Rainfall Variations of the Asian Summer Monsoon:
Diurnal and Intraseasonal Oscillations
Tsing-Chang (Mike) Chen, Shih-Yu Wang, and Wan-Ru Huang
Department of Geological and Atmospheric Sciences
Iowa State University, Ames, Iowa, U.S.A. e-mail: tmchen@iastate.edu

1. Introduction

Time series of 3-day mean PG (thick-solid line),

One of the major goals of the GAME is toenhance our understanding of the hydrological cycle ofthe Asian summer monsoon. However, it is also equallyimportant for us to understand the role played by theAsian monsoon hydrological cycle in the global context.
It was revealed from previous studies that the Asiansummer monsoon undergoes diurnal and intraseasonal(12-24 day and 30-60 day) variations. As depicted by thepotential function of water vapor flux (χQ), the global
diurnal variation mode propagates westward, opposite to
the Earth’s rotation, and the global 12-24 day and 30-60
day modes propagate eastward. In view of propagation
properties of these global hydrological modes, thefollowing questions are raised:(1) What is the contribution of the Asian monsoon to theglobal hydrological cycle?
(2) How does the Asian monsoon hydrological cycle
respond/link to these global propagating modes?
The GAME hydrological data, several global precipitation
datasets (TRMM, GPCP, GPI), and NCEP/NCAR
reanalyses were analyzed to search for answers to thesetwo questions.

PAA (thin-solid line), and PEA (thin-dash line) are shownin Fig. 2a. Variations of PAA and PEA are more or lessout-of-phase. However, variation of PG almost follows~that of PAA . The 12-24 day (^) and 30-60 day ( )intraseasonal signals emerge from power spectra of thesevariables (Figs. 2d-f). Periods of 1979-1980 and 19892001 were analyzed, but time series of various variables
in 1979 are shown in Fig. 2 as an example for illustration.
~ ~Time series of ( P̂G , P̂AA , and P̂EA ) and ( PG , PAA , and~PEA ) are shown in Figs. 2d and c, respectively. In-phase~variations exist between P̂G and P̂AA and between PG~
and PAA , while out-of-phase variations appear between~~P̂G and P̂EA and between PG and PEA . As diurnal
variation, intraseasonal variations of the global rainfall
are primarily determined by the Asian monsoon, throughthe AA-hemisphere rainfall.

2. Global Rainfall Variation
It was shown by our previous studies (Chen et al.
1995a, b) that intraseasonal rainfall variations of theAsian-monsoon (AA; 60oE-120oW) and extra-AAmonsoon (EA; 120oW-60oE) hemispheres are out-ofphase. Let us represent the global, AA- and EAhemisphere rainfall by PG, PAA, and PEA, respectively.
Departures of these three variables (∆PG, ∆PAA, and ∆PEA)
at four synoptic times (00, 06, 12, 18UTC) from their
daily mean values are shown in Figs. 1a and b. The daily
variation of ∆PG is almost in-phase with that of ∆PAA, butout of phase with ∆PEA. The diurnal variation signal ( )’dominates power spectra of all three variables (Figs. 1d′ , and PEA′ behave the samef). Variations of PG′ , PAA
way as those of ∆PG, ∆PAA, and ∆PEA. Evidently, theAsian summer monsoon plays a crucial role indetermining the diurnal variation of the global rainfall.

,PAA,

PEA

Fig. 1Fig.2

3. Variation of Asian monsoon rainfall
The global diurnal (Chen et al. 2004) andintraseasonal [12-24 day (Chen et al. 1995a) and 30-60
day (Chen et al. 1995b)] modes of streamfunction andpotential function of water vapor flux, ψ Q and χQ ,exhibit relatively regular westward and eastward
propagations, respectively. On the other hand, the Asianmonsoon hydrological cycle responds to the global
propagating modes in complicated ways:
a. Diurnal Variation: Diurnal components of χQ andrainfall [blue (positive) and pink (negative) areas], χ′Qand P ′, undergo a clockwise rotation around the Asiancontinent in response to the westward propagation ofthe global χ′
Q mode, as shown in Fig. 3.

c. 30-60 day Variation: The East Asian monsoon lifecycle(onset-active-break-revival)
isbasicallyestablished by the northward migration of the 30-60
day monsoon trough/ridge (inferred from composite~ of eight phases in Fig. 5) from the Equator (ChenψQet al. 2000) in response to the global 30-60 day modeas shown in Fig. 5b. Monsoon rain is primarily
generated by fronts [blue (positive) and pink (negative)
areas in Fig. 5b] along the Meiyu-Baiu rainbelt before
the monsoon break, and by TCs/TDs (red dots in Fig.
5a) after the break. The modulation of the global 3060 day mode on the East/Southeast Asian monsoon
rainfall is accomplished through the effect of the 30-60
day monsoon mode on the frontal and TC/TD activities(Fig. 5a).

Fig. 3b. 12-24 day Variation: The 12-24 day monsoon mode
propagates northwestward along the Southeast Asiawestern tropical Pacific monsoon trough to East China.
The eastward propagation of cold fronts before theEast Asian summer monsoon break (not shown) is
regulated by this intraseasonal mode, while the tropical
cyclone (TC)/tropical depression (TD) activity(indicated by red dots in Fig. 4) in East/Southeast Asia
is modulated by this intraseasonal mode (depicted bycomposites ψ̂ Q of six phases) after the East Asianmonsoon break (shown in Fig. 4). Because fronts andTCs/TDs are major rainfall producers, theEast/Southeast Asian summer monsoon rainfall isaffected by the global 12-24 day mode through thisdownscale process.

4. Remarks

Fig.4

Fig.5

Impacts of the global diurnal and intraseasonalmodes on the Asian monsoon rainfall are establishedthrough the downscale processes described in Section 3.
On the other hand, the contribution of the Asian monsoon
to the global hydrological cycle is reflected by thecoherent variations between the global and monsoonhemispheric precipitation, and the variation in the water
vapor flux exchange between the monsoon- and extramonsoon hemispheres.

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