Pan-Asian monsoon and its definition, principal modes of precipitation, and variability features

SCIENCE CHINA Earth Sciences

© Science China Press and Springer-Verlag Berlin Heidelberg 2012 earth.scichina.com www.springerlink.com

*Corresponding author (email: [email protected]) †Equal contributor (email: [email protected])

• RESEARCH PAPER • May 2012 Vol.55 No.5: 787–795

doi: 10.1007/s11430-012-4382-7

Pan-Asian monsoon and its definition, principal modes of precipitation, and variability features

GAO Ya1,2,3* & WANG HuiJun1,3†

1 Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China;

3 Climate Change Research Center, Chinese Academy of Sciences, Beijing 100029, China

Received September 30, 2011; accepted February 23, 2012; published online April 12, 2012

Here we propose a new concept, the Pan-Asian monsoon, and use empirical orthogonal function (EOF) analysis and linear re-gression approach to define it and to analyze the monsoon-related rainfall variability. The Pan-Asian monsoon is referred to as the monsoon occurred over the great region (60°E–140°E, 10°S–35°N), consisting of the Indian monsoon, Southeast Asian monsoon, East Asian monsoon, and Western North Pacific monsoon. The Pan-Asian monsoon region is the principal region of the summer rainfall over the Asian-Pacific monsoon region and is also water vapor channel connecting several Asian-Pacific sub-monsoon systems. The first EOF mode of the Pan-Asian monsoon precipitation (PAMP_F) shows a meridional tripole pattern with more (less) rainfall zonal belt over the Bay of Bengal (BOB), the Indo-China Peninsula, South China, the South China Sea (SCS), Philippines and the Philippine Sea, and less (more) rainfall on both sides. The first rainfall mode is associat-ed with the weakened Somali cross-equatorial flows, enhanced southerly over the eastern coast of Australia, and strengthened westerly over the tropical Pacific. The first EOF rainfall mode shows a close relationship with the simultaneous El Niño- Southern Oscillation (ENSO) and Pacific South America (PSA). The preceding spring and simultaneous summer Antarctic Oscillation (AAO) in the western Hemisphere (AAO in Pacific) has a connection with the first summer rainfall mode of the Pan-Asian monsoon. Because the main influence factors are over the Pacific, the first rainfall mode is named as the Pacific mode. The second mode of the Pan-Asian monsoon precipitation (PAMP_S) shows a dipole pattern from northeast to south-west, which is associated with the weakened Somali cross-equatorial flows, enhanced easterlies over the Maritime Continent, and weak easterly over the tropical Pacific. The second rainfall model has a close relationship with the atmospheric convection activity and the sea surface temperature variability over the Maritime Continent and South Indian Ocean. Because the influ-ence factors are mainly over the eastern Hemisphere, the second rainfall mode of the Pan-Asian monsoon is named as the In-dian Ocean mode.

Pan-Asian monsoon precipitation, ENSO, teleconnections

Citation: Gao Y, Wang H J. Pan-Asian monsoon and its definition, principal modes of precipitation, and variability features. Sci China Earth Sci, 2012, 55: 787–795, doi: 10.1007/s11430-012-4382-7

Monsoon is referred to as the phenomenon with seasonal reversal of the prevailing wind direction and seasonal alter-nation of the wet climate and dry climate. Asian monsoon is an important component of global monsoon and is also one

of the dominant summer rainfall regions over the world. Consequently, it is highly relevant to investigate the rainfall spatial-temporal feature and its causes over the Asian mon-soon region. In the 1980s, Tao et al. [1] proposed that Asian summer monsoon included two subsystems: the Indian monsoon and East Asian monsoon. Recently, Murakami et al. [2] addressed that there is an independent monsoon over

788 Gao Y, et al. Sci China Earth Sci May (2012) Vol.55 No.5

the western North Pacific. Later, Wang [3] pointed out that the Asian-Pacific can be divided into three sub-monsoon regions based on the rainfall characteristics, which are the Indian monsoon, East Asian monsoon, and western North Pacific monsoon. The Indian monsoon and western North Pacific monsoon are the tropical monsoon, whereas the East Asian monsoon is the subtropical monsoon. These three sub-monsoons are independent of each other and, at the same time, interact with each other. The previous re-searches on the Asian-Pacific monsoon have been well re-viewed by He [4]. Due to the limitation of observation data, early studies and predictions of the Asian monsoon variabil-ity focus on the each monsoon’s local features. Webster and Yang [5] defined an index using the zonal wind shear be-tween 850 and 200 hPa over the South Asian region to de-scribe the South Asian monsoon circulation and its variabil-ity; Goswami et al. [6] defined the Indian monsoon index and the Hadley circulation index based on the meridional wind shear between 850 and 200 hPa. Wang and Fan [7] presented an zonal wind shear index and an outgoing longwave radiation (OLR) index computed over the Indian monsoon region and the Southeast Asian monsoon region to describe the two submonsoon systems separately. They found two main convection centers forcing the East Asian summer monsoon, which are located in the Bay of Ben-gal-India-Arabian Sea and the South China Sea (SCS)- Philippine Sea. Based on the low-level wind, Wang [8] de-fined an East Asian monsoon index and explored the rela-tionship between the East Asian monsoon and ENSO, re-vealing the instability of their relation. Huang et al. [9] found that the intraseasonal variability of the East Asian summer monsoon rainfall has a close relationship with the heating condition of the western Pacific warm pool. The warmer western Pacific warm pool corresponds to more rainfall over the Yangtze River valley in summer, and vice versa. Sun et al. [10] revealed the Arabian Peninsula-North Pacific Oscillation (APNPO) and found that the APNPO has a profound impact on the East Asian and South Asian sum-mer monsoon, reflecting these two Asian submonsoons’ general features.

During the prevailing period of the Asian summer mon-soon, the maximum rainfall centers are located over the Indochina peninsula, Bay of Bengal (BOB), northeastern Indian Ocean, and southeastern Maritime Continent (figure not shown). The anomalous monsoon rainfall can result in considerable disaster, such as droughts or floods, over the region. A number of researches have studied the persistence and variability of these Asian submonsoon systems Howev-er, the submonsoon systems over the Asian-Pacific region have close linkages, involving the ocean-atmospheric inter-action, water vapor channels, etc. It is necessary to study the monsoon on a greater scale and better understand the varia-bility of the Asian-Pacific monsoon. Therefore, here we propose a new concept, the Pan-Asian monsoon, which dominates South Asia, southern East Asia, Southeast Asia,

the Indian Ocean, and western North Pacific Ocean (60°E–140°E, 10°S–35°N). By investigating the rainfall variability and corresponding atmospheric and oceanic con-ditions, we want to present the major features of the Pan-Asian monsoon.

1 Datasets and method

The primary data set used in this study is from the NCEP-NCAR (National Centers for Environmental Predic-tion-National Center for Atmospheric Research) reanalysis from 1979 to 2009 [11]. The monthly NCEP-NCAR data set has a horizontal resolution of 2.5° latitude by 2.5° longitude and 12 pressure levels vertically. This study focuses on summer from May to September. The preceding spring means from March to May, and the preceding winter indi-cates from December of the previous year to February of this year. The precipitation is derived from the Global Pre-cipitation Climatology Project Version 2.1 monthly precipi-tation analysis (GPCP) [12], which has a horizontal resolu-tion of 2.5° latitude by 2.5° longitude. The sea surface tem-perature (SST) is provided by NOAA (NOAA Extended Reconstructed Sea Surface Temperature V3b) [13, 14]. The SST data set has a horizontal resolution of 2° latitude by 2° longitude. The OLR is also provided by NOAA (NOAA Interpolate Outgoing Longwave Radiation) [15], which has a horizontal resolution of 2.5° latitude by 2.5° longitude. All above monthly data sets are first calculated to the seasonal mean, and then they are detrended and normalized before the analysis.

2 Pan-Asian monsoon and its dominant mode of summer precipitation

Given the prevailing atmospheric circulation and water va-por transportation characteristics, the region of 60°E–140°E, 10°S–35°N is defined as the Pan-Asian monsoon region, which encompasses several submonsoons over the Asian- Indian Ocean-Pacific region. Figure 1 shows that temporal evolution of the monthly mean precipitation, OLR and zon-al wind at 850 hPa averaged over the Pan-Asian monsoon region from 1979 to 2008. All these three time series pre-sent a significant seasonal variation. The Pan-Asian mon-soon precipitation (PAMP) mainly occurs over the summer from May to September. According to the monsoon defini-tion with the seasonal reversal of the prevailing wind direc-tion and seasonal alternation of the wet climate and dry cli-mate, the Pan Asian monsoon (PAM) is proposed in this study. Because of the particularity of PAM region, it is es-sential to study the variation characteristics of Pan Asian monsoon summer precipitation.

To investigate the dominant mode of the PAMP, the EOF analysis is applied to the GPCP dataset. Figure 2 shows the

Gao Y, et al. Sci China Earth Sci May (2012) Vol.55 No.5 789

Figure 1 Temporal evolution of the monthly mean precipitation, OLR and 850 hPa zonal wind averaged over the Pan-Asian monsoon region from 1979 to 2008.

Figure 2 The first EOF mode of the Pan-Asian Monsoon summer precipitation and its corresponding principal component over the period of 1979–2009. The first mode explained 15.92% of total variance; the red solid lines represent negative rainfall centers and the blue solid line represents positive rainfall centers.


first EOF mode of the PAMP (PAMP_F) in summer, which explains 15.92% of the total variance. The PAMP_F fea-tures a tripole pattern from north to south, which indicates that when the rainfall over the Tibetan Plateau and the Yangtze River and Huaihe River valleys decreases, the rainfall increases over the BOB, Indo-China Peninsula, South China Sea (SCS), Philippines, and the Philippine Sea and decreases over southern India, the eastern tropical In-dian Ocean and Maritime Continent. These results are con-sistent with that of Zhou et al. [16]. They displayed that after the onset of summer monsoon, abundant water vapor is transported to the Asian monsoon region from the Southern Hemisphere, resulting in principal water vapor convergence over the BOB, Indo-China Peninsula, and SCS region (fig-ure not shown). The rainfall over the Yangtze River valley has a negative correlation with that in the western North Pacific (10°N–20°N) and a positive correlation with that in southwestern India, similar to the result of Wang et al. [17], Liu et al. [18], and Ding et al. [19]. Over the region to the west of 80°E, the rainfall displays a reverse distribution related to the region to the east of 80°E. When the rainfall is less over the Arabian Sea and Indian Peninsula, it is more

over the Iran Plateau and the western tropical Indian Ocean. In addition, it is worth to point out that the principal com-ponent (PC) of the PAMP_F experienced a significant de-cadal change around the beginning of the 1990s before it is de-trended.

The second EOF mode, the PAMP (PAMP_S) (Figure 3), accounting for 9.11% of the total variance, shows that when the rainfall is enhanced over the Arabian Sea, India Penin-sula, BOB, Indo-China Peninsula, southern China, and Mari- time Continent in Southeast Asia, it is suppressed over the Tibetan Plateau, SCS and eastern tropical Indian Ocean. Besides, the PAMP_S has a decadal change at the beginning of the 21st century before it is de-trended.

3 Variation characteristics of the Pan-Asian summer monsoon precipitation (PAMP) domi-nant modes

3.1 Variation characteristic of the PAMP_F

As the interannual variability is focused in this study, all the following analysis is based on the de-trended data. Fu et al.

Figure 3 The second mode of the Pan-Asian Monsoon summer precipitation and its corresponding principal component over the period of 1979–2009. The second mode explained 9.11% of total variance; the red solid lines represent negative rainfall centers and the blue solid line represents positive rainfall cen-ters.


[20], Huang et al. [21], and Chen [22] pointed out that the EASM precipitation anomaly is related to the ENSO cycle. On the developing phase of ENSO, floods usually occurred in the Yangtze River and Huaihe River region and droughts in northern China; on the decaying phase of ENSO, the sit-uation is reversed. Figure 4 shows regression pattern of the SST against the PAMP_F PC. The SST anomalies are the typical ENSO pattern, with warmer SST over the tropical eastern Pacific SST and colder SST over the tropical west-ern Pacific. The simultaneous correlation of PAMP_F PC with Niño 3.4 index during 31 years reaches 0.74, exceed-ing the 99% significance level (Figure 5). On 850 hPa wind field, as shown in Figure 6, the anomalous circulation ex-hibits a negative phase of Walker cell. Anomalous westerly exists over the tropical western Pacific and anomalous anti-cyclone is over the eastern equatorial Indian Ocean and Mari- time Continent, which is corresponding to divergent anomaly at the lower level and convergent anomaly at the upper level over the Walker cell subsidence region. Such changes of cir-culations weaken the convection activity and decrease sum-mer rainfall over the region. Meanwhile, the water vapor flow from the eastern Indian Ocean converges with the northwest-erly over the Indian Peninsula and northeasterly from the East Asia coastal area, favoring more rainfall over the region from the BOB to Indo-China Peninsula. In addition, the southerly along the coastal region of eastern Australia converges with the northerly in the SCS-Philippines Sea, favoring more rainfall over the SCS-Philippines Sea. Because of the con-vergence circulation, the region (east of 80°E, 10°N–25°N) from the BOB, Indo-China Peninsula, southern China, SCS, Philippines and the Philippine Sea has more precipitation, while the region (south of 10°N) from the equatorial Indian

Ocean to the Maritime Continent has less precipitation, re-sulting in the leading mode of the Pan-Asian monsoon rain-fall. Owing to the close linkage between interannual varia-tion of PAMP_F and ENSO cycle, the PAMP_F pattern can be also named as “Pacific mode”.

Furthermore, the PAMP_F_related atmospheric circula-tions are analyzed. The SLP anomalies display a zonal asymmetry structure of the Antarctic Oscillation (AAO). This result is consistent with that of Fan [23] and Wang et al. [24]. They revealed that the AAO has a strong zonal asym-metry feature. Xue et al. [25] found that in a strong AAO spring, the Mascarene and Australia highs are strengthened, and the rainfall will increase over the region from the Yangtze River valley to Japan in summer. Yang et al. [26] indicated that the Southern Hemisphere subtropical high can induce the change in the western Pacific subtropical high over the Northern Hemisphere. Fan and Wang [27–30] suggested that the AAO is associated with East Asian at-mospheric circulations. Sun et al. [31] showed that the bo-real spring AAO has impact on the summer rainfall over the Yangtze River valley through changing the convection ac-tivity over the Maritime Continent. In addition, Sun et al. [32] and Sun [33] further revealed that the boreal spring AAO can also influence the West Africa and North America summer monsoons. Fan [34] indicated that there is a rela-tionship between the Atlantic tropical hurricane frequency and the Antarctic Oscillation in the western Hemisphere. Based on the study of Fan [23], the western Hemispheric AAO (AAOWH) index is defined as the normalized SLP difference between 40°S and 60°S averaged in 180°–0°W. The correlation coefficients of simultaneous summer and previous spring AAOWH indices with the PAMP_F PC are

Figure 4 Simultaneous regression pattern of de-trended SST field with the PAMP_F PC. The heavy shading and light shading denote regions with correla-tions positively and negatively significant at the 95% confidence level, respectively.


Figure 5 Normalized PAMP_F PC and Niño 3.4 SST index (averaged over the region of 4°S–4°N, 170°–120°W).

Figure 6 Simultaneous regression pattern of de-trended 850 hPa wind field with the PAMP_F PC. The shading denotes regions significant at the 95% confidence level.

Figure 7 Simultaneous regression pattern of de-trended SLP field with the PAMP_F PC. The heavy shading and light shading denote regions with correlations positively and negatively significant at the 95% confidence level, respectively.

0.65 and 0.46, respectively, which are both significant at the 99% confidence level.

Actually, the SLP anomalies are stronger over the south-ern Pacific (Figure 7). If the EOF analysis is applied to the SLPs over the Southern Hemisphere (south of 20°S, 0°– 360°), the second mode is highly similar to the SLP distri-bution in Figure 7. The correlation coefficient between the simultaneous SLP PC2 and PAMP_F PC is 0.78 and the correlation coefficient of the preceding spring SLP PC2 (April to June) with the PAMP_F PC reaches 0.63. These results indicate that the AAOWH in the preceding spring and simultaneous summer has a significant correlation with the PAMP_F in summer, which is similar to the ENSO. Besides, the ENSO is also closely related to the AAOWH variability [35].

After analyzing the middle and high levels pressure structures, Mo [36], Ghil [37], and David [38] found the Pacific South America pattern (PSA), which has strong in-traseasonal and interannual variations, and the PSA is the response to the central Pacific heating, showing a close re-lationship with the Southern Oscillation. The zonal asym-metry AAO is largely affected by the Southern Oscillation. The second EOF mode of the normalized 500 hPa height is the PSA pattern and the corresponding PC is used as the PSA index. We calculate the correlation coefficients be-tween the PSA index with the PAMP_F PC and the Niño 3.4 index. The correlation coefficients are 0.62 and 0.65, respectively, which indicates that the Pan-Asian summer monsoon precipitation is closely associated with the PSA and ENSO.

3.2 Variation characteristic of the PAMP_S

In this section, the second mode of Pan-Asian summer monsoon precipitation (PAMP_S) is briefly analyzed. As described in the last section, the PAMP_F is closely related to the ENSO, named as the Pacific mode. Because the ENSO is the strongest interannual signal in global climate, the PAMP_S therefore explains smaller parts of total vari-ance than PAMP_F, with a value of 9.11%.

The SST anomalies associated with the PAMP_S show two significant regions to the northwest and southwest of Australia (Figure 8(a)). Both of the SST indices over these two regions are significantly correlated with the PAMP_S PC at the 99% confidence level. Figure 8 shows the convec-tion activity associated with PAMP_S. There are negative OLR anomalies over the tropical western Indian Ocean and north of this region. The negative convection activity favors more rainfall over the region. The OLR anomalies are posi-tive over the eastern equatorial Indian Ocean and Australia. The weakened convection results in more rainfall over the region. And the OLR anomalies are negative over both south and north side of Australia, which brings more rainfall to the region. Thus, the EOF analysis is applied to the de-trended and normalized OLR over the key region (50°S–35°N, 40°–160°E), and three leading modes are ob-tained, which account for 13.86%, 10.78%, and 10.46% of


Figure 8 Simultaneous regression pattern of de-trended SST field (a), OLR field (b), SLP field (c), 850 hPa wind field (d) with the PAMP_S PC. The heavy shading and light shading denote regions with correlations positively and negatively significant at the 95% confidence level, respectively.

total variance, respectively. We find that the spatial distri-bution of the third EOF mode resembles Figure 8(b). The simultaneous correlation coefficient of the PC of the third OLR EOF mode with the PAMP_S PC is 0.82. Therefore, the simultaneous atmospheric convection over the Indian Ocean, Australia and western Pacific Ocean can reflect the PAMP_S structure during summertime. The SST anomalies to the northwest of Australia and the see-saw pattern of the OLR anomalies between the tropical eastern and western Indian Ocean (associated with the Indian Ocean dipole mode [39, 40]) can result in a meridional anomalous pattern to the east of Australia. Under the positive phase of the pat-tern, two anomalous lows are over both the north and south sides of Australia, and an anomalous high over the Australia region (Figure 8(c)). This anomalous circulation pattern is related to the AAO pattern over the southern Indian Ocean. So we compute the de-trended and normalized SLP differ-

ence between 35°S and 70°S averaged in 0°–120°E and name it as the Indian Ocean AAO index. The simultaneous correlation coefficient between the Indian Ocean AAO in-dex and PAMP_S PC is 0.36, significant at the 95% confi-dence level. This result indicates that the mass exchange between the middle and high latitude over the South Indian Ocean has a linkage with the PAMP_S. In the positive phase of PAMP_S, the lower level cross-equatorial flows (Figure 8(d)) are strengthening over the Indian Ocean region, and anomalous southeasterly appears over the tropical eastern Indian Ocean and northern Australia. The southeasterly shifts to the southwesterly over the Arabian Sea-India Pen-insula-BOB, making a convergence with the easterly from the equatorial Pacific over the Indo-China Peninsula and then resulting in increased rainfall extending from the India Peninsula to the Maritime Continent and decreased rainfall over southwestern China and the southern tropical Indian


Ocean. In short, the PAMP_S is strongly linked to the at-mospheric and oceanic anomalies over the Indian Ocean region and weakly linked to those associated to the ENSO over the Pacific. Hence, the PAMP_S can be named as the Indian Ocean mode.

Then, how about the relationship of the PAMP_F and PAMP_S with summer precipitation over China? The PAMP_F is mainly linked to the precipitation over southern China and the middle and lower reaches of Yangtze River Valley and Huaihe River region. In other words, we should focus more on the anomalous precipitation over the Yangtze River Valley and Huaihe River region if the PAMP_F is considered. However, the PAMP_S is related to the precip-itation anomaly over southwestern China.

4 Conclusions and discussion

The rainfall and circulation anomalies over the great region, including several Asian monsoon subsystems over the western Pacific Ocean, tropical Indian Ocean and low lati-tude Asia, have a remarkable spatiotemporal structure and strong variability. Thus, after the principal variability mode of the summer rainfall over the region is analyzed, the Pan-Asian monsoon is proposed in this study, and further the principal rainfall mode related atmospheric and oceanic features are explored.

The PAMP_F shows a “+, , +”anomalies from north to south, which means the precipitation over the BOB-Indo- China Peninsula-south China-SCS-Philippines-Philippine Sea is opposite to the precipitation on both sides. Its corre-sponding circulations indicate that the lower level Somali cross-equatorial flows are weakened, and the southerly over the east coast of Australia is strengthened, as strong west-erly anomaly appears over the tropical Pacific. The PAMP_S shows a “+, , +” anomalies from northeast to southwest during the boreal summer, which is associated with the weakened Somali jet, enhanced easterly over the Maritime Continent, and weak easterly over tropical Pacific.

The PAMP_F is primarily modulated by the ENSO and major atmospheric mode over the south Pacific (AAOWH and PSA), which is the typical Pacific mode. The PAMP_S is mainly modulated by the SST anomalies over the Indian Ocean, the atmospheric principal pattern over the South Indian Ocean (Indian Ocean AAO), and the convection ac-tivity over tropical Indian Ocean, which is the obvious In-dian Ocean mode. The variance of PAMP_S is smaller than that of the PAMP_F, that is, the variance of Indian Ocean mode is smaller than that of the Pacific mode. The atmos-pheric circulations, thermal conditions of the SST, and the convection activities over the Maritime Continent are crucial to both the PAMP_F and PAMP_S. The key summer rainfall regions in China associated with the PAMP_F and PAMP_S during summer are over the Yangtze River valley-Huaihe River region and southwestern China, respectively.

This work was supported by National Basic Research Program of China (Grant No. 2009CB421406), National Natural Science Foundation of China (Grant No. 40905041), Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. KZCX2-YW-QN202), and National Key Scientific Research Project of Global Changes Research of China (Grant No. 2010CB950304).

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Pan-Asian monsoon and its definition, principal modes of precipitation, and variability features