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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L14S07, doi:10.1029/2008GL033429, 2008

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ArticleOn the roles of the northeast cold surge, the Borneo vortex, theMadden-- Julian Oscillation, and the Indian Ocean Dipole during theextreme 2006//2007 flood in southern Peninsular Malaysia
Fredolin T. Tangang,1 Liew Juneng,1 Ester Salimun,1 P. N. Vinayachandran,2
Yap Kok Seng,3 C. J. C. Reason,4 S. K. Behera,5 and T. Yasunari6
Received 27 January 2008; revised 18 March 2008; accepted 16 April 2008; published 31 May 2008.

[1] The mid-December 2006 to late January 2007 flood insouthern Peninsular Malaysia was the worst flood in a
century and was caused by three extreme precipitation
episodes. These extreme precipitation events were mainly
associated with strong northeasterly winds over the South
China Sea. In all cases, the northeasterlies penetrated
anomalously far south and followed almost a straight
trajectory. The elevated terrain over Sumatra and southern
Peninsular Malaysia caused low-level convergence. The
strong easterly winds near Java associated with the
Rossby wave-type response to Madden-Julian Oscillation(MJO) inhibited the counter-clockwise turning of thenortheasterlies and the formation of the Borneo vortex,
which, in turn, enhanced the low-level convergence over
the region. The abrupt termination of the Indian Ocean
Dipole (IOD) in December 2006 played a secondary role as
warmer equatorial Indian Ocean helped in the MJO
formation. Citation: Tangang, F. T., L. Juneng, E. Salimun,
P. N. Vinayachandran, Y. K. Seng, C. J. C. Reason, S. K. Behera,
and T. Yasunari (2008), On the roles of the northeast cold surge,
the Borneo vortex, the Madden-Julian Oscillation, and the Indian
Ocean Dipole during the extreme 2006/2007 flood in southern
Peninsular Malaysia, Geophys. Res. Lett., 35, L14S07,
doi:10.1029/2008GL033429.

1. Introduction[2 ] The flood that occurred in southern Peninsular
Malaysia during mid-December 2006 to late January 2007
(hereafter referred as the 2006/2007 flood) was the worst
occurrence in a century. Exacerbated by the conditions on
the ground (e.g., poor drainage) in most areas, the durationof the flood extended over a month. The number of people
evacuated exceeded 200,000 with 16 reported deaths. The
initial estimate of economic losses due to the flood was
1
School of Environmental and Natural Resource Sciences, Faculty of
Science and Technology, National University of Malaysia, Bangi, Selangor,
Malaysia.
2
Centre for Atmospheric and Oceanic Sciences, Indian Institute of
Science, Bangalore, India.
3
Malaysian Meteorological Department, Petaling Jaya, Selangor,
Malaysia.
4
Department of Oceanography, University of Cape Town, Rondebosch,South Africa.
5
Frontier Research Center for Global Change, JAMSTEC, Yokohama,Japan.
6
Hydrospheric-Atmospheric Research Center, Nagoya University,
Nagoya, Japan.

Copyright 2008 by the American Geophysical Union.
0094-8276/08/2008GL033429$05.00

reported to be around USD 500 million. Although floods in
Peninsular Malaysia during the northeast monsoon season
are relatively common, the 2006/2007 flood was consideredunusual. This is because the usual flood locations during thisseason are in northeastern Peninsular Malaysia whereas the
2006/2007 flood was concentrated in the south [e.g., Juneng
et al., 2007]. In fact, almost the entire southern part ofPeninsular Malaysia was inundated during the peak period.
The 2006/2007 flood also occurred during the peak period of
the 2006/2007 El Niño event. Climatologically, Peninsular
Malaysia experiences normal or slightly above normal rainfall during the peak of an El Niño event [Juneng andTangang, 2005]. For several months from mid-September to
mid-December 2006, the Maritime Continent region experienced anomalously low precipitation associated with the
2006/2007 El Niño and the 2006 Indian Ocean Dipole(IOD) event [Vinayachandran et al., 2007].
[3] The 2006/2007 flood was caused by three episodes ofheavy precipitation events (Figure 1a). The duration of each
episode was only several days but the accumulated rainfall
recorded during each episode at most rain-gauge stations in
the area was several times higher than climatologicalmonthly means. For example, the Malaysian Drainage andIrrigation Department Air Panas station (2.47°N, 103.05°E)
recorded about 782 mm of rainfall during a four-day period
from 18– 21 December 2006 which is about four times the
December average. The occurrence of the three heavy precipitation episodes was associated with three dominant factors namely, the strong northeast cold surge; the absenceof the Borneo vortex; and the influence of eastward
propagating Madden-Julian Oscillation (MJO) disturbances.
These extreme precipitation episodes were basically partof the deep cumulus and heavy precipitation system in
the Maritime Continent during the boreal winter. The interaction between these systems i.e. the synoptic-scale Borneo
vortex, the northeast cold surge and the MJO, largely determines the variability of the organized convection, particularly
over the western part of the region [Chang et al., 2005].
Interestingly, the flood also occurred following the termination of the 2006 Indian Ocean Dipole event. The abrupt
termination of the IOD event in December 2006 could also
have provided a favorable condition for the extreme weather
event to occur. This study is intended to investigate the rolesof these circulation systems in contributing to the occurrenceof the heavy rainfall episodes that led to the 2006/2007 flood.

2. Data[ 4 ] Tropical Rainfall Measuring Mission (TRMM)
data [Huffman et al., 2007] were used to assess precipitation

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L14S07rate during the episodes. The seasonal and monthly
precipitation data are from the Climate Anomaly
Monitoring System – OLR Precipitation Index (CAMSOPI) data set [Janowiak and Xie, 1999]. The low-levelwinds were obtained from the National Center
Environmental Prediction (NCEP) reanalysis [Kalnay etal., 1996] and the sea surface temperature (SST) data arefrom TRMM Microwave Imager (TMI) (http://www.ssmi.
com). The outgoing long-wave radiation (OLR) data are
obtained from the National Oceanic Atmospheric Agency(NOAA) Interpolated OLR [Liebmann and Smith, 1996].

3. Results and Discussion
3.1. The Role of Northeast Cold Surges[5] The three extreme precipitation episodes that led to
the 2006/2007 flood occurred during 17 – 20 December
2006, 24– 28 December 2006 and 11 – 14 January 2007,
respectively (Figure 1a). The spatial distributions ofaccumulated rainfall indicate that in all episodes, southern
Peninsular Malaysia consistently received higher rainfall(Figures 1b – 1d). These extreme precipitation episodes
occurred during a period of stronger northeasterly winds
over the southern South China Sea. Figures 2a– 2c shows
the 925hPa wind vectors during the beginning of each
episode i.e. 00Z December 17, 2006, 00Z December 26,
2006 and 00Z January 11, 2007. During these periods,
anomalously strong northeasterly winds over the central
South China Sea penetrated further south to southern
Peninsular Malaysia and the Sumatra region in a straight
trajectory. However, it was only during the first episode thatthe strongest equatorward spread of the cold surge fromeastern Asia was evident. This cold surge strengthens thenortheasterly winds near the surface and the regional
topography acts to restrict the flow as it is channeledtowards the equator [Chang et al., 2005]. Although thesurge from the north is relatively cooler and drier, it
becomes moister as it travels over the warmer parts of thesouthern South China Sea [Johnson and Houze, 1987]. It
plays an important role in the episodes of enhanced deep
convection over the equatorial South China Sea [e.g.,
Cheang, 1977].
[6] During the second and third episodes, the cold surge
outflow was weak. However, in all three episodes, there
were strong easterly winds from the western Pacific. These
easterly flows were also restricted and channeled towards
the equator and hence contributed to the strengthening of
the northeasterly winds over the southern South China Sea.
In these cases, the northeasterly winds penetrated further
south, directly into southern Peninsular Malaysia
and Sumatra in a straight trajectory. The low-level winds
interacted with the terrain over the region and this resulted
in a low-level convergence and widespread deep convection

Figure 1. (a) Area-averaged (1°N – 2.5°N, 102.5°E –105°E) precipitation rate (mm/hr) for a period of
1 December 2006 – 21 January 2007. The thick red line
indicates the five-day running means, (b –d) Accumulated
rainfall for first, second and third extreme precipitation
episodes, respectively. Unit is in mm.
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Figure 2. The 925 hPa winds (ms 1) and divergence (shaded, 10 5s 1) during the beginning of each extreme episode(a) 00Z 17 December 2006, (b) 00Z 26 December 2006 and (c) 00Z 11 January 2007. (d) The 925 hPa winds anddivergence during 00Z 30 December 2006 when there was no heavy precipitation over southern Peninsular Malaysia.
over Sumatra and southern Peninsular Malaysia, eventuallyleading to heavy precipitation over the region.
3.2. The Role of the Borneo Vortex[7] The northeasterly cold surge often interacts with thesynoptic scale disturbance known as the Borneo vortex
which could result in the strengthening of the system. The
9 – 11 December 2004 extreme precipitation that caused
severe flood over the northeastern coast of the Peninsular
was associated with such an interaction [Juneng et al.,2007]. The formation of the rare typhoon Vamei was also
associated with this interaction [Chang et al., 2003]. However, during the 2006/07 flooding, the Borneo vortex wasnoticeably absent. There appears to be a relationshipbetween the trajectory of the winds, the tendency forcounter-clockwise turning and the formation of a Borneo
vortex. The distribution of enhanced deep convection was
related to this relationship. The absence of the Borneo
vortex and the lack of counter-clockwise turning of thenortheasterly winds inhibited low-level moisture from being

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transported over the west coast of Borneo and reduced
deep convection in this area. On the other hand, strong
counter-clockwise turning of the northeasterly winds andthe presence of the Borneo vortex helps to enhance theconvection over western and southern parts of Borneo.
A comparison with other periods of equally strong northeasterly winds over the South China Sea (e.g., 00Z
30 December 2006, Figure 2d) indicated that the counterclockwise turning of the wind was more pronounced in these
periods and a Borneo vortex forms with its center located justoff western tip of Borneo. The lack of heavy rainfall
over southern Peninsular Malaysia and Sumatra during
30 December 2006 is an indication of the low-level moisturetransport being intercepted by the Borneo vortex circulation
and also transported by the strong cross-equatorial flow.
This is indicated by the strong low-level moisture flux
convergence over western and southern parts of Borneo
during this period (not shown). The pronounced counterclockwise turning of the northeasterly wind and the formationof Borneo vortex during this period were associated withenhanced westerly winds over and north of Java (Figure 2d).
During the first episode (Figure 2a), strong easterly winds
over and south of Java prevailed and this inhibited thecounter-clockwise turning of the northeasterly wind andindirectly inhibited the formation of Borneo vortex. The
existence of the strong easterly winds over and south ofJava, as we show below, was MJO related.
3.3. The Role of the Madden-Julian Oscillation[8] Despite the similarity of the low-level convergenceover southern Peninsular Malaysia in the three episodes,
there were notable differences in the large-scale circulation
patterns especially over the eastern-central Indian Ocean.
These differences were attributed to the eastward propagation of the MJO disturbances over the region during these
periods. On the intra-seasonal timescale, the large-scale
circulations over the Indian Ocean and Maritime Continent
were very much influenced by the MJO event which waspresent during the episodes. Depending on the phase of the
MJO, the anomalous large-scale circulations associated with
the Rossby wave-type responses may act to strengthen or
weaken a cold surge event. Generally, the frequency of cold
surges and vortex days is reduced during periods when MJO
is present.
[9] Chang et al. [2005] have indicated that the primary
impact of MJO is to inhibit weak cold surges and to impact
on the Borneo vortex. The chance of a cold surge occurring
during MJO phases 1 and 2 (refer to Chang et al. [2005] fora definition of these phases) is about one half that of phase 3
and 4 and that of non-MJO cases. This tendency is due tothe existence of anomalous southerly winds during MJO
phases 1 and 2 over the South China Sea. These anomalous
winds weaken or inhibit the development and southward
progression of cold surges. However, the conditions during
the 2006/07 precipitation episodes were exceptional as an
MJO disturbance emerged over the Indian Ocean during
mid-December 2006 and propagated eastwards across the
Maritime Continent.
[10] As shown by the 30 to 60 days filtered outgoing
long-wave radiation (OLR) anomaly (Figure 3a), an
enhanced deep convection emerged over the Indian Ocean
and propagated towards the Maritime Continent. However,

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the MJO weakened considerably as it approached thewestern side of the Maritime Continent. Nevertheless, we
postulate that the MJO played an important role in theextreme precipitation episodes by interacting with
the northeasterly winds over the South China Sea. During
the first episode, the center of the enhanced deep convection
region was located over the central Indian Ocean (notshown). The strong easterly winds south of Java andsouth-western Sumatra are consistent with the Chang etal. [2005] composite of phase 1 of MJO. Strong easterly
winds over Java during this period helped inhibit thecounter-clockwise turning of the northeasterly winds andindirectly inhibited the formation of the Borneo vortex. The
strong easterly winds over the western Pacific helped
strengthen the northeasterly winds over the southern South
China Sea. Hence we hypothesize that the presence of the
MJO over the Indian Ocean contributed to the first episodeof precipitation in two ways. First, the presence of the MJO
helped to strengthen the northeasterly wind over the South
China Sea although the cold surge outburst from the north
was the prime cause of the strong northeasterly wind.
Secondly, the existence of strong easterly winds south ofJava opposed the penetration of the cross-equatorial winds
from the southern part of the South China Sea. As a result,the counter-clockwise turning of the northeasterly winds wasinhibited leading to the direct penetration of northeasterly
winds into the southern Peninsular and Sumatra. Indirectly,
this condition also inhibited the formation of the Borneo
vortex.
[11] The large-scale circulation over an area west of
Sumatra during the second episode was markedly different
compared to that during the first episode. During thisperiod, the circulation was dominated by twin cyclones
west of Sumatra with strong westerly winds in between thecyclones [Perreira et al., 1996]. As the system approached
Sumatra and southern Peninsular Malaysia, the strong
westerly jet in between the twin-cyclones eventuallytriggered strong westerly winds over an area north of Java.
This led to the counter-clockwise turning of the northeasterly
winds over the southern South China Sea and helped toform the Borneo vortex and strong cross-equatorial flows(Figure 2b). These circulations eventually helped intercept
and transport the low-level moisture to the western andsouthern parts of Borneo. During this period, deep convection over Sumatra and southern Peninsular Malaysia was
reduced while that over western and southern parts ofBorneo was enhanced.
[12] For a period of about two weeks after the second
episode (i.e. from 29 December 2006 – 10 January 2007),there was no significant heavy precipitation recorded oversouthern Peninsular Malaysia. During this period theremnant of the MJO was over the Maritime Continent(not shown). However, the strong westerly winds over andnorthof Java during thisperiod promoted the counter-clockwise
turning of the northeasterly winds and avoided direct
penetration of the northeasterly winds to southern Peninsular
Malaysia and Sumatra. The formation of the Borneo vortex
during this period helped to shift the enhanced deep
convection region to western and north Borneo. It wasreported that certain locations in northern Borneo wereaffected by floods during this period. However, during thethird episode of the extreme precipitation event, the remnant

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Figure 3. Time – longitude plot of the 5°S – 5°N averaged of (a) 30 to 60 day band pass filtered OLR anomalies (Wm 2).
The daily anomalies of OLR were filtered using a Lanczos filter with 121 weights. (b) Daily TMI SST anomaly (°C),(c) The difference of the daily mean interpolated OLR from a value of 200 Wm 2. Red indicates enhanced convection.
of the MJO was over the eastern edge of the Maritime
Continent (not shown). The strong northeasterly wind overthe central South China Sea originated mostly from a strong
easterly component over the western Pacific. Over and northof Java, the westerly winds were considerably weaker thanthat during 00Z 30 December 2006 and such a condition
was favorable for a direct penetration of northeasterly winds
to the southern Peninsular Malaysia, hence producing
similar conditions to that during the first episode.
3.4. The Role of the Termination of the 2006 Indian
Ocean Dipole[13 ] The anomalous conditions during the extreme
episodes could also be related to the abrupt termination of
the 2006 IOD. The anomalously cooler sea surface temperature (SSTA) of the southeastern Indian Ocean associated withthis event was at its peak in September-October-November
2006 (Figure 3b). However, the negative anomaly was
abruptly eroded in December 2006. In fact, during thisperiod the entire equatorial Indian Ocean and regional seassurrounding the Maritime Continent were warmer thanusual. Associated with this change was the eastward
propagation of the active deep convection (Figure 3c).

During November 2006, the active deep convection was
confined to the Indian Ocean. However, during December
2006, the active convection spread over the entire equatorial
Indian Ocean and the Maritime Continent, indicating an
eastward propagation (Figure 3c). Interestingly, in normal
years, during this period, the active convection is mostly
confined in the east. The prolonged convection was mostprobably due to the warm SST anomalies that persisted in
the west. As suggested by Figure 3c, the convective phase
continued until mid-January and failed to switch to a nonconvective phase. The role of convection from Indian
Ocean, however, only had a secondary role in the extreme
precipitation episodes. It was the northeast cold surge thatinteracted with the MJO related circulation that mostly
contributed to the extreme events. Convection over thewarmer SST in the entire equatorial Indian Ocean provided
a favorable condition for these events to occur.

4. Summary[14] The 2006/2007 flood over Peninsular Malaysia wascaused by extreme precipitation events which occurred inthree episodes i.e., between 17– 20 December 2006, 24–

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28 December 2006 and 11 – 14 January 2007. These
extreme precipitation episodes were mainly due to thestrong northeasterly winds over the South China Sea thatinteracted with the large-scale circulation associated with
the MJO. In all cases, strong northeasterly winds penetrated
anomalously far south to southern Peninsular Malaysia andSumatra in a straight trajectory. The blocking effect of theelevated terrain of Sumatra caused low-level convergenceand enhanced deep convection over southern Peninsular
Malaysia and hence the flood. The lack of counterclockwise turning of the northeasterly winds and theabsence of Borneo vortex were mainly due to the enhanced
easterly over and north of Java. These enhanced easterly
winds over this region and also north of Australia and in thewestern Pacific were associated with Rossby wave-type of
responses to the MJO deep convection over the Indian
Ocean and were most pronounced during the first episode.
The scenario during the second episode was similar to thatof the first. However, the circulation west of Sumatra wasdominated by the presence of twin-cyclones over the region.
The presence of strong westerly jets over the equator in
between the twin-cyclones eventually promoted the counterclockwise turning of the northeasterly winds and theformation of the Borneo vortex. The circulation associatedwith the vortex helped to intercept low-level moisture fromthe regions of Sumatra and southern Peninsular Malaysia towestern and southern parts of Borneo. During the third
episode, the remnants of MJO disturbances were at theeastern edge of the Maritime Continent. During this period,
the enhanced deep convection was also associated with thedirect penetration of northeasterly winds to a region ofelevated terrains over Sumatra and Peninsular Malaysia.
The abrupt termination of the IOD in December 2006 also
played a role. The anomalously cooler SST in southeastern
Indian Ocean was at its peak in September-OctoberNovember 2006. However, in December 2006 thesenegative anomalies eroded abruptly. Warmer SST in theequatorial Indian Ocean helped in the MJO formation.
[15] Acknowledgments. This research is funded by the Malaysian
Government Sciencefund grants 04-01-02-SF413, 04-01-02-SF0437, andthe Universiti Kebangsaan Malaysia Research University Grant OUPUKM-OUP-ASPL-5/2007.

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