impacts of the basin-wide indian ocean ssta on the south china sea summer monsoon onset

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 28: 1579–1587 (2008)Published online 30 January 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/joc.1671

Impacts of the basin-wide Indian Ocean SSTA on the SouthChina Sea summer monsoon onset

Yuan Yuan,a,c,d Wen Zhou,a,b Johnny C. L. Chana,b* and Chongyin Lica City U-IAP Laboratory for Atmospheric Sciences, City University of Hong Kong, Hong Kong, China

b Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, Chinac LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

d Graduate School of Chinese Academy of Sciences, Beijing, China

ABSTRACT: This article explores the impacts of the Indian Ocean basin-scale sea surface temperature anomaly (SSTA)on the South China Sea (SCS) summer monsoon onset. Basin-wide warming in the tropical Indian Ocean (TIO) is foundto occur in the spring following an El Nino event, and the opposite occurs for a La Nina event. Such changes of theIndian Ocean SSTA apparently prolong the El Nino-Southern Oscillation (ENSO) effects on the subsequent Asian summermonsoon, mainly through modifying the strength of the Philippine Sea anti-cyclone.

Warming in the TIO induces an anomalous reversed Walker circulation over the tropical Indo–Pacific Ocean, whichleads to descending motion, and hence suppressed convection in the western Pacific. The intensified Philippine Sea anti-cyclone in May and June advances more westward and prevents the extension of the Indian Ocean westerly flow into theSCS region, thereby causing a late SCS monsoon onset. The case is opposite for the TIO cooling such that the PhilippineSea anti-cyclone weakens and retreats eastward, thus favouring an early onset of the SCS monsoon. Copyright 2008Royal Meteorological Society

KEY WORDS Indian Ocean basin-wide SSTA; South China Sea summer monsoon onset; Philippine Sea anti-cyclone

Received 25 April 2007; Revised 15 November 2007; Accepted 29 November 2007

1. Introduction

The El Nino-Southern Oscillation (ENSO) exhibits agreat influence on the inter-annual variability of theglobal climate (Webster et al., 1998), including the Indianmonsoon (Khandekar and Neralla, 1984), the summerprecipitation over east Asia (Nitta, 1986; Huang and Wu,1989; Li, 1990a), and the east Asian winter monsoon(Li, 1990b; Zhang et al., 1996). Further, Wang et al.(2000) documented that ENSO effects can persist intothe following summer, causing rainfall anomalies overEast Asia through the Pacific-East Asian teleconnection,in which an anomalous anti-cyclone over the PhilippineSea is forced by El Nino and maintained by local air-seainteractions.

It has also been widely recognized that basin-widewarming in the tropical Indian Ocean (TIO) lags behinda mature phase of an El Nino event by a few months,while a cooling occurs after a La Nina event (Nigam andShen, 1993; Tourre and White, 1995; Chambers et al.,1999). Based on the ‘atmospheric bridge’ concept (Kleinet al., 1999), recent studies proposed that associated withan El Nino, warm sea surface temperatures (SSTs) in theTIO appear to be primarily driven by surface heat flux

* Correspondence to: Johnny C. L. Chan, Department of Physics andMaterial Science, City University of Hong Kong, Tat Chee Avenue,Kowloon, Hong Kong, China. E-mail: [email protected]

anomalies, such as reduced latent heat loss from the oceanand increased solar radiation due to decreased wind speedand suppressed convective activity respectively (Venzkeet al., 2000; Lau and Nath, 2003; Shinoda et al., 2004). Inaddition, Xie et al. (2002) and Huang and Kinter (2002)stated that much of the southwest Indian Ocean SSTanomaly (SSTA) is caused by oceanic Rossby waves thatpropagate from the east. Hence, through both atmosphericand oceanic processes, ENSO may induce SST variationsover the TIO.

It is, therefore, logical to expect that the basin-wideIndian Ocean SSTA might play a role in prolongingENSO effects into the following year. One such possibleeffect is on the South China Sea (SCS) summer mon-soon onset that occurs in early summer and marks thebeginning of the east and southeast Asian summer mon-soon (Tao and Chen, 1987; Lau and Yang, 1997; Dingand Chan, 2005). The objective of this study is, therefore,to investigate the possible impacts of the Indian OceanSSTA on the SCS monsoon onset.

The data and methodology are described in Section2. A TIO index is then defined in Section 3 to identifythe extreme cases of Indian Ocean warming and cooling.Section 4 analyses the composite low-level circulationsduring the SCS summer monsoon onset for the extremewarming and cooling cases. Physical mechanisms arefurther investigated in Section 5 and Section 6 gives asummary and discussion.

Copyright 2008 Royal Meteorological Society

1580 Y. YUAN ET AL.

2. Data and methodology

The data used in this study are mainly from the 55-year (1948–2002) National Centers for Environmen-tal Prediction/National Center for Atmospheric Research(NCEP/NCAR) reanalysis, including the monthly and 5-day (or pentad) mean wind and geopotential heights atstandard pressure levels, with a horizontal resolution of2.5° latitude by 2.5° longitude (Kistler et al., 2001). Themonthly mean SST data on a 1° latitude–longitude gridare derived from the Hadley Centre of the UK Mete-orological Office, with a 55-year period of 1948–2002(Rayner et al., 2003).

Inter-annual anomalies are obtained by deviations fromthe monthly climatology. To isolate the basin-wide SSTAin the TIO, an empirical orthogonal function (EOF)analysis is applied on the spring [(March-April-MayMAM)] SSTA in the TIO (40 °E–120 °E, 20 °S–20 °N).Composite analyses are performed on the extreme TIOwarming and cooling case, with the significance of theresults tested using the classical Student’s t-test (Chervinand Schneider, 1976).

3. Tropical Indian Ocean basin-wide SSTA

As mentioned in the Introduction, mainly through chang-ing the ocean latent heat flux and solar radiation, ENSOcould force a basin-wide warming/cooling in the TIO(Venzke et al., 2000; Lau and Nath, 2003), which usuallypeaks in the boreal spring and lasts till the summer (Panand Oort, 1983; Nigam and Shen, 1993).

The first principal component (PC1) in the EOF anal-ysis of the spring (MAM) SSTA in the TIO shows abasin-scale variation of the same sign, which explainsnearly 60% of the total variance (Figure 1(a)). A lineartrend is evident in the time series of the PC1 coeffi-cients (Figure 1(b)), the reason for which is beyond thescope of this paper. The normalized detrended coeffi-cient is defined as the TIO index to represent the basin-scale SSTA structure over the Indian Ocean in spring(Figure 1(c)). Extreme warming (cooling) cases are cho-sen to be those during which the TIO index value is above0.75 (below −0.75) of the standard deviation (Table I).It is clear from Table I that 10 out of 12 warming springsfollow an El Nino event, and 8 out of 13 cooling springsfollow a La Nina event, further confirming the possiblelagged response of the Indian Ocean SSTA to a previousENSO event.

The SCS summer monsoon onset pentads (based onWang et al., 2004 and Zhou and Chan, 2007) for eachwarming/cooling case suggest that most warming caseshave a monsoon onset after pentad 28 (the climatologi-cal mean onset pentad is 28 (May 16–20), Ding, 1994;Ding and Chan, 2005), except 1952 and 1953. On theother hand, in most cooling cases (except 1963, 1965and 1968), the SCS summer monsoon occurs earlier thanthe climatological mean. The TIO warming (cooling) thusappears to be related to a late (an early) SCS monsoononset.

Figure 1. (a) The first principal component (PC1) of the spring (MAM)SSTA in the TIO (40 °E–120 °E, 20 °S–20 °N). (b) The time series ofthe PC1 coefficients (open circles), with its linear trend (long-dashedline). (c) The normalized detrended time series of the PC1 coefficients(open circles). The long-dashed lines indicate ±0.75 of the standard

deviation of the time series.

Recently, Zhou and Chan (2007) pointed out that awarm (cold) ENSO event tends to induce a late (an early)SCS monsoon onset in the following year through thepropagation of cold (warm) sub-surface water into thewestern North Pacific. The current result suggests thatTIO warming/cooling may be another contributing fac-tor in prolonging the ENSO effects on the SCS summermonsoon onset.

Copyright 2008 Royal Meteorological Society Int. J. Climatol. 28: 1579–1587 (2008)DOI: 10.1002/joc

INDIAN OCEAN SSTA AND THE SOUTH CHINA SEA SUMMER MONSOON ONSET 1581

Table I. Extreme warming and cooling cases in the TIO withcorresponding onset pentad of the SCS summer monsoon.The warming/cooling springs are selected based on ±0.75 ofstandard deviation of the TIO index, which is defined as thenormalized detrended time series of the first EOF mode onthe spring SSTA over the TIO (40 °E–120 °E, 20 °S–20 °N).An asterisk indicates an El Nino (a La Nina) that occurs in thewinter prior to a warming (cooling) year, as defined in Trenberth(1997) and Wang and Gong (1999). The shaded years are notincluded in the composites because of the abnormal onset dates

from each group.

Warmingyears

SCS monsoon onsetpentads

Coolingyears

SCS monsoon onsetpentads

1952∗ 27 1951∗ 25

1953∗ 26 1963 30

1958∗ 29 1965∗ 29

1959 30 1968 301969∗ 29 1971∗ 251970∗ 32 1972∗ 261973∗ 33 1974∗ 261983∗ 31 1975 231987∗ 32 1976∗ 261988∗ 29 1984 241991 32 1986 271998∗ 29 1989∗ 28

2000∗ 2610/12 8/13

4. South China Sea summer monsoon onsetThe composite low-level circulations during the SCSsummer monsoon onset for the extreme warming/coolingcases show similar features, but with those of the warm-ing cases lagging by about two pentads. In the warm-ing composite, strong easterlies extend from the westernPacific to the southern SCS region, along with an intenseanti-cyclone covering the SCS and Southeast China inpentads 27 and 28 (Figure 2(a), (b)). Even though theAsian monsoon circulation has formed over the IndianOcean in pentad 28, it cannot advance into the SCS butonly northward to the Indo–China Peninsula (ICP). Thenin pentad 29 (Figure 2(c)), the anti-cyclone retreats east-ward, followed by the Indian Ocean southwesterly windsextending to the northern SCS. The full establishment ofthe SCS summer monsoon occurs around pentad 30–31(not shown), later than the climatological mean.

For the cooling composite, the main body of the low-level Philippine Sea anti-cyclone has already movedout of the SCS region early in pentads 25 and 26(Figure 3(a), (b)), with weak westerlies in the southernSCS. The Indian Ocean monsoon flow gets stronger inthe next pentad, and most of the ICP, SCS, and SoutheastChina are occupied by the westerly/southwesterly winds(Figure 3(c)), indicating an earlier monsoon onset.

The evolutions for the warming and cooling casesdescribed above suggest that the anti-cyclone over thePhilippine Sea may be important in linking the IndianOcean SSTA and the timing of the SCS summer monsoononset. This will be examined further in the next section.

5. Physical mechanisms

As expected, the composite SSTA in May-June (MJ) forwarming cases shows a basin-wide warming over mostof the Indian Ocean, along with a dissipating El Nino inthe eastern equatorial Pacific (Figure 4(a)). The coolingcomposite has a nearly opposite pattern, with a decayingLa Nina and negative SSTA in the TIO (Figure 4(b)).

The warm SSTA structure would likely result in low-level winds towards the Indian Ocean, as illustrated bythe strong (and statistically significant) 850-hPa east-erly anomalies across the Maritime Continent, and west-erly anomalies in the equatorial western Indian Ocean(Figure 5(a)). An anomalous low-level cyclone can beidentified in the central Indian Ocean around (75 °E,5 °N), suggesting the possibility of enhanced convec-tion caused by significant anomalous ascending motionabove the warm water (Figure 6(a)). At 150 hPa, anoma-lous westerly winds are also significant from the easternIndian Ocean to the western Pacific (Figure 7(a)). Ananomalous reversed Walker circulation thus forms overthe Indo–Pacific Ocean, featured as low-level easterlywinds from the western Pacific to the TIO, rising motionover the TIO, upper-level westerlies, and then subsidencein the western Pacific (Figure 6(a)). The Philippine Seaanti-cyclone should therefore be intensified.

The condition for the cooling composite is just theopposite, with anomalous low-level westerlies (Figure5(b)) and upper-level easterlies (Figure 7(b)) overly-ing the equatorial eastern Indian Ocean and westernPacific, and also subsidence over the TIO cold water(Figure 6(b)). Hence, in the western Pacific, abnormal ris-ing motion, though relatively weak (Figure 6(b)), mightweaken the Philippine Sea anti-cyclone to some extent.

The 500-hPa anomalous geopotential heights in MJfurther confirm the variation of the Philippine Sea anti-cyclone caused by the Indian Ocean SSTA. In the warm-ing composite (Figure 8(a)), the tropical Indo–PacificOcean is dominated by positive anomalies of the 500-hPageopotential height, with maximum values of ≥10 gpmin the ICP, SCS and the Philippines. The intensified sub-tropical high exhibits a centre of 5880 gpm over thewestern Pacific with the 5860-gpm contour extendingwestward to the west of the ICP. For the cooling compos-ite (Figure 8(b)), negative anomalies are significant in thetropical Indo–Pacific. The western Pacific sub-tropicalhigh is relatively weak, with no 5880-gpm contour andthe western edge of the 5860-gpm contour only reachingthe Philippines, almost 30° longitude eastward comparedwith that of the warming case.

These results suggest that warm SSTs in the TIOcould induce an anomalous reversed Walker Cell fromthe Indian Ocean to the western Pacific, with subsidenceand suppressed convection over the latter ocean so as toreinforce the sub-tropical high. The westward-extendinganti-cyclone along with the enhanced low-level easterlywinds to its south flank would inhibit the extension ofthe Indian Ocean westerly flow into the SCS region, inturn, causing a late SCS monsoon onset. The situationfor cooling cases will just be the opposite.


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Figure 2. Composite pentad mean 850-hPa winds for warming cases in pentad (a) 27, (b) 28 and (c) 29. Shaded areas indicate that either zonalor meridional wind is significant above the 95% confidence level.

6. Summary and discussions

This article examines the impacts of the TIO basin-wideSSTA on the SCS summer monsoon onset. It is found thatwarming (cooling) in the TIO apparently leads to a late(early) SCS monsoon onset by enhancing (weakening)the Philippine Sea anti-cyclone. As most warm (cold)cases mature in the spring following an El Nino (a La

Nina) event, Indian Ocean SSTA is suggested to prolongENSO influences on the SCS summer monsoon onset inthe subsequent year.

We further investigate the possible mechanisms forthe variation of the critical Philippine Sea anti-cyclonecaused by the Indian Ocean SSTA. After the maturephase of an El Nino event, positive SSTA prevailsin the Indian Ocean during the following MJ, which



Figure 3. As in Figure 2, but for cooling cases in pentad (a) 25, (b) 26 and (c) 27.

likely triggers upward motion and increased convectionthere. An anomalous reversed Walker Cell thus formsover the tropical Indo–Pacific Ocean. The associatedanomalous subsidence and suppressed convection in thewestern Pacific help strengthen the sub-tropical high.On the contrary, following a previous La Nina event,TIO cooling in the following MJ would weaken thesub-tropical high. Consequently, the westward-advancing

(eastward-retreating) sub-tropical high prevents (favours)the extension of the equatorial westerly monsoon flowfrom the Indian Ocean into the SCS region, leading to alate (an early) SCS summer monsoon onset.

Through numerical simulations, Wang et al. (2000) andLau and Nath (2000) suggested that the persistence ofthe Philippine Sea anti-cyclone is forced by El Nino andmaintained by local air-sea interactions. More recently,


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Figure 4. Composite SSTA structure in May-June (MJ) for (a) warming and (b) cooling cases, with solid lines (dots) referring to positive(negative) SSTA. Contour interval: 0.2 °C.

Figure 5. Composite 850-hPa anomalous winds in MJ for (a) warming and (b) cooling cases, with shaded areas for zonal or meridional windsignificant above the 95% confidence level and ‘C’ (‘A’) for anomalous cyclone (anti-cyclone).



Figure 6. Composite anomalous zonal-vertical circulations in MJ averaged in (Equator–10 °N) for (a) warming and (b) cooling cases, with shadedareas for vertical motion significant above the 90% confidence level.

Zhou and Chan (2007) documented that the strength ofthe anti-cyclone can be enhanced (suppressed) by thepropagation of cold (warm) sub-surface water into thewestern North Pacific. The current analyses provide anadditional mechanism for the strengthening or weakeningof the sub-tropical high caused by the Indian Oceanwarming or cooling, in agreement with the modelingresults of Annamalai et al. (2005). As the Philippine Seaanti-cyclone is an important system in the Pacific–EastAsian teleconnection (Wang et al., 2000), its maintenancemay involve complicated physical processes, with theabove three dynamical processes all possibly contributingto its persistence.

It should be noted that the current results are derivedfrom ‘extreme’ warming/cooling springs. Even then, notevery TIO warming case has a late SCS monsoon onset,

and not all delayed monsoon onsets are associated withpositive SSTA over the Indian Ocean. The same can besaid for the cooling cases and early monsoon onsets. Sev-eral TIO warming/cooling cases are also not associatedwith previous ENSO events. Therefore, further analysesare still required to investigate the impacts of the IndianOcean SSTA on the SCS summer monsoon onset, and theteleconnection of the ENSO forcing in the Indian Ocean.

Furthermore, our additional research suggests that theanomalous Philippine Sea anti-cyclone resulted from theIndian Ocean SSTA can last till the following summer.As the strength and position of the sub-tropical high areclosely related with the summer rainfall in East Asia(Yang and Sun, 2003), we will examine the possibleinfluences of the Indian Ocean SSTA on the summerrainfall pattern over China in a future study.


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Figure 7. As in Figure 5, but for the 150-hPa anomalous winds.

Figure 8. Composite 500-hPa anomalous geopotential heights averaged in MJ for (a) warming and (b) cooling cases, with the thin solid(long-dashed) lines for positive (negative) anomalies, and the thick lines for the 5860 and 5880-gpm contours as the boundary of the sub-tropical

high. Contour interval for anomalies is 5 gpm, and shaded areas indicate significance above 95% confidence level.



Acknowledgments

A large part of the work by the first author is completedas a joint collaborative work between City University ofHong Kong and the Institute of Atmospheric Physics,Chinese Academy of Sciences. The authors acknowledgethe support from the China National 973 Program (GrantNo. 2006CB403600) and City University of Hong KongGrants 7001825 and 9610021.

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impacts of the basin-wide indian ocean ssta on the south china sea summer monsoon onset

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