Characteristics, Evolution and Mechanisms of the Asian Summer
Monsoon Onset over the Southeast Asia
Zuqiang Zhang1*

Johnny C. L. Chan2 and Yihui Ding1

1

National Climate Center,China Meteorological Administration, Beijing, China* Email: Zhangzq@cma.gov.cn
2

Department of Physics and Materials Science,

City University of Hong Kong, Hong Kong, China

ABSTRACT
Based on the 1979-1995 mean pentad reanalysis data, the climatologicalcharacteristics and physical mechanism of the Asian summer monsoon (ASM) onset
are investigated. A special focus is whether the ASM onset first commences over theIndochina Peninsula and why the ASM is established earlier over Southeast Asia thanover India. The thermal forcing of the landmass in the lower latitudes of Asiancontinent on the seasonal transition is also investigated.
Keywords: Asian summer monsoon, South China Sea, Indochina Peninsula, Rainfall
1. Introduction

The time for the onset of various
components of the ASM has long beenof great interests. According to theinvestigation of previous scientists, theASM onset first commences over theSouth China Sea in early to mid May,
which was considered to be the earliest
in the tropical Asia. However,somerecent studies also suggested that theearliest onset of the ASM may notnecessarily commence over the SCS.
Indeed,heavyrainfall
beginsclimatologically over the Indochina
Peninsula in early May, about 10-15
days earlier than the SCS (Lau and Yang
1997; Matsumoto 1997; Webster et al.
1998; Wu and Zhang 1998; Wu andWang 2000). However, the occurrenceof heavy rainfall during the seasonaltransition period does not necessarily
represent the ASM onset. Simply using
rainfall and/or outgoing longwave
radiation (OLR) to argue for the ASM
onset first occurring over the Indochina

Peninsula may therefore not besufficient. In fact, the ASM onset isquite a complicated process that includes
not only an abrupt increase in rainfall,but also a seasonal reversal of thelarge-scale ocean-land thermal contrast
and the associated reversal ofatmospheric circulation. Therefore, it
seems reasonable to re-examine theASM onset date from the physicalmechanism
perspective,i.e.
byexamining
thedynamicandthermodynamic variations in theplanetary circulation rather than only therainfall or convection.
Here theNCEP/NCAR reanalysis data isemployed
toinvestigate
theclimatologicalcharacteristics
andphysical mechanism of the Asiansummer monsoon (ASM) onset.
2. Result
An examination of thermodynamic
and dynamic quantities confirms theprevious result that the ASM onset
1commences first over the Indochina
Peninsula. The characteristics not only
include the earliest active convection
and rainfall, but also the earliest reversalof meridional gradient of temperature
throughout the entire troposphere andthe corresponding establishment of aneasterly vertical wind shear (Fig.1). The
convergence of southwesterly flow fromthe Bay of Bengal (BOB) vortex andeasterly winds associated with thesubtropical anticyclone over the South
China Sea (SCS) accounts for the firstcharacteristic. The last two features are
responsible of the upper-level warming
due to the vertical transport of latent
heat
through
cumulusand
turbulence-induced
eddiesinconvection.
A new criterion for the tropicalASM onset is proposed by consideringboth the rainfall and the reversal of thepart of planetary-scale circulation, andthen the advance of ASM in the tropicalregion is obtained (Fig.2). It appears
that the ASM onset occurs first over thesouthwestern Indochina Peninsula inearly May, and then advances to thenorth on May 16-20. Over the SCS, theonset starts in the fourth pentad of May
and the monsoon covers the entire
region by the end of May. A similar
onset process is found to commenceover the eastern Arabian Sea, India andwestern BOB in the last pentad of May,and the complete establishment of ASM
over India is accomplished in mid June.
In the process of the onset of eachcomponent of the ASM, the overturningofupper-level
planetary-scale
circulation strongly depends on thereversal of the meridional temperature
gradient. Over the Indochina Peninsula,
the seasonal transition of upper-level
temperature
results
fromconvection–induced diabatic heating,while over western Asia it is attributed
to the subsidence warming induced by
the active ascending motion over theformer region.
The steady increase in surface
sensible heating over the Indian
subcontinent and the latent heating over
the tropical Indian Ocean in April toearly May appear to be the major
impetus for the development of thecyclonic vortex over the BOB. A similar
enhancement over the Arabian Peninsulaand the surrounding regions is also
identified to be crucial to thedevelopment of the cyclone over theArabian Sea, and then ultimately to theASM onset over India. Due to its later
seasonal transition versus the tropicalregion, the thermal forcing of theTibetan Plateau appears to have little
influence on the ASM onset over theIndochina Peninsula in early May.
Reference
Lau K. M., and S. Yang, 1997.

Climatology andinterannual variability of the Southeast Asian summer
monsoon. Adv. Atmos. Sci., 14: 141-162.
Masumoto, J., 1997.

Seasonal transition of summerrainy season over Indochina and adjacent region.
Adv. Atmos. Sci., 14: 231-245.
Webster, P. J., et al., 1998.

Monsoon process,predictability, and the prospects for prediction.

J.

Geophys. Res., 103: 14451-14510.
Wu, G. and Y. Zhang, 1998.

Tibetan Plateau forcing

and timing of the monsoon onset over South Asia andSouth China Sea.

Mon. Wea. Rev., 126: 913-927.

2Fig. 1.

Longitude-pressure cross-sections of pentad mean meridional gradient of temperature (MGT, contours,ooC per 2.5 lat) and wind (arrows, m s-1) over 5o-20oN from (a) April 26-30 to (h) June 1-5 in sequence. Light anddark shadings denote MGT per 2.5 olat > 0 oC and > 1 oC, respectively.

The contour or ± 0.1oC per 2.5 olat isshown in particular.

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Fig. 2.

Climatological advance of the Asian summer monsoon onset denoted by closed circles andprecipitation rate (mm day-1) from May 1-5 (a) to June 6-10 (h) in sequence. Contour interval for precipitation
rate is 5 mm day-1, with light and dark shading indicating the values > 5 mm day-1and > 10 mm day-1, respectively.
Data used here is the Climate Prediction Center Merged Precipitation(CMAP).

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