Interdecadal and Interannual Variationsof the South China Sea Summer Monsoon:
JOHNNY C L CHAN* AND WEI ZHOU#Laboratory for Atmospheric Research, Dept. of Physics & Materials Science
City University of Hong Kong, Hong Kong, China
1. Introduction
It has been found that the South China Sea summer monsoon (SCSSM) has variations
with periods ranging from interdecadal to intraseasonal. This paper presents a summary ofthe interdecadal and interannual variations and their possible physical mechanisms/processes.
The intraseasonal variations have been explored by some other studies (e.g. Mao and Chan2004) and will therefore not be considered in this paper.
2. Interdecadal variations
A prominent interdecadal signal is the Pacific Decadal Oscillation (PDO) (e.g.
Mantua et al. 1997). In addition, the El Niño/Southern Oscillation (ENSO) has also been
observed to go through interdecadal changes (e.g. Wang 1995). These two oscillations arefound to be related to the monsoon rainfall over South China (SC), the results of which arepresented in this section. These rainfall data are used as a proxy for that over the South
China Sea (SCS) because the latter does not have such long records.
2.1. Data
The rainfall data include those at Hong Kong, Macao and Guangzhou for 92 years
from 1910-2001. Monsoon rainfall is defined as the average of the May and June rainfall forthese three stations (SCMR). Only these two months are chosen because of the “hybrid”
nature of the rain-bearing systems in April. The choice of these three stations is because thecorrelation coefficients of these three stations with the Guangdong monthly rainfall averaged
over 47 stations are significant above the 99% confidence level.
The PDO index is taken from Mantua’s website (see Mantua et al. 1997) and is equal
to the leading empirical orthogonal function (EOF) of SST anomalies (January to March) in
the North Pacific Ocean poleward of 20oN. A Niño-3 SST index is defined as the January toMarch SST averaged over the central Pacific (5oS-5oN, 90-150oW) to represent the effect ofENSO. The SST data for 1910-1949 are extracted from U.K. Meteorological Office
GISST2.3 dataset, and for 1950-2001 from the US Climate Prediction Center. Reanalyses ofmonthly 500-hPa geopotential height from 1950-2001 are produced from the NCEP/NCAR
reanalyses.
2.2. Interdecadal variations of SCMR
Based on the Z-index (Ju et al. 1997), and using half a standard deviation (|Z| > 0.5)
as the threshold (the Z-index is normalized to make the category simple), the 92 years can be
stratified into three groups: 31 dry, 37 normal and 24 wet. According to the PDO shifts(1925, 1947 and 1977) (e.g., Mantua et al. 1997), the number of dry or wet monsoon years isfound to be different during these three epochs (Table 1). Note the existence of aninterdecadal variation, from a more frequent occurrence of dry monsoon years during 19251946 to the opposite in the next epoch (1947-1976), and then back to more dry monsoon*Corresponding author address: Johnny Chan, Dept. of Physics & Materials Science, City University of Hong
Kong, Tat Chee Ave., Kowloon, Hong Kong, China. Email: Johnny.chan@cityu.edu.hk#Current affiliation: Institute of Atmospheric Physics, Chinese Academy of Sciences

years again after 1977. It is obvious that these changes match the PDO shifts very well, witha high PDO phase corresponding to more dry monsoon years, and vice versa. A wavelet
analysis of the SCMR suggests that during the entire 92 years, most of the wavelet transform
coefficients that alternate between positive and negative values are around 16-32 yr except
for 1920s-1940s when the main peak is larger than 32 yr (not shown).
TABLE 1. Dry or wet monsoon years during each of the three PDO epochs. El Niño years:
parentheses, La Niña years: underlined, EN+1 years: boldfaced and LN+1 years: italicized.

EpochWet

1925-1946
1925, 1926, 1929,1930, 1932, 1933,1935, 1937, 1938,1939, 1945
1927, 1940, 1944

Ratio of dry/wet

11/3

Dry

1947-1976
1949, 1954, 1958, 1961,(1963), 1967
1950, 1951, 1955,(1957), 1959, 1966,
1970, (1972), 1973, 1975
6/10

1977-1995
1977, 1980,1985, 1988,1990, (1991),(1994), 1995
(1982), 1989,1992, 1993
8/4

2.3. Possible relationships among SCMR, ENSO and PDO
The results in Table 1 suggest the existence of a possible linkage among the SCMR,ENSO and PDO. Based on the wavelet analysis, the scale-weighted sums of the wavelet
transform coefficients within periods of 16-32 yr and 32-64 yr of PDO, ENSO and SCMR areexamined (Fig. 1). The important feature is that PDO and ENSO are strongly correlated (0.71)
on decadal timescales. Indeed they are nearly phase locked. It appears that SCMR has a close
association with ENSO/PDO at both periods, being –0.59/-0.71. Similar results are obtained
for the 32-64 yr period (not shown). These results suggest the SCMR may be highly related
to PDO and ENSO at decadal timescales such that a high PDO/El Niño (low PDO/La Niña)
phase pattern is associated with a deficient (above-normal) SCMR.

Fig. 1. The normalized reconstructed time series of SCMR (solid with *), PDO (solid) and ENSO(dash) at period of 16-32 yr.

To verify this hypothesis, for each of the years between 1950 and 1995, the ENSO
condition and the category of SCMR (wet, normal or dry) are examined. It is found that thebest relationship exists for the years after an El Niño (EN+1) or La Niña (LN+1) event (Table2). The variation of the SCMR is more (less) “predictable” when both ENSO and PDO are inphase (out of phase). When an in-phase situation occurs, the SCMR tends to be below
normal or normal during high PDO phase/EN+1 events, but above normal or normal duringlow PDO phase/LN+1 events. On the contrary, the trend of SCMR has no preference in an
out-of-phase situation (i.e. high PDO/LN+1 or low PDO/EN+1). These results suggest astrong coupling between PDO and ENSO in affecting the SCMR. It appears that when these
two forcings are in phase, the SCMR is highly related to these forcings. However, when theyare out-of-phase, one forcing can be stronger than the other or their effects can cancel eachother. As a result, the “predictability” becomes low.
TABLE 2. Comparison of the conditions of SCMR during different ENSO/PDO phases. A, N and B
denote the above-normal, normal and below-normal SCMR. EN+1 and LN+1 represent the year after
an El Niño and a La Niña event respectively.

EN+1

LN+1

A:
N:
B:
A:
N:
B:

Low PDO
2/7 (1966, 1973)
4/7 (1952, 1958, 1964, 1969)
1/7 (1954)
5/8 (1950, 1951,1955,1957,1975)
3/8 (1965, 1971, 1974)
0/8

A:
N:
B:
A:
N:
B:

High PDO
1/9 (1992)
3/9 (1978, 1983, 1987)
5/9 (1977, 1980, 1988, 1994, 1995)
1/3 (1989)
1/3 (1996)
1/3 (1985)

2.4. Physical mechanism
The reason for the SCMR being more predictable in some years than in other years
relative to the phase relationship between ENSO and PDO is likely to be in the intensity ofthe subtropical high associated with these two oscillations. Consider the high PDO phasesituation. In this case, the North Pacific SSTA is below normal (e.g. Mantua et al. 1997). Ifa warm ENSO event occurs during the high PDO phase, the SSTA over the westernequatorial Pacific as well as the Philippine Sea is also below normal. Thus, these twoanomalies superpose on each other to give a below-normal SSTA over the Philippine Sea andthe tropical western North Pacific. Associated with this distribution of SSTA are theanticyclonic anomalies associated with the mature phase of the warm ENSO event (Wang etal. 2000) and the PDO that will be enhanced through the mechanism discussed in Chang et al.
(2000). Since the subtropical high tends to persist for a relatively long period of time, theenhanced subtropical high will likely extend westward into the SCS in the early summer,
which would result in a weak summer monsoon over SC.
On the other hand, if a cold ENSO event occurs, the SSTA over the westernequatorial Pacific as well as the Philippine Sea are above normal. Therefore, depending on
which one dominates, the SSTA could either be positive or negative. Furthermore, because
cyclonic anomalies are associated with the mature phase of the cold event, but for the highPDO phase, the flow should be anticyclonic, the subtropical high is weaker. In fact, the twocirculation anomalies might cancel each other, which results in a low predictability of theintensity of the subtropical high, and hence the intensity of the summer monsoon over SC.
The scenarios for the low PDO phase are just the reverse. That is, during the LN+1
situation, the cyclonic anomalies of the PDO are superposed onto those associated with thecold ENSO event so that the subtropical high is very weak, which then results in a stronger
summer monsoon, and hence an above-normal SCMR.
2.5. Summary
These results suggest that the interdecadal variability of the SCMR is determined atleast partly by the phase relationship between PDO and ENSO. However, only when thesetwo oscillations are in phase can such a variability be more deterministic.

3. Interannual variability
The purpose of this part is to investigate how the onset time of the SCSSM may beaffected by remote SST anomalies over the Pacific associated with ENSO.
3.1. Data
The data used in this study are mainly from the NCEP/NCAR reanalysis. The ENSO
is represented by the Niño3.4 SST anomalies (5oS-5oN, 170-120oW). The monthly mixedlayer ocean temperature (0/400m) data on a 2o latitude x 5o longitude grid are obtained fromthe Scripps Institute of Oceanography. From the ocean dynamics point of view, thethermocline depth could be roughly represented by the 20oC isotherm within the thermocline
layer (Kessler 1990). To estimate the depth of this isotherm, the mixed-layer ocean
temperature (0/400m) datasets are interpolated to 1 m x 1 m vertical resolution using cubicsplines, and the ocean heat content (OHC) above the thermocline could then be obtained.
3.2. Monsoon onset date
The 850-hPa zonal winds over the central and southern part of the SCS (110-120ºE, 515ºN), is used to determine the onset pentad, which is defined as the first pentad after midApril when the 5-day mean value of the 850-hPa zonal winds is westerly over the central andsouthern part of SCS and lasts for more than two pentads. An early or late monsoon onset
date (MOD) can then be classified using pentad 28th (P28) as the threshold (Table 3).
TABLE 3. Classification of MOD. EE, NE, NN, NL and EL represent extreme early onset (< 27pentad)), early to normal onset (=27 pentad), normal onset (=28 pentad), late to normal onset (=29pentad), and extreme late onset (> 29 pentad)).

EE (<P27)
NE (=P27)
NN (=P28)
NL (=P29)
EL (>P29)

1956, 1960, 1966, 1971, 1972, 1974, 1976, 1984, 1985, 1994, 1996, 1999,2000, 2001
1957, 1961, 1979, 1980, 1981, 1986, 1995
1962, 1964, 1977, 1989, 1990, 1992, 1997
1955, 1958, 1965, 1967, 1969, 1978, 1983, 1988, 1998
1959, 1963,1968, 1970, 1973, 1975, 1982, 1987, 1991, 1993

3.3. ENSO cycles associated with OHC and MOD
The longitude-time sections of OHC composites along the equator show that, for theearly onset cases (Fig. 2a), in year –21, positive (negative) OHC anomalies exist over theeastern (western) equatorial Pacific, which is typical of the mature phase of an EN event.
The negative OHC anomalies then propagate eastward and reach the central equatorial
Pacific (CEP) by the winter of year -1 (Dec -1 to Feb 0), signifying the mature stage of La
Niña (LN). At this time, OHC anomalies over the western equatorial Pacific (WEP) havebecome positive. These positive anomalies begin to propagate eastward towards the end ofyear 0. These results suggest that early onset cases mostly occur from an EN type in year -2
or -1 to the LN type in year 0. Almost the exact reverse occurs for the late onset cases (Fig.
2b), although the eastward propagation of the positive OHC anomalies is not as strong. Infact, the maximum positive OHC anomalies appear to remain in the central equatorial Pacificduring the winter of year -2 and year -1. Nevertheless, it is apparent that these late MOD
cases are associated with an EN type that follows from a LN type.

1

Year 0 is defined as the year in which the MOD of a particular type is considered. Year (-n) refers to n year(s)before, and year (n) n year(s) after, year 0.

Fig. 2. Longitude-time section of OHC composites (unit: 108 J m-2) averaged over 4oS-4oN for (a) E
onset years and (b) L onset years. Year 0, +1, and –1 (–2) refer to the monsoon onset year, the year
after, and the year (two years) before. Shaded areas indicate negative values.

This conclusion is further demonstrated from the fact that 11 out of 14 early onset
cases occurred either during an LN year or the year after an LN event (LN+1) (Table 4), and8 out of 10 late onset cases are associated with either an EN event or the year after an EN
event (EN+1). These results suggest that changes in the SCSSM onset date may be tuned tothe ENSO cycle, with late and early onsets taking turns. In other words, variations of MOD
appear to follow the ENSO cycle. For example, from an LN event to an EN event, theSCSSM experiences an NE (normal to early) or E (early onset) onset and then an NL (normalto late) or L (late) onset, e.g. 1956-58, 1971-73, 1974-78, 1984-87, and 1995-98. Likewise,from an EN event to an LN event, the onset time sequence reverses, e.g. 1968-71, 1982-85,1986-89, 1991-96, and 1997-2001.
TABLE 4. ENSO event and MOD. LN, LN+1, EN, EN+1, neutral refer to, respectively, the La Niña
year, the year after La Niña, the El Niño year, the year after El Niño year, and non-ENSO year. The
years in parentheses are those in which a switch to the other phase of the ENSO occurs.

EN/EN+1

Early onset (14 cases)
1956, 1971, 1974, (1972), 1976,1984, 1985, 1996, 1999, 2000, 2001
1966, 1994

Neutral1960

LN/LN+1

Late onset (10 cases)
1975
1963, 1968, (1970), (1973),
1983, 1987, 1991, 1993
1959

3.4. Physical mechanism
A possible mechanism for the interannual variation of the onset date of the SCSSM
could be as follows. Consider a case of a late monsoon onset in year 0. Two years before(year –2), it is related to a pre-EN onset situation (see Fig. 2b) so that warm sub-surface waterover the WEP propagates eastward while sub-surface water over the eastern equatorial
Pacific tends to be cold. As the warm sub-surface water continues to move eastward as
Kelvin waves, the cold water is displaced poleward, so that the negative correlation along theSouth American coast decreases. The cold water then propagates westward as a Rossbywave response. When the cold water reaches the western North Pacific, it weakens the warm
pool and hence intensifies the subtropical high. The strengthening of the subtropical highleads to an enhancement of the easterlies and hence stronger upwelling and cold sub-surface
water. Persistence of the strengthening of the subtropical high also results in its westward
extension and hence a weak winter monsoon (the case for EN+1; Li and Mu 2002) and then alate summer monsoon onset (year 0, EN+1). A persistent subtropical high results in anincrease in insolation and hence an increase in SST. This would weaken the subtropical high,or the development of cyclonic anomalies. The other half of the cycle then repeats.
3.5. Summary
These results suggest that the interannual variability of the MOD very much relates tothe ENSO cycle. Further, the physical mechanism presented is consistent with that from thelast section in that the subtropical high appears to be a dominant factor in controlling theSCSSM. Any mechanism that can modify the strength of the subtropical high is likely to
produce an effect on the SCSSM.
4. Summary and discussion
This paper shows the existence of large interdecadal and interannual variabilities of
the SCSSM. The former is apparently related to the coupling between the forcings from PDO
and ENSO while the latter is largely due to variations in ENSO. The proposed mechanisms
related to ENSO are consistent with each other. The results suggest that in certain situations,
it is possible to predict the onset and intensity of the SCSSM.
Acknowledgments. The authors would like to thank following individuals andinstitutions: NCEP-NCAR for the reanalysis data and global SST data, Dr. N. J. Mantua for
the PDO index, Drs. C. Torrence and G. P. Compo for the Nino-3 SST index and the wavelet
analysis software at URL: http://paos.colorado.edu/research/wavelets/, the respective weather
centers for monthly rainfall, NOAA for the OLR data (1975-2000), the Scripps Institution ofOceanography for the monthly mixed-layer ocean temperature (0/400m), and Y. Liu of South
China Sea Institute of Oceanology of Chinese Academy of Sciences for the OHC calculation
method.. This research was partially supported by the City University of Hong Kong
Research Grants 7001212, 7001336 and 7010010.
References
Chang, C.-P, Y. S. Zhang, and T. Li, 2000: Interannual and Interdecadal variations of the east Asiansummer monsoon and tropical Pacific SSTs. Part II: Meridional structure of the monsoon. J.
Climate, 13, 4326-4340.
Ju, X., X. Yang, L. Chen and Y. Wang, 1997: Research on determination of station indexes anddivision of regional flood/drought grades in China, Quarterly Journal of Applied Meteor., 8, 2633 (in Chinese).
Kessler, W. S., 1990: Observations of long Rossby waves in the northern tropical Pacific. J. Geophys.
Res., 95, 5183-5217.
Li, C. and M. Mu, 2002: A further study of the essence of ENSO. Chinese J. Atmos. Sci., 4, 309-328.
Mantua, N. J., S. R. Hare, Y. Zhang, J. M. Wallace, and R. C. Francis, 1997: A Pacific interdecadal
climate oscillation with impacts on salmon production. Bull. Amer. Meteor. Soc., 78, 1069-1079.
Mao, J. and J. C. L. Chan, 2004: Intraseasonal variability of the South China Sea summer monsoon.
Submitted to J. Climate (under revision)
Wang, B. 1995: Interdecadal changes in El Niño onset in the last four decades. J. Climate, 8, 267–285.
Wang, B., R. Wu, and X. Fu, 2000: Pacific–East Asian teleconnection: How does ENSO affect East
Asian Climate? J. Climate, 13, 1517-1536.