The Monsoon of East Asia and Its Global Associations—A Survey

The Monsoon of East Asia and its Ka-Ming Lau1 Global Associations—A Survey and Mai-Tsun Li2

Abstract is by no means a regular, nor stationary feature in space and time. There are large intraseasonal as well as interannual vari- Observations concerning the summer and winter monsoons of East abilities associated with its transition, maintenance, and ex- Asia and their global associations are reviewed. The seasonal mean tent of remote influence or teleconnections. structure, transient variation, including intraseasonal to interan- In this paper, first we review some observations that define nual, and synoptic to planetary scale fluctuations are discussed sepa- rately for the two monsoon components. Similarities and differences the basic structure including the spatial and temporal scales between the East Asian monsoon and that of India are also surveyed. of the regional and planetary scale aspects of both the We also present a description of the current status of monsoon re- summer and the winter monsoon of East Asia. We then dis- lated observational and theoretical research and highlight important cuss the anomalies in both components of the monsoon and scientific problems. The importance of understanding the long-term anomalies of the monsoon is stressed, and an attempt is made to put their possible relationships with atmospheric teleconnection the East Asian monsoon in a global perspective with a view towards and long range predictions. The main objectives of this paper identifying with the problems of long-range weather forecasting or are: 1) to serve as an introduction to the many facets of the short-term climate prediction, in general. Finally, some future direc- East Asian monsoons and 2) to review studies that have been tions of research are suggested. conducted in China and elsewhere in order to delineate potential scientific problems, and provide a stimulus for further research in these areas. 1. Introduction

Bordered by the Tibetan Plateau to the west, the Euroasian 2. The summer monsoon land mass to the northwest, and the vast Pacific Ocean to the south and east, East Asia has experienced one of the most a. Large-scale mean circulation pronounced monsoon climates of the globe for centuries. Monsoon related droughts and floods have had enormous The large-scale mean circulation of the East Asian summer social and economic impacts on the people of China and the monsoon is characterized by strong cross-equatorial flows countries of southeast Asia, which form a large segment of from the southern to the northern hemisphere in the lower the world's population. The study of the monsoon is now the troposphere, near the longitudes of the east coast of Africa number-one priority in the national program for long-range and the maritime continent of Borneo and Indonesia, as well forecasting and climate research of the government of the as a strong westerly flow over South India and southwesterly People's Republic of China. flow over China (Fig. la). This circulation is superposed at The monsoon, by definition, is a reversal of wind with the 200 mb (Fig. lb) by an intense anticyclone centered over the season. The East Asian monsoon, in particular, exhibits a Tibetan Plateau and strong easterly winds near 10°N stretch- distinct summer and winter component. This is different ing from Indochina to west Africa. The divergent part of the from the Indian monsoon in that the winter monsoon of circulation consists of a gigantic, thermally-direct circula- India is not known to be present to any significant extent. tion with hot air rising over Tibet and Indochina, and cold air During the summer, the bulk of central and eastern Asia is sinking over the adjacent areas (Fig. lc). Figs. 2a and b show under the influence of southwest surface winds accompanied the vertical structure of the divergent N-S and E-W circula- by heavy rain, and a generally hot and humid climate. Dur- tion associated with the summer monsoon. The local Hadley ing the winter, the prevailing surface wind over the same area circulation between 75°E-110°E is manifested in the pole- is northeasterly, originating from a huge anticyclonic circu- ward surface flow toward the elevated Tibetan heat source, lation over Siberia. This circulation brings cold dry continen- with equatorward return flow aloft (Fig. 2a). At 10°N, the tal air mass to the eastern and southern coastal region of center of rising motion of the Walker-type circulation is China. Although an annual event, the monsoon of East Asia found at 500 mb over the Bay of Bengal, with the strongest sinking motion over the Arabian Sea and East Africa (Fig. 2b). A strong sinking motion is found over vast areas of the central Pacific at 25°N (not shown).

1 Laboratory for Atmospheric Sciences, NASA, Goddard Space Flight Center, Greenbelt, MD 20771. 2 Center for Meteorology and Physical Oceanography, Massachu- setts Institute of Technology, Cambridge, MA 02139. Permanent af- b. Tibetan heat source filiation: Institute of Atmospheric Physics, Academia Sinica, Beij- ing, People's Republic of China. The importance of the Tibetan Plateau as an elevated heat source during the northern summer has been noted by many © 1984 American Meteorological Society authors (e.g., Flohn, 1957, 1968; Koteswaram, 1958, Mura-

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FIG. 1. Mean summer streamline pattern at 850 mb (upper panel) and 200 mb (middle panel) and 200 mb velocity potential (lower FIG. 2. a) Vertical cross-section of wind vectors between 75-110°E panel) including divergent wind vectors. Adapted from Webster et showing streamline flow of the Hadley circulation during the East ai, (1977). Asian summer monsoon, b) Vertical cross-section of wind vectors along 10°N, showing streamline flow of the Walker circulation during the East Asian summer monsoon. kami, 1958, 1981; Yeh, 1981 and Luo and Yanai, 1983). Situ- strong low-level westerly flow from the Arabian sea to ated on the leeside of the plateau, East Asia comes under the southwest China. Recent studies (e.g., Wang and Li, 1982) direct influence of the thermal and dynamical forcing by this suggest that by the time this surface westerly flow over India huge elevated land mass. Sensible heat flux over the semi-arid reaches China, its heat and moisture content are mostly de- western region of the plateau and latent heat release above pleted and, therefore, its influence on the monsoon rain over the Himalayas, contribute to a strong tropospheric heat South China will only be marginal, at best. It is now known source which maintains the large scale Asian monsoon circu- that a large part of the summer monsoon climate over south- lation. Not only does the plateau determine the large scale ern and central China, including the Yangtze basin is circulation, but it also has profound influence on synoptic strongly controlled by the low-level southerly surface flow scale events over monsoon China. Tao and Ding (1981) over the maritime continent of Borneo and Indonesia (see Fig. showed that severe convective storms and heavy rain activity la). This flow, which crosses the equator near 100°E, appears over the middle and lower Yangtze basin often have their to originate from anticyclonic outflow over Australia, and origin near the plateau. During the mid-summer monsoon becomes the prevalent surface circulation over China. As the months of July to August, low-level cyclonic vortices gener- peak monsoon period approaches, this southerly or south- ated in the boundary layer over the rugged terrain of the pla- westerly flow penetrates progressively further into northern teau are frequently steered into the monsoon low-level China and Japan. As a counterpart to the Somali jet which trough, where they intensify and develop into major convec- heralds the Indian summer monsoon (Findlater, 1969, 1972, tive systems. 1974), a low-level southerly jet also is observed near 20°N, The above description is only a very brief outline of the ef- 110°-115°E, reaching to a maximum of 8 m/s near 2 km in fect of the plateau on the weather and climate of East Asia. July (Fig. 3). The meteorology of the plateau itself and other associated The onset of heavy summer monsoon rain over the phenomena are diverse and extremely interesting, but are Yangtze basin occurs around late June following the build- outside the scope of this survey. For a more comprehensive up of this low-level jet in a similar manner to the onset of account of the subject, the reader is referred to the work of heavy rain over western India after the intensification of the Yeh and Gao (1979). Somali jet in late May to early June. A major difference between the Chinese and the Indian monsoon is that while the former takes about two months to spread over an extensive area from southern to northern China, the monsoon rain- c. Low-level cross-equatorial flow band associated with the latter takes only about one to two Traditionally, the Chinese and the Indian monsoons were re- weeks to migrate from southern to northern India (Sikka and garded as one and the same system characterized by the Gadgil, 1980). However, it should be made clear that the In-

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ageostropic jet is suggested to be a result of the downward transport of horizontal momentum by cumulus convection (Akiyama, 1973, and Ninomiya and Akiyama 1971). Re- cently Chou et al. (1982) showed that the low-level jet may be a result of momentum transport by the secondary meridional circulation induced by diabatic heating. It is now generally agreed that the "Mei-Yu" front is a result of the interaction between the south and southeastward intrusion of the East Asian polar fronts and the seasonal northward migration of the monsoon trough, and, therefore, possesses mixed tropical and midlatitude characteristics.

e. Transitions The summer monsoon of East Asia does not undergo a smooth seasonal transition, but occurs in distinct stages with abrupt changes (Murakami, 1958, Yeh et al., 1959 and Tao and Ding, 1981). From April to early May, most of the pre- monsoon rain activity is restricted to Southern China. The transition into the "Mei-Yu" regime (mid-May to mid-June), is marked by a rapid northward penetration of low-level westerlies and upper-level easterlies to the foothills of the Himalayas and the Yunan and Kweichow highlands (Fig. 5a). Associated with this transition, an abrupt northward shift of the upper level westerlies over eastern China and Japan becomes evident (Fig. 5b). At the same time, the monsoon trough over China migrates from a position south of the Yangtze to north of it (Fig. 6). The transition typically takes place within a few days to a week. By mid-July, the monsoon trough again undergoes a further abrupt northward migra- FIG. 3. Vertical cross-section of the meridional wind component tion into northern China and the Yellow River Basin, where along 25°N over south China during July, showing the low-level southerly jet with maximum wind speed at about 2 km, near 110°E. heavy rain prevails for one to two months, until late Sep- Units are in m • s-1. tember or early October. The retreat of the large-scale monsoon, and the re-establishment of the monsoon trough south of 10°N occur in October, almost as abruptly as their onset dian and Chinese monsoons and their associated low-level jet and northward migration. structures are not completely independent systems, as both are affected by the same large heat source over Tibet and sim- The abrupt change in the large-scale circulation, however, ilar heat sink distributions over the tropical Pacific and the is not a feature unique to the monsoon. Charney and DeVore Indian Ocean. (1979), Charney and Straus (1980), and Li and Lo (1983) showed that multiple equilibria of the large-scale atmospheric flow and abrupt transitions can occur as a result of d. The early phase: "Mei-Yu" regime nonlinear effects. The presence of inhomogeneities at the earth's surface due to orographic or thermal effects can read- During the early phase of the summer monsoon (around ily cause transition into one of such preferred equilibrium mid-May to mid-June) a quasi-stationary front establishes states. In the case of the East Asian monsoon, the presence of itself in a southwest to northeast direction from southern the Tibetan Plateau will have a profound influence on the China to Japan (Fig. 4). Throughout this entire period, re- configuration and stability of such states. gions south of the Yangtze River experience both intermittent and continuous rain spells lasting from several days to weeks. This period is called the "Mei-Yu" (Plum rain) in f. Intra-seasonal oscillations China or "Bam" in Japan (Chen and Chang, 1980 and Mat- sumoto etal., 1970, 1971). Rainfall rates of as high as several The planetary scale monsoon is strongly modulated by sub- hundred millimeters per day frequently are observed during seasonal scale oscillations. These oscillations lead to periods this period. of intense activity ('active' monsoon) separated by distinct Recent studies have shown that the "Mei-Yu" rainband is lulls ('break' monsoon). Krishnamurti and Bhalme (1976) a mixed synoptic and mesoscale complex where the tempera- showed that these subseasonal scale oscillations are related ture gradient is weak, but the moisture gradient is quite to a quasi-biweekly oscillation of the Indian monsoon system strong across the frontal zone. A most interesting feature is including the Tibetan High, the cross-equatorial low level jet, the formation of a transient low-level jet near 700 mb along the tropical easterly jet, monsoon cloud cover and rainfall. the frontal zone associated with the heavy rain episodes. This Oscillations of a similar time scale are found in the summer

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FIG. 4. Streamline chart at 3 km during 26-31 May, 1956. The "Mei-Yu'1 trough, whose southern tip is south of the Yangtze River, is seen to be separated from the monsoon trough over India. monsoon rainfall and surface wind fluctuation over south- Lorenz (1983), using an air-sea coupled, low-order spectral western China, and the middle and lower Yangtze basin. Fig. model, showed that a global scale, low frequency oscillation 7 shows the mean coherence spectrum of the individual co- between two stable climatic states in a simple monsoon sys- herence of the zonal and meridional wind at different levels tem can occur. They suggested that this oscillation is a with the surface wind over the lower Yangtze basing during "natural oscillation" of the monsoon system arising from the "Mei-Yu" period of 1970-72. It can be seen that in gen- nonlinearity of the larg-scale flow which gives rise to multi- eral monsoon activities are rather shallow and restricted to ple climatic states in concert with a given air-land-sea below 850 mb, for most time scales. A strong coherence with distribution. the 200 mb wind occurs around the 12-15 day period suggesting a preferred quasi-biweekly time scale for deep convective systems during the "Mei-Yu" regime. While the physical mechanisms leading to the quasi-bi- 3. Winter monsoon weekly oscillation are still a matter of debate, there have been a number of hypotheses that have been proposed to "ex- a. Large-scale mean circulation plain" such an oscillation. Krishnamurti and Bhalm (1976) suggested that the interaction of solar radiation and cloud During the winter monsoon, the low-level circulation over cover may be responsible for this oscillation. This view is Asia is dominated by a huge anticyclone over Siberia giving supported by Webster et al. (1979), and Webster and Chou rise to strong northerly and northeasterly winds covering the (1980), who used a simple model to show that intraseasonal entire region of the central and northern China and the South oscillations of the moosoon rainfall can occur as a result of China Sea. This circulation appears to be connected with the interactions between solar radiation, cloud cover and the hy- corresponding summer monsoon cyclonic circulation over drological cycle. They further pointed to the importance of northern Australia and New Guinea by a strong cross-equa- air-sea coupling in the interactive process. Recently, Li and torial flow near 100°E (Fig. 8a). The low-level flow over East

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FIG. 5. a) Five-day mean cross-section of zonal wind along 90°E from May to June, 1956 showing the sudden northward intrusion of subtropical upper (lower) level easterlies (westerlies) to the latitudes of the Himalayas during the "Mei-Yu" regime. Units are in m -s-1. b) Five-day mean cross-section of zonal wind along 125°E over the East China Sea from May to June, 1956, showing the abrupt northward shift of the upper level westerlies. Units are in m*s-1. and Southeast Asia is almost an exact reversal of that of the icantly from a simple mirror image of its summer counter- summer monsoon. At the upper level, the circulation is dom- part. Heavy precipitation and, therefore, condensation heat inated by strong anticyclonic flow over the Philippines with a source, is centered over the near-equatorial regions of the strong midlatitude westerly jetstream over Japan (Fig. 8b). maritime continent of Indonesia and Borneo. A strong Although the energetics of the thermally direct circulation north-south thermal contrast exists between this heat source is basically similar to that of the summer monsoon, the win- and the very cold Asian continent to the north. This heating ter monsoon exhibits a distinct character which differs signif- gradient produces perhaps the most energetic of all heat en-

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FIG. 6. As in Fig. 4 except for 6-10 June, 1954. Southern tip of the "Mei-Yu" trough is now north of the Yangtze River.

gines known today, which drive the general circulation along the east coast of China tightens causing cold air to (Ramage, 1968, 1975; Chang and Lau, 1980). At 200 mb burst out of the continent towards southern China and the (Fig. 8c), the divergent circulation that results from this South China Sea. The sudden outburst of cold air is called a thermal contrast is near-global with rising motion over the "cold surge." The structure and energetics of midlatitude maritime continent and extensive sinking motion over Siberia, disturbances associated with cold surges during Winter South Australia, the tropical central Pacific, and West MONEX have been reported in a recent study by Lau and Africa. Fig. 9 shows that the vertical structure of the mean Lau (1984). The rapid widespread influence of cold surges Hadley circulation between 55°E-140°E has completely re- over the South China Sea is suggestive of surface gravity- versed from its summer counterpart (Fig. 2a). wave characteristics (Chang et al., 1979 and Lim and Chang, 1981). The effect of increased surface northeasterly winds associated with a cold surge can often be felt in changes in con- b. Cold surges vective activity in near-equatorial latitudes. (Cheang, 1977, Chang et al., 1979, Chang and Lau, 1982). Typically, cold During the winter monsoon season, excessive radiative cool- surges occur during the peak monsoon months of December ing coupled with cold air advection and the blocking effect of to February, at intervals of several days or weeks. Cold the Tibetan Plateau to the southwest, cause the build-up of a surges are also modulated by intermittent lulls like the huge mass of cold surface and tropospheric air over northern summer monsoon breaks. The period of modulation appears China. This creates a strong baroclinic zone between the cold to have a broad peak at 15-30 days (Murakami, 1980). polar air mass and the warm tropical air mass to the south. Passages of upper troughs in midlatitudes often trigger intense cyclogenesis over Japan and anticyclogenesis over c. The East-Asian upper level westerly jet stream eastern China (Danielson and Ho, 1969; Chang and Lau, 1980 and Lau et al., 1983). As the anticyclone over eastern The East Asian jetstream is the most intense large-scale China moves south and southeastward, the pressure gradient westerly flow known to exist within the troposphere. The jet

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FIG. 9. Vertical cross-section of wind vectors along 55-140°E, showing the northern winter monsoon Hadley circulation.

core stretches from northern China, across Japan to the central North Pacific. Its quasi-stationarity in position and large fluctuations in intensity are indicative of climatological as well as transient forcings unique to East Asia. Blackmon et al., (1977) showed that the winter mean East Asian jet is maintained largely by the Coriolis torque exerted on the upper-level divergent poleward flow at the jet entrance re- FIG. 7. Coherence spectra between wind at different levels with gion. This divergent flow arises as a result of the thermally that at the surface over the lower and middle Yangtze basin for direct circulation induced by the winter monsoon heat source 1970-72, respectively. over the maritime continent. In the transient case, a similar control by the Hadley circulation at the jet entrance region exists. As a result of the direct effect of baroclinic development on jet accelerations, monsoonal cold surges at the surface over South China and Southeast Asia are reflected as transient fluctuations in the intensity and position of the jet core. Using Winter MONEX data, Lau et al. (1983) showed that the fluctuations of the jet are also related to the intensity of midlatitude-tropical interactions and midlatitude downstream developments. The study of the dynamics of the maintenance of the East Asian jetstream, therefore, is crucial to the understanding not only of the East Asian winter monsoon, but also the mechanics of planetary scale iriteractions in general.

d. Midlatitude-tropical planetary scale interactions

Midlatitude-tropical interactions associated with baroclinic disturbances that trigger cold surges, can often lead to intensification of convection in the maritime continent of Borneo and Indonesia. This increased convection, in turn, leads to planetary-scale divergent outflow in the upper troposphere, enhancing the Walker and Hadley circulations. Fig. 10 shows the schematic of a sequence of events leading to and following a cold surge, including some possible correlation with a secondary jet over West Asia. Recently Lau et al. (1983), using Winter-MONEX data, FIG. 8. Mean winter streamline pattern at 850 mb (upper panel) and 200 mb (middle panel) and 200 mb velocity potential (lower showed that during the period (Dec. 1978-Feb. 1979), in ad- panel) including divergent wind vectors. Adapted from Webster et dition to enhanced tropical convection resulting from cold al. (1977). surges, midlatitude trough and ridge developments also be-

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FIG. 11. Total eddy kinetic energy along 35-45°N over the central North Pacific at a) 200 mb and b) 850 mb as a function of the phase (hours) of cold surges over South China. Strong intensification of eddy activity downstream of East Asia is indicated by the almost doubling of eddy kinetic energy 2-3 days after a cold surge onset (0 hour).

cific between 25°-35°N was increased more than two-fold, 2-3 days after a cold surge onset over north China. In weather maps, this downstream teleconnection can some- times be seen as wave trains crossing the North Pacific from the east coast of northern China to the west coast of United States. Some of these wave developments can be traced all the way westward to Eurasia (Joung and Hitchmann, 1982). There are now evidences to suggest that this wave train development may lead to changes in tropical convection over the central Pacific, which may in turn influence midlatitude synoptic and quasi-stationary systems further downstream. A schematic picture of the interaction is depicted in Fig. 12. Thus the influences of the monsoon cold surges on other cir- FIG. 10. Schematic diagram showing a sequence of events during a typical cold surge cycle, showing the possible tropical influence culation regimes, both tropical and midlatitude are quite of winter monsoon surges. Adapted from Chang and Lau (1980). profound. The physical mechanisms for such influence still remain unknown. Recent theories of atmospheric teleconnection (Webster, 1982; Hoskinsand Karoly, 1981; Simmons came enhanced downstream from the East Asian jet. Fig. 11 et al., 1983; Lim and Chang, 1983; and Lau and Lim, 1984) shows that the total eddy kinetic energy over the central Pa- are beginning to provide some answers to this problem.

FIG. 12. Schematic showing possible midlatitude-tropical and downstream interactions associated with low level monsoonal cold surges over East Asia.during Winter-MONEX. Dashed lines represent the domi- nant thermally direct circulations.

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gion over Indonesia and North Australia have been known to occur. During the Winter-MONEX, a case of convection enhancement over the Java Sea was detected two to three days after a cold surge onset over the South China Seas. The area of enhanced convection subsequently moved eastward before it disappeared over the Solomon Islands (Williams, 1979). Using satellite cloud imagery, Lau (1982) showed that eastward and westward propagating cloud clusters generated over the maritime continent and the Southern Hemisphere tropics by cold surges can be identified with equatorially trapped Kelvin and Rossby waves modes. Research in this area is still rather limited, largely due to the lack of reliable data. Winter-MONEX has provided a rich source of data from which this problem can be studied.

FIG. 13. a) Time series of percentage deviation of anomalous 4. Anomalies in the East Asian monsoons "Mei-Yu" rainfall amount (R%)and sea surface temperature anomaly of the Kuroshio (TS°C) during the antecedent winter, b) Correla- tion coefficient between "Mei-Yu" anomalous rainfall and SSTA As mentioned in the introduction, the monsoon of East Asia along the Kuroshio current (120°E, 20°N-165°E, 50°N) as a function is associated with a large interannual variability (I AV). Often of the calendar month. Values of 0.46, 0.54 and 0.61 correspond to the amplitude of the IAV can be as large as the mean state. the 95%, 99% and 99.9% confidence levels, respectively. Damage in properties, crop yield, and famine caused by ex- cesses and deficits of monsoon rain, have put severe socio- economic hardships on the population of China and countries of Southeast Asia. Long-range prediction of the e. Interhemispheric interactions monsoons is of extreme practical importance. In the follow- Interaction between cold surges originating over northern ing sections, we will discuss briefly some scientific issues as- China and the south hemisphere summer monsoon in the re- sociated with the anomalies of the East Asian monsoon.

FIG. 14. Spatial distribution of correlation pattern ofMei-Yu" rainfall with snowcover amount over the Tibetan Plateau, showing strong positive correlation over Southern China. Contour values of 0.45,0.55 and 0.65 correspond approximately to the 95%, 99% and 99.9% confidence levels respectively.

Unauthenticated | Downloaded 10/05/21 10:14 AM UTC Bulletin American Meteorological Society 123 a. Summer monsoon convection over the equatorial central Pacific would likely provide a more strong interaction between tropical midlati- The correlation between summer monsoon rain over China tude systems over the central Pacific. The observed below- and sea surface temperature has been a subject of intense normal intensity of cold surges during Winter-MONEX and study by Chinese meteorologists. The IAV of Indian summer the enhanced downstream teleconnection support this view monsoon rain has been linked to sea surface temperature (Lau et al., 1983). Since the 1975-76 El Nino, convection over anomalies (SSTA) both local and remote (Shukla, 1975; the equatorial central Pacific remained above normal and Shukla and Misra, 1977; Rasmusson and Carpenter, 1983). never quite returned to its normal position over the maritime For the East Asian summer monsoon, a similar relationship continent until the end of 1981. In a sense, the Winter- exists. Time series of percentage monthly rainfall deviation MONEX period was still within the LOW/WET phase of the from normal over large areas of the middle and lower Yangtze Southern Oscillation (Van Loon and Madden, 1978). basin shows a remarkable correlation with preceding winter As a further note, the El Nino of 1982-83 appears to be SSTA over the Kuroshio current (Fig. 13a). Notably, for the associated with climatic anomalies worldwide, the monsoons monsoon flood of 1954 which caused a major disaster over of Asia, India, and Australia in particular. It remains to be central China, large positive SSTA in the region of the Kuro- seen how the anomalous patterns fit into the above picture. If shio were already well-developed during the preceding win- these anomaly patterns are proved to be correlated, the mon- ter. Lagged cross correlation of the "Mei-Yu" rain with a soon is truly part of a global phenomenon. The IAV of the SSTA over a cross-section along the Kuroshio again con- East Asian monsoon both summer and winter is then an in- firms that the SSTA of the southern portion of this warm cur- tegral part of the world-wide changes occurring in the ocean- rent from 120°E, 20°N to 140°E, 30°N, has significant corre- atmosphere system during the El Nino/Southern Oscillation. lation starting from the preceding fall to early spring (Fig. 13b). Such lag correlation statistics has been extremely useful in the prediction of monsoon rainfall over China. Observations also suggest that the "Mei-Yu" is closely re- 5. Concluding remarks lated to tropical eastern Pacific SSTA, through teleconnection via the atmosphere and/or the ocean. Li (1983) showed We have surveyed observations of the monsoons of East Asia that during El Nino year, the "Mei-Yu" is generally deficient and highlighted some interesting scientific problems asso- and the cross-equatorial low-level jet is much reduced. The ciated with the summer and the winter monsoons respec- correlation between "Mei-Yu" and SSTA over the tropical tively. Compared with the Indian monsoon, the East Asian eastern Pacific is as high as —0.63 significant at the 99.9% monsoon has received much less attention by the scientific level. community. We hope this review will serve as an introduc- Correlation of "Mei-Yu" rainfall amount with snow cover tion to the subject and stimulate interests in research in these over the Tibetan Plateau also shows a significant positive areas. The discussions above are by no means exhaustive. Yet correlation suggesting enhanced precipitation with above we have seen there is a wide spectrum of monsoon related normal snow cover over the plateau (Fig. 14). Numerical phenomena ranging from subsynoptic to planetary and glo- studies by Yeh and Chang (1974) supported this finding. On bal scales. In the latter scales, as we have shown, the issue is the other hand, the pioneer studies by Blanford (1884), no longer just for the East Asian monsoon but rather should Walker (1910) and later supported by Hahn and Shukla be regarded as a prerequisite for understanding in general the (1976), suggested that the amount of snow cover over Tibet problem of planetary scale interactions and long range and rainfall over India are negatively correlated. Consider- forecasting. ing that the Indian and the Chinese monsoon are governed by the same large-scale forcing, it is intriguing that such different relationships exist. Obviously more work has to be done to confirm or refute these observations. Acknowledgments. The first author (K.-M.L.) would like to ac- knowledge partial support by the National Oceanic and Atmos- pheric Administration under Contract NA82AAG02003 and by the National Science Foundation under grant ATM80-13153 both to the Naval Postgraduate School for travel and helpful discussions with b. Winter monsoon Professors C. P. Chang and G. T. Y. Chen. Part of this work was done while the second author (M.T.L.) was on sabbatical leave Very little is known of the IAV of the winter monsoon. How- (1981-1983) from the Academia Sinica, People's Republic of China ever, based on recent and some currently unpublished work, to the Department of Meteorology and Oceanography, Massachu- we may find some clues to a plausible pattern of variation. setts Institute of Technology. Encouragement and helpful discus- The following discussion is somewhat speculative, but is in- sions throughout this period provided by Professor E. N. Lorenz are gratefully acknowledged. tended to be so to provoke interests in research in this area. During the winter following an El Nino, surface northeaster- lies are greatly reduced over the western Pacific (Rasmusson and Carpenter, 1982) and the winter monsoon near-equator- References ial convective system shifts from the maritime continent to the vicinity of the dateline (Lau and Chan, 1983a, b). The re- Akiyama, T. 1973: The large scale aspects of the characteristic fea- duced convection over the maritime continent suggests a tures of the Baiu front. Paper. Meteor. Geophys., 24, 157-188. weakened East Asian Hadley circulation and hence dimin- Blackmon, M. L., J. M. Wallace, N. C. Lau and S. M. Mullen, 1977: ished intensity of the East Asian cold surges. Yet enhanced An observational study of the Northern Hemisphere winter time

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circulation. J. Atmos. Sci., 34, 1040-1053. , 1982: Equatorial response to northeasterly cold surges as in- Blanford, H. F.„ 1884: On the connection of the Himalayan snow- ferred from satellite cloud imagery. Mon. Wea. Rev., 110,1306-1313. fall and seasons of drought in India. Proc. Roy. Soc. London, 37, and H. Lim, 1982: Thermally induced motions in an equatorial 3-22. (3-plane: Hadley and Walker circulation during the winter mon- Chang, C. P. and K. M. Lau, 1980: Northeasterly cold surges and soon. Mon. Wea. Rev., 110, 336-353. near-equatorial disturbances over the Winter-MONEX area dur- and P. H. Chan, 1983a: Short-term climate variability and at- ing 1974. Part II: Planetary scale aspects. Mon. Wea. Rev., 108, mospheric teleconnection as inferred from satellite derived out- 293-312. going longwave radiation Part I: simultaneous relationships. J. and , 1982: Short-term planetary scale interaction over the Atmos. Sci., (in press). tropics and midlatitudes. 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Flood research report the procedures for message formulation and diffusion; pursue a program of long-term comparative studies on flood victims, An Illinois State Water Survey report, the result of an 18-month including specific measures of individual physical and mental survey, concludes that future research on flood hazard mitigation health, and interpersonal relationships; and identify instances of should have, as its goal, an increase in the efficient use of flood- successes and failures of state and local governments to obtain prone lands, not merely the reduction of national flood losses. This authority and funds for planning and acting on flood mitigation. was one of many conclusions in the report, which describes 115 A major conclusion of the assessment was that much more tasks to which research priority should be given in the coming research attention should be given to economic, geographic, soci- years. The survey involved eight disciplines in both the physical ological, and political science research, rather than physical sci- and social sciences, and as a result, a majority of the issues des- entific work. Single copies of the report, A Plan for Research on ignated as priorities in the report, call for interdisciplinary work. Floods and Their Mitigation in the United States, are available Some of the research recommendations in the report are to at no charge while supplies last from the Illinois State Water Sur- develop reliable methods for predicting the effect of land use on vey, Box 5050, Station A, Champaign, IL 61820. flood peaks, volume, and sediment production; investigate the sources of large amounts of debris in streams and ways to avoid formation of debris dams; determine the effect of flooding on groundwater quality, including bacteriological, virological, and Environmental Science and Engineering Fellows chemical contaminants; carry out an economic evaluation of Program existing public policies and institutions for flood hazard mitigation; examine the adequacy of integrated warning systems and The American Association for the Advancement of Science (AAAS) announces the availability of six summer environmental science and engineering fellowships, which would run from 11 1 Notice of registration deadlines for meetings, workshops, and seminars, deadlines for submittal of abstracts or papers to be pre- sented at meetings, and deadlines for grants, proposals, awards, nominations, and fellowships must be received at least three months before deadline dates.—News Ed. (

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