Role of the Asian Monsoon on the Interannual Variability of the

mausam.hyarc.nagoya.u.ac.jp

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 to1988.ong>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 lines.ong>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. This study hasbeen supported by ong>theong> Grant-In-Aid for ScientificResearch from ong>theong> Ministry ong>ofong> Education, Scienceand Culture No. 01460055.ReferencesBarnett, T. P., 1985: Variations in near-global sea levelpressure, J. Atmos. Sci., 42, 478-501.Barnett, T. P., 1988: Variations in near-global surfacepressure. Anoong>theong>r view. J. Climate, 1, 225-230.Barnett, T. P., L. Dumenil, U. Schlese, F. Roeckner andM. Latif, 1989: The effect ong>ofong> Eurasian snow coveron regional and global climate variations. J. Atmos.Sci., 46, 661-685.Bjerknes, J., 1969: Atmospheric teleconnections fromong>theong> equatorial Pacific. Mon. Wea. Rev., 97, 163-172.Blackmon, M. L., J. E. Geisler and E. J. Pitcher, 1983: Ageneral circulation model study ong>ofong> January climateanomaly patterns associated with interannual variationong>ofong> equatorial Pacific sea surface temperatures.J. Atmos. Sci., 40, 1410-1425.Charney, J. and J. Shukla, 1981: Predictability ong>ofong> monsoon.ong>Monsoonong> Dynamics. edi. by R. P. Pierce, CambridgeUniv. Press, 99-109.Dey, B. and O. S. R. U. Bhanu Kumar, 1984: An apparentrelationship between Eurasian spring snow cover andong>theong> advance period ong>ofong> ong>theong> Indian summer monsoon.J. Appl. Meteor., 21, 1929-1933.Dickson, R. R., 1984: Eurasian snow cover versus Indianmonsoon rainfall-An extension ong>ofong> ong>theong> Hahn-Shuklaresults. J. Appl. Meteor., 223, 171-173.Hahn, D. G. and J. Shukla, 1976: An apparent relationshipbetween Eurasian snow cover and Indian monsoonrainfall. J. Atmos. Sci., 33, 2461-2462.Horel, J. D. and J. M. Wallace, 1981: Planetary-scale atmosphericphenomena associated with ong>theong> Souong>theong>rnOscillation. Mon. Wea. Rev., 109, 813-829.Hoskins, B. J. and D. J. Karoly, 1981: The steady linearresponse on a spherical atmosphere to ong>theong>rmal andorographic forcing. J. Atmos. Sci., 38, 1179-1196.Hoskins, B. J., H. H. Hsu and F. F. Jin, 1990: The 1985/86Intraseasonal Oscillation and ong>theong> ong>Roleong> ong>ofong> ong>theong> Extratropics.J. Atmos. Sci., 47, 823-839.Japan Meteorological Agency, 1990: Monthly report onClimate System, No. 90-7, 32.Kodera, K. and M. Chiba, 1989: Western Siberian springsnow cover and east ong>Asianong> June 500mb height. PapersMeteor. Geophys., 40, 51-54.Krishnamurti, T. N., 1971: Tropical east-west circulationsduring ong>theong> norong>theong>rn summer. J. Atmos. Sci.,28, 1342-1347.Krishnamurti, T. N., W. J. Koss and J. D. Lee, 1973:Tropical east-west circulations during ong>theong> norong>theong>rnwinter. J. Atmos. Sci., 30, 780-787.Lorenz, E. N., 1963: Climatic determinism. Meteor.Monogr., 8, 1-3.Lorenz, E. N., 1990: Can chaos and intransitivity lead tointerannual variability? Tellus, 42A, 378-389.Meehl, G. A., 1987: The annual cycle and interannualvariability in ong>theong> tropical Pacific and Indian Oceanregion. Mon. Wea. Rev., 115, 27-50.Mooley, D. A. and J. Shukla, 1987: Variability and forecastingong>ofong> ong>theong> summer monsoon rainfall over India.ong>Monsoonong> Meteorology, C. P. Chang andT. N. Krishnamurti, eds., Oxford Univ. Press, 26-59.Morinaga, Y. and T. Yasunari, 1987: Interactions betweenong>theong> snow cover and ong>theong> atmospheric circulationin ong>theong> norong>theong>rn hemisphere. IAHS. PublicationsNo. 166, 73-78.Morinaga, Y., 1992: Interactions between ong>theong> Eurasiansnow cover and ong>theong> atmospheric circulations in ong>theong>norong>theong>rn hemisphere. Ph. D. Thesis, University ong>ofong>Tsukuba,82 pp.Palmer, T. N. and D. A. Mansfield, 1984: Response ong>ofong> twoatmospheric general circulation models to sea surface57


188 Journal ong>ofong> ong>theong> Meteorological Society ong>ofong> Japan Vol. 70, No. 1temperature anomalies in ong>theong> tropical East and WestPacific. Nature, 310, 483-485.Parthasarathy, B., 1987: Droughts/floods in ong>theong> summermonsoon season over different meteorological subdivisionsong>ofong> India for ong>theong> period 1871-1984. J. Climatol.,7, 57-70.Philander, S. G. H., 1985: El Nino and La Nina. J. Atmos.Sci., 42, 2652-2662.Philander, S. G. H., T. Yamagata and R. C. Pacanowski,1984: Unstable air-sea interactions in ong>theong> tropics. J.Atmos. Sci., 41, 604-613.Rasmusson, E. M. and Carpenter, T. H., 1982: Variationsin tropical sea surface temperature and surface windfields associated with ong>theong> Souong>theong>rn Ocscillation/ElNino. Mon. Wea. Rev., 110, 354-384.Walker, G. T. and E. W. Bliss, 1932: World weaong>theong>r V.Mem. Roy. Met. Soc., 4, 53-84.Wallace, J. M. and D. S. Gutzler, 1981: Teleconnectionsin ong>theong> geopotential height field during ong>theong> Norong>theong>rnHemisphere winter. Mon. Wea. Rev., 109, 784-812.Webster, P. J., 1981: Seasonality in ong>theong> local and remoteatmospheric response to sea surface temperatureanomalies. J. Atmos. Sci., 39, 41-52.Yamazaki, K., 1989: A study ong>ofong> ong>theong> impact ong>ofong> soil moistureand surface albedo changes on global climateusing ong>theong> MRI-GCM-I. J. Meteor. Soc. Japan, 67,123-146.Yasunari, T., 1987: Global structure ong>ofong> ong>theong> El Nino/Souong>theong>rn Oscillation. Part II. Time evolution. J.Meteor. Soc. Japan, 65, 81-102.Yasunari, T., 1988: Global teleconnections associatedwith Indian monsoon and ENSO. Meteor. Res. Report,Univ. ong>ofong> Tokyo, 88-1, 30-38.Yasunari, T., 1990a: Impact ong>ofong> Indian monsoon on ong>theong>coupled atmosphere/ocean system in ong>theong> tropical Pacific.Meteor. & Atmos. Phys., 44, 29-41.Yasunari, T., 1990b: ong>Monsoonong> and ENSO-A coupledocean/land/atmosphere system. Proc. Int. Sci.Conf. TOGA, July 1990, Honolulu, Hawaii. WMOTD No. 379, 111-120.Yasunari, T., 1991: "The monsoon year"-A new conceptong>ofong> ong>theong> climatic year in ong>theong> tropics. Bull. Amer.Meteor. Soc., 72, 1331-1338.Yasunari, T., A. Kitoh and T. Tokioka, 1991: Local andremote responses to excessive snow mass over Eurasiaappearing in ong>theong> norong>theong>rn spring and summerclimate-a study with ong>theong> MRI-GCM-. J. Meteor.Soc. Japan, 69, 4, 473-487.Yasunari, T. and K. Ueno, 1992: Seasonal and interannualvariability ong>ofong> teleconnection patterns in ong>theong>norong>theong>rn hemisphere. To be submitted.Yeh, T. C., R. T. Weong>theong>rald and S. Manable, 1983: Amodel study ong>ofong> ong>theong> short-term climatic and hydrologiceffects ong>ofong> sudden snow cover removal. Mon.Wea. Rev., 111, 1013-1024.

More magazines by this user
Similar magazines