brunei climate

8/2/2019 Brunei Climate

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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)


WIND ROSE PLOT:

Station #96315 - BIA, BSB, BNWINDROSE (BIA) SW MONSOON ( June to October, 2005 - 2007)

DATE:

1/12/2008

PROJECT NO.:

1

COMMENTS:


MODELER:

CDIC

COMPANY NAME:


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:


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.


WIND ROSE PLOT:

Station #96315 - BIA, BSB, BNWINDROSE (BIA) INTER NE MONSOON ( November, 2005 - 2007)

DATE:

1/12/2008

PROJECT NO.:

3

COMMENTS:


MODELER:

CDIC

COMPANY NAME:


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:


Figure 10: Pre-Northeast Monsoon Wind Rose (November, 2005-2007)

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WIND ROSE PLOT:

Station #96315 - BIA, BSB, BNWINDROSE (BIA) NE MONSOON ( December to March, 2005 - 2007)

DATE:

1/12/2008

PROJECT NO.:

2

COMMENTS:


MODELER:

CDIC

COMPANY NAME:


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:


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)


Department of Civil Aviation

Ministry of Communications

Brunei Darussalam

Updated by Dr. Hj Sidup bin Hj Sirabaha (March, 2008)

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