to variations of Asian winter monsoon activities as suggested by Watanabe, and dipole pattern between the western tropical Pacific Ocean and Indian Ocean. The latter dipole pattern may be related with that found by Nicholls, suggesting the involvement of the Indian Ocean through a possible TBO mechanism. Oceanographers need to know from meteorologists where the monsoon circulation of the atmosphere is most sensitive to those SST anomalies. As suggested by Yamagata, the Indonesian Through flow (ITF) may certainly affect the dipole pattern by providing a pathway of oceanic heat from the western tropical Pacific to the eastern tropical Indian Ocean. The ITF is, in turn, affected in delicate manners by ENSO and other climate variations through wind variations. Since the surface circulation is in general driven by winds; high-quality satellite wind data set as shown by Halpern is quite useful even in estimating the thermal budget of the upper ocean using a data assimilation system as described by Mikawa.

The SST is not a simple oceanic variable and it is affected by both oceanic and atmospheric complex processes that are different from location to location, as typically shown by Watanabe for the western North Pacific. Upper ocean heat content is more clearly related to oceanic dynamics. Therefore, process studies on air-sea heat flux and oceanic thermal structure of sufficient accuracy in places where those prominent signals appear is highly recommended to further clarify the major processes which determines the above SST patterns. Modeling is also quite useful to check hypothesis as well as to interpolate and extrapolate data both in space and time.

In order to predict the Monsoon variations, we need to fully understand the Pacific ENSO event and the turnabout mechanism. This is not only because the western tropical Pacific SST is dominated by the ENSO signal but also the remote atmospheric and oceanic conditions are closely linked with the ENSO event. Xie discussed the meridional asymmetry of the Pacific cold tongue and northern hemisphere ITCZ, suggesting that the Andes which may block the zonal winds and the cooling by southerly- induced coastal upwelling off South America are necessary elements for the Bjerknes feedback hypothesis to work in the eastern tropical Pacific.

A possible interplay among the Asian Monsoon and the decadal climate variations such as the well-known mid-1970's and late 1980's regime shifts is another challenging topic which should be addressed more for a long-term prediction of the Asian Monsoon. Nakamura showed that the anomalous SST pattern is decomposed into two components. The subtropical signal is more directly related to the tropical phenomena but the subarctic signal is an outcome of more or less in situ ocean-atmosphere interaction. The latter is associated with the meridional migration of the strong oceanic thermal front north of the Kuroshio Extension discussed by Maximenko using drifter data.

Mikowa addressed the issue of sparse ocean thermal structure observations in the Asian-Australian monsoon regions, suggesting the value of an ocean data assimilation system to make useful fields. Although some robust features of the ocean response to monsoon forcing emerge, more ocean observations are clearly necessary particularly in the Indian Ocean Vinayachandran compared the seasonal cycle of the OGCM to observations in the Bay of Bengal, find ing some key features, but not all, are reproduced. Observations are again sparse in space-time in this important bay. Experiments show that wind stress distribution in the bay is dominant except during the southwest Monsoon, where remote equatorial forcing is important to force the equatorward boundary flow along the east coast of the India.

Using a diagnostic model calculation, Miyama showed that the warm pool of the western Pacific and Indian Ocean is a result of a balance between surface heat flux and horizontal advection. The residence time of the water parcels in the warm pool is short, only 1.3 years. this result is for the long-term mean seasonal cycle, and further work is needed to determine the role of ENSO in this mean result. Qu presented evidence for the inclusion of the Luzon Undercurrent and Mindanao Undercurrent as features of the general ocean circulation in the western Pacific. The Mindanao Current transport is consistent with Sverdrup theory, but observations of such low-latitude western boundary currents are also insufficient to determine its variability, including the annual cycle.

Reports on Radar and Satellite Observation over Monsoon Asia
by Tsuyoshi Nitta (CCSR, The University of Tokyo)

9 papers were presented in this session. Among them 5 papers described satellite observations dealing with clouds and SST measurements (2 papers) and hydrology such as snow cover and soil moisture (3 papers). 4 papers discussed surface radars such as MU radar and boundary-layer radar.

Through presentations and discussions it became clear that since satellite data give us uniform data over land and ocean, they are very useful for the studies on Asian monsoon. Satellite data are able to observe various types of convective systems with wide range of spatial and temporal scales from diurnal to interannual variations which play important roles in Asian monsoon variabilities. However, more quantitative precipitation data and 3-D heating profiles should be obtained in future to validate numerical atmospheric models. In this regards, TRMM which will be launched next year will provide us valuable quantitative data of precipitation and diabatic heating over Asian monsoon regions from space.

Satellite data are also powerful for monitoring of interannual signals in the Asian monsoon regions by measuring SST. Since the Asian monsoon is a coupled system between atmosphere, ocean and land, monitoring of SST is a quite important for understanding of intrerannual variabilities of monsoon and their relationships with ENSO and QBO. In future accuracy of SST estimation by satellites should be more improved. Also more time resolved SST data such as 5-day means or 10-day means may be needed for understanding of more detailed interaction processes between atmosphere and ocean.

Algorithms have been developed to derive hydrological parameters over land surface by using satellite microwave measurements. Interactions between atmosphere and land surfaces as well as interactions between atmosphere and ocean are essential for understanding of the Asian monsoon system. Further algorithm development for getting quantitative values of snow amounts and soil moisture is required in near future and these data will be needed to accomplish the objectives of GAME.

Surface radar observations can catch up fine structures of various kinds of weather systems which pass over the radar stations. These data are also useful for validation of satellite observations. Larger-scale networks of radar observations may be needed in future to obtain spatial structures as well as vertical structures of weather systems in the monsoon regions.

Finally it would be recommended that the 4-D Data Assimilation including both the satellite and radar data will be needed in future for quantitative understanding of Asian monsoon system which consists of interactions between atmosphere, ocean and land surface.

3. Recommendation

RECOMMENDATIONS adopted by
INTERNATIONAL WORKSHOP ON THE CLIMATE SYSTEM OF MONSOON ASIA
December 3-7, 1996

[1] Name of the project: Relationship between the Asian-Australian monsoon and ENSO, and its influence on the intraseasonal to interannual climate variability in Asia.

[2] When conducting the future research mentioned above, the workshop recommends the following major observational and data handling aspects to be implemented.

  1. Long-term monitoring of the interaction between atmosphere, ocean and land surface, including the biosphere, in the Asian-Australian monsoon area.
  2. Four dimensional data assimilation and reanalysis of observations of atmospheric, oceanic and land surface parameters, including satellite data.
  3. Establishment of an international data management and archive system, including data analysis centres for earth observation data.
  4. Recovery and analysis of climate data from historical records and paleoclimate study.

[3] When conducting the future research mentioned above, the workshop recommends the following modeling activities.

  1. Studies on the interaction between ENSO and the Asian monsoon by using coupled atmosphere-ocean-land GCMs at climate computing centres.
  2. Regionalisation of climate variability for monsoon Asia and prediction, to the possible extent, of seasonal to interannual climate anomalies.

[4] The workshop recommends the following guidelines to enhance international cooperation.

  1. To work within the framework of GEWEX GAME and CLIVAR GOALS and some regional international project such as SCSMEX in order to profit from and to contribute to the internationally coordinated projects of the World Climate Research Programme.
  2. To seek involvement of the inter-governmental network such as APN in order to involve more nations of the region in monsoon studies.

4. Acknowledgment

As an editor, I express my thanks to the International Programme Committee members, the local organizing committee members, and all the participants in the workshop. I am especially grateful to the session chairpersons for summarizing their sessions.


Dr. Nobuo Yamazaki
Typhoon Research Department
Meteorological Research Institute
1-1 Nagamine, Tsukuba-shi
Ibaraki-ken 305, Japan
E-mail: nyamazak@mri-jma.go.jp