SOLA, 2007, Vol. 3, 093 096, doi:10.2151/sola.2007 024

93

The Impact of Trans-equatorial Monsoon Flow on the Formationof Repeated Torrential Rains over Java Island
Peiming Wu1, Masayuki Hara1, Hironori Fudeyasu1, Manabu D. Yamanaka1, Jun Matsumoto1,
Fadli Syamsudin2, Reni Sulistyowati2, and Yusuf S. Djajadihardja2
1
Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
2
Agency for the Assessment and Application of Technology, BPPT, Indonesia

Abstract
Torrential rains that repeatedly occurred over Java
Island causing widespread floods in late January andearly February 2007 coincided with a strong and persistent trans-equatorial monsoon flow from the Northern
Hemisphere. While convections develop frequently overthe island’s mountainous areas in the afternoon, convections over the northern plains are active during thenight and morning hours. The strong trans-equatorial
monsoon flow with an upper southeasterly windproduces a strong low-level vertical shear of wind anddry mid-level environment over the island. These conditions allow the severe convections to occur repeatedly
for days and to sustain for an extended period of time.
The results suggest that the trans-equatorial monsoon
flow plays a principal role in the formation of therepeated torrential rains. The probability of occurrenceof a strong and persistent trans-equatorial monsoon
flow that causes torrential rains and widespread floods
over Java Island is estimated to be once every 5 10
years.

Fig. 1. Sea surface wind vectors measured by NASA’s Quick
Scatterometer for 31 January 2007.

1. Introduction
erly winds over the Karimata Strait and over the JavaSea in the Southern Hemisphere. As will be shown insection 2, this strong trans-equatorial monsoon flow persisted for more than one week in late January and earlyFebruary 2007.
Torrential rains repeatedly occurred over Java
Island during the period from late January to earlyFebruary 2007, resulting in one of the worst flooding
events in recorded history in the Indonesian capital of
Jakarta and surrounding areas. Prior analyses of thecold surges were focused mainly on the structure andevolution of the surges in different regions. How thetrans-equatorial monsoon flow affects the occurrenceof torrential rain over Java Island in the Southern
Hemisphere is unknown. Relevant phenomena include
the Madden and Julian Oscillation (MJO), which significantly affects tropical weather, especially in the Indian
and western Pacific Ocean regions. An active MJO cancause enhanced convection over the island for an
extended period of time. However, the period duringlate January to early February 2007 when the torrential
rains occurred over the island was not in a MJO active
phase. In this study, we investigate the influence ofthe trans-equatorial monsoon flow on the formation of
the repeated torrential rains over the island using
QuikSCAT sea surface winds, GMS infrared images,
radar observation and balloon sounding data for theperiod from late January to early February 2007.

During the Asian winter monsoon cold-air outbreaks
from the Eurasian Continent accompanied by rapid
drop in air temperature and increase of surface pressure
across much of Southeast Asia are an important aspectof the intraseasonal variability of the winter monsoon
system (e.g., Ramage 1971; Garreaud 2001). Often thecold surges enhance convection over the South China
Sea, in the region of north of Borneo and along the IndoChina coast because of the intense low-level wind convergence along their leading edge (Compo et al. 1999;
Chang et al. 2005). Love (1985) suggested that winterhemisphere subtropical cold surges enhance monsoon
westerly flow in the summer hemisphere. On rareoccasions, a strong and persistent cold surge can help
to create a large background cyclonic vorticity at theequator in the equatorial tropical cyclone formation(Chang et al. 2003).
As the cold surge moves southward into the nearequatorial tropics, strong surface heat-fluxes weaken
the cold air anomalies, and the surge may lose its ‘cold’
character. Previous analyses on the cold surges showedthat the pressure surges originated from the Northern
Hemisphere mid-latitude can be traced only to as far
south as 10°N (e.g., Compo et al. 1999). However, strong
northerly wind can blow to lower latitudes and caneven penetrate into the Southern Hemisphere. The sea
surface wind vectors measured by NASA’s Quick
Scatterometer (QuikSCAT) on 31 January 2007 areshown in Fig. 1. A strong northeasterly winter monsoon flow with a wind speed greater than 10 m s 1 wasobserved over the South China Sea, and a strong north-

2. Occurrence of trans-equatorial monsoon
flow from the Northern Hemisphere

Corresponding author: Peiming Wu, Japan Agency for MarineEarth Science and Technology, Natsushima-cho 2-15,
Yokosuka-City, Kanagawa 237-0061, Japan. E-mail: pmwu@
jamstec.go.jp. ©2007, the Meteorological Society of Japan.

We investigated the occurrence of the trans-equatorial monsoon flow from the Northern Hemisphere in the
boreal winter season using QuikSCAT sea surface winds
for the 8-years from October 1999 to March 2007. (The94

Wu et al., The Impact of Trans-equatorial Monsoon

QuikSCAT sea surface winds are available only since1999). In almost every year during 1999 to 2007, atrans-equatorial monsoon flow occurred during theperiod from late December to mid-February. In 2002 and2007, a similarly strong and persistent trans-equatorial
monsoon flow occurred in late January and earlyFebruary. In both years, the trans-equatorial monsoon
flow caused torrential rains and widespread floods overJava Island. However, except for 2002 and 2007, thetrans-equatorial monsoon flow was relatively weak, andcontinued only for 2 3 days each time (not shown).
Therefore the probability of occurrence of a strong andpersistent trans-equatorial monsoon flow that causes
torrential rains and widespread floods over Java Islandis estimated to be once every 5 10 years.
Time variation of the Asian winter monsoon flow
over the South China Sea, over the Karimata Strait and
the Java Sea in the Southern Hemisphere is shown in
Fig. 2 as a time-latitude diagram of the meridional
winds from QuikSCAT sea surface winds along 108°E
for the period from October 2006 to March 2007. This
longitude is representative of the region from 106 to
110°E. The strong northerly wind events occurred fivetimes over the South China Sea: in late October to earlyNovember, in the beginning and middle of December
2006, in the middle of January, and in late January toearly February 2007. Among them, in the first fourtimes the strong northerly winds occurred only over theSouth China Sea. However, in the last and strongest
event, which occurred from 28 January, the strong
northerly monsoon flow blew across the equator, andpenetrated into the northern part of Java Island in theSouthern Hemisphere. This strong trans-equatorial
monsoon flow persisted for more than one week during
the period from late January to early February 2007.

Fig. 2. Time-latitude diagram showing the meridional
winds from QuikSCAT sea surface winds along 108°E.

3. Spatiotemporal variation of severe convection/rainfall over Java Island
As mentioned previously, intense precipitation repeatedly occurred over Java Island in late January andearly February 2007. Figure 3 shows the hourly rainfall
observed at Pondok Betung Meteorological Observatory, Jakarta in Java Island (Fig. 4) during 29 January to
3 February 2007. The four-day total rainfall during
30 January to 2 February reached 589.0 mm. It is noteworthy that the rainfall exhibits a pronounced diurnal
cycle. The precipitation was regularly initiated in thenight during 2000 to 2200 LT, and continued for 4 5hours. The rains stopped falling in the early morning,occurred again after 3 4 hours and continued until
around noon on 1 and 2 February 2007.
We examined the spatiotemporal variation of convection in and around Java Island using infrared (IR)images from the Geostationary Meteorological Satellite(GMS) MTSAT-1R. The three-day images for 31 January
to 3 February, 2007 that typify the diurnal cycle areshown in Fig. 5. It can be seen that by 1200 LT in themorning, there are few clouds over the land on mostparts of Java Island (the top panels). As the day progresses, convections are very active over the southern
mountain areas of the island (1700 LT, the middle
panels). At the later time, convections occur over thenorthern plains during night (the bottom panels). The
convections migrate to the Java Sea during the latenight to early morning period.
The above results from the MTSAT-1R infrared (IR)images showed that while a number of convections
develop frequently over the mountainous areas of Java
Island in the afternoon, convections are active overthe plains during the night and early morning. GMS
infrared (IR) images cannot provide a distribution of
rainfall on the ground. However, radar observation cangive a detail distribution of rainfall near the ground andevolution of storms. The X band radar Plan Position

Fig. 3. Hourly rainfall observed at Pondok Betung
Meteorological Observatory, Jakarta in Java Island during
29 January to 3 February 2007.

Indicator (PPI) reflectivity display at the 0.5-degree elevation angle obtained at Pondok Betung MeteorologicalObservatory, Jakarta for 1600, 1900, 2000 LT on 1
February and 0300 LT on 2 February 2007 is shown in
Fig. 6.
Initially, in the afternoon (1600 LT) the rains werepredominantly over the mountainous areas of Java
Island, approximately 40 70 km south of the Pondok
Betung radar site. By 1900 LT the rains over the mountains are decreasing, and a broken line of convections
initiated over the northern plains near the mountainfoot. By 2000 LT the rains over the mountains disappeared, and convections over the plains had solidified
into a line orientated west-east, extending for about 100
km. The precipitation system was tracked within 〜10
km south and approaching the Pondok Betung radarsite. Very heavy rainfall with intensities of 53.0, 50.0
and 49.0 mm h 1 was measured on the ground at the
Pondok Betung radar site at 2200, 2300 and 0000 LT, respectively. Subsequently, the reflectivity pattern continuously showed a line of storms, migrating northward
to the Java Sea during the nighttime. The propagation
speed of the precipitation system was close to 〜3 m s 1,
much slower than the speed observed in typical midlatitude squall lines. In addition to the example shown
above in this section, a similar result of the regular occurrence of afternoon rains over the mountainous areasof Java Island, night and morning rains over the plains

SOLA, 2007, Vol. 3, 093 096, doi:10.2151/sola.2007 024

Fig. 4. The topography of Java Island and location of
Pondok Betung (black circle), Jakarta, where radar observations were conducted. The shades denote terrain elevation.

Fig. 5. GMS satellite infrared (IR) images over Java Islandfor 31 January to 3 February 2007. These images weremade using the infrared channel of MTSAT-1R image,which has a spatial resolution of 5 km.

Fig. 6. The X band radar Plan Position Indicator (PPI) reflectivity display at the 0.5-degree elevation angle obtainedat Pondok Betung Meteorological Observatory, Jakarta fora) 1600, b) 1900, c) 2000 LT on 1 February and d) 0300 LT
on 2 February 2007.

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were also observed on 31 January 2007 (not shown).
A wind blows up the slope of a hill or mountain due
to solar heating of the land during daytime. An upslope
wind forms because the air over a mountain becomes
warmer than the surrounding atmosphere at the samelevels. The upslope wind produces a wind convergence,
which in turn induces an enhancement of moisture andrising atmospheric motion, causing the formation ofcloud over the mountain in the late afternoon. As shownin Fig. 5 and 6, a number of convections developed overthe southern mountain areas of Java Island in the lateafternoon. This afternoon convection over the mountains may result from the enhancement of moisture bythermally induced local circulations, similar to thatwhich occurred over Sumatra Island (Wu et al. 2003).
Tucker and Crook (1999) suggested cold air flowing
down the lee slope as a mechanism for the generation ofmesoscale convective system, the cold air in their study
originated as the outflow from convections that developed earlier over the lee slope of the Rocky Mountains.
Satomura (2000) reported a similar triggering andpropagation of late afternoon and evening squall lines
near the lee-side foot by the outflow from previous
mountain convections over the Indo-China Peninsula. It
is expected that when convections over the mountainsof Java Island dissipate in the late afternoon and evening, cold surface outflow that is induced by the convections flows down the mountain slope. These surface
outflow winds create an intensive low-level wind convergence, lift the warm moist air near the surface, andinitiate the convections over the northern plains near
the mountain foot, similar to those that occurred overthe Indo-China Peninsula (Satomura 2000). As a result,the severe convections developed repeatedly over thenorthern plains of the island in the nighttime as
depicted by the radar observations shown in Fig. 6.

4. Influence of trans-equatorial monsoon
flow on the formation of severe, long-lived
convection over Java Island
The results from radar observations in the previous
section indicated that the nighttime convection was
triggered over the northern plains near the mountainfoot in Java Island. The well-organized convections
were quite severe and sustained for an extended periodof 8 9 hours until the early morning of the next day.
The severe, long-lived convections propagated northward, bringing about the heavy rainfall over thenorthern plains of the island during the night andmorning hours.
Balloon soundings were performed twice a day at
Soekarno-Hatta International Airport, Jakarta in Java
Island. The Skew-T Log-P diagrams for 1200 UTC (1900
LT) on 31 January and 1 February 2007 are shown in
Fig. 7. The 1200 UTC soundings on 31 January and 1
February indicated the Convective Available Potential
Energy (CAPE) values of 1651.0 and 2202.8 J kg 1 , respectively. The high CAPE represents the potential fora high maximum speed of updraft leading to the strong
convections. Both the two dewpoint profiles indicatedthe existence of a distinct dry layer at the mid-level of〜600 300 hPa. The existence of the mid-level dry layer
represents a strong convective instability over theisland. Meanwhile, the wind profiles indicated thatwhile southeasterly wind prevailed in the upper atmosphere (higher than about 700 hPa), there was a layer ofnorthwest winds in the levels lower than about 800 hPa.
The wind directions at the altitudes of 900 and 700 hPa
are almost in the opposite directions, with a large
vertical wind shear of 15 m s 1 through the 900 700 hPa
layers. The low-level northwesterly winds were a resultof the strong trans-equatorial monsoon flow from theNorthern Hemisphere, as shown previously in Figs. 1
and 2.

96

Wu et al., The Impact of Trans-equatorial Monsoon
Northern Hemisphere. The precipitation/convection
over the island exhibits a pronounced diurnal cycle.
While convections develop frequently over the island’s
mountainous areas in the afternoon, convections overthe northern plains are active during the night andmorning hours. The strong trans-equatorial monsoon
flow and the upper southeasterly wind produce a strong
low-level vertical shear of wind and a strong dry-air
intrusion at the mid-level over the island. The strong
low-level vertical shear of wind, wet lower and dry midlevel layer conditions allow the severe convections tooccur repeatedly for days and to sustain for an extended
period of time. The results suggest a possibility that thetrans-equatorial monsoon flow interacts with thermallyand convectively induced diurnal changes in the boundary-layer wind over the island, enhancing the convections over the mountains in the afternoon, and over theplains in the night and morning. The trans-equatorial
monsoon flow plays a principal role in the formation of
the repeated torrential rains over Java Island.

Acknowledgments

Fig. 7. The Skew-T Log-P diagrams obtained at Jakarta for1200 UTC (1900 LT) on (upper) 31 January and (lower) 1
February 2007. The right line in each of the plots is thetemperature profile, and the left line is the dewpoint profile.
The winds are plotted as wind barbs with height on theright edge of the plots.

Low-level wind shear is a favorable and necessary
condition for convection maintained by downdraft.
Rotunno et al. (1988) suggested that squall-line strengthand longevity is most sensitive to the strength of thecomponent of low-level (0 3 km AGL) ambient verticalwind shear perpendicular to squall-line orientation.
Usually, during the boreal winter the vertical shear ofwind over Java Island is relatively weak in the absenceof a trans-equatorial monsoon flow from the Northern
Hemisphere. However, as shown in Fig. 7, results fromballoon soundings obtained at Jakarta indicated astrong vertical wind shear in the low-levels on 31
January and 1 February 2007. As described previously,
the strong trans-equatorial monsoon flow caused the
low-level northwesterly winds over the island. Thisstrong and persistent trans-equatorial monsoon flow
induced an intensive low-level wind convergence along
its leading edge over the island. Simultaneously, thelow-level northwesterly flow with the upper southeasterly wind produced a strong low-level vertical shear ofwind over the island for days. Theses conditions allow
the convections to be organized and to sustain for an
extended period of time. As a result, the torrential rains
occurred repeatedly for several days over the island,
which resulted in the widespread floods over Jakarta
and surrounding areas in late January and earlyFebruary 2007.

5. Summary
Torrential rains that repeatedly occurred over Java
Island causing widespread floods during late January
and early February 2007 coincided with a strong andpersistent trans-equatorial monsoon flow from the

We are deeply grateful to Drs. Jie Li, Hiroki
Tokinaga and Dennis G. Dye of Japan Agency for
Marine-Earth Science and Technology, for their invaluable comments and support on this study. Thanks arealso due to Messrs Urip Haryoko, Suhatno and Marjuki,
the Meteorological and Geophysical Agency ofIndonesia (BMG) for providing the rain data and radar
images used in Figs. 3 and 6. The infrared Satellite data
used in Fig. 5 are provided by Kochi University, Japan.
The balloon sounding data used in Fig. 7 are provided
by University of Wyoming. Data are available atweather.uwyo.edu. QuikScat data are produced by
Remote Sensing Systems and sponsored by the NASA
Ocean Vector Winds Science Team. Data are available atwww.remss.com.

References
Chang, C.-P., C. Liu, and H. Kuo, 2003: Typhoon Vamei: An
equatorial tropical cyclone formation, Geophys. Res.
Lett., 30(3), 1150, doi:10.1029/2002GL016365.
Chang, C. P., P. A. Harr, and H. J. Chen, 2005: Synoptic disturbances over the equatorial South China Sea andwestern Maritime Continent during boreal winter.
Mon. Wea. Rev., 133, 489 503.
Compo, G. P., G. N. Kiladis, and P. J. Webster, 1999: The horizontal and vertical structure of east Asian wintermonsoon pressure surges. Quart. J. Roy. Meteor. Soc.,
125, 29 54.
Garreaud, R. D., 2001: Subtropical cold surges: Regional
aspects and global distribution. Int. J. Climatol., 21,1181 1197.
Love, G., 1985: Cross-equatorial influence of winter hemisphere subtropical cold surges. Mon. Wea. Rev., 113,
1487 1498.
Ramage, C. S., 1971: Monsoon Meteorology. Academic
Press, 296 pp.
Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory
for strong, long-lived squall lines. J. Atmos. Sci., 45,
463 485.
Satomura, T., 2000: Diurnal variation of precipitation overthe Indo-China Peninsula: Two-dimensional numericalsimulation. J. Meteor. Soc. Japan, 78, 461 475.
Tucker, D. F., and N. A. Crook, 1999: The generation of amesoscale convective system from mountain convection. Mon. Wea. Rev., 127, 1259 1273.
Wu, P., J. Hamada, S. Mori, Y. I. Tauhid, M. D. Yamanaka,and F. Kimura, 2003: Diurnal variation of precipitablewater over a mountainous area of Sumatra Island. J.
Appl. Meteor., 42, 1107 1115.
Manuscript received 30 May 2007, accepted 29 August 2007
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