On the onset of the Asian summer monsoon
Ding Yihui
National Climate Center,
China Meteorological Administration
E-mail: dingyh@cma.gov.cn

Extended Abstract
The onset of the Asian summer monsoon is a key indicator characterzing theabrupt transition from the dry season to the rainy season and subsequent seasonal
march. Numerous investigators have studied this problem from the regional
perspectives. It is to some extent difficult to obtain a unified and consistent picture of
the climatological onset dates of the Asian summer monsoon in different regions due
to differences in data, monsoon indices and definitions of monsoon onset used inthese investigations. Recently, Wang and Lin (2002) have identified two phases in theevolution process of the Asian summer monsoon. The first phase or the onset phasebegins with the rainfall surges over the South China Sea in mid-May, which
establishes a planetary-scale monsoon rainband extending from the South Asian
marginal seas (the Arabian Sea, the Bay of Bengal, and the SCS) to the subtropicalwestern North Pacific (WNP). The second phase of the Asian monsoon onset ischaracterized by the synchronized initiation of the Indian rainy season and theMeyu/Baiu in early to mid-June. The peak rainy seasons tend to occur primarily inthree stepwise phases, in late June over the Meiyu/Baiu regions, the northern Bay ofBengal and the vicinity of the Philippines; in late July over India and northern China;
and in mid-August over the tropical WNP (10°-22°N, 120°-160°E). The rainy season
retreats southward in East Asia during late August and early September while theIndian summer monsoon and the WNPSM withdraws southward after mid-September.
Ding (2004) has summarised the climatological dates of the onset of the Asiansummer monsoon in different tropical regions based on various sources, with dividing
the whole onset process into four stages: (1) Stage 1 (late in April or early in May):
the earliest onset is often observed in the central Indochina Peninsula late in April andearly in May, but in some cases, the onset may first begin in the southern part or thewestern part of the Indochina Peninsula. (2) Stage 2 (from mid to late May): this stage
is characterized by the areal extending of the summer monsoon, advancing northward
up to the Bay of Bengal and eastward down to the SCS. (3) Stage 3 (from the first
dekad to second dekad of June): this stage is well known for the onset of the Indiansummer monsoon and the arrival of the East Asian rainy season such as the Meiyu
over the Yangtze River Basin and the Baiu season in Japan. (4) Stage 4 (the first orsecond dekad of July): the summer monsoon at this stage can advance up to North
China, the Korean Peninsula (so-called Changma rainy season) and even North Japan.
During the first pentad of May, the summer monsoon is established only overSumatra. In the next two pentads, the tropical monsoon advances up to the land bridge,
first establishing itself over the southwestern Indochina Peninsula and then expanding
to the entire southern peninsula. During the pentad of May 16-20, the build-up of thesummer monsoon is observed over the central Indochina Peninsula. At the same time,
the onset location extends into the central and southern SCS, accompanied by arainfall rate of ＞5 mm day-1 over the entire SCS. In the next pentad, onset expands1quickly and almost covers the entire SCS. On the other hand, the Asian summer
monsoon also advances northwestward to the Indian monsoon region from thenear-equatorial East Indian Ocean and the Indochina Peninsula starting from mid-May.
Earliest onset of the Asian summer monsoon in this region may be observed over thesouthern tip of the Indian subcontinent. In early June, the Asian summer monsoon
rapidly advances northwestward, arriving in the central Indian subcontinent.
Meanwhile, the onset over the Arabian Sea and the western coast of the Indian
subcontinents is observed, due mainly to the enhancement of the cross-equatorial
airflow off the Somali coast and the development of the onset vortex in the central andnorthern Arabian Sea (Krishnmaurti et al., 1981; Ding, 1981). This date is generally
believed to be normal onset dates for the Indian summer monsoon. So, the onset of theEast-and Southeast Asian summer monsoon and the South Asian summer monsoon isclosely interrelated in the context of the Asian summer monsoon. However, theearliest onset of the Asian summer monsoon occurs over the Indochina Peninsula andthe SCS.
Similar to the onset vortex over the Arabian Sea, a twin cyclone is often observed
to develop in the near-equatorial East Indian Ocean (near Sri Lanka) prior to the onsetof the summer monsoon over Indochina Peninsula and the South China Sea. Theequatorial westerlies are acceclerated and extend northeastward into the Indo-China
Peninsula and the northern SCS, thus greatly enhancing the low-level westerlies there.
Therefore, a pre-condition of the first onset of the Indochina Peninsula and SCS
summers monsoon is the development of a twin cyclone over equatorial East IndianOcean. The climatological and case study both have shown that this pre-condition
generally exists (Lau et al., 1998, 2000; Zhang et al., 2001). For 13 cases of the first
monsoon onset of the northern SCS out of 47 years from 1953 to 1999, the twin
cyclones over the equatorial East Indian Ocean were always observed, although theirdevelopmental processes may have some differences.
Another pre-condition for the first summer onset of Indochina Peninsula and theSouth China Sea (especially northern SCS) is the southward intrusion of strong cold
air accompanied by Chang and Chen (1995) an obvious cold front. As pointed out by
Chang et al. (2000), and Ding and Liu (2001) ,this process from mid-latitudes may be
a triggering mechanism for the monsoon onset in the northern SCS, especially for
rapid amplification of regional precipitation and convection in these regions. Thiscondition is different from that of the Indian summer monsoon whose onset isbasically forced by tropical impacts. Recently, Liu et al. (2002) suggest that thesouthward intrusion of midlatitide frontal systems is related to the downstream
propagation of the Rossby wave excited by convective activity in Bay of Bengal.
One important indicator characterizing the onset of the Indochina Peninsula andthe SCS is the of precipitation and convection. Based on observations made by thedual-Dpppler radars deployed in the northern SCS by SCSMEX, the organized
convections in the monsoon trough were very vigorous after the onset, which often
organized into meso-scale rainbands or meso-scale convective systems (MCSs). These
MCSs are closely related to the synoptic processes and systems (Ding, Li and Liu,2004). In particular, the establishment of the low level monsoon trough and associatedwind shear line during the monsoon onset provides the favorable synoptic anddynamic condition for formation and maintenance of MCSs. The latter will in turncreate a significant feedback effect on large-scale circulation and synoptic system inwhich are embedded the MCSs, through vertical heat and moisture transport andrelease of latent heat. Johnson and Ciesielski (2001) have recently studied thecharacteristics of the onset of the summer monsoon over the northern SCS utilizing
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observation from the May – June 1998 SCSMEX. They have shown that the onset
occurred in the northern SCS in mid-May with a rapid increase in deep convection
over a week to ten-day period, thus producing the significant vertical transports ofheat and moisture. During the undisturbed pre-onset period (May 6-12), there isupper-level convergence, low-level divergence and deep subsidence, consistent withthe mostly clear skies and high values of OLR. Q1 (the apparent heat source) is
negative at all levels with values in excess of -2.5K day-1 in the upper troposphere.
The profiles of divergence and vertical motion during the onset (May 16-22) andpost-onset periods (June 3-9) are dramatically different from the pre-onset period.
Low-level convergence, upper-level divergence and strong upward motion occur
during both periods. Deep convergence extends to 300-500 hPa, Q1 and Q2 (theapparent moisture sink) profiles are characteristic of deep convection during theseperiods, with a peak heating rate of 5 K day-1 located at about 400 hPa and aconsiderable separation of the two curves, suggesting more vigorous deep convection
at that time. Thus, the intense heating can led to the surface pressure in the monsoon
trough further to fall down and tropical low-level southwesterlies to monsoon troughwas enhanced. This positive feedback process is also favorable for eastward retreat of
the subtropical high out of the SCS region.
The onset of the monsoon is the most anxiously awaited weather singularity inthe sub-continent as it heralds the rainy season and marks the end of the hot summer(Ding and Sikka, 2004). Over continental India, the drama of the onset of the South
Asian summer begins first across Kerala coast, normally by 31 May (Ananthakrishnanand Soman,1988) when heavy rains lash the coastal state after the cross equatorial low
level jet (LLJ) is established across the Somali coast into near-equatorial Arabian Sea.
This phenomenon is usually accompanied by the formation of a mid-troposphereshear zone across central Bay of Bengal to SE Arabian Sea in which may beembedded a cyclonic vortex. The vortex may even intensify into a cyclonic storm
either in the Bay of Bengal or SE Arabian Sea. The cyclonic storm forming in the SEArabian Sea is known as the Monsoon Onset Vortex (Krishnamurti et al., 1981) after
the event which occurred on 11 June 1979 during the Summer MONEX year. The
vortex is formed to the north of the LLJ, in the zone of maximum cyclonic shear in
the lower tropospheric zonal winds. In association with the northward movement of
the vortex, the large scale monsoon current also advances northward along the West
Coast of India. On the average, the onset vortex forms in nearly 50 percent of theyears and in other years the onset is accompanied by either mid-tropospheric shear
line or formation of an off-shore trough along 10 to 150N off the Kerala coast of India
with an embedded weak low pressure area. Prior to the onset the cross-equatorial flow
increases in strength, the moisture fields builds upto the mid-tropospheric level 7 to10
days in advance (Pearce and Mohanty, 1984), the near-equatorial cloud band in theArabian sea expands eastward and the upward motion field in the troposphere near theequator enhances which eventually moves northward with the advance of themonsoon.
Yin (1949) was the first to link the process of monsoon onset to the displacementof westerly troughs in the circumpolar westerlies and shift of STJ to the north of theHimalayan periphery. Pearcl (1983) examined the monsoon onset over India during
1979 from the perspective of planetary scale circulation features and noted theirgradual setting up prior to the formation of a transient disturbance off the Kerala coast.
Murakami and Ding (1982) have opined that the onset is related to the warming of theEurasian region by diabatic heating. Yanai et al. (1992) have linked the onset to theeffect of the Tibetan Plateau. Thus the onset of monsoon over India is linked to a3

combination of regional and planetary scale changes over the entire Indian Ocean
region. There exists a variety in the linkages of the onset process with the seasonal
developments or transitions in the regional and planetary scales features. Lack ofuniqueness among them points to the initiation of chaotic dynamics with theformation of the weather system at Kerala coast which heralds the onset process.

Reference
Ananthakrishnan, R. and M.K. Soman, 1988: The onset of SW monsoon over Kerala,
Int. J. Climatol. 8, 283-296.
Chang, C.-P., and G.T.-J. Chen, 1995: Tropical circulation associated with southwest
monsoon onset and westerly surge over the South China Sea. Mon. Wea. Rev.,
123, 3221-3267.
Chang, C.-P., Y. Lan, and G.T.J. Chen, 2000: A numerical simulation of vortex
development during the 1992 East Asian summer monsoon onset using theNavys Regional Model. Mon. Wea. Rev., 1604-1631.
Ding, Y.H., 1981: A case study of formation and structure of a depression over theArabian Sea. Chinese J.Atmos.Sci., 5, 267-280 (in Chinese).
Ding, Y.H., and Y.J. Liu, 2001: Onset and the evolution of the summer monsoon over
the South China Sea during SCSMEX field experiment in 1998. J. Meteor. Soe.,
Japan, 79, 255-276.
Ding, Y.H., C.Y. Li, and Y.J. Liu, 2004: Overview of the South China Sea MonsoonExperiment. Adv. Atmos. Sci., 21, 343-360.
Ding, Y.H., 2004, Seasonal march of the East Asian summer monsoon in The East
Asian Monsoon, ed. by C.P. Chang, World Scientific Publisher, Singapore. (Inprint)
Ding, Y.H. and D.R. Sikka, 2004: Synoptic Systems and Weather in the AsianMonsoon, Ed. By Bin Wang. To be published.
Johnson, R.H., and P.E. Ciesielski, 2002: Characteristics of the 1998 summer
monsoon onset over the northern South China Sea, J. Meteor. Soc. Japan, 80,561-578.
Krishnamurti, T.N., P. Ardanuy, Y. Ramanathan, and R. Pasch, 1981: The onset-vortexof the summer monsoon. Mon. Wea. Rev., 109, 344-363.
Lau, K.-M. and P.C. Chan, 1986: Aspects of 40-50 day oscillation during the northernsummer as inferred from outgoing long wave radiation. Mon. Wea. Rev., 114,1354-1367.
Lau, K.-M., Y.H. Ding, J.T. Wang, R. Johnson, R. Cifelli, J.G.erlach, O. Thjiely,T.Rikenbach, S.C. Tsay and P.H. Lin, 2000: A report of the field operation andearly results of the South China Sea Monsoon Experiment (SCSMEX). Bull.
Amer. Meteor. Soc., 81, 1261-1270.
Liu, Y.M., J.C.L. Chan, J.Y. Mao and G.X. Wu, 2002: The role of Bay of Bengal
convection in the onset of the 1998 South China Sea summer monsoon. Mon.
Wea. Rev., 130, 2731-2744.
Murakami, T., and Y.H. Ding, 1982: Wind and temperature changes over Erasia
during the early summer of 1979, J. Meteor. Soc., Japan, 60, 183-196.
Pearcl, R.P., and O.C. Mohanty, 1984: Onset of the Asian summer monsoon 1979-82.
J. Atmos. Sci., 41, 1620-1639.
4Wang, B., and H. Lin, 2002, Rainy season of the Asian-Pacific summer monsoon, J.
Climate, 15, 386-396.
Yanai, M., C. Li and Z. Song, 1992: Seasonal heating of the Tibetan Plateau and its
effects on the evolution of the Asian summer monsoon, J. Meteoro. Soc., Japan,70, 319-351.
Yin, M.T., 1949: A Sysmatic aerological study of onset of the summer monsoon overIndia and Burma, J. Meteor. 6, 393-400.
Zhang, X.Z., J.L. Li, Y.H. Ding and J.Y. Yan, 2001: Research of circulation
charachteristics and index of South China Sea summer monsoon, Acta
Meteorologica Sinica, 15, 450-464.

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