Interannual variability of summer
monsoon precipitation over the Indochina
Peninsula in association with ENSO
Fei Ge, Xiefei Zhi, Zaheer Ahmad Babar,
Weiwei Tang & Peng Chen
Theoretical and Applied Climatology
ISSN 0177-798X
Theor Appl Climatol
DOI 10.1007/s00704-015-1729-y
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Author's personal copy
Theor Appl Climatol
DOI 10.1007/s00704-015-1729-y
ORIGINAL PAPER
Interannual variability of summer monsoon precipitation
over the Indochina Peninsula in association with ENSO
Fei Ge 1,2 & Xiefei Zhi 1 & Zaheer Ahmad Babar 1,3 & Weiwei Tang 4 & Peng Chen 5
Received: 14 May 2015 / Accepted: 29 December 2015
# Springer-Verlag Wien 2016
Abstract The interannual variability of summer monsoon
precipitation (1979–2011) over the Indochina Peninsula
(ICP) is characterized using the first empirical orthogonal
function of 5-month total precipitation (May to September).
The leading mode, with a monopole pattern, accounts for
30.6 % of the total variance. Dynamic composites and linear
regression analysis indicate that the rainy season precipitation
over the ICP is linked to El Niño–Southern Oscillation
(ENSO) on interannual scales. The preceding winter
[D(−1)JF(0)] negative sea surface temperature (SST) over
the Niño-3.4 region is predominantly correlated with the rainy
season precipitation over the ICP. Notably, the simultaneous
correlation between remote SST anomalies in the Niño-3.4
region and the rainy season precipitation over the ICP is weak.
The interannual variation of tropical cyclones modulated by
ENSO is a significant contributing factor to the rainy season
precipitation over the ICP. However, this relationship is not
homogeneous over the ICP if ENSO is considered. Before
removing the ENSO signal, enhanced precipitation is present
over the northeastern part of the ICP and reduced precipitation
appears in the western ICP, especially in coastal areas. In
contrast, after removing ENSO, only a minor significant pos* Fei Ge
[email protected]
1
Collaborative Innovation Center on Forecast and Evaluation of
Meteorological Disasters; KLME, Nanjing University of Information
Science and Technology, Nanjing, China
2
Max Planck Institute for Meteorology, Hamburg, Germany
3
Pakistan Meteorological Department, Islamabad, Pakistan
4
Chongqing Institute of Environmental Science, Chongqing, China
5
Chongqing Meteorological Observatory, Chongqing, China
itive precipitation anomaly occurs over the northeastern part
of the ICP and the negative anomaly appears particularly in
the western and eastern coastal regions. The results obtained
through the present study are useful for our understanding of
circulation mechanisms and provide information for assessing
the ability of regional and global climate models in simulating
the climate of Southeast Asia.
1 Introduction
The interannual variability of the Asian monsoon has long
been a central theme of climate research. The Indochina
Peninsula (ICP) is located between the Indian subcontinent
and the western North Pacific and is influenced by the South
Asian, East Asian, and Australian monsoons. The climate
change over the ICP affects not only the local economy but
also the agriculture of this region. Hence, to fully understand
the impacts of climate variability on interannual and
interdecadal time scales is of primary importance for the countries in and around the ICP.
The interannual variability in precipitation over the ICP has
been summarized in many previous studies (Matsumoto 1997;
Wang and Fan 1999; Wu and Wang 2000; Takahashi and
Yasunari 2006; Caesar et al. 2011; Zhou et al. 2011; Hsu et
al. 2014). Chen and Yoon (2000) showed that the Indochina
summer monsoon precipitation is related to the variations in
sea surface temperature (SST) over the eastern tropical Pacific
and the occurrence frequency of westward-propagating
weather disturbances in the western tropical Pacific monsoon
trough. Chen et al. (2012) suggested that the interannual variation of autumn rainfall in central Vietnam is influenced by
rain-producing weather systems. Nguyen et al. (2014) investigated rainfall and temperature variability for the whole of
Vietnam from 1971 to 2010, revealing relationships between
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F. Ge et al.
El Niño–Southern Oscillation (ENSO) and temperature and
rainfall variability.
The activity of tropical cyclones (TCs) also contributes to
the occurrence of heavy rainfall/flood events in this region.
Climatologically, there are two seasonal rainfall peaks over
the ICP during the monsoon period. Takahashi and Yasunari
(2006) found that the first rainy peak (from May to June) is
closely linked with strong monsoon southwesterlies, while the
second rainy peak (from August to September) is mainly contributed to by TC activities. Fudeyasu et al. (2006) suggested
that westward-propagating TCs often cross over the ICP and
cause severe flooding in August and September. It has also
been found that nearly 70 % of the total precipitation over
Thailand is related to TCs in September (Takahashi and
Yasunari 2008).
It is widely known that ENSO is the predominant mode of
interannual variability in the tropical Pacific, exerting significant influence on global weather and climate (Rasmusson and
Carpenter 1982; Fraedrich and Müller 1992; Fraedrich 1994;
Zhang and Levitus 1997; Zhi 2001; Wang 2002; Zhi et al.
2010; Zhang et al. 2014; Zhang et al. 2015). A number of
studies have reported the influence of ENSO on the TC activities over the western North Pacific (WNP). The interannual
variation of TCs ultimately affects the rainy season precipitation over the Asian monsoon regions. Chan (1985), Wu and
Lau (1992), and other authors suggested that, during El Niño
years, the sinking motion of anomalous Walker circulation
weakens the monsoon trough, which is accompanied by decreasing TC activity over the WNP. Wang and Chan (2002)
further investigated how strong ENSO events affect TC activity over the WNP. They found that, during La Niña years, the
local positive SSTA increases equatorial convective heating in
the WNP and induces significant easterly anomalies in the
equatorial central-eastern Pacific. The anomalous ascending
motion concentrates in the northwest quadrant of the WNP,
whereas strong descending motion appears in the southeast
quadrant. The meridional wind shears associated with the
equatorial easterly anomalies enhance low-level vorticity in
the northwest quadrant and provide an essential energy source
for the generation of TCs.
In addition to exploring the interannual variability in
precipitation during the rainy season over the ICP, it is
also necessary to investigate the physical mechanisms underlying the remote SST forcing and the associated
atmospheric processes. In fact, it is still a controversial
issue as to how ENSO affects the precipitation over the
ICP. Goswami et al. (1999) examined the precipitation
variations in the Asian monsoon region, including the
ICP. The results showed that the correlation between
ENSO and the precipitation variations over the ICP is
weak. Takahashi et al. (2015) reported that the statistical
relationship between SST over the eastern tropical Pacific
and the interannual variability in precipitation during the
rainy season over the ICP is unclear. These results are
different to those of Chen and Yoon (2000), implying that
this issue remains a significant scientific challenge and
concern. Therefore, it is necessary to further investigate
the association between the variability in precipitation
during the rainy season over the ICP and in SST modes
over the WNP and Niño-3.4 regions.
In this study, our analyses focus on the interannual variability of the ICP rainy season precipitation, with the aim to identify the atmospheric circulation and oceanic patterns concerned. The rest of the paper is organized as follows: Section
2 describes the datasets and the methods of analysis used in
this study. In Section 3, we illustrate the interannual variability
in precipitation during the rainy season over the ICP. Also in
this section, we present the ICP rainy season precipitation
anomalies associated with large-scale atmospheric, SST and
TC activities. A conclusion and further discussion are provided in Section 4.
1.1 Data and methods
The observational precipitation field obtained from the
Climate Prediction Center (CPC) Merged Analysis of
Precipitation (CMAP) (Xie and Arkin 1997) and the
high-resolution precipitation dataset (CRU TS3.1) derived
from the Climatic Research Unit (CRU) (Mitchell and
Jones 2005; Harris et al. 2014) for the period 1979–2011
are used in this work. The other variables include circulation fields from the National Centers for Environment
Prediction–National Center for Atmospheric Research
(NCEP–NCAR) reanalysis (Kalnay et al. 1996) for the period 1979–2011, and the SST fields from the Hadley
Centre Sea Ice and Sea Surface Temperature dataset
(HadISST) for the period 1978–2011 (Rayner et al.
2003). The Niño-3.4 Index is the average SST anomalies
in the region (5° S–5° N, 120°–170° W) (Trenberth 1997).
The time series of SSTAs are used to stratify the strength of
El Niño (or La Niña). The China Meteorological
Administration (CMA)’s tropical cyclone best-track
dataset (CMABST) is also used to analyze the interannual
variation in TC activities (Ying et al. 2014).
All the datasets of the above fields are computed by removing the long-term linear trends. Here, we use empirical orthogonal function (EOF) analysis to identify the dominant pattern
of rainy season precipitation in the ICP. The principal component (PC) given by the EOF analysis indicates the interannual
variability of the ICP rainy season precipitation. When examining the statistical significance of the correlation and linear
regression, the effective number of degrees of freedom is considered, following Davis (1976). All statistical significance
tests for correlations are analyzed by employing the twotailed Student’s t test.
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Interannual variability of summer monsoon precipitation
2 Results
2.1 Interannual variations of the rainy season
precipitation over the ICP
To investigate the temporal and spatial features of precipitation over the ICP, we show that the first leading modes of
the EOF analysis, based on CRU and CMAP, account for
30.6 and 21.8 % of the total variance, respectively (Fig. 1).
The spatial patterns are similar, with a uniform monopole
over the ICP, as shown in Fig. 1a, b. The corresponding time
coefficients (PCs) are also highly correlated to the first
mode, with correlation coefficients of 0.62 (figures not
shown). Both of the PCs indicate large interannual and
interdecadal variability. In order to examine the year-toyear variability of rainy season precipitation in the ICP,
the correlation of normalized rainfall index and the corresponding PC-1 derived from CRU TS3.1 is shown in
Fig. 1c. This dataset is chosen because it has a higher resolution compared to the CMAP dataset. The correlation coefficient between the normalized rainfall index and PC-1 is
0.93, with a 95 % confidence level. This indicates that the
signal variation between them is simultaneous, implying the
corresponding PC-1 characterizes the variation of the interannual rainy season precipitation of the ICP. The correlation
coefficient of PC-1 with the area-averaged SSTA over the
Niño-3.4 region is shown in Fig. 1d. Note that the year -1
Fig. 1 Spatial distribution of
rainy season rainfall (MJJAS)
during 1979–2011 and
correlations between PC1,
normalized rainfall index, and
Niño3.4 index: a first EOF of
CRU dataset; b first EOF of
CMAP dataset; c correlation
between PC1 and normalized
rainfall index; d correlation
between PC1 and Niño3.4 index
indicates the previous year but not to ENSO cycle. The PC-1
is significantly negatively correlated with the Niño-3.4 index from JJA(−1) to MAM(0), especially in winter
[D(−1)JF(0)], with a value of −0.41 passing the 90 % confidence level. Notably, after the correlation coefficient
reaches its peak in D(−1)JF(0), the correlation between
them increases rapidly and becomes positive, but its statistical significance does not pass the 90 % confidence level.
This indicates that the preceding SSTA over the Niño-3.4
region relates strongly to the rainy season precipitation in
the ICP.
Composite analysis is used to examine the associated SST
anomalies of anomalous precipitation in the ICP on interannual time scales. We classify all anomalous positive and negative years from 1979 to 2011 based on one standard deviation
of PC-1. Consequently, four wetter than normal years (1994,
2000, 2006, and 2011) and seven dryer than normal years
(1983, 1985, 1988, 1992, 1993, 1998, and 2010) are identified. The spatial map of rainy season precipitation during dry
(wet) years is shown in Fig. 2a. The precipitation is more
intense than normal over the ICP from May to September in
wet years and vice versa. The composite analysis of SST in
D(−1)JF(0) (Fig. 2b) shows that when the precipitation is
more intense than normal, anomalously warm SST appears
in the western Pacific and WNP and cold SST occurs in the
equatorial central-eastern Pacific and vice versa. This anomalous SST mode over the tropical Pacific Ocean closely
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F. Ge et al.
Fig. 2 Composites of anomalies
of rainfall in MJJAS and SST in
the preceding winter [D(−1)JF(0)]
during dry and wet years: a CRU
(mm); b SST (°C)
resembles the La Niña (or El Niño) pattern. This indicates that
ENSO events are associated with the rainy season precipitation anomalies over the ICP.
2.2 Possible causes of the rainy season precipitation
over the ICP
a) Relationship with SST
To illustrate the possible causes of the interannual variability of rainy season precipitation over the ICP, we examine the
SST and 850 hPa wind difference of strong minus weak years,
respectively (Fig. 3a, b). The SST anomalies indicate that a
significant negative correlation is observed in the equatorial
Pacific, whereas a positive correlation appears in the WNP
from SON(−1) to D(−1)JF(0). This anomalous SST over the
tropical Pacific is similar to the typical La Niña pattern. The
negatively correlated areas in the tropical central-eastern
Pacific almost disappear in MAM(0). No significant SST signals are observed in JJA(0) over the western Pacific or equatorial Pacific. This implies that the cooling (warming) of SST
over the tropical central-eastern Pacific during the preceding
autumn and winter is relatively in phase with the strengthening (weakening) of rainy season precipitation over the ICP,
whereas the relationship between SST over the Niño-3.4 region and precipitation during the rainy season over the ICP is
unclear.
Many previous studies have demonstrated that ENSO not
only impacts upon the oceanic variability of the Pacific, but
also influences the climate around the globe, especially in the
Asian monsoon area (Barnett et al. 1991; Webster et al. 1998;
Wang et al. 2000, 2001). Lau et al. (2000) studied that the
relationship between SST and the Southeast Asian Monsoon.
They found that the SSTA patterns related to monsoon indices
are similar to ENSO evolution. Nguyen et al. (2007) investigated the connections between the monthly precipitation over
the central highlands of Vietnam and the SSTA over the
tropical Pacific and Indian Ocean. They suggested that the
rainfall, such as in April, October, and November, was
highly correlated to ENSO. In addition, Takahashi et al.
(2015) showed a weak relationship between SST over the
Niño-3.4 region and rainy season precipitation over the ICP.
Our results are consistent with these previous studies, as
shown in Fig. 3a. The remote SST forcing from the tropical
Pacific on the rainy season precipitation over the ICP is weak.
However, it is significantly correlated to the SST over the
Niño-3.4 region from SON(−1) to D(−1)JF(0).
b) Anomalous atmospheric circulation
An anomalous cyclone generates over the WNP and enhances rapidly from SON(−1) to D(−1)JF(0) during the La
Niña mature phase (Fig. 3b). The anomalous cyclone over
the Philippine Sea, with prevailing northeasterly winds over
East China, influences the wintertime wind variability of the
ICP. The westerlies govern the tropical Indian Ocean (TIO),
while easterly anomalies are observed over the equatorial
Pacific. In MAM(0), the anomalous cyclone persists over
the Philippines and the westerlies are stronger than in the
preceding winter. Notably, the anomalous cyclone almost disappears in summer. The westerlies govern the Indian subcontinent and south of the ICP, which probably implies that the
above-normal precipitation over the ICP is not directly linked
to the enhanced monsoon westerlies. To further illustrate the
anomalous atmospheric circulation impact, we show the difference of 200 hPa velocity potential and divergence wind
fields in Fig. 4. In SON(−1), the upper-level divergences occur
over the WNP, Maritime Continent, and Indian Ocean, while
the enhanced convergences appear over the tropical centraleastern Pacific. In D(−1)JF(0), the upper-level divergences are
primarily located over the WNP and south of the Indian
Ocean, and the convergences over the tropical central-
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Interannual variability of summer monsoon precipitation
eastern Pacific still exist. These patterns are closely associated
with La Niña forcing. In MAM(0), the WNP is controlled by
an upper-level divergent center, and though the convergence
persists over the tropical central-eastern Pacific, its statistical
significance does not pass the 90 % confidence level. Owing
to decaying La Niña conditions in JJA(0), the upper-level
convergence completely disappears in the central-eastern
Pacific, whereas the strong divergence appears along the monsoon trough. The diagnosis of upper-level velocity potential
and low-level anomalous circulation also reveals the rainy
season precipitation over the ICP is highly correlated to the
SSTA in the preceding winter, but it is not associated with the
SST forcing in JJA(0).
c) TC activities
The rainy season precipitation over the ICP concerned
with TC activities has been reported in previous studies.
Fig. 3 The difference of
composite SST (°C) (a) and
850 hPa wind field (m/s) (b)
between wet and dry anomalous
years (shaded areas are
statistically significant at the 90 %
confidence level)
The northwestward-heading TC tracks are a probable factor determining the summer monsoon precipitation over
the ICP (Chen and Yoon 2000; Takahashi and Yasunari
2008; Chen et al. 2012; Takahashi et al. 2015). Although
TCs can occur throughout the whole year, the May–
September (MJJAS) period reflects the most frequent TC
season over the ICP. The composites of outgoing
longwave radiation (OLR) anomalies and TC tracks for
the wettest years and driest years are shown in Fig. 5.
As revealed from the OLR and TC track distributions,
more TCs are observed along the monsoon trough in the
northwest quadrant of the WNP and east of the ICP in wet
years compared with fewer TCs in dry years. The cumulus
convection activity (inferred from OLR) is vigorous along
the monsoon trough in the Indian subcontinent, the head
of the Bay of Bengal, Indochina, and the WNP in wet
years. It should be mentioned that the northwestwardpropagating TCs (inferred from TC tracks) primarily land
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F. Ge et al.
Fig. 4 As in Fig. 3 but for 200 hPa velocity potential (contours; 10−6 m2/s) and divergence wind (vectors; m/s)
in the northeastern region of the ICP. This implies that the
precipitation would significantly increase in the northeastern part of the ICP.
It is well known that ENSO has a strong influence on the
generation of TCs over the Pacific Ocean. To confirm if and
how the TC activities can affect the rainy season precipitation
over the ICP, we employ regression analysis to remove the
influence of ENSO on TC formation, as shown in Fig. 6.
According to Clark et al. (2000) and Li et al. (2014), the
regression equation is
σð yÞ
;
yr ¼ y−cx
σð xÞ
where σ(x) and σ(y) are the standard deviation of x and y,
respectively; c is the correlation coefficient between the time
series x and y; and yr is the resulting value, from which the
impact of x has been removed. The standardized time series of
the total number of TC formations and TC formations with
ENSO removed are shown in Fig. 6. The standardized time
series of TC-original indicates the year-to-year variation of TC
Fig. 5 Composites of OLR
(W/m2) anomalies and TC tracks
in MJJAS during anomalously a
dry and b wet years (shaded areas
indicate OLR anomalies; black
lines are TC tracks)
formation over the WNP (Fig. 6a). After removing the ENSO
signal, although the variance of TC formation with ENSO
removed resembles TC-original, the interannual variation is
not significant.
To further elucidate the influence of TC changes on the
rainy season precipitation over the ICP, we examine the regression patterns of rainy season precipitation with respect to
TC-original and TC formation with ENSO removed (Fig. 7).
For the TC-original part, the precipitation reduces in the western ICP, especially in coastal areas. Due to the landing position
of TCs in the northeastern part of the ICP, significant positive
anomalies occur in this region (Fig. 7a). After removing the
ENSO signal, the precipitation decreases over most areas of
the ICP, especially in the southwestern and southeastern coastal regions. Only a minor positive anomaly appears in the
northeast (Fig. 7b). The results indicate that the impacts of
TCs’ activities are not homogeneous over the ICP. The rainy
season precipitation significantly increases in the northeastern
part of the ICP and reduces in the southwestern part, if ENSO
is considered. In contrast, the precipitation apparently decreases in the southwestern and southeastern part of the ICP
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Interannual variability of summer monsoon precipitation
Fig. 6 The standardized time
series of TC number over the
WNP in MJJAS: a original TC
formation number; b TC
formation number with ENSO
removed
after removing ENSO impacts. Thus, the linear regression
analysis reveals that ENSO-modulated TC activities could
partially influence rainy season precipitation over the ICP, as
well as ENSO-independent TC activities. We can further conclude that ENSO partially compensates for the negative TC
influence in the southeast of the ICP.
3 Conclusion and discussion
Based on CRU TS3.1 data, the interannual variability of the
summer monsoon rainfall over the ICP is investigated in this
study through EOF analysis. The first EOF of the total precipitation from May to September over the ICP describes
30.6 % of the spatiotemporal variability. The normalized
PC-1 is used to monitor the interannual variability of the
wetness and dryness in the ICP. Using correlation and composite methods, we have examined the relationships of the
preceding SSTA forcing from the Niño-3.4 region with the
following rainy season precipitation over the ICP. To further
determine whether ENSO events can influence the rainy
season precipitation over the ICP, we also investigated the
relationship between TC formation number and the interannual precipitation pattern over the ICP. Before removing the
impact of ENSO, the rainy season precipitation is predominantly enhanced in the northeastern part of the ICP.
However, the positive anomalies apparently weaken in the
northeastern part of the ICP and decreased precipitation appears in the southeastern part of the ICP if the ENSO signal
Fig. 7 Precipitation patterns over
the ICP for linear regression with
different indices: a the
standardized time series of TC
formation; b the standardized
time series of TC formation with
ENSO removed (shaded areas are
statistically significant at the 90 %
confidence level)
is removed. This analysis indicates that the interannual variation of TC modulation by ENSO is a significant contributing factor to the rainy season precipitation over some
areas of the ICP.
Nevertheless, it is noteworthy that one question remains open: Why does weakened precipitation appear in
the southeastern part of the ICP when the ENSO signal is
removed? After removing the impact of ENSO, it is acceptable that the difference of precipitation occurs in the
northeastern part of the ICP because of the ENSO-induced
TC activities. But the variation of precipitation in the
southeastern region is not synchronized with that in the
northeastern part. One hypothesis is that the
northwestward-propagating TCs transport cyclonic vorticity poleward, thereby reducing the ascending motion and
convection temporarily over the southern part of the
monsoon trough in the subsequent days, whereas the
northeastern part of the ICP is under the direct influence
of the heavy precipitation accompanying TCs. Therefore,
the rainy season precipitation would significant decrease
in the southeastern region of the ICP and slightly increase
over the northeastern regions being passed by TCs. The
study of Holland (1995) also provides certain evidence in
support of this notion. However, the physical mechanism
of the ENSO influence on the weakening precipitation in
the southeastern part of the ICP is still unclear. In addition, the variations and influences of ENSO-Modoki,
which have been investigated widely in recent years,
should also be considered (Larkin and Harrison 2005;
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F. Ge et al.
Ashok et al. 2007; Kao and Yu 2009; Kug et al. 2009; Yu
et al. 2011). These issues will be the subject of forthcoming research.
Acknowledgments We are grateful to the two anonymous reviewers
for their comments and suggestions, which helped to improve the paper.
We thank Frank Sielmann and Andrea Schneidereit for inspiring discussions on TC activity patterns. This study acknowledges the support of the
Jiangsu Planned Projects for Postdoctoral Research Funds (1402004B),
the National Basic Research B973^ Program of China (2012CB955200),
the National Natural Science Foundation of China (No. 41405036), a
Max Planck Institute for Meteorology (MPI-M) fellowship, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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