Brunei ClimateThe 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.