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CLIMATE OF BRUNEI DARUSSALAM
Introduction
The climate of Brunei is governed by the location on the northwest coast of Borneo
within the equatorial tropics, and the wind systems of Southeast Asia which result
from the atmospheric pressure distribution over the region as a whole.
The pressure distribution can be simply describe as an area of low pressure at the
Equator, with areas of high pressure over the subtropics in each hemisphere. The low
pressure trough at the Equator is often termed the Inter Tropical Convergence
Zone or ITCZ, emphasizing its importance as the area where air masses from the
northern and southern hemispheres converge (Figure 1). The ITCZ is not a stationary
feature, its position oscillating with respect to the zenithal position of the sun, but
lagging some two months behind, and the magnitude of the latitudinal oscillation is
reduced to about half that of the sun. The annual movements of the ITCZ and theassociated Trade wind fields produce two main seasons in Brunei, separated by two
transitional periods.
Figure 1: MTSAT-IR Source: Kochi University, Japan
During the Northern Hemisphere winter months from December to March, the
northeast monsoon winds affect the South China Sea and Borneo, re-curving through
the ITCZ to become northwesterly winds over the Indonesia. The average position of
the ITCZ is the zone between the latitudes of 5S and 10
S after having moved
southwards across Borneo and Brunei during late December. The period is commonly
termed the Northeast Monsoon (Figure 2A). During the months from June to
September the ITCZ is situated at latitude of around 15N to east of the Philippines
but to west the ITCZ becomes a monsoon trough. The southeast trades from the
southern hemisphere re-curve across the Equator to become the broad southwesterly
flow over the Southwest Monsoon. The first transitional periods occurs in April and
May while the second one in October and November.
On inter-annual timescale (3 to 7 years) the climate of Brunei is influenced by El
Nio Southern Oscillation (ENSO). The warm (cold) episode or El Nio (La Nia) is
normally associated with prolonged dry conditions (wetter than normal) in BruneiDarussalam.
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Figure 2: (A) Mean Asian monsoon circulation at 850hPa and surface pressure (hPa)
during Winter Monsoon (1948-99); (B) as for (A) except for Summer Monsoon.
The term monsoon is derived from the Arabic word mausim meaning season,
and was originally used to indicate the constancy of the winds in many parts of East
and Southeast Asia. However, the term has become synonymous with the rainy season
associated with the southwest monsoon. In common with other low latitude areas the
atmospheric pressure in Brunei exhibits a quite regular semi-diurnal cycle. The origin
of this cycle is still controversial, but appears to be related to daily temperature, cloud
and rainfall (Riehl, 1979).
Rainfall
The annual rainfall total exceeds 2300mm throughout the country. Figure 3 gives a
general guide to the mean annual rainfall distribution in Brunei Darussalam. Notably,
the Brunei International Airport Meteorological Station (BIA) makes the most
complete range of meteorological observations at half-hourly intervals conforming to
standard international procedures spelt out by the World Meteorological Organisation
(WMO) that started in 1966-current. Figure 3 gives the monthly and yearly rainfall
statistics of Brunei International Airport (1966-2006).
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MEAN MONTHLY AND ANNUAL RAINFALL
1966 - 2006
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
Jan
Feb
Mac A
prM
ay Jun
July
Aug
Sept
Oct
Nov
Dec
MONTH
RAINFALLAMOUNT(mm)
273.
0
139.
5
129.
3194.
8 247.
9
219.
7
216.
7
219.
6
2
57.
6 314.
0
321.
9
346.
2
Figure 3: (above); Mean monthly and annual rainfall (1966-2006) and (below); Brunei
International Airport (BIA) monthly rainfall for the respective year (January-
February:1966-2007 and March-December up to 2006).
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Figure 4: Map of average rainfall and seasonal wind directions (source: The Curriculum
Development Department, Ministry of Education, Brunei Darussalam)
The mean monthly rainfall indicate certain seasonal patterns with two maxima andtwo minima. The first maximum is from October (in fact in late October) to January
(in fact in early January) with December being the wettest month. The second minor
maximum is from May to July with May being relatively wetter. This seasonality is a
reflection of the two monsoon seasons in conjunction with the related movements of
the ITCZ and the influence of the localized land-sea circulations. The lowest
minimum occurs in February (in fact starts in late January) and March and the next
minor minimum is from June to August.
The concept of dry month or dry season in Brunei Darussalam (as well as in the
equatorial tropical areas) is relative and should not be taken in its literal sense.
Normally there is some rainfall at any time in the year. In particular years, however,dry periods of different durations do occur. In fact some heavy rainfalls with short
durations have occurred during the dry periods.
The orographic effect on rainfall in Brunei is notable, particularly in Temburong
District. The stations of Semabat and Selangan both have mean annual rainfall totals
of over 4000mm compared to the stations nearer the coast such as Puni and Bangar
with annual means of around 3600 mm (the respective figure is not shown here).
An important feature of rainfall is that it is characterized by high intensities (measured
in mm per hour) very large amounts falling over, sharply delimited areas at short
time intervals in contrast to prolonged rainfalls associated with large-scale synoptic
systems such as tropical cyclones (or typhoons in the Western Pacific) (Figure 5) and
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fronts in the higher latitudes. Such rainfall regime in the ITCZ are common features
and, as pointed out earlier, they are directly related to the origin from vigorous, small
to mesoscale convections cells of thunderstorms and squall lines.
Figure 5: Typhoon tracks during 2005 Pacific typhoon season
Figure 6 shows the diurnal variations of the probability of thunderstorms occurrences
(per 1000) for Brunei International Airport for the period of years 1979-2006. Two
peaks are evident the higher peak in April and May from late afternoons to early
hours of the morning and secondary peak from September to November, mostly from
late afternoons to just around midnight. The mean monthly rainfall amounts given onthe extreme right hand end of Figure 6 may be taken as indicator of the total rainfall
contribution from all rain producing weather systems rains accompanied by
thunderstorms and rain from showers (without thunderstorms) etc. It is evident that
more rainfalls are received in the form of rains and showers from September to early
January during the secondary peak of thunderstorm activities (from September to
December) than those during the higher peak of thunderstorms (in April and May).
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Figure 6: Diurnal variations of probability of thunderstorms (per1,000)observed at BIA (WW=17+95+97;1979-2006)
Temperature
The temperature regime is notable for its uniformity with only small variations both
seasonally and in different parts of the country. Figure 7 shows mean diurnal
variations of surface temperature recorded at BIA for the period of 1979-2006. Higher
temperatures are generally recorded during the months of March to September with
higher solar heating and decreased cloudiness and rainfall compared to other months.
Cold air surges originating from Siberia/China area during the northeast monsoon
season affect Brunei Darussalam resulting in lower minimum temperatures. Normally,
the minimum temperature is recorded near sunrise and the maximum temperature in
the early hours of the afternoon, in particularly around 1300 to 1400 LT. The highest
(screen) temperature may reach 34C to 38C, while the lowest may fall to 18C to
21C.
Figure 7: BIA average diurnal variations of surface temperature (1979-2006).Unit: Degree Celsius (C)
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Relative Humidity
Diurnal variation of relative humidity is presented in Figure 8. Its inverse correlation
with temperature at the corresponding hour is reflected in that high humidities are
found from around midnight to sunrise. The average daily value is around 85% withthe high peaking up to 95 to 98% and the lows reaching up to 62% on cloudless
afternoons.
Figure 8: BIA diurnal average relative humidity (1999-2006). Unit: Percentage (%)
Sunshine and Evaporation
Once again higher sunshine and evaporation are nearly coincident with maximum
heating and decreased cloudiness and rainfall during the months of February to May
(figure is not shown).
Wind
Based on mean annual wind rose of Brunei International Airport for the years of
1969-1988 (not shown here), most wind speeds are low with about 42% of time being
calm (less than 1 knot) during the night and early morning hours immediately after
sunrise. The surface wind over inner coastal of Brunei Darussalam as represented by
hourly average wind observations in a form of wind rose diagrams in Figure 9 , 10
and 11, which cover the northeast and southwest monsoon seasons as well two
transitional monsoon seasons for the years of 2005-07. The seasonal influence of the
wind direction is evident in that the southerly wind dominates about 25% of time
during the southwest monsoon months, Figure 9 (below). Interestingly, both northerly
and southerly winds prevail about 18% and nearly 16% of time during the northeast
monsoon months (Figure 11).
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WRPLOT View - Lakes Environmental Software
WIND ROSE PLOT:
Station #96315 - BIA, BSB, BNWINDROSE (BIA) INTER SW MONSOON (April to May, 2005 - 2007)
DATE:
1/14/2008
PROJECT NO.:
3
COMMENTS:
Extract from B0501 - B0712no. of calm wind =43
MODELER:
CDIC
COMPANY NAME:
Brunei Meteorological Service
NORTH
SOUTH
WEST EAST
5%
10%
15%
20%
25%
WIND SPEED
(Knots)
>= 16
10 - 16
6 - 10
3 - 6
1 - 3
Calms: 0.98%
TOTAL COUNT:
4392 hrs.
CALM WINDS:
0.98%
DATA PERIOD:
2005-2007Apr 1 - May 3100:00 - 23:00
AVG. WIND SPEED:
8.58 Knots
DISPLAY:
Wind SpeedDirection (blowing from)
WRPLOT View - Lakes Environmental Software
WIND ROSE PLOT:
Station #96315 - BIA, BSB, BNWINDROSE (BIA) SW MONSOON ( June to October, 2005 - 2007)
DATE:
1/12/2008
PROJECT NO.:
1
COMMENTS:
Extract from B0501 - B0712no. of calm wind =87
MODELER:
CDIC
COMPANY NAME:
Brunei Meteorological Service
NORTH
SOUTH
WEST EAST
6%
12%
18%
24%
30%
WIND SPEED
(Knots)
>= 16
10 - 16
6 - 10
3 - 6
1 - 3
Calms: 0.69%
TOTAL COUNT:
11015 hrs.
CALM WINDS:
0.69%
DATA PERIOD:
2005-2007Jun 1 - Oct 3100:00 - 23:00
AVG. WIND SPEED:
8.75 Knots
DISPLAY:
Wind SpeedDirection (blowing from)
Figure 9: Pre-Southwest Monsoon Wind Rose (April-May, 2005-2007)
and Southwest Monsoon proper (Jun-October, 2005-2007) (below)
During pre-southwest and northeast monsoon proper northwesterly wind has a little
domination in term of time percentage around 7% and 10% respectively, being theresult of diurnal variations associated with sea breeze. It has even lesser time
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percentage domination during southwest monsoon proper around 4%, due to strong
wind components from southerly to westerly during this season. Sea breeze is a
significant feature that extends up to at least 1000m above sea level, contributing to a
sea breeze front that develops over the interior hinterland areas of the country. Large
cumulonimbus cloud cells with thundery showers form on these fronts in the
afternoons and evenings. These fronts move westwards with the prevailing upper(steering) level winds at around 3000m resulting in heavy burst of rains and strong
gusty winds along the trajectory.
The strongest winds are usually recorded during squalls which are often accompanied
by thunder and heavy rains. They are the result of convergence and intense convection
at sea and often appear as line squalls during the southwest monsoon season. They
traverse northeastwards with the prevailing upper (steering) level wind and may cause
much destruction to structures as well as, in some cases, affect offshore and nearshore
operations. Their durations are limited to one or two hours and wind speeds can reach
up to 50 to 60 km/h.
WRPLOT View - Lakes Environmental Software
WIND ROSE PLOT:
Station #96315 - BIA, BSB, BNWINDROSE (BIA) INTER NE MONSOON ( November, 2005 - 2007)
DATE:
1/12/2008
PROJECT NO.:
3
COMMENTS:
Extract from B0501 - B0712no. of calm wind =9
MODELER:
CDIC
COMPANY NAME:
Brunei Meteorological Service
NORTH
SOUTH
WEST EAST
6%
12%
18%
24%
30%
WIND SPEED
(Knots)
>= 16
10 - 16
6 - 10
3 - 6
1 - 3
Calms: 0.42%
TOTAL COUNT:
2160 hrs.
CALM WINDS:
0.42%
DATA PERIOD:
2005-2007Nov 1 - Nov 3000:00 - 23:00
AVG. WIND SPEED:
9.11 Knots
DISPLAY:
Wind SpeedDirection (blowing from)
Figure 10: Pre-Northeast Monsoon Wind Rose (November, 2005-2007)
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WRPLOT View - Lakes Environmental Software
WIND ROSE PLOT:
Station #96315 - BIA, BSB, BNWINDROSE (BIA) NE MONSOON ( December to March, 2005 - 2007)
DATE:
1/12/2008
PROJECT NO.:
2
COMMENTS:
Extract from B0501 - B0712no. of calm wind =87
MODELER:
CDIC
COMPANY NAME:
Brunei Meteorological Service
NORTH
SOUTH
WEST EAST
4%
8%
12%
16%
20%
WIND SPEED
(Knots)
>= 16
10 - 16
6 - 10
3 - 6
1 - 3
Calms: 1.00%
TOTAL COUNT:
8711 hrs.
CALM WINDS:
1.00%
DATA PERIOD:
2005-2007Check Date Range Report00:00 - 23:00
AVG. WIND SPEED:
9.42 Knots
DISPLAY:
Wind SpeedDirection (blowing from)
Figure 11: Northeast Monsoon Wind Rose (December-March, 2006-2007)
Tropical cyclones in the West Pacific are common during the southwest monsoon
season and are identified according to the maximum wind speeds near their centres
tropical depression, tropical storms and typhoons. They originate over the water in the
equatorial and tropical areas of Southwest Pacific and generally traverse
northwestwards and may pass South China Sea just to the north of Brunei waters, as
demonstrated by Figure 5. The strong southwesterly to westerly winds over the
Brunei waters during the passage of these tropical cyclones may cause severe squall
lines to form over that area. The seas generated by these strong winds and the tropical
cyclones formed can significantly affect local swell conditions and usually lead to
rough sea state as well as occasional crossing of squally showers across the nation
from southwest (west) to northeast (east).
Compiled by Abraham David
and Hj Sidup bin Hj Sirabaha (1996)
Brunei Meteorological Service
Department of Civil Aviation
Ministry of Communications
Brunei Darussalam
Updated by Dr. Hj Sidup bin Hj Sirabaha (March, 2008)
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