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Journal of the Meteorological Society of Japan, Vol. 80, No. 2, pp. 213--227, 2002 213

Moisture Circulation over East Asia during El Nino Episode

in Northern Winter, Spring and Autumn

Renhe ZHANG

Chinese Academy of Meteorological Sciences, Beijing, China

and

Akimasa SUMI

Center for Climate System Research, University of Tokyo, Tokyo, Japan

(Manuscript received 8 February, in revised form 25 December 2001)

Abstract

Diagnostic analysis is made to investigate features of the moisture circulation over East Asian duringthe El Nino episode in northern winter, spring and autumn. It is found that in all these seasons, theanomalies of precipitation in China, atmospheric precipitable water, water vapor transport and moisturedivergence over East Asia in the El Nino mature phase, differ from those in the rest of the phases. In theEl Nino mature phase, positive precipitation anomalies occur in the southern part of China. More north-eastward water vapor transport appears around the southeastern coast of East Asia, where moistureconverges, and precipitable water is above normal, which are consistent with the precipitation anoma-lies. The physical process through which El Nino affects the East Asian climate, is also identified. Dif-fering from the rest of the phases in the El Nino episode, the mature phase is characterized by strongconvective cooling anomalies in the atmosphere, in the area (0–15�N, 110�E–150�E) over the westerntropical Pacific. As a Rossby wave response of the tropical atmosphere to the cooling anomalies, ananomalous low-level anticyclone forms to the north of the maritime continent. This anticyclonic anomalynot only transports more water vapor to the area around the southeastern coast of East Asia, but alsostrengthens the western Pacific subtropical high, and shifts it to the south of the mainland China, whichare favorable for more precipitation in the southern part of China.

1. Introduction

Numerous studies have shown the significanteffects of El Nino on global weather and climatenot only in the tropical region (Rasmusson andCarpenter 1982), but also in the extratropics

through observational data analyses (e.g.,Horel and Wallace 1981; Van Loon and Mad-den 1981; Hamilton 1988; Fraedrich 1994) andnumerical modeling (e.g., Shukla and Wallace1983; Lau 1985; Held et al. 1989). The Asianmonsoon system is one of the most importantcirculation system in the general circulationof the global atmosphere (Krishnamurti 1971;Krishnamurti et al. 1973). The influence of ElNino on the summer climate over East Asiahave been studied by some investigators (e.g.,Nitta 1986, 1987; Huang and Wu 1989; Huang

Corresponding author: Renhe Zhang, ChineseAcademy of Meteorological Sciences, 46 Zhong-guancun South Road, Haidian District, Beijing100081, China.E-mail: [email protected]( 2002, Meteorological Society of Japan

et al. 1993). By examining the circulation fea-tures during the El Nino episode, Zhang et al.(1996, 1999) (hereafter referred to as ZSK),found that the impact of El Nino on the EastAsian climate is significant in the El Ninomature phase. The influence can be significantnot only in northern summer, but also in otherseasons. ZSK pointed out that in the lower tro-posphere, an anomalous anticyclone appearedto the north of maritime continent in the west-ern tropical Pacific in the El Nino maturephase, which can significantly affect the circu-lation over East Asia. Recently, Wang et al.(2000a) also pointed out that this anomalousanticyclone acts as a bridge to connect the warmevents in the eastern tropical Pacific in its ex-treme phase, and the weak East Asian wintermonsoon.

The moisture circulation is one of the mostimportant components in monsoon Asia. Innorthern summer, the water vapor over theEast Asian monsoon area, mainly comes fromthree areas, namely, from Indian monsoon areaacross the Bay of Bengal, and then northeast-ward to East Asia, from western tropic Pacificalong the western periphery of the western Pa-cific subtropical high, and from Siberia, whichare important in forming the summer climatein East Asia. (e.g., Murakami 1959; Akiyama1973; Asakura 1973; Murakami et al. 1984).Summer monsoon flows bring large amount ofwater vapor to the monsoon area. Water vaportransport anomalies, and its divergence areclosely related with the precipitation anoma-lies, and thus with droughts and floods inChina (Huang et al. 1998; Yatagai and Yasu-nari 1998). Tian and Yasunari (1998) showedthat in northern spring the water vapor trans-port, associated with low-level southwesterliesto southern China, is responsible for the per-sistent rains over southern China.

As mentioned above, the climate variabilityover East Asia is closely related with anomaliesof the moisture circulation. The variability ofthe East Asian climate exists not only in thenorthern summer, but also in other seasons,which often cause droughts or floods in China(Huang 1996). El Nino is the most outstandinginterannual variability in the tropical Pacific.Its impact on the moisture circulation over EastAsia in northern winter, spring and autumn isnot yet clarified. Therefore, in this paper we

investigate anomalous features of moisture cir-culation over East Asia during the El Nino epi-sode, and how moisture circulation over EastAsia are affected by El Nino events in northernwinter, spring and autumn by using much moreextensive datasets. The data used in the pres-ent study are described in Section 2. In thisSection, the definitions of an El Nino and itsmature phase are also given, based on the seasurface temperature anomalies (SSTA), aver-aged on the NINO3 area (150�W–90�W, 5�S–5�N). The features of the precipitation in China,the moisture circulation over East Asia and theoutgoing longwave radiation (OLR) in the trop-ics during the El Nino episode are investigatedin Section 3, Section 4 and Section 5, respec-tively. In Section 6, the physical process throughwhich El Nino affects the East Asian climate isdiscussed. Summary, and concluding remarks,follow in Section 7.

2. Data and analysis methods

The precipitation data at 35 stations inChina from 1880 to 1996 are used in the pres-ent study. The data are seasonal mean fornorthern winter (December–February), spring(March–May), summer (June–August) and au-tumn (September–November). The data camefrom both instrument-based observations, andthe historical documentary evidence before1951, while it was based on instrument-basedprecipitation observations after 1951. The de-tails of this dataset can be found in Wang et al.(2000b). The station locations of the data asgiven in Figure 1 suggest that the data repre-sent the precipitation in the eastern part ofChina.

Two kinds of temporal resolutions of theNCEP/NCAR Reanalysis Data (Kalnay et al.1996), are used in the present study. The twicedaily (00 and 12UTCs) data from 1958 to 1996are used to compute the water vapor transport,moisture convergence, and precipitable water inthe atmosphere. The data of winds at 850 hPaand geopotential heights at 500 hPa are fromthe monthly NCEP/NCAR Reanalysis Data,from 1949 to 1996. In this study, the water va-por transport and precipitable water are verti-cally integrated for the troposphere from sur-face to 100 hPa based on the twice daily data.The monthly sea surface temperature data forthe period between 1903 to 1994 are from Global

Journal of the Meteorological Society of Japan214 Vol. 80, No. 2

Sea-Ice, and Sea Surface Temperature Data(GISST2.2) compiled by Hadley Centre forClimate Prediction & Research, MeteorologicalOffice of UK (Rayner 1996) and those for 1995and 1996 are from the monthly NCEP/NCARReanalysis Data. Monthly outgoing longwaveradiation (OLR) data for the period between1974 and 1996 from NOAA, are also utilized inthe present study.

Same as ZSK, in the present study an ElNino is defined according to a standard index,i.e., the averaged SSTA over an area calledNINO3 (150�W–90�W, 5�S–5�N). The monthlyNINO3 SSTA are averaged to be the seasonalmean for four seasons as those for the precipi-tation data. Then, we define the El Nino to bethe period when the seasonal NINO3 SSTA isgreater than 0.5�C. The El Nino mature phaseis defined as follows. For the strong El Ninoevents, the seasons when seasonal NINO3SSTA are greater than 1.0�C are taken to bethe mature phase of these El Ninos. For the

weak El Nino events with seasonal NINO3SSTA less than 1.0�C, we then choose the sea-son with the highest seasonal NINO3 SSTA, tobe the mature phase. Figure 2 shows the evo-lution of the seasonal NINO3 SSTA with themarking of the mature phase for each El Nino.According to our definition, from 1903 to 1996,within the El Nino episode are 25 winters in1902/03, 1904/05, 1905/06, 1911/12, 1918/19,1925/26, 1930/31, 1940/41, 1941/42, 1957/58,1963/64, 1965/66, 1968/69, 1969/70, 1972/73,1976/77, 1977/78, 1979/80, 1981/82, 1982/83,1986/87, 1987/88, 1990/91, 1991/92, 1994/95,11 springs in 1905, 1926, 1931, 1957, 1958,1969, 1972, 1983, 1987, 1992, 1993, and 22autumns in 1904, 1905, 1911, 1925, 1930, 1941,1951, 1953, 1957, 1963, 1965, 1968, 1969, 1972,1976, 1979, 1982, 1986, 1987, 1991, 1993,1994. Among these seasons, in the El Nino ma-ture phase are 13 winters in 1902/03, 1911/12,1918/19, 1925/26, 1930/31, 1940/41, 1957/58,1965/66, 1972/73, 1977/78, 1982/83, 1991/92,

Fig. 1. Locations (crosses) of the 35 stations for seasonal mean precipitation data in China.

R. ZHANG and A. SUMI 215April 2002

Fig. 2. Evolution of the seasonal mean SSTA in NINO3 area (Unit: �C). M stands for the maturephase of each El Nino event.


1994/95, 4 springs in 1983, 1987, 1992, 1993,and 13 autumns in 1930, 1941, 1951, 1953, 1957,1963, 1965, 1969, 1972, 1976, 1979, 1982, 1987.

3. Precipitation in China

Figure 3 shows composites of the precipita-tion anomalies in the El Nino mature phase fornorthern winter (Fig. 3a), spring (Fig. 3b) andautumn (Fig. 3c). It can be seen that the fea-tures of the anomalies are quite similar in eachof these three seasons, i.e., the positive anoma-lies appear in the southern part of China. Inorder to check if these precipitation anomaliesare statistically significant, a t-test is made bycomparing the average of the precipitationanomalies in the El Nino mature phase asshowed in Fig. 3 with that over the rest of thedata period. The statistically significant areasexceeding 95% level are shaded in Fig. 3. Evi-dently, the positive anomalies in the southernpart of China in northern winter, spring andautumn are statistically significant.

Figure 4 shows the composites of the precipi-tation anomalies in rest of the phases during

the El Nino episode for northern winter (Fig.4a), spring (Fig. 4b) and autumn (Fig. 4c). Al-though the anomalies showed in Fig. 4 are alsoin the El Nino episode, the features are quitedifferent from those in the mature phase asshowed in Fig. 3. The striking positive anoma-lies of precipitation in the El Nino maturephase in the southern part of China do not ap-pear in the rest of the phases. In winter (Fig.4a), anomalies are very weak. In spring (Fig.4b), in the southern part of China, the patternis complicated. The negative anomalies existto the west of 114�E, and to the east of 117�E,respectively, and in between is a small area ofpositive anomalies. In autumn (Fig. 4c), evennegative anomalies appear. The results heredemonstrate the importance of the impact of ElNino on the East Asian precipitation in its ma-ture phase.

4. Moisture features over East Asia

In the above section, precipitation in Chinaduring the El Nino episode is discussed. Mois-ture circulation is one of the most important

Fig. 3. Composites of precipitation anomalies (Unit: mm) in the El Nino mature phase for northernwinter (a), spring (b) and autumn (c). The contour interval is 40 mm. The shadings are the areaswith statistical significance exceeding the 95% level.


components in the East Asian climate, and itis closely related to the precipitation. There-fore, in this section the moisture featuresduring the El Nino episode over East Asia areexamined.

4.1 Atmospheric precipitable waterPrecipitation is closely related to the atmo-

spheric precipitable water. More precipitablewater is favorable for more precipitation. Fig-ure 5 shows the composites of the atmosphericprecipitable water anomalies in the El Ninomature phase for northern winter (Fig. 5a),spring (Fig. 5b) and autumn (Fig. 5c). It is clearthat positive precipitable water anomalies ap-pear around the southeastern coast of EastAsia in all three seasons. The patterns of pre-cipitable water anomalies are quite similar tothose of precipitation anomalies in all threeseasons. Positive precipitation anomalies inthe southern part of China correspond to atmo-spheric positive precipitable water anomalies.Therefore, positive anomalies of the precipit-able water are responsible for more precipita-tion in the southern part of China.

4.2 Water vapor transportThe increase of the atmospheric precipitable

water should be accompanied with more water

vapor to be transported to the place where theprecipitable water increases. Figure 6 showsthe composites of the anomalous water vaportransport vectors in the El Nino mature phasefor northern winter (Fig. 6a), spring (Fig. 6b)and autumn (Fig. 6c). In Fig. 6, similar featurescan be observed over East Asia in all threeseasons, namely, northeastward water vaportransport anomaly vectors exist around thesoutheastern coast of East Asia, where positiveprecipitable water anomalies and more precipi-tation appear. In order to show the statisticalsignificance of the northeastward vectors aroundthe southeastern coast of East Asia, projectionof the vectors in Fig. 6 to the northeast direc-tion was made, and the results are given in Fig.7. It can be seen from Fig. 7 that the positivenortheastward water vapor transport anoma-lies appear around the southeastern coast ofEast Asia, in the El Nino mature phase in allthree seasons. We apply the same t-test as thatin the above section. The areas with the statis-tical significance exceeding 95% level areshaded in Fig. 7. The results of the t-test indi-cate that the positive northeastward water va-por transport anomalies around the southeast-ern coast of East Asia, in the El Nino maturephase are statistically significant.

Fig. 4. Same as Figure 3, but in the rest of the phases during the El Nino episode.


Figure 8 shows the composites of the north-eastward water vapor transport anomalies inthe rest of the phases during the El Nino epi-sode for northern winter (Fig. 8a), spring (Fig.8b) and autumn (Fig. 8c). It can be seen that nosimilar features can be observed around thesoutheastern coast of East Asia, as those in theEl Nino mature phase. In northern winter (Fig.

8a), the anomalies are quite weak. In spring(Fig. 8b), weakly positive anomalies exist aroundthe southeast coast of East Asia, and they arenot statistically significant. In autumn (Fig.8c), even negative northeastward water vaportransport anomalies appear. Here we can seethat, during the El Nino episode, water vaportransport anomalies around the southeastern

Fig. 5. Composites of atmospheric pre-cipitable water anomalies (Unit: mm)in the El Nino mature phase for north-ern winter (a), spring (b) and autumn(c). The contour interval is 0.5 mm.

Fig. 6. Composites of water vapor trans-port anomaly vectors in the El Ninomature phase for northern winter (a),spring (b) and autumn (c).


coast of East Asia, in the El Nino maturephase, are distinct from those in the rest of thephases.

4.3 Water vapor divergenceAlthough more water vapor is transported to

the area around the southeastern coast of East

Asia in the El Nino mature phase, the conver-gence of the water vapor is still needed to formpositive anomalies of precipitable water andprecipitation. Figure 9 shows composites of thewater vapor divergence anomalies in the ElNino mature phase for northern winter (Fig.9a), spring (Fig. 9b) and autumn (Fig. 9c). Itcan be seen that in all three seasons, negativeanomalies exist around the southeastern coastof East Asia although the anomalies in autumnare weaker than those in winter and spring.

Fig. 7. Composites of northeastwardwater vapor transport anomalies (Unit:kg/s/m) in the El Nino mature phase fornorthern winter (a), spring (b) andautumn (c). The contour interval is15 kg/s/m. Shadings are the areas withstatistical significance exceeding the95% level.

Fig. 8. Same as Figure 7, but in the restof the phases during the El Nino epi-sode.


Since the negative anomalies represent theanomalous convergence of moisture, and theyappear at the area where positive anomalies ofthe precipitation and atmospheric precipitablewater exist, more precipitation and precipitablewater appear around the southeastern coast ofEast Asia.

From the above analyses, the positive pre-cipitation anomalies in the south part of China,can be well explained by anomalous features of

the atmospheric precipitable water, northeast-ward water vapor transport and moisture con-vergence. During the El Nino mature phase,more water vapor is transported to the regionaround the southeastern coast of East Asia.The water vapor converges there, and moreprecipitable water forms, which are favorablefor more precipitation in the southern part ofChina.

5. Outgoing longwave radiation (OLR)in the tropics

The tropical atmosphere is heated mainly bythe latent heat associated with the convectiveactivities. In order to study the features of theconvective activities during the El Nino epi-sode, the OLR data are examined. In the trop-ics, OLR can serve as a proxy index of theamount of convection (Morrissey 1986). The de-creased OLR corresponds to the increased cov-erage of cold (high) cloud tops and increasedconvection.

Figure 10 shows the composites of the OLRanomalies in the El Nino mature phase fornorthern winter (Fig. 10a), spring (Fig. 10b)and autumn (Fig. 10c). In the tropics, the pat-tern of the OLR anomalies is quite similar ineach of these seasons. Negative OLR anomaliesappear in the central and/or eastern tropicalPacific, where the convections are strength-ened. In the western tropical Pacific, there arepositive OLR anomalies and thus the con-vections are suppressed there. Since the OLRdata only cover the period from 1974 to 1996,from Fig. 2 we can see that during this periodEl Ninos appeared in northern spring are all intheir mature phases. Therefore, In Fig. 11 weshow the composites of the OLR anomalies inrest of the phases during the El Nino episodeonly for northern winter (Fig. 11a), and autumn(Fig. 11b). The pattern of the OLR anomalies inthe rest of the phases, is different from those inthe mature phase. Negative OLR anomaliesmainly appear in the central tropical Pacificin both northern winter and autumn. In thewestern tropical Pacific, although positive OLRanomalies also appear, they are very weak. Theconvections are not suppressed so greatly asthose in the mature phase.

In Section 2 we define the El Nino maturephase as the peak period of seasonal NINO3SSTA. From Fig. 10, we can see the El Nino

Fig. 9. Composites of water vapor diver-gence anomalies (Unit: 10�5 kg/s/m2) inthe El Nino mature phase for northernwinter (a), spring (b) and autumn (c).


mature phase is characterized by a notabledipolar distribution of the OLR anomalies inthe tropical Pacific, i.e., the positive anomaliesin the western tropical Pacific and negativeanomalies in the central and/or eastern tropicalPacific. This distribution of OLR anomalies isdistinct from that in the rest of the phasesduring the El Nino episode. Such a dipolardistribution indicates that in the atmosphere

there exists anomalous convective heating overthe central and/or eastern tropical Pacific, andanomalous convective cooling over the westerntropical Pacific. The pattern of the OLR anoma-lies suggests that the effect of the El Nino onthe atmosphere over the western tropical Pa-cific should be the largest in its mature phase.

6. Possible physical process throughwhich El Nino affects the East Asianclimate

In order to understand the physical reasonfor the moisture circulation anomalies overEast Asia in the El Nino mature phase, wemake the composites of wind anomalies at850 hPa in the mature phase for northern win-ter, spring and autumn, as shown in Fig. 12.In all three seasons, easterly anomalies pre-vail over the maritime continent. Anticyclonicanomalies appear to the north of the maritimecontinent. Comparing with Fig. 6, the anti-

Fig. 10. Composites of OLR anomalies(Unit: W/m2) in the El Nino maturephase for northern winter (a), spring (b)and autumn (c). The contour intervalis 5 W/m2. Heavy and light shadingsindicate OLR anomalies less than�10 W/m2, and greater than 10 W/m2,respectively.

Fig. 11. Composites of OLR anomalies(Unit: W/m2) in rest of the phases dur-ing the El Nino episode for northernwinter (a) and autumn (b). The contourinterval and shadings are the same asFigure 10.


cyclonic anomalies are quite similar to thewater vapor transport anomaly vectors. Almostthe same anticyclonic anomalies appear to thenorth of the maritime continent. Evidently, thewater vapor transport anomaly vectors are as-sociated with this anticyclonic wind anomaly in

the lower troposphere, which transports morewater vapor to the area around the southeast-ern coast of East Asia.

By examining the ’86/87, and ’91/92 El Ninoevents, Zhang et al. (1996) explained the anti-cyclonic wind anomaly in the lower troposphereto the north of the maritime continent as aRossby wave response to the anomalous con-vective cooling around the maritime continent.Wang et al. (2000a) proposed that this anti-cyclone results from a Rossby wave response tosuppressed convective heating, which is in-duced by both the in situ ocean surface cooling,and the subsidence forced remotely by the cen-tral Pacific warming. In fact, according to Gill(1980), an anomalous cooling symmetric aboutthe equator, can force atmospheric Rossby waveresponse in the lower troposphere, i.e., a pair ofanticyclone anomalies to both the south andthe north of the cooling area. The Rossby waveresponse to the heating anti-symmetric aboutthe equator, can induce an anticyclone over thecooling area, and a cyclone over the heatingarea. Figure 13 shows the composite of the OLRanomalies, and the wind anomalies at 850 hPain the El Nino mature phase during northernwinter, spring and autumn. We can see that onboth sides of the equator, a pair of anticyclonicanomalies can be clearly observed with theircenters located around north Australia and thesouth part of the Philippine Islands, respec-tively. It is tempting to explain this pair ofanticyclone anomalies to be the Rossby waveresponse of the tropical atmosphere to the con-vective cooling anomaly around the equator.Meanwhile, it can be seen that the anticyclonicanomaly to the north of the equator is muchstronger. It seems that the Rossby wave re-sponse of the lower troposphere to the con-vective cooling anomalies around the southernpart of the Philippines strengthens the anti-cyclonic anomaly to the north of the equator.Anomalous southwesterlies, associated withthis anticyclone anomaly, stretch to the areaaround the southwest coast of East Asia, whichexert significant effect on the East Asian cli-mate.

Tian and Yasunari (1998) studied the year toyear fluctuations of the spring persistent rains(SPR) in the southern part of China. Theyfound that the east-west thermal contrast inspring between the Indochina Peninsular and

Fig. 12. Composites of wind anomalies inthe El Nino mature phase at 850 hPafor northern winter (a), spring (b) andautumn (c).


the western North Pacific to the east of thePhilippines, plays the primary role on SPR. Inthe El Nino mature phase, the remarkable con-vective cooling anomalies around the southernpart of the Philippines in northern spring (Fig.10b), are favorable for increasing such thermalcontrast, and can lead to more precipitation inthe southern part of China. Our results here innorthern spring are in agreement with the re-sults by Tian and Yasunari (1998).

The precipitation over the East Asian regionis greatly affected by the western Pacific sub-tropical high. The rainfall area generally ap-pears along its northwestern edge, where thewarm and humid flow from the south meets thecold and dry flow from the north (Chen et al.1991). In order to investigate the physical rea-sons for the seasonal precipitation anomalyin the El Nino mature phase, we examine thecorresponding composites of the geopotential

heights at 500 hPa for northern winter, springand autumn, which are shown in Fig. 14. Itis clearly seen that the subtropical high isstrengthened, and shifts to the south of themainland of China in the El Nino mature phasein all three seasons, which is favorable for moreprecipitation in the southern part of China asshowed in Fig. 3. The strengthening of the sub-tropical high, and its appearance to the southof the mainland of China, can also be explainedby the anticyclonic anomaly to the north of themaritime continent.

7. Summary and concluding remarks

In this paper, anomalous features of mois-ture circulation over East Asia are investigated.It is found that in northern winter, spring andautumn, positive precipitation anomalies ap-pear in the southern part of China in the ElNino mature phase, which is distinct from those

Fig. 13. Composites of OLR anomalies (contours; Unit: W/m2) and wind anomalies at 850 hPa (vec-tors) in the El Nino mature phase for northern winter, spring and autumn. The contour interval is5 W/m2. Shadings indicate OLR anomalies greater than 10 W/m2.


in the rest of the phases during the El Nino ep-isode. By examining the atmospheric precipit-able water, water vapor transport and moistureconvergence, we find that their anomalies havegood agreements with the precipitation anoma-lies. In the El Nino mature phase, there existsignificant northeastward water vapor trans-

port anomalies around the southeastern coastof East Asia. The anomalies of the water vaporconvergence, and precipitable water over EastAsia, also show the similar feature as the pre-cipitation anomalies, i.e., moisture convergenceand positive precipitable water anomalies ap-pear around the southeastern coast of EastAsia in the El Nino mature phase. Our resultsshow that the impact of El Nino on the EastAsian climate is significant in the El Nino ma-ture phase, which are consistent with our pre-vious studies in ZSK.

In the El Nino mature phase, the precipita-tion anomalies in the southern part of Chinain northern winter, spring and autumn can bewell explained by the moisture circulationanomalies over East Asia. The El Nino maturephase is characterized by the strong convectivecooling anomalies over the western tropicalPacific from the area to the north of Australiaaround the equator to that around the southpart of the Philippines. Both the Rossby waveresponses of atmosphere, to the cooling anoma-lies around equator and to those around thesouth part of the Philippines, lead to the ap-pearance of an anticyclonic anomaly to thenorth of the maritime continent in the lowertroposphere. Southwesterly anomalies asso-ciated with this anticyclonic anomaly transportmore water vapor to the area around the south-eastern coast of East Asia, where the moistureconverges and more atmospheric precipitablewater appears. On the other hand, the anti-cyclonic anomaly makes the western Pacificsubtropical high strengthened, and appeared tothe south of mainland China, which are alsofavorable for the appearance of positive pre-cipitation anomalies in the southern part ofChina.

Acknowledgements

The authors thank Prof. X.B. Zeng at theUniversity of Arizona, and Prof. Bin Wang atthe University of Hawaii for reading the manu-script and providing many valuable sugges-tions. We also thank Dr. R. Kawamura and twoanonymous reviewers for their constructivecomments. This work is partly supported bythe China National Key Program for Develop-ing Basic Sciences (G1998040900, Part 1) andNSFC Excellent National Key Laboratory Re-search Project (49823002).

Fig. 14. Seasonal mean climatology (solidlines), and El Nino mature phase com-posites (dashed lines) of geopotentialheights (Unit: gpm) at 500 hPa fornorthern winter (a), spring (b) and au-tumn (c).


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