on the west coast of Sulawesi therefore tend to have their highest rainfall in December whereas those on the east coast have their wettest month around May. One might expect to find intermediate areas with two dry seasons (a bimodal distribution) and this is indeed the case; Pendolo and Pinrang in the middle of the southwest peninsula are examples.
Where the orientation of a range of mountains is more or less at right angles to the prevailing winds, the rainfall is higher on the windward side because the water in the air rises and cools as it climbs over the mountains, and this moisture is released as rain. Thus Maros receives over 500 mm per month between December and February but towns on the leeward side of the peninsula receive little rain. Valleys orientated in a north-south direction are in a rain shadow for virtually the whole year and the sheltered nature of the central western coast results in the Palu valley being one of the driest areas of Indonesia with less than 100 mm of rain, on average, falling in each month and an annual total of less than 600 mm.
Various authors have mapped the climatic zones of Sulawesi. The map which corresponds closest to the distribution of vegetation is that which uses the ratio between dry and wet periods (fig. 1.13) (Schmidt and Ferguson 1951; Whitmore 1984a, b).
A second map based on suitability criteria for growing rice has also been devised (fig. 1.14) (Oldemann and Darmiyati 1977), in which five major zones are recognized.
| Zone A | - | an area with ten to twelve consecutive wet months and two or less consecutive dry months; |
| Zone B | - | an area with seven to nine consecutive wet months and three or less consecutive dry months; |
| Zone C | - | an area with five or six consecutive wet months and three or less consecutive dry months; |
| Zone D | - | an area with three or four consecutive wet months and two to six consecutive dry months; |
| Zone E | - | an area with zero to two consecutive wet months and up to six consecutive dry months. |
'Wet' and 'dry' are defined as more than 200 mm and less than 100 mm of rain per month respectively.
Sulawesi has a greater percentage of its area in agroclimatic Zone E than have the islands around it, but more in Zones B and C than Borneo (most of which is in Zone A), Nusa Tenggara and Bali, or the Moluccas (table 1.4).
The most recent and complex climatic map of Sulawesi concerns bioclimate (Fontanel and Chantefort 1978) and recognizes many zones determined by three criteria: mean temperature of the coldest month, mean annual rainfall (fig. 1.15), and number of dry months9 (fig. 1.16). One of the major differences between this map and that on agroclimatology is that it takes into account altitude effects, although not in an absolute sense since the degree of exposure to prevailing winds has a significant effect (p. 21).
The maps discussed above are based on long-term averages but uncommon climatic events, particularly periodic drought, can be extremely important in determining the distribution of certain animals and plants. Most crops, for example, experience stress after only about four days without rain. The variation in annual rainfall is quite considerable (fig. 1.17) with the total for one year sometimes being twice the total of another. Meteorological records normally extract maximum rainfall figures but these are not particularly meaningful ecologically because above a certain quantity, rain will simply run off saturated soils to rivers. Lack of water, with its associated cloudlessness, high temperatures and low humidity, is a much more potent factor and an examination of rainfall minima and their distributions reveal that these too are extremely variable (fig. 1.18). Dry seasons can in fact only be defined by probability since, for the stations examined, at least six different months were recorded as 'driest months' in at least one year. A single location (e.g., Mapanget or Watampone) can have minimum monthly rainfalls between years ranging from 0 to 100+ mm.
Figure 1.13. Rainfall types based on dry/wet period ratios.
After Schmidt and Ferguson 1951; Whitmore 1984a, b
Figure 1.14. Agroclimatic zones.
After Oldemann and Darmiyati 1977
* The figures in the text do not add up to 100%.
After Oldemann and Darmiyati 1977; Oldemann et al. 1980
Figure 1.15. Areas with different mean annual rainfall.
After Fontanel and Chantefort 1978
Figure 1.16. Areas with different numbers of dry months.
Alter Fontanel and Chantefort 1978
Figure 1.17. Annual rainfall at Mapanget (Manado airport), Watampone, Tobea Besar (an island between Butung and the mainland) and Palu.
Data obtained from the Directorate of Meteorology, Jakarta
Figure 1.18. Percentage of years' driest months occurring in each month with agroclimatic zone, average annual rainfall in parentheses, and years of complete data in brackets.
From data obtained from the Directorate of the Meteorology, Jakarta
Droughts in Sulawesi seem to occur about every 20-30 years but information on their ecological effects have not been found. The grassy Sook Plain in Sabah, however, was formed when fire burned rainforest and the peaty topsoil on which it grew after an exceptionally dry period in 1915 (Cockburn 1974). In 1972-73 a prolonged dry spell caused changes on Mt. Kinabalu, Sabah, which it has been estimated will take a century to be reversed (Whitmore 1984a). In most areas of the world, but particularly around the Pacific region, 1982-83 was exceptionally warm and dry, and weather patterns were most unusual. For example, Watampone had 5, and Tobea Besar 6, consecutive rainless months at the end of 1982. This was related to the anomalous sea surface temperatures which in parts of the Pacific rose by as much as 8°C above normal. Around Sulawesi, however, the sea was only about 0.2°C warmer than normal. The sea surface temperature anomaly first appeared in the region of the Sulawesi Sea during the middle of 1982 but its major effects were not felt until December 1982-February 1983 (Barber and Chavez 1983; Cane 1983; Gill and Rasmusson 1983; Rasmusson and Wallace 1983; Chavez et al. 1984; Whitmore 1984a). Although 1982 was generally a very dry year on Sulawesi, 1972 seems to have been even drier.
VEGETATION
Palaeovegetation
Impressions of leaves of grassy plants and rattans have been found in rocks in Minahasa (Koorders 1895) but the vast majority of information regarding palaeovegetation is derived from the careful analysis of pollen remains
