GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L03803, doi:10.1029/2011GL050540, 2012

Advanced Asian summer monsoon onset in recent decades
Yoshiyuki Kajikawa,1 Tetsuzo Yasunari,1 Shuhei Yoshida,2 and Hatsuki Fujinami1
Received 8 December 2011; revised 7 January 2012; accepted 10 January 2012; published 9 February 2012.

[1] In this paper, we first elucidate the significant seasonality in long-term trends in the Asian monsoon on a monthly
mean basis. Advanced monsoon onsets over the Bay of
Bengal and the western Pacific were evident in recent decades. Increasing rainfall in May along 10°N reflected theadvanced monsoon onset. Decreasing rainfall trends in Junealong 10°N were also detected. Because the rainfall trends
in July and August showed less significance, the monsoon
transition phase should be discussed in the context of climatechange rather than boreal summer mean field. The advanced
monsoon onset and weakening of the monsoon during earlysummer are most likely to be attributed to the heat contrast
between the Asian landmass and the tropical Indian Ocean.
The heating trend over the Asian landmass primarily contributes to the heat contrast variability with the persistent
SST increase in the Indian Ocean throughout the season.
Citation: Kajikawa, Y., T. Yasunari, S. Yoshida, and H. Fujinami(2012), Advanced Asian summer monsoon onset in recent decades,
Geophys. Res. Lett., 39, L03803, doi:10.1029/2011GL050540.

1. Introduction[2] The long-term variability of the Asian monsoon underincreases in greenhouse gases and anthropogenic aerosols
has been a vital issue in recent decades because of theregion’s high population. Many studies have revealed longterm change in the Asian summer monsoon rainfall, especially over China [Ding et al., 2008; Yu and Zhou, 2007;
Zhao et al., 2010]. It has been suggested that the trends inmonsoon rainfall in China and India were due to increases inblack carbon and sulphate aerosol [Menon, 2002; Ramanathan
et al., 2001]. Most previous studies have focused and assessedon the boreal summer mean field (e.g., averaging from June to
September). However, the Asian monsoon climate is characterized by strong seasonal variations with dry and rainyseasons. The seasonal march of the Asian summer monsoon
displays a stepwise northward and northeastward migrationof rainfall, with abrupt onset during boreal spring and summer [Tao and Chen, 1987; Wu and Wang, 2001]. Becauseof the large seasonal variability, the long-term trend of the
Asian monsoon should exhibit seasonally dependent featuresthat must be taken into consideration. Here, we analyze thetrends of the Asian monsoon rainfall and water vapor flux
during 1979–2008 on a monthly mean basis to clarify theseasonality of the Asian monsoon trend. We focus on thetransition phase from boreal spring to summer and elucidate
the nature of the advanced monsoon onset in recent decades.

1
Hydrospheric Atmospheric Research Center, Nagoya University,
Nagoya, Japan.
2
Hitachi Solutions, Ltd., Tokyo, Japan.

Copyright 2012 by the American Geophysical Union.
0094-8276/12/2011GL050540[3] We mainly used the Climate Prediction Center (CPC)
Merged Analysis of Precipitation (CMAP) [Xie and Arkin,1997], wind circulation, temperature and humidity datafrom the National Centers for Environmental Prediction(NCEP) DOE reanalysis II data [Kanamitsu et al., 2002], andthe Hadley Center sea surface temperature (SST) dataset[Rayner et al., 2003] for the period 1979–2008, chosen forsatellite data availability. The non-parametric Mann-Kendall
test was applied for assessment of the trends.

2. Seasonality of Asian Monsoon Long-Term
Trend[4] We calculated the long-term trends of the Asian summer monsoon rainfall and water vapor flux during 1979–
2008 on a monthly mean basis. Significant rainfall trends
over the Asian monsoon area occur in May and June. In May,an increasing rainfall trend over the Arabian Sea, Bay of
Bengal, South China Sea, and western Pacific was detected(Figure 1a). A 4–6 mm/day increase in the past 30 years
accounts for about 50% of the climatology. A decreasing
rainfall trend over the southern Indian Ocean was also found.
In contrast, rainfall over the areas where an increasing trend
was seen in May had a significant decreasing trend in June
(Figure 1b). The increasing rainfall trend over southern
China was also remarkable in June. These rainfall trends
were consistent with the monsoon circulation and water
vapor flux trends in the lower troposphere, especially westerly wind anomalies accelerated along the 10°N line in May
and strong anti-cyclonic circulation anomaly over the western Pacific in June. Interestingly, rainfall and water vapor
flux trends in May were similar to seasonal evolution from
May to June (Figure 1c). This means that the rainfall andcirculation pattern in May during recent decades is becoming
closer to that in June during previous decades, suggestingthat the Asian summer monsoon has started earlier inrecent decades. On the other hand, the rainfall trends in Junewere quite similar to that in boreal summer mean (JuneSeptember; JJAS), but the amplitude was more than twice(Figure 1d). This implies the large contribution of rainfall
long-term variability in June to that in boreal summer (JJAS)
mean. Incidentally the rainfall trends in seasonal mean
between May and September showed less significance alongthe 10–15°N (figure not shown). Above rainfall trends arealso robust in other rainfall data sets (Figure S1 in theauxiliary material).1[5] Figure 2 shows a Hovmöller diagram of rainfall trends
along 10–15°N, climatologically peak area of monsoonal
rainfall, superimposed with the climatology. Increasing
rainfall trends in May and decreasing rainfall trends in June
are prominent. However, the Asian monsoonal rainfall in
1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL050540.

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L03803Figure 1. (a) Linear trends of rainfall (CMAP: Shading: mm/day) and vertical integrated water vapor flux (NCEP DOE
reanalysis II: vector: kg m 1 s 1) in May. All values were multiplied by 30 (years) for the period 1979–2008. Only those thatare significant at the 95% level following the Mann-Kendall test are plotted. (b) Same as Figure 1a but for June. (c) Climatological difference in rainfall and vertical integrated water vapor flux between June and May. (d) Linear trend of rainfall in
boreal summer (June-September) mean. Contour denotes climatological summer mean rainfall (mm/day).

Figure 2. (left) Latitude-time section of trend (shading: mm/day) and climatology (contour: mm/day) of CMAP rainfall
along 10–15°N. Shadings are the same as in Figure 1. (right) Time series of trend (shading: top axis) and climatology(line: bottom axis) of rainfall averaged over the area (10–15°N, 60–140°E).
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L03803Figure 3. Date of monsoon onset by climatological pentad mean rainfall during 1979–1993 (Digit). The onset pentad wasdetermined by the method of Wang and LinHo [2002]; the Julian pentad in which the relative rainfall rate, minus the January
mean, exceeds 5 mm/day is defined as the onset pentad. The shading denotes the difference in the monsoon onset between
1994–2008 and 1979–1993 and is shaded for differences more and less than 2 pentads.
July and August does not have a significant trend, although asmall decreasing trend in August is visible. This correspondswith the rainfall trend in boreal summer mean (Figure 1d).
Another increasing trend over the Arabian Sea in October isalso significant. Thus, the Asian monsoon rainfall had amainly significant trend during the transient phase, especiallyfrom boreal spring to summer.
[6] We examined the monsoon onset variability based on
the climatological pentad mean rainfall data. We divided thewhole analysis period into two equal periods: 1979–1993 and
1994–2008. The definition of the monsoon onset date was
the first Julian pentad in which monsoonal rainfall exceeds
5 mm/day following the method of Wang and LinHo [2002]
The monsoon onset dates over the Bay of Bengal, Indochina

Peninsula, and western Pacific around 120–140°E haveshifted to approximately 10–15 days earlier in recent decades(Figure 3). Conversely, the monsoon onset over the westernmiddle Pacific along 160°E has become later by 10–15 days.
These advanced onsets of the Southeast Asian summer monsoon (SEAM) and the western North Pacific summer monsoon (WNPM) during recent decades reflect the increasing
rainfall trends in May.

3. Possible Factors of the Advanced Monsoon
Onset[7] To analyze possible factors causing the advanced monsoon onset in recent decades, we examined the dynamical

Figure 4. (a) Longitude-time section of the trend (shading: m) and climatology (contour: m) of the difference in atmosphericthickness (200–500 hPa) between land (30°N) and ocean (5°N) areas. Trends are multiplied by 30 (years) for the period 1979–
2008. Only those that are significant at 95% level following the Mann-Kendall test are plotted. (b) Same as Figure 4a but for
SST along 5°N. (c) Same as Figure 4a but for along 30°N.
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heating contrast between land and ocean. The atmosphericthickness between 200 and 500 hPa is an effective index
of the meridional thermal contrast in the monsoon region[Li and Yanai, 1996; Ueda and Yasunari, 1998]. Figure 4ashows the trend of the thermal contrast between land
(30°N) and ocean (5°N). A significant warming trend wasfound mainly in May over the broad scale of the Asianmonsoon area, especially along the Tibetan Plateau. Taking
into consideration the zero line of this index in the climatology, these warming trends shifted the timing of the heat
contrast overturning between land and ocean advance.
Hence, the large-scale monsoon likely had an advanced
onset. In contrast, after the climatological heat contrast
changed sign in June, the trend of heat contrasts turned tocooling in the Asian monsoon region, especially over China.
A cooling trend in the upper troposphere over China and thetendency toward increased flood in Yangtze River valley(107°E 120°E, 27°N 32°N) have been reported [Yu et al.,2004] which is consistent with our result in Figure 1d.
[8] Research has also suggested that the trend and longterm variability of the Asian monsoon are closely related to
the warming SST trend of the El Niño–Southern Oscillation
mode over the tropical Pacific and Indian Ocean [Yang andLau, 2004]. However, these SST warming trends lastthroughout the year, with little significant seasonal variation(Figure 4b). The heating trends over land (along 30°N) had
striking seasonal differences. Warming trends dominatedalong 40°E and 80°E in May and cooling trends dominatedalong 110–120°E during boreal summer (Figure 4c). Therefore, we concluded that heating trends over the land area
mainly contributed to the trend in the thermal contrast
between land and ocean, which reflects continental largescale monsoon onset variability.

4. Concluding Remarks and Discussions[9] We examined the long-term trend of the Asian summer
monsoon during 1979–2008, with a focus on its strong seasonality. The increasing rainfall trend in May was remarkable
over the Arabian Sea, Bay of Bengal, and southeastern
monsoon region, which have experienced advanced monsoon onset in recent decades. The trends were, however,nearly reversed in June over the abovementioned region. Of
interest is that the Asian monsoonal rainfall in July andAugust did not show a clear significant trend. The Asianmonsoon had a significant trend during the transient phasefrom boreal spring to summer, in particular. Therefore, thelong-term trend of boreal summer mean rainfall significantly
differed according to whether it involved May. These longterm trends were also detected in common by using threedifferent rainfall datasets (Figure S1).
[10] The advanced onset and weakening of the SEAM and
WNPM during early summer were most likely due to the heat
contrast between the Asian landmass and tropical Indian
Ocean. The heating trend over the Asian landmass primarily
contributed to the heat contrast variability with the persistent
SST increase in the Indian Ocean throughout the season. One
plausible factor for the warming trend would be dust aerosol
loading along the Himalaya-Tibetan Plateau. The accumulation of desert dust and soot aerosols over northern India andthe foothills of the Himalayas can enhance the heating in
the mid-upper troposphere by the aerosol direct effect (solar
absorption) and strengthen the meridional thermal contrast

L03803[Lau and Kim, 2006; Lau et al., 2006]. The warming temperature trend in the troposphere during recent decades wasobserved in high-resolution satellite data and was interestingly prominent in the pre-monsoon season with peak in May[Gautam et al., 2009]. This aerosol loading drops with thearrival of monsoon rainfall due to the washout.
[11] We speculate that the thermal contrast induced by dust
and aerosols mainly have caused the recent change in the
SEAM onset, however, the WNPM and the East Asianmonsoon trends could have other possible factors. For
instance, the enhancements of the intraseasonal variability
and tropical cyclone activity, which are attributed to SST
warming over the western Pacific, could be also a stronger
trigger of the South China Sea summer monsoon onset[Kajikawa and Wang, 2012]. The East Asian monsoon
variability could be affected by the thermal condition nearsurface during late spring [Zhang and Zuo, 2011] andanomalous water vapor transport by the easterly associatedwith the northwestern Pacific subtropical high [Simmonds
et al., 1999]. The decreasing rainfall trend in June could
be due to the slowing down of northward propagation of thefirst ISV [Goswami et al., 2010]. Further investigations areneeded (1) to elucidate the possible mechanism that produces
seasonality in the long-term variation in each monsoon systems and (2) to address whether the remarkable SST increase
was due to anthropogenic or natural forcing.
[12] We first noted the significant seasonality in the longterm rainfall trend: the Asian monsoon onset has advanced inrecent decades. Our results strongly suggest that the responses of the Asian monsoon to natural and/or anthropogenic
forcing may differ largely from season to season. Hence,
we recommend that attention be given to the seasonality andseasonal transition phase in future climate change research,
including the assessment of state-of-the-art climate model
simulations. The use of both seasonal mean and monthly
mean data is also recommended for a better understanding
and assessment of the seasonal transition phase, which isimportant for clarifying the impact on human society.
[13] Acknowledgments. The authors are grateful to the editor ofGeophysical Research Letters and anonymous reviewers for their usefuland critical comments. They also thank to Hironari Kanamori for observational data handling. This work is supported by the Global Environment
Research Fund of the Ministry of the Environment, Japan (A0902).
[14] The Editor thanks the two anonymous reviewers for their assistancein evaluating this paper.

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Y. Kajikawa, T. Yasunari, and H. Fujinami, Hydrospheric AtmosphericResearch Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya,
Aichi 464-8601, Japan. (ykaji@hyarc.nagoya-u.ac.jp)
S. Yoshida, Hitachi Solutions, Ltd., 4-12-7 Higashishinagawa,Shinagawa-ku, Tokyo 140-0002, Japan.

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