Role of the Asian Monsoon on the Interannual Variability of the

Role of the Asian Monsoon on the Interannual Variability of the

February 1992 T. Yasunari and Y. Seki 177ong>Roleong> ong>ofong> ong>theong> ong>Asianong> ong>Monsoonong> on ong>theong> Interannual Variabilityong>ofong> ong>theong> Global Climate SystemBy Tetsuzo Yasunari and Yuji Seki1Institute ong>ofong> Geoscience, University ong>ofong> Tsukuba, Tsukuba, Ibaraki 305, Japan(Manuscript received 9 September 1991, in revised form 3 December 1991)AbstractThe role ong>ofong> ong>theong> ong>Asianong> summer monsoon on ong>theong> interannual variability ong>ofong> ong>theong> global climate systemparticularly relevant to ong>theong> ENSO time scales is discussed, by examining ong>theong> statistical and dynamicallinks between ong>theong> ong>Asianong> summer monsoon, ong>theong> atmosphere/ocean system in ong>theong> tropics and ong>theong>westerly flow regimes in ong>theong> extratropics.The ong>Asianong> monsoon, ong>theong> ocean and ong>theong> atmosphere in ong>theong> tropical Pacific are tightly linked togeong>theong>ras one climate system, named here as ong>theong> MAOS (ong>Monsoonong> and ong>theong> Coupled Atmosphere/OceanSystem). The MAOS prominently shows ong>theong> biennial oscillatory nature which tends to have anomalousstates starting in ong>theong> norong>theong>rn summer monsoon season and persisting for about one year (Yasunari,1990a: 1991).The anomalous state ong>ofong> ong>theong> MAOS produces ong>theong> anomalous atmospheric circulation over ong>theong> subtropicsand ong>theong> extratropics ong>ofong> ong>theong> north Pacific during summer through ong>theong> early winter, throughong>theong> modulation ong>ofong> ong>theong> subtropical high and ong>theong> stationary Rossby wave propagation mechanism. Inong>theong> mid winter, this anomalous circulation over ong>theong> north Pacific is evolved to ong>theong> hemispheric winteranomalous circulation with wavenumber-one and/or-two structure.The anomalous circulation over Eurasia associated with this hemispheric anomalous flow regimeseems to provide a favorable condition for ong>theong> extensive (or diminished) snow cover area over centralAsia, which in turn is responsible for ong>theong> reversed anomalous state ong>ofong> ong>theong> next ong>Asianong> summer monsoonand ong>theong> MAOS. That is, ong>theong> biennial nature ong>ofong> ong>theong> climate system in ong>theong> norong>theong>rn hemisphere maybe due, at least partly, to this two-way interactions between ong>theong> tropics and ong>theong> extratropics. In ong>theong>seprocesses, ong>theong> ong>Asianong> monsoon plays a key role as a transmitter ong>ofong> climate signals between ong>theong> tropicsand ong>theong> extratropics through ong>theong> land/atmosphere/ocean interaction in ong>theong> seasonal cycle.In addition, it is strongly suggested that ong>theong> North Atlantic Oscillation (NAO), in reality, playsa crucial role in ong>theong> timing ong>ofong> ong>theong> occurrence ong>ofong> ong>theong> ENSO event, by stochastically amplifying ordamping ong>theong> biennial oscillation ong>ofong> this coupled climate system. That is, ong>theong> more or less irregularENSO cycle may result from this interaction between ong>theong> MAOS and ong>theong> NAO, where ong>theong> formerseems to have ong>theong> nature ong>ofong> an almost-intransitive climate system, while ong>theong> latter seems to representong>theong> more chaotic nature ong>ofong> ong>theong> westerly flow regime.1. Introduction The ong>Asianong> summer and winter monsoon form hugecirculation systems in ong>theong> general circulation ong>ofong> ong>theong>global atmosphere (Krishnamurti, 1971; 1973). Simplyspeaking, this system is characterized as a zonalasymmetry as well as a meridional asymmetry inong>theong> tropics. Figure 1 shows ong>theong> monthly mean sealevel pressure and surface wind vectors (a) and ong>theong>monthly mean velocity potential and divergent windvector at 200mb (b) in July 1990 (JMA, 1990).This figure apparently shows a predominant systemwith ong>theong> zonally-oriented east-west (or Walker) circulationover south Asia through ong>theong> souong>theong>rn subtropicalPacific, as well as ong>theong> meridionally orientedmonsoon circulation over ong>theong> south Asia throughong>theong> Indian Ocean. This remarkable zonal asymmetryong>ofong> ong>theong> circulation has proved to be maintainedand modulated through ong>theong> interaction betweenong>theong> ong>Asianong> monsoon and ong>theong> coupled atmosphere/oceansystem in ong>theong> equatorial Pacific (Yasunari,1990a). This interaction seems to play akey role on ong>theong> mechanism ong>ofong> ong>theong> El Nino/Souong>theong>rnOscillation (ENSO), as has been discussed by Barnett(1985, 1988) and Yasunari (1987). That is, aweaker (stronger) than normal ong>Asianong> summer monsoonis very favorable for triggering ong>theong> El Nino(anti-El Nino or La Nina (Philander, 1985)) state ong>ofong>ong>theong> equatorial Pacific through ong>theong> weaker (stronger)east-west circulation in ong>theong> tropics.

February 1992 T. Yasunari and Y. Seki 179Fig. 2. Interannual variation ong>ofong> all-India monsoon rainfall. The ENSO years are indicated with black bars.(Mooley and Shukla, 1987)Fig. 3. Time series ong>ofong> Indian monsoon rainfallanomaly (thick solid line) and sea watertemperature anomaly at 20m (thickdashed line) and 100m (thin dashed line)depth averaged for 137E line (2N-10N) inong>theong> succeeding January. (Yasunari, 1990a)However, Meehl (1987) and Yasunari (1990a) notedthat ong>theong> Indian monsoon activity seems to play anactive raong>theong>r than a passive role in determining ong>theong>anomalous state ong>ofong> ong>theong> warm water pool in ong>theong> westernPacific through ong>theong> anomalous east-west circulationin ong>theong> following autumn and winter seasons,as shown in Fig. 3. These studies furong>theong>r substantiatedong>theong> tight link between ong>theong> ong>Asianong> summer monsoonfluctuation and ong>theong> anomalous state ong>ofong> ong>theong> coupledocean/atmosphere system in ong>theong> tropical Pacific,and suggested that ong>theong> ong>Asianong> summer monsoonand ong>theong> coupled system in ong>theong> tropical Pacificshould be understood as a climate system combinedtogeong>theong>r in ong>theong> tropics. Hereafter, we refer tothis climate system by ong>theong> abbreviation MAOS (i. e.,ong>Monsoonong>/Coupled Atmosphere Ocean System).A prominent feature ong>ofong> ong>theong> MAOS is its oscillatorynature on ong>theong> quasi-biennial time scale. In addition,an anomalous state ong>ofong> this MAOS shows astrong phase lock to ong>theong> seasonal cycle, as is apparentlyseen in Fig. 4. This strong seasonality ong>ofong>ong>theong> anomalous state ong>ofong> ong>theong> MAOS has prompted usto introduce a concept ong>ofong> ong>theong> "monsoon year" (Yasunari,1991) as a unit climatic year in ong>theong> tropics.In this context, an ENSO event may be understoodas a considerably amplified phase ong>ofong> ong>theong> oscillationin ong>theong> MAOS.Fig. 4. Lag-correlations between ong>theong> Indianmonsoon rainfall anomaly and ong>theong> sea surfacetemperature anomaly in ong>theong> western(0-8N, 130E-150W) and ong>theong> eastern (0-8N, 170W-150W) Pacific. The referencemonsoon season is shown with a thickblack bar. Y(0) denotes ong>theong> year ong>ofong> referencemonsoon year and Y(-1) (Y(+1))denotes ong>theong> year before (after) Y(0). (Yasunari,1990a)3. Influence ong>ofong> ong>theong> extra-tropics on ong>theong>MAOSThe ong>Asianong> summer monsoon is forced primarily byong>theong> differential heating between ong>theong> Eurasian continentand ong>theong> surrounding oceans, though ong>theong> moistprocess in ong>theong> atmosphere is essential for maintenanceong>ofong> ong>theong> gigantic monsoon circulation. However,this monsoon circulation is supposed to be very sensitiveto ong>theong> surface heating condition (Charney andShukla, 1981). Blanford (1884) first noted this aspect,by examining ong>theong> relationship between ong>theong> Himalayansnow cover in winter and ong>theong> following Indiansummer monsoon. Hahn and Shukla (1976),Dickson (1984), Dey and Bhanu Kumar (1984)and oong>theong>rs have re-examined this Eurasian snowcover-Indian monsoon connection by using ong>theong> dataong>ofong> satellite-derived snow cover extent. Morinaga(1992) pointed out that ong>theong> snow cover over centralAsia near ong>theong> Caspian Sea in late winter throughspring shows significant negative correlation with

180 Journal ong>ofong> ong>theong> Meteorological Society ong>ofong> Japan Vol. 70, No. 1Fig. 5. Time series ong>ofong> ong>theong> snow cover extentanomaly over central Asia in April(Kodera and Chiba, 1989) and ong>theong> sea watertemperature anomaly at 125m depthin ong>theong> western tropical Pacific in ong>theong> followingJanurary for ong>theong> period ong>ofong> 1967>theong> Indian monsoon rainfall (IMR) (Parthasarathy,1987).The anomalous state ong>ofong> ong>theong> surface conditions(e. g., snow cover, soil moisture) ong>ofong> ong>theong> continentin winter and spring, ong>theong>refore, should affect ong>theong>anomalous state ong>ofong> ong>theong> MAOS for each monsoonyear. The apparent time-lag correlation between ong>theong>snow cover extent in central Asia in April (Koderaand Chiba, 1989) and ong>theong> oceanic mixed layer temperatureong>ofong> ong>theong> tropical western Pacific in ong>theong> succeedingJanuary as shown in Fig. 5, strongly supportsthis idea. Undoubtedly, this time-lag correlationshould physically be based upon ong>theong> anomalousstate ong>ofong> ong>Asianong> monsoon and ong>theong> associated coupledocean/atmosphere system in ong>theong> tropical Pacific persistingfrom summer through winter, which may beinduced by ong>theong> anomalous snow cover over ong>theong> continent.There seem to be two physical processes responsiblefor ong>theong> time-lag correlation between ong>theong> wintersnow cover and ong>theong> summer monsoon. One is ong>theong>albedo effect and anoong>theong>r is ong>theong> snow-hydrologicaleffect, as shown schematically in Fig. 6. The lattereffect involves ong>theong> melting ong>ofong> an anomalous snowmass, moistening ong>ofong> ong>theong> soil and an anomalousevaporation, which substantially reduces ong>theong> heatingong>ofong> ong>theong> atmosphere from ong>theong> ground surface inong>theong> warmer seasons. The importance ong>ofong> this processwas noted by Yeh et al. (1983), and was furong>theong>rsubstantiated in ong>theong> GCM experiments by Yamazaki(1989), Barnett et al. (1989) and Yasunariet al. (1991). These recent GCM studies also notedthat ong>theong> combined albedo/snow hydrological effectFig. 6. Schematic diagram for ong>theong> albedo feedbackand ong>theong> hydrological feedback ong>ofong> snowcover during ong>theong> seasonal march from winterto summer. (Yasunari et al., 1991)is actually important for this "time-lagged" effect.On ong>theong> oong>theong>r hand, Morinaga and Yasunari (1987)deduced ong>theong> atmospheric circulation pattern in winterresponsible for ong>theong> maximum (minimum) snowcover extent over central Asia. Figure 7 shows ong>theong>lag-correlation pattern between ong>theong> snow cover extentover central Asia in February and ong>theong> 500mbheight anomaly ong>ofong> ong>theong> norong>theong>rn hemisphere in ong>theong>preceding December. This figure indicates that ong>theong>heavy snowfall (and ong>theong> large snow cover extent)over central Asia is associated with ong>theong> anomalousdeep trough over ong>theong>re combined with ong>theong> anomalousridges over Europe and east Asia. This patternmay be identified as a sort ong>ofong> "Eurasian (EU)" pattern(Wallace and Gutzler, 1981). That is to say,ong>theong> anomalous winter circulation pattern over ong>theong>Eurasian continent shown here seems to be a precursorsignal affecting ong>theong> anomalous state ong>ofong> ong>theong>MAOS in ong>theong> following monsoon year.4. Subtropical and extratropical responses toong>theong> MAOSSince Bjerknes (1969), it has been noted thatong>theong> El Nino event, or more exactly, ong>theong> anomalousstate ong>ofong> ong>theong> atmosphere over ong>theong> equatorialcentral/eastern Pacific associated with ong>theong> sea surfacetemperature (SST) anomaly ong>theong>re, producesong>theong> anomalous circulation over ong>theong> north Pacificthrough ong>theong> north American sector. Horel and Wallace(1981) demonstrated ong>theong> characteristic circula-

February 1992 T. Yasunari and Y. Seki 181Fig. 7. Lag-correlationmap ong>ofong> ong>theong> 500mbgeopotential height anomaly in Decemberand ong>theong> snow cover extend anomaly,over central Asia in ong>theong> following February.(Morinaga and Yasunari, 1987)tion patterns (e. g., PNA, NAO, WP, Eu etc.) associatedwith ong>theong> ENSO events. Hoskins and Karoly(1981) and Webster (1981) gave a ong>theong>oretical basisfor ong>theong>se observations as a forced Rossby wavepropagation from ong>theong> tropics. Many GCM studies(e. g., Blackmon et al., 1976 etc.) have verified ong>theong>extra-tropical responses to ong>theong> SST anomalies in ong>theong>equatorial Pacific with ong>theong> El Nino condition.However, ong>theong> extra-tropical responses to ong>theong> SSTanomalies in ong>theong> equatorial Pacific should be examinedmore generally with reference to each anomalousstate ong>ofong> ong>theong> MAOS, since ong>theong> extra-tropical responseis not only limited to ong>theong> El Nino conditions.Palmer and Mansfield (1984), for example, showedthat ong>theong> atmospheric response over ong>theong> north Pacificis as strong as, and even stronger during ong>theong> LaNina conditions (namely, higher SST anomalies inong>theong> western Pacific) than ong>theong> El Nino conditions.Figure 8 shows ong>theong> monthly wind stress anomalyin ong>theong> norong>theong>rn Pacific subtropics and mid-latitudes,composited for weak summer monsoon years minusstrong summer monsoon years, to show moreclearly ong>theong> features for ong>theong> weak monsoon year. Itis apparent that cyclonic circulation anomalies are Gating ong>theong> weaker subtropical high during summerthrough ong>theong> following autumn. This result is wellcorrelated with ong>theong> westerly anomalies along ong>theong>equatorial belt, which implies a weaker state ong>ofong> ong>theong>MAOS with ong>theong> weaker east-west circulation (i. e.,weaker trade wind and upper tropospheric wind sys-Fig. 8. Anomaly surface wind stress vectorscomposited for July through December forweak Indian monsoon years minus strongIndian monsoon years. (Yasunari, 1990b)tem) in ong>theong> tropics. The persistent cyclonic circulationanomaly in ong>theong> same region in Novemberand December, on ong>theong> oong>theong>r hand, corresponds to astronger than normal, or more equatorward shift ong>ofong>ong>theong> Aleutian low.To deduce ong>theong> dominant regional circulation patternsin ong>theong> norong>theong>rn mid-latitudes associated withong>theong> anomalous state ong>ofong> ong>theong> MAOS, ong>theong> time coefficientsong>ofong> some localized dominant circulation (orteleconnection) patterns are correlated to ong>theong> Indiansummer monsoon index. These patterns are obtainedby applying ong>theong> varimax-rotated empirical or-

182 Journal ong>ofong> ong>theong> Meteorological Society ong>ofong> Japan Vol. 70, No. 1Fig. 9. Examples ong>ofong> dominant teleconnection patterns deduced by varimax rotated EOFs ong>ofong> monthly 500mb geopotential height anomalies for 45 years from 1946 to 1990.Fig. 10. Lag-correlations between ong>theong> Indian monsoon rainfall anomaly and time coefficients ong>ofong> ong>theong> principalteleconnection patterns shown in Fig. 9. Correlations with values less than 0.2 are not shown.thogonal functions (EOFs) analysis to ong>theong> monthly500mb height anomaly data for ong>theong> period ong>ofong> 1946to 1990 (Yasunari and Ueno, 1992). Figure 9 showsong>theong> spatial patterns ong>ofong> six dominant circulation patterns.This objective method could successfully deducemost ong>ofong> ong>theong> well-known teleconnection patternssuch as NAO, PNA, WP, EU (Wallace and Gutzler,1981). Figure 10 shows ong>theong> time sequence ong>ofong> lag correlationbetween ong>theong> Indian monsoon rainfall indexand ong>theong> time coefficients ong>ofong> some dominant teleconnectionpatterns for each month before through afterong>theong> reference monsoon year. It is noteworthy tostate that ong>theong> significant correlations (above ong>theong> 95% level) related to ong>theong> Indian summer monsoon (or52

February 1992 T. Yasunari and Y. Seki 183Fig. 12. Same as Fig. 11 except for Decemberong>ofong> (a) strong monsoon year and (b) weakmonsoon year.Fig. 11. Composite anomalies ong>ofong> 500mbgeopotential height from August toNovember ong>ofong> strong (left) and weak (right)Indian monsoon years. Contours are 10gpm and negative values are shown withdashed>theong> MAOS) persistently appear during and after ong>theong>summer monsoon seasons (i. e., autumn and winter)in ong>theong> area ong>ofong> ong>theong> north Pacific and ong>theong> north Americansector, as shown in ong>theong> series ong>ofong> lag-correlationsfor ong>theong> PNA, WP and PNA-II patterns. This strongseasonality ong>ofong> ong>theong> persistent correlations seems tobe consistent with ong>theong> concept ong>ofong> ong>theong> correspondingmonsoon year in ong>theong> topics. Oong>theong>r significant correlationpeaks ong>ofong> ong>theong> EU, PNA and PNA-II patternsappear in ong>theong> previous autumn through early winterong>ofong> Y(-1), which seems to be closely associatedwith ong>theong> pattern for ong>theong> snow cover anomaly overEurasia, as shown in Fig. 7.To find out ong>theong> hemispheric anomalous circulationpatterns associated with ong>theong> anomalous stateong>ofong> ong>theong> MAOS, ong>theong> monthly 500mb height anomaliesare composited for each month during and afterong>theong> weak and strong summer monsoon years,respectively, as shown in Fig. 11. As is readily deducedfrom this figure, in ong>theong> weak monsoon years(Fig. 11 right), for example, ong>theong> PNA pattern (ong>ofong>negative height anomaly over ong>theong> north Pacific andong>theong> norong>theong>astern part ong>ofong> north America with positiveanomaly over ong>theong> Alaska/Rockies) is significantin August through October, implying an intensifiedAleutian low and ong>theong> meandering flow on ong>theong> downstreamside over ong>theong> north American sector. In ong>theong>strong monsoon years, in contrast, ong>theong> reversed PNApattern is dominant in ong>theong> same period, implying ong>theong>weakened Aleutian low with more zonal flow overthis region. These (reversed) PNA patterns in thisseason with ong>theong> intensified (or weakened) Aleutianlow may not necessarily be due to ong>theong> Rossby wavepropagation mechanism, but may be more directlyrelated to ong>theong> weakened (or intensified) north Pacifichigh in ong>theong> subtropics, as shown in Fig. 8. Thisweakened (or intensified) high may facilitate (or obstruct)ong>theong> southward intrusion ong>ofong> ong>theong> westerly flowover ong>theong> north Pacific.5. Feedback processes from ong>theong> extratropicsto ong>theong> MAOSInterestingly, ong>theong> anomalous circulation with ong>theong>(reversed) PNA pattern in late autumn seems to developinto ong>theong> anomalous planetary-scale flow regimewith wavenumber one plus two through ong>theong> courseong>ofong> ong>theong> seasonal evolution ong>ofong> ong>theong> polar air mass inearly winter (November and December), as shown inFigs. 11 and 12. That is, in ong>theong> weak monsoon year,ong>theong> PNA pattern over ong>theong> north American sectortends to develop ong>theong> strong seasonal trough over ong>theong>norong>theong>ast north America (and that over east Asia)while more zonal flow develops over ong>theong> Eurasiancontinent. In ong>theong> strong monsoon years, by con-

184 Journal ong>ofong> ong>theong> Meteorological Society ong>ofong> Japan Vol. 70, No. 1Fig. 13. Schematic diagram ong>ofong> ong>theong> interannual variation ong>ofong> ong>theong> climate system through ong>theong> interactionsbetween ong>theong> MAOS in ong>theong> tropics, and ong>theong> westerly flow regime in ong>theong> extratropics, by means ong>ofong> ong>theong>Rossby-wave propagation and land-surface processes over Eurasian continent in ong>theong> seasonal cycle .trast, more zonal flow develops over ong>theong> north Pacificthrough ong>theong> north American sector, while an anomaloustrough develops over ong>theong> central part ong>ofong> ong>theong>Eurasian continent. These anomalous flow regimesin winter may be due partly to ong>theong> linear stationaryRossby-wave response to ong>theong> persistent forcing ong>ofong>ong>theong> anomalous MAOS, but also to ong>theong> non-linear interactionbetween ong>theong> changing basic-state flow andtransient eddies through ong>theong> seasonal march.Now, it should be noted that this anomalous patternin early winter (Fig. 12 left) is nearly identicalto ong>theong> pattern favorable for a large snow cover extentover central Asia, as shown in Fig. 7. Namely, ong>theong>strong (or weak) summer monsoon condition tendsto produce ong>theong> weak (or strong) summer monsooncondition for ong>theong> next year, through ong>theong> anomalouswesterly flow regime and ong>theong> associated anomaloussnow cover condition in ong>theong> following winter. Inoong>theong>r words, ong>theong> biennial nature ong>ofong> ong>theong> MAOS mayowe its mechanism at least partly to ong>theong> two-way interactionsbetween ong>theong> tropics and ong>theong> extra-tropicsin different seasons ong>ofong> ong>theong> year, in which a negativefeedback exists in ong>theong> propagation ong>ofong> climatic signals.The interactions ong>ofong> ong>theong> climate system betweenong>theong> MAOS in ong>theong> tropics and ong>theong> extra-tropics, andbetween ong>theong> seasons with a biennial oscillatory natureare summarized schematically in Fig. 13. Onepersistent anomaly in ong>theong> MAOS from summer towinter seems to be responsible for ong>theong> correspondingwinter anomaly in ong>theong> westerly flow regime in ong>theong>norong>theong>rn extra-tropics, which provides ong>theong> anomalousland-surface condition (i. e., snow cover andpossibly soil moisture) over ong>theong> Eurasian continent.This anomalous land-surface condition, in turn, producesong>theong> opposite anomaly ong>ofong> ong>theong> following norong>theong>rnsummer monsoon (and ong>theong> MAOS) to ong>theong> previousyear.6. ong>Roleong> ong>ofong> ong>theong> NAO on ong>theong> MAOS and ong>theong>ENSOIn reality, however, ong>theong> year-to-year variabilitiesong>ofong> ong>theong> MAOS as well as ong>theong> westerly flow regimesdo not necessarily show ong>theong> biennial nature exactly,as is seen, e. g., in Fig. 5. The ENSO event (or ong>theong>ENSO cycle), in fact, shows a preferred periodicityong>ofong> 4 to 6 years, raong>theong>r than 2 to 3 years (Rasmussonand Carpenter, 1982 etc.), though it seems to basicallycontain ong>theong> nature ong>ofong> ong>theong> biennial oscillation.We will furong>theong>r examine this problem.Here, to focus on ong>theong> difference ong>ofong> ong>theong> circulationpatterns between ong>theong> ENSO years and ong>theong>non-ENSO years among ong>theong> weak summer monsoonyears, ong>theong> time coefficients ong>ofong> some dominant patterns(Fig. 9) are composited for three years centeredby ong>theong> weak monsoon year (Y(0)) with ong>theong>ENSO event (a) and without ong>theong> ENSO event (b)(Yasunari, 1988), as is reproduced here in Fig. 14.The SOI is also composited in ong>theong> same manner andincluded in this diagram. It should be noted that ong>theong>SOI apparently shows a biennial cycle in ong>theong> periodong>ofong> Y(-1) through ong>theong> beginning ong>ofong> Y(+1) particularlyin ong>theong> case ong>ofong> ong>theong> ENSO. Associated with thisfeature, a remarkable difference is also seen in ong>theong>SOI tendency from ong>theong> winter ong>ofong> Y(-1)/Y(0) to ong>theong>summer ong>ofong> Y(0) between ong>theong> two cases.To examine ong>theong> teleconnection patterns associatedwith this remarkable feature, we focus on ong>theong> differenceong>ofong> ong>theong> dominant patterns appearing in ong>theong> winterong>ofong> Y(-1)/Y(0). The negative phase ong>ofong> PNA-II pattern, with negative height anomaly over ong>theong>norong>theong>ast north America and positive anomaly overong>theong> norong>theong>ast Pacific seems to be a common featurein both cases. This may presumably be relatedto ong>theong> strong or non-weak summer monsoonong>ofong> Y(-1), as is suggested in Figs. 11 and 12. Aremarkable difference between ong>theong> two cases is ong>theong>phase or polarity ong>ofong> ong>theong> NAO pattern, as noticed

February 1992 T. Yasunari and Y. Seki 185Fig. 15. Composite anomalies for 500mbheight (north ong>ofong> 20N) for ong>theong> precedinglate winter (January and February) ong>ofong> ong>theong>ENSO event among ong>theong> weak Indian monsoonyears. The contour interval is 5gpmand negative values are dashed. 5% (orless) significant area is shaded.Fig. 14. Composite time coefficients ong>ofong> somedominant teleconnection patterns associatedwith (a) ong>theong> ENSO years and (b)non-ENSO years among ong>theong> years ong>ofong> weakong>Asianong> summer monsoon. Time sequence ong>ofong>composite SOI for each case is also plotted.(Yasunari, 1988)in ong>theong> sign ong>ofong> ong>theong> time coefficients in ong>theong> same period.In ong>theong> case ong>ofong> ong>theong> ENSO years, negative valuesare significant, which implies ong>theong> phase ong>ofong> ong>theong> NAOwith north/negative and south/positive anomalies.In ong>theong> case ong>ofong> ong>theong> non-ENSO years, in contrast, largepositive values are noticeable.It is noteworthy to state that since Walker andBliss (1932) ong>theong> NAO has been noted as an oscillationindependent from ong>theong> Souong>theong>rn Oscillation(SO). A more sophisticated complex EOF analysis ong>ofong>ong>theong> near-global sea level pressure date made by Barnett(1985) also shows ong>theong> apparent orthogonality ong>ofong>ong>theong> NAO to ong>theong> SO. Certainly Fig. 10 shows no significantlinear correlation between ong>theong> Indian monsoonrainfall index and ong>theong> NAO, though ong>theong> NAOis one ong>ofong> ong>theong> most dominant teleconnection patternsin ong>theong> norong>theong>rn hemisphere. However, this does notnecessarily imply that ong>theong> NAO is not physicallylinked with ong>theong> SO. Figure 14 suggests, on ong>theong> contrary,that ong>theong> polarity ong>ofong> ong>theong> NAO, coupled with ong>theong>negative phase ong>ofong> ong>theong> PNA-II may play a crucial rolein ong>theong> occurrence or non-occurrence ong>ofong> ong>theong> ENSOeven later on. The seasonal 500mb height anomalycomposited for ong>theong> late winter (Jan. to Feb.) ong>ofong> Y(0) with ong>theong> ENSO event (Fig. 15) apparently showsa large positive anomaly over ong>theong> Aleutian low areaand ong>theong> middle ong>ofong> ong>theong> north Atlantic and large negativeanomalies over ong>theong> norong>theong>rn part ong>ofong> north Americaand central Asia. This pattern is, indeed, verysimilar to ong>theong> typical anomalous pattern for largesnow cover extent over central Asia (Fig. 7), exceptfor ong>theong> prominent NAO pattern over ong>theong> north Atlanticand ong>theong> more southward extent ong>ofong> negativeanomaly toward ong>theong> Arabian Sea. This anomalouspattern strongly suggests that in ong>theong> winter precedingong>theong> ENSO event a more-zonally oriented strongjet is predominant over north America through ong>theong>north Atlantic while a deep trough with a southwardintrusion ong>ofong> ong>theong> jet is located over central Asia.A question may arise as to how this slight differenceong>ofong> ong>theong> preceding winter circulation anomalyover ong>theong> north Atlantic through ong>theong> Eurasian continenttriggers ong>theong> ENSO event in later seasonsor no, though both conditions are undoubtedly responsiblefor a weaker summer monsoon condition.One speculative explanation may be a difference ong>ofong>55

186 Journal ong>ofong> ong>theong> Meteorological Society ong>ofong> Japan Vol. 70, No. 1Rossby-wave train path and associated storm tracksover central and south Asia, which may (or maynot) facilitate ong>theong> cyclogenesis and westerly windburst over ong>theong> equatorial Indian Ocean in late winterthrough ong>theong> pre-monsoon season, possibly throughong>theong> mechanism suggested by Hiskins et al. (1990).Namely, ong>theong> NAO seems to play a dynamical roleong>ofong> modulating ong>theong> intensity and distribution ong>ofong> ong>theong>jet stream over ong>theong> north Atlantic, which in turnchanges ong>theong> Rossby wave propagation characteristicson ong>theong> downstream-side, i, e., over Eurasia andong>theong> Indian Ocean.We could hypoong>theong>size, in any event, that if onlyone anomalous state ong>ofong> ong>theong> NAO with ong>theong> polarityong>ofong> negative/north and positive/south is additionalin ong>theong> preceding winter to ong>theong> anomalous state ong>ofong>ong>theong> MAOS with ong>theong> weak ong>Asianong> summer monsoon,a very favorable condition for triggering ong>theong> ENSOevent may be produced.7. Concluding remarksThe role ong>ofong> ong>theong> ong>Asianong> monsoon on ong>theong> global-scaleclimatic variability with ong>theong> biennial and ENSOtime scales is discussed, by examining ong>theong> two-wayinteractions between ong>theong> tropics and ong>theong> extratropicsin ong>theong> seasonal cycle. In this process, ong>theong>ong>Asianong> monsoon functions as part ong>ofong> one climate systemreferred to as ong>theong> MAOS (ong>Monsoonong> and ong>theong>coupled Atmosphere/Ocean System) over souong>theong>astAsia through ong>theong> tropical Pacific.The MAOS prominently shows a biennial oscillatorynature which tends to have one anomalous statestarting in ong>theong> norong>theong>rn summer monsoon seasonand persisting for about one year (i. e., one "monsoonyear") with ong>theong> mature phase in winter (Yasunari,1990a; 1991). This anomalous state ong>ofong> ong>theong>MAOS is combined with ong>theong> anomalous atmosphericcirculation over ong>theong> subtropics and ong>theong> extratropics(i. e., PNA or reversed PNA) ong>ofong> ong>theong> north Pacificfrom late summer through winter, through ong>theong> modulationong>ofong> ong>theong> subtropical high and ong>theong> stationaryRossby wave propagation mechanism.The anomalous circulation over ong>theong> north Pacificthus produced in ong>theong> early winter seems toevolve to ong>theong> hemispheric anomalous circulationregime with wavenumber-one and/or-two structurein late winter. That is, ong>theong> PNA pattern (negativeheight anomalies over ong>theong> north Pacific andong>theong> norong>theong>ast north America with positive anomalyover Alaska/Rockies), followed by ong>theong> weak summermonsoon, tends to produce on anomalous coldtrough over norong>theong>ast north America with morezonal flow over Eurasia. By contrast, ong>theong> reversedPNA pattern, followed by ong>theong> active monsoon, tendsto produce more zonal flow over ong>theong> north Pacificthrough north Atlantic sector with an anomalouscold trough over Eurasia.The anomalous circulation thus produced overFig. 16. Schematic diagram ong>ofong> ong>theong> interactionsbetween ong>theong> MAOS and ong>theong> chaoticwesterly flow regime (ong>theong> Chaos), similarto Fig. 13 but focusing more on ong>theong> spatialstructure. The role ong>ofong> ong>theong> NAO is alsospecified.Eurasia seems to provide a favorable condition forong>theong> extensive (or diminished) snow cover area andmass over central Asia, which in turn is responsiblefor ong>theong> reversed anomalous state ong>ofong> ong>theong> followingong>Asianong> summer monsoon and ong>theong> MAOS, presumablythrough ong>theong> albedo and snow-hydrological effect ong>ofong>anomalous snow cover. That is, ong>theong> biennial natureong>ofong> ong>theong> MAOS and ong>theong> climate system in ong>theong> norong>theong>rnhemisphere may be due, at least partly, to thistwo-way interaction between ong>theong> MAOS in ong>theong> tropicsand ong>theong> westerly flow regime in ong>theong> extratropicsin ong>theong> seasonal cycle.In addition, ong>theong> significant polarity change ong>ofong> ong>theong>NAO in ong>theong> preceding winter depending on ong>theong>ENSO or ong>theong> non-ENSO event suggests that ong>theong>NAO over ong>theong> north Atlantic seems to play a rolein ong>theong> climatic chaos, by stochastically amplifyingor damping ong>theong> biennial nature ong>ofong> ong>theong> MAOS. Thetiming ong>ofong> ong>theong> ENSO event or ong>theong> ENSO cycle may,ong>theong>refore, be modulated by this interaction betweenong>theong> MAOS and NAO, both ong>ofong> which are basicallyindependent ong>ofong> each oong>theong>r. The MAOS seems to behavemore or less as a quasi-transitive or almostintransitiveclimate system (Lorenz, 1968), whileong>theong> NAO may represent ong>theong> chaotic behavior ong>ofong> ong>theong>westerly regime. The whole view ong>ofong> ong>theong> MAOS, ong>theong>chaos in ong>theong> westerly regime and ong>theong>ir interactionsthrough ong>theong> Rossby wave propagation and ong>theong> landsurfaceprocesses (snow cover, soil moisture etc.)over ong>theong> Eurasian continent is schematically shownin Fig. 16.The mechanism ong>ofong> ong>theong> interannual variability ong>ofong>ong>theong> climate system discussed here should be comparedto ong>theong> recent model result by Lorenz (1990).He argued that ong>theong> interannual variability ong>ofong> ong>theong> climatesystem could result from an interaction betweenong>theong> chaotic winter circulation and ong>theong> intransitivesummer circulation but shows a totallychaotic behavior because ong>ofong> a randomly-renewed ini-

February 1992 T. Yasunari and Y. Seki 187tial condition for summer by ong>theong> chaotic winter condition.This conceptual model for ong>theong> propagationong>ofong> climatic signals in ong>theong> seasonal cycle may bepartly compatible with ong>theong> observational evidencepresented here. However, in reality, ong>theong> anomalousstate ong>ofong> ong>theong> MAOS seems to give persistent anomalousforcing to ong>theong> westerly regime from ong>theong> norong>theong>rnsummer to ong>theong> following winter, to produce a particularplanetary flow pattern as a "climatic signal",though it is more or less disturbed by ong>theong> chaotic natureong>ofong> ong>theong> westerly flow regime. This climatic signalin winter is, in turn, transmitted to ong>theong> followingsummer, through ong>theong> memory ong>ofong> land-surface (andpossibly ocean-surface) processes during winter andspring. As a consequence, ong>theong> variability ong>ofong> ong>theong> realclimate system seems to have one maximum powerspectra in ong>theong> biennial through ong>theong> ENSO time scale,raong>theong>r than ong>theong> red or white noise spectra.These results suggest strongly that ong>theong> ong>Asianong>monsoon is manifested as a global-scale land/atmosphere/ocean interaction between ong>theong> largestcontinent and ong>theong> largest ocean on ong>theong> earth, andplays a key role as a transmitter ong>ofong> climate signalsbetween ong>theong> tropics and ong>theong> extratropics inong>theong> seasonal cycle. There are, however, still manyproblems to be solved. Particularly, ong>theong> interactivephysical processes ong>ofong> ong>theong> ong>Asianong> monsoon withong>theong> coupled ocean/atmosphere system in ong>theong> Pacific/Indianocean sector and with ong>theong> land-surfacecondition (e. g., snow cover, soil moisture etc.) overong>theong> Eurasian continent may be a central issue tobe intensively studied both observationally and ong>theong>oretically.These problems should be undoubtedlyone ong>ofong> ong>theong> main targets for ong>theong> ongoing TOGA andong>theong> forthcoming GEWEX.AcknowledgementsThe present authors wish to thank Ms. YukiMorinaga for computing assistance and helpful discussion.They are also indebted to Dr. M. Koderaong>ofong> ong>theong> Meteorological Research Institute (MRI) forsupplying ong>theong> arranged snow cover data. Thetwo anonymous reviewers' comments were extremelyhelpful in ong>theong> revision ong>ofong> ong>theong> paper. 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