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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L18811, doi:10.1029/2007GL030480, 2007

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ArticlePropagating diurnal disturbances embedded in the Madden-Julian
Oscillation
Hiroki Ichikawa1 and Tetsuzo Yasunari1,2
Received 24 April 2007; revised 31 July 2007; accepted 24 August 2007; published 26 September 2007.

[1] Fine structure of large-scale intraseasonal disturbanceassociated with the Madden-Julian Oscillation (MJO) was
analyzed using the 3-hourly (3B42) TRMM rainfall data.
Over the Maritime Continent, diurnal cycle becomespronounced during the passage of MJO, and the eastward
propagating diurnal disturbances (PDDs) dominate as aninternal structure of large-scale convection system of MJO.
Importantly, fast PDDs that penetrate through the islands areobserved within the slowly propagating MJO system. This
eastward penetration of PDDs through the islands result in asudden shift of the convection center from the western partof the islands to the east, causing an overall propagation of
the MJO from the Indian Ocean to the Pacific. Our results
demonstrate that the diurnal cycle of convection over andaround the major islands thus plays an active role in the
propagation of the MJO over the Maritime Continent.
Citation: Ichikawa, H., and T. Yasunari (2007), Propagating
diurnal disturbances embedded in the Madden-Julian Oscillation,
Geophys. Res. Lett., 34, L18811, doi:10.1029/2007GL030480.

1. Introduction[2] The deep cumulus convection and heavy precipitation
over the Maritime Continent [Ramage, 1968] including theIndonesian Archipelago play an essential role in the Earth’s
climate system through a huge latent heat release. This
world convection center is subject to the intraseasonal timescale atmospheric disturbance called Madden-Julian Oscillation (MJO) [Madden and Julian, 1971, 1972] with aperiodicity of about 30– 60 days. The MJO originates inthe equatorial Indian Ocean and moves eastward across the
Maritime Continent into the western Pacific at about 5 m s1,
accompanying deep convective activity in the eastern hemisphere. Previous studies indicates that the eastward propagation of the MJO shows a peculiar tendency over the
Maritime Continent; the large-scale convection systems
associated with the MJO become stationary over the islands,
and exhibit a sudden shifting from the western part of the
Maritime Continent to the eastern part [Hsu and Jin, 1990;
Weickmann and Khalsa, 1990]. However, the detail processes of MJO propagation through the complex distributed
islands has not been fully understood yet.
[3] In addition to the significant intraseasonal-timescalevariability, the diurnal cycle in convection and precipitation
is pronounced over the Maritime Continent that is charac1
Hydrospheric Atmospheric Research Center, Nagoya University,
Nagoya, Japan.
2
Also at Frontier Research System for Global Change, JAMSTEC,
Yokohama, Japan.

Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL030480$05.00

terized by distinct land-sea contrast. Furthermore, offshore
and onshore propagation of the diurnal signal is one of theprominent features over that region [Liberti et al., 2001;
Mori et al., 2004; Sakurai et al., 2005; Ichikawa andYasunari, 2006; H. Ichikawa and T. Yasunari, Intraseasonalvariability in diurnal rainfall over New Guinea and thesurrounding oceans during austral summer, submitted to
Journal of Climate, 2006, hereinafter referred to as Ichikawa and Yasunari, submitted manuscript, 2006). The
diurnal phase propagation is particularly significant overthe major islands that vary in association with the largescale circulation field changes, specifically associated withthe low-level prevailing wind [Ichikawa and Yasunari,
2006; Ichikawa and Yasunari, submitted manuscript,
2006]. The pronounced diurnal cycle would be modulatedby the large-scale convection associated with the MJO [e.g.,
Sui and Lau, 1992]. Recently, Tian et al. [2006] indicated thatthe significant enhancement in diurnal cycle in rainfall over
the Maritime Continent during the convectively active
phase of MJO. However, how the MJO propagate throughthe islands where the strong diurnal cycle exists has notbeen revealed. In this study, we investigated the finestructures associated with the diurnal variation embedded
within the intraseasonal disturbances that propagated during
January through February in 2001. During this particularboreal winter season, the large-scale convection was enhanced all over the Maritime Continent. Through thedetailed analysis, we address the linkage between diurnal
cycle and large-scale intraseasonal disturbance, and clarify
the multi-scale organization in convection by the interaction
process between these two.

2. Data[4] The present study used Tropical Rainfall Measuring
Mission (TRMM) 3B42 (version 6) rainfall product, containing estimated rain rate (mm h1) based on a combination of infrared radiation (IR), passive microwave, and radar
data from TRMM and IR data from the geostationary
satellites. The data are consecutive both temporally andspatially in the domain within the 50°S – 50°N for every
3 hour, with horizontal resolution of 0.25°  0.25°. Furthermore, in order to describe large-scale convection andcirculation field, the data sets of outgoing longwave radiation (OLR) from NOAA satellite and NCEP-DOE AMIP-II
reanalysis (R-2) were used, with horizontal resolution of
2.5°  2.5° in a daily average.

3. Results[5] An eastward propagation of large-scale anomalous
convection was observed early in 2001 over the Maritime

L18811

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L18811ICHIKAWA AND YASUNARI: DIURNAL CYCLE AND THE MJO

Figure 1. Time sequences of the horizontal distribution of
OLR and wind at 850 hPa from late January throughFebruary, 2001.
Continent. Figure 1 shows the sequence of time-averaged
OLR plots during late January to February, with daily winds
at 850-hPa. As is seen in the OLR field, the center ofconvection was located over the Indian Ocean and extended
to the western part of the Maritime Continent in late
January. The large-scale convection reached the Maritime
Continent on 1 – 10 in February, and spread over there. After
February 10 –20, the deeper convection propagated further
eastward, and was located around New Guinea toward
the Pacific Ocean on 21– 28 February. In association withthe enhanced convection, the anomalous westerly spreadin the latitudes near the equator enclosed by the cyclonic
circulation in both hemispheres (Equatorial Rossby wave
pattern). In contrast to the double cyclonic circulation
around the convection, the prevailing easterlies dominate
at all latitude zones to the east of the convection (Kelvinwave pattern). The wind field described above is known as
the ‘Matsuno-Gill pattern’ [Matsuno, 1996; Gill, 1980], thatappears in response to the latent-heat release associated withthe deep convection imposed on the Equator, and is often
observed in relation to the MJO disturbance [e.g., Madden,1986].
[6] In order to clarify prominent frequencies of precipitation under the large-scale convection of the MJO, thepower spectrum of the TRMM merged rainfall at 6 regions
near Equator were investigated during the period from

L18811January 20 to February 28 in 2001 (i.e. 320 data points
were used for calculation with maximum lag number of
44 data, having a band width of 1/320 cycle per 3hours)(Figure 2, left). The time series of actual rainfall (bar),
diurnal amplitude (solid line) and OLR (crossed line) were
described also in Figure 2 (right). Over the Indian Ocean(Figure 2a), relatively longer time-scale (more than 5 day)variability was dominant. In contrast, it is evident that the
pronounced variations in rainfall occurred with periods ofabout 1 day over and around the islands (Figures 2b– 2f).
Particularly over the islands (Figures 2c, 2d, and 2e), wherestrong diurnal cycle exists, the MJO low-frequency variance
was greatly weakened in the spectra, and rainfall occurred
continuously in the time series. Interestingly, the temporal
evolution of rainfall over the islands through this active
MJO period was different from island to island. Over
Borneo (Figure 2c), the diurnal signal became rather
amplified as the large-scale convection center reached tothe island in Feb 5 – 15. In the case of New Guinea(Figure 2e), the diurnal cycle was prominent throughout
this period while the diurnal amplitude tend to be enhanced
before the MJO convection matured around the island (lateJanuary) due to the heavier diurnal rainfall over the southwestern part of the island under the lower easterly (Ichikawa
and Yasunari, submitted manuscript, 2006). The diurnal
cycle over the northern part of that island became strongas the MJO passed through. Over the offshore region, on theother hand, the significant signal of the MJO low-frequency
variability could be observed also as well as the prominent
diurnal signal (Figures 2b and 2f). This is because theoccurrence of offshore rainfall was strongly related to thepassage of deep convection portion of MJO, and wasfavored by strong diurnal cycle.
[7] In order to elucidate more detail space-time structureof MJO system, the time-longitude diagrams of 3-hourly
TRMM rainfall (color contour) averaged between 4°S – 1°S
during the end of January through February in 2001,
superimposed on the OLR (shaded) and zonal wind at
600 hPa (white contour) are shown in Figure 3. Because
the diurnal cycle was the most dominant mode in rainfall
variability, a high-pass filter was applied to the data in order
to extract the diurnal cycle component for TRMM rainfall
data. The eastward propagation of deep convection (OLR)
followed by the 600-hPa level westerlies were apparent at aphase speed of about 3 – 4 m s1. Under the large-scaleconvection, the high-frequency variability in rainfall
exhibited a significant eastward or westward propagationthat is likely related to the background low-mid level wind.
At the evolving stage of the MJO (end of January), aside
from the standing oscillating rainfall over the western Indian
Ocean (60°E), the westward propagating disturbances dominated to the west of the Maritime Continent (80° – 120°E),
intruding on the eastern Indian Ocean. These westward
propagating disturbances at about 1 – 2 day would be
associated with the previous finding [Nakazawa, 1988],
and is presumably related to the convectively coupledinertio-gravity wave [Takayabu, 1994b]. Part of the westward propagating signal was triggered by each island, and isassociated with the diurnal rainfall over the islands.
[8] In February, on the other hand, the eastward propagating diurnal disturbance (PDD) became pronounced over
the whole Maritime Continent from the Indian Ocean to the

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L18811ICHIKAWA AND YASUNARI: DIURNAL CYCLE AND THE MJO

L18811Figure 2. The power spectrum of 3-hourly TRMM rainfall from January 20 through February in 2001 by using (left)
maximum entropy method and (right) time series of actual rainfall (5day running mean; bar) and diurnal cycle filteredrainfall anomaly (solid line) at 6 regions area averaged over (a) 90°E– 92.5°E, 3.75°S– 1.25°S, (b) 113.5°E–116°E, 6.5°S–
4°S, (c) 113°E – 115.5°E, 1°S– 1.5°N, (d) 120°E – 122.5°E, 3.75°S – 1.25°S, (e) 140.5°E – 143°E, 6.5°S– 4°S and(f) 142.5°E– 145°E, 3.75°S– 1.25°S. Time series of OLR averaged over 10°  10° grid box centered around eachlocation are described by crossed line in the right plot also.
Pacific as the center of the large-scale convection reachedthe major islands and the westerlies prevailed. It should benoticed that the significant PDDs, which cut through the
Maritime Continent from 100°E to 160°E, appeared during
6 to 11 February (black solid line). The inferred phase speedof the propagating disturbances was about 15– 20 m s1,which were close to the 600-hPa wind speed. These
eastward PDDs are likely to play an important role in the
propagation of the large-scale MJO convection. In association with the penetration of PDDs through the islands, thecenter of rainfall activity was shifted from the western partof the Maritime Continent to the east. Associated with this,the PDDs starting from around 130°E– 140°E and propagating toward the Pacific became pronounced after 16 February.
On the other hand, the rainfall over the islands became

rather scarce following the pervasiveness of PDDs after
10 February between the significant rainfall over the Indian
Ocean and the Pacific. The other noticeable feature is thatthe westerly area expanded eastward incrementally over thewestern Pacific in association with the eastward movementsof the PDDs.
[9] Figure 4 shows the composite structure of the PDDs,
described by the diurnal rainfall averaged between 5 –20 in
February when the eastward PDDs are prominent. The
PDDs appeared sequentially with the diurnal cycles inwhich (1) the strong diurnal signal developed in the afternoon to night (09-16UTC; 17-00LT at 125°E) over the
major islands, and then (2) propagated eastward from midnight through the next day morning (16-03UTC; 00-11LT)
offshore toward the neighboring islands to the east. The

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L18811ICHIKAWA AND YASUNARI: DIURNAL CYCLE AND THE MJO

L18811Figure 3. Time-longitude cross section of diurnal cycle filtered 3 hourly rainfall anomaly (color contour; 0.2, 0.4 mm h1)averaged between 4°S and 1°S from late of January through February, 2001. OLR (shaded) and zonal wind at 600 hPa(contour; solid-2 m s1, dashed-10 m s1) averaged between 5°S and equator are described also. The island terrain
averaged between 4°S and 1°S is denoted at the bottom. Black solid line corresponds to the penetrating diurnal disturbances(see text for detail).

rainfall signals over and around the islands greatly faded out
at 03-06UTC (11-14LT) as a local calm hours when thediurnal phase of rainfall shifts from sea to island areas [Tianet al., 2006]. The characteristics of the developing PDDs (ordiurnal rainfall) over each major island area correspond wellto the previous findings [Liberti et al., 2001; Mori et al.,
2004; Sakurai et al., 2005; Ichikawa and Yasunari, 2006,
submitted manuscript, 2006]. Over Sumatra, the northwestsoutheast oriented rainfall disturbance developed along theisland and propagated eastward by 12-15UTC. This PDD
further progressed eastward over Jawa Sea reaching thewest coast of Borneo between 18-00UTC, then graduallydiminished at 03 through 06 UTC (the local calm hours). It,
however, revived at 09UTC over Borneo island, whichdeveloped at local afternoon to midnight hours (09UTC to
15UTC), shifting its maximum eastward to the offshore
region between Borneo and Sulawesi island. An interesting
feature may be a near-synchronous evolution of rainfall overBorneo and Sulawasi but with some time lagging. The PDD
over Borneo starts to develop at 09– 18 UTC, whereas thatover Sulawesi starts to develop at 12 to 21UTC. The PDD
originating over Borneo tends to be weakened at 21 to
00 UTC, and be transferred to that over Sulawesi at 03UTC.
The PDD originating over Sulawesi, once weakened at the
local calm hours (03-06UTC), re-appeared over the northwestern tip of New Guinea at 09UTC, which merged withthe diurnal rainfall over the central mountain range in
12-15UTC. The PDD over New Guinea gradually propagated offshore northeastward around 18UTC. The pronounced diurnal disturbance then propagated from thenorthern coast of New Guinea toward the western Pacificby 21UTC to 18UTC of the next day.
[10] It should be noted that the three PDD series arelikely to exist as is shown in Figure 4; Sumatra series,
Borneo-Sulawesi series and New Guinea series. The twopairs of the three series (i.e., Sumatra and Borneo-Sulawesi,

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L18811ICHIKAWA AND YASUNARI: DIURNAL CYCLE AND THE MJO

L18811Figure 4. Spatial distribution of composite propagating diurnal disturbance described by diurnal rainfall anomaly
averaged between 5– 20 in February, 2001. Local standard time (LST) is at midmost longitude (125°E). The 09UTC is
described at twice for both top and bottom for a reference. The rainfall disturbances are indicated by diamond (fromSumatra), triangle (from Borneo), plus (from Sulawesi), and cross (from New Guinea). Dotted circle denotes the diurnal
disturbances from Borneo and Sulawesi that propagate as a packet.

and Borneo-Sulawesi and New Guinea) are connected byconveying rainfall signals in the morning (00-03UTC)
eastward to rainfall signals in the afternoon (09UTC),
crossing over the local calm hours. Thus, the diurnal rainfall
signals appear as a long successive series of eastward
propagating disturbances throughout the islands and seasin the Maritime Continent as shown in Figure 3.

4. Discussion[11] A previous study suggested a dynamical blocking of
the propagations of deep convection associated with the
MJO due to the elevated orograghy over the islands,
particularly over Sumatra [Nitta et al., 1992]. Based on
our study, the MJO disturbance, in turn, propagates throughthe complex orography of the Maritime Continent under thestrong, highly-active diurnal cycle of convection/rainfall

associated with the unique land-sea distribution. This has
been supported by the recent observation of TRMM indicating the overall amplification of the diurnal cycle ofrainfall over and around the Maritime Continent associatedwith the MJO propagation [Tian et al., 2006]. The PDDs
could also be identified in other cases of the MJO event
during the boreal winter season (1998 – 2005).
[12] As shown in Figure 3, the evolution of the MJO over
the Maritime Continent is characterized not by smoothpropagation but by the sudden eastward shift as was also
observed previously [Hsu and Jin, 1990; Weickmann andKhalsa, 1990]. Here, we note that the PDDs are likely toplay a crucial role in this sudden shift of the main MJO
convection system. The dynamics of these PDDs across the
Maritime Continent are not clear yet, but a clue may be theadvective process by the westerly wind associated withthe large-scale MJO disturbances. The phase speed of the

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L18811ICHIKAWA AND YASUNARI: DIURNAL CYCLE AND THE MJO

PDDs is about 15– 20 m s1, which is similar to the lowmid level (600 hPa) zonal wind speed. The long-lasting
propagation of the diurnal signals for over more than a few
days suggests that the atmospheric environment (e.g. moistened air) suitable for the occurrence of diurnal convection
was advected by the ambient wind across the Maritime
Continent. Interestingly, the significant eastward PDDs canalso be observed over Indian Ocean. The pronounced
appearance of PDDs over that ocean would be consistent
with the results of Tian et al. [2006] who indicated thestrong enhancement in diurnal amplitude over the Indian
Ocean associated with the passage of MJO. And thus, someof the diurnal signal preferred for convection might be
triggered over Indian Ocean, propagating to the islands.
[13] In addition to the advection by the backgroundwesterlies, the dynamical interaction between the diurnal
cycle and the large-scale atmospheric wave trapped nearequator might occur. Interestingly, as is noted by Masunaga
et al. [2006], the timing of the PDDs (6 – 11 February, 2001)
was consistent with a passage of the planetary-scale Kelvinwave over the Maritime Continent, suggesting that thepenetration of the PDDs was triggered by the large-scaleequatorial wave. Importantly, previously observed equatorial wave coupled with convection often exhibit a equivalent
depth in the range of about 20– 50 m [Takayabu, 1994a;
Wheeler and Kiladis, 1999], corresponding to thepgravity
ﬃﬃﬃﬃﬃ
wave with phase speed of about 14– 22 m s1 ( gh; g:gravity acceleration, h: equivalent depth), which is similar
to the phase speed of PDDs (15 – 20 m s1). In addition, thezonal space scale of the PDDs (of one day period) is about
20° in longitude, which would correspond to the highfrequency mode of some equatorial waves (i.e. Kelvin wave
and inertio-gravity wave) in the wavenumber-frequency
domain [Takayabu, 1994a; Wheeler and Kiladis, 1999].
Interestingly, this zonal scale of about 20° in longitude
corresponds to the mean distance between the major islands(i.e., Sumatra, Borneo-Sulawesi, and New Guinea), which islikely to constrain the zonal scale of the PDDs. Thus, thelarge-scale atmospheric wave disturbance associated with
the MJO might be re-organized to series of wave-packets of
the PDDs through multi-scale interaction between the largescale waves and the diurnal convection over the complex
island/sea system in the Maritime Continent. Further investigation on the dynamics of this interaction is reserved for
future study.
[ 14 ] Acknowledgments. The authors thank K. Nakamura,
H. Masunaga and H. Fujinami for helpful discussions and advice, and
M. K. Yamamoto for assistance processing data. The authors also deeply
thank T. Matsuno for excellent comments. The TRMM 3B42 data were
distributed by NASA Goddard Distributed Active Archive Center (DAAC).

L18811This study was supported by the 21st Century COE Program ‘‘Dynamics of
the Sun-Earth-Life Interactive System (SELIS)’’ of Nagoya University,
Japan.

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H. Ichikawa and T. Yasunari, Hydrospheric Atmospheric ResearchCenter, Nagoya University, Furo-cho, Chikusa-ku, 464 – 8601 Nagoya,Japan. (ichikawa@hyarc.nagoya-u.ac.jp; yasunari@hyarc.nagoya-u.ac.jp)

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