Borneo has an equatorial climate characterised by uniformly high temperature and rainfall throughout the year. Rainfall can exceed 4000 mm per annum in many parts of the region.
There is no distinct seasonality on land but the seas, especially the South China Sea on the West Coast of Borneo with its long north to south fetch, are noticeably affected by the monsoon winds. The Sulu Sea and the Sulawesi Sea on the East Coast of Borneo, rarely experience large waves and the major climatic influence is the daily land-sea breeze cycle with it's associated rainfall.
Relative humidity is invariably over 70 per cent, and air temperatures fluctuate between 20 - 36 degrees Celcius.
Surface waters in the South China Sea range from 21 - 29 degrees Celcius.
Over the South China Sea, the monsoons are generated by the low-pressure troughs in the intertropical convergence zone which moves North or South following the sun through the seasons.
During the North-East monsoon from November to March, there is an atmospheric high pressure centred over China, a low over Japan and the winds in the South China Sea blow from the North-East. During the early part of the monsoon, up to the end of January, the North-Easterly winds come from between Japan and mainland China and these winds from a cold cyclonic area bring stormy weather.
During this time of the year, the Northeast monsoon is subject to surges or rapid increases in strength and these bring rapid deterioration of weather and sea conditions. A Northerly swell generated in the northern part of the South China Sea will usually take 24 hours to reach the West coast of Borneo, but it will produce rough seas very quickly upon reaching shallow water.
The second type of weather experienced during the North-East monsoon occurs during February and March when the winds come predominantly from the Pacific ocean. These winds are dry and the weather is mainly dry and sunny which makes February the driest month of the year in Northern Borneo
During the transition between monsoons in March - April, winds are light and weather is controlled mainly by the land and sea breezes with localised showers and thunderstorms. During the Southwest monsoon from May to September, the atmospheric equatorial trough moves North while a heat created low pressure area forms over China. The trade winds then blow East from Northern Australia, curving to the northwest as they cross the Java Sea and the equator to blow from the Southeast along the coast of Borneo. These winds are dry and subsiding and produce hot weather with calm seas. However, this is the time for cyclones to form in the northern parts of the South China Sea and these create westerly winds from the Indian Ocean bringing unsettled wet weather with frequent squalls.
The monsoon winds also combine with the daily land and sea-breezes to create localised weather effects. The sea breeze starts around 11.00 hrs and combined with the Northwest monsoon creates heavy rain or thunderstorms wherever the breeze is forced over hills. During the night, the land breeze blows at right angles to the North-West monsoon wind, creating a line of convergence parallel to the coast which creates a line of showers or thunderstorms about 8 km off the coast. This rain rapidly dies away at dawn as the land breeze weakens.
During the Southwest monsoon (strong winds blowing from the South-West to the North-East), the land and sea-breezes are both at right angles to the monsoon wind so while the rain dies away at dawn further showers form near the coast as the sea breeze develops late in the morning. These rain showers move inland with the sea breeze and often develop into thunderstorms over high land or later in the afternoon.
In Sabah, two seasons are generally distinguished: the rainy/wet season, and the dry season. The wet season starts in November, with the onset of the North-East monsoons in Peninsular Malaysia and ends towards March. During the months of April to November, when typhoons frequently develop over the West Pacific and move westwards across the Philippines, South-Westerly winds over the North-West coast of Sabah and Sarawak region may strengthen, reaching 20 knots or more. Sabah is not affected by the typhoons - hence being known as the "Land Beneath The Wind".
Sabah has uniform temperatures throughout the year. The annual variation is less than 2 degrees C, with temperatures rarely exceeding 31 degrees C, and rarely dropping below 23 degrees C. The mean monthly relative humidity falls within 70% to 90%, varying from place to place and from month to month.
Mt Kinabalu is South East Asia’s highest mountain and at the top (4095.2 metres), temperatures can drop to 0 degrees Celsius. At Laban Rata and the other mountain rest houses (situated around 3300 metres), the early morning temperature hovers around 10 degrees Celsius. The Mountain also tends to make its own climate, and winds, clouds and rainfall can be expected in the afternoon. For safety reasons all climbers must depart from the Kinabalu National Park Head Quarters by, or before noon.
On the whole, the climate in Sarawak is very similar to Sabah. The temperature is relatively uniform within the range of 23 degrees C to 32 degrees C throughout the year. During the months of March to September, the weather is generally dry and warm.
Humidity is consistently high on the lowlands ranging from 85% to 95 %. The average rainfall per year is between 3,300 mm and 4,600 mm, depending on locality, and the wettest months are from November to February.
The coastal areas of Sarawak and North-East Sabah experience a rainfall regime of one maximum and one minimum. While the maximum occurs during January in both areas, the occurence of the minimum differs. In the coastal areas of Sarawak, the minimum occurs in June or July, while in the northeast coastal areas of Sabah, it occurs in April. Under this regime, much of the rainfall is received during the northeast monsoon months of December to March. In fact, it accounts for more than half of the annual rainfall received on the western part of Sarawak.
Inland areas of Sarawak generally experience quite evenly distributed annual rainfall. Nevertheless, slightly less rainfall is received during the period June to August which corresponds to the occurence of prevailing southwesterly winds. It should be noted that the highest annual rainfall area in Malaysia may well be found in the hill slopes of inland Sarawak areas. Long Akah, by virtue of its location, receives a mean annual rainfall of more than 5000 mm.
The North-West coast of Sabah experiences a rainfall regime of which two maxima and two minima can be distinctly identified. The primary maximum occurs in October and the secondary one in June. The primary minimum occurs in February and the secondary one in August. While the difference in the rainfall amounts received during the two months corresponding to the two maxima is small, the amount received during the month of the primary minimum is substantially less than that received during the month of the secondary minimum. In some areas, the difference is as much as four times.
In the central parts of Sabah, where the land is hilly and sheltered by mountain ranges, the rainfall received is relatively lower than other regions and is evenly distributed. However, two maxima and two minima are noticed, though somewhat less distinct. In general, the two minima occur in February and August while the two maxima occur in May and October.
Southern Sabah has evenly distributed rainfall. The annual rainfall total received is comparable to the central part of Sabah. The period February to April is, however slightly drier than the rest of the year.
Brunei Darussalam lies around 4 degrees North of the Equator and around 114 degrees East and is sandwiched between Sabah
to the North and Sarawak to the East and South. The climate is very similar to that of its neighbours. Its Equatorial
type of climate shares the sunshine and rainfall all the year round. The seasonal and the two types of
diurnal wind systems - the land and sea breezes along the coast and the katabatic and anabatic wind further
South, are prevailing in Brunei. The local weather is generally affected by the combination of these local winds
and the orographic nature etc. It is is also influenced by the North-East Monsoon during December to March and
the South-West Monsoon during May to September. April and October are the Inter-Monsoon period or the
Transition periods. Ocassionally, the effects of the tropical cyclones around the area also contribute to the
local weather pattern.
Weather information coutesy of Tropical Research And Conservation Centre (TRACC) in Kuching and the Malaysia Meteorological Service