Brunei Climate
The climate of Brunei is generally hot and wet throughout the year
and 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. 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 the
associated "Trade" wind fields produce two main seasons in Brunei,
separated by two transitional periods.
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 5 °S and 10 °S after having moved southwards across
Borneo and Brunei during late December. The period is commonly
termed the "Northeast Monsoon". During the months from June to
September the ITCZ is situated at latitude of around 15 °N 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.
The mean monthly rainfall indicate certain seasonal patterns with
two maxima and two 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.
On inter-annual timescale (3 to 7 years) the climate of Brunei is
influenced by El Niño Southern Oscillation (ENSO). The warm (cold)
episode or El Niño (La Niña) is normally associated with prolonged
dry conditions (wetter than normal) in Brunei Darussalam.
