Abstract
The intertropical convergence zone (ITCZ) has been considered to be an important factor to which the rainfall distribution in the low latitudes ascribes. But the ITCZ has not yet been clearly analyzed and has been defined differently by many authors. It was defined in terms of air streams, cloudiness or even rainfall distribution. But the cloud distribution does not always show a good correspondence with the air stream analysis. Consequently, it is necessary to clarify the relationship among the rainfall distribution and the air streams and the cloud distribution.
The purpose of this paper is to clarify the climatic factors which account for the rain-fall distribution over Indonesia. The author proposed a model of the general circulation of the lower troposphere based on the data of Geostationary Meteorological Satellite, and discussed the relationship among the rainfall distribution, the general circulation in the lower troposphere, and the cloud distribution.
The following results are obtained:
1. Three main air streams dominate over Indonesia; the tropical easterlies which origi hate in the Northern Pacific Ocean, the tropical easterlies which originate in the Southern Pacific Ocean, and the westerlies which originate in the Indian Ocean. In January the westerlies flow zonally (Fig. 7), but in the other three months (April, July, October) the two tropical easterlies are separated by a wedge of the westerlies (Figs. 8_??_10). So there are two zones of convergence. When the westerlies dominate mainly in the Northern Hemisphere (Figs. 9 and 10), the northern convergence zone is clear and the southern one is not clear. The latter is explained by the fact that the southern one separates the westerlies and the deflected winds of the tropical easterlies. Therefore, the author defined the north-ern one as the “main ITCZ” and the southern one as the “sub ITCZ”. When the west-erlies dominate mainly in the Southern Hemisphere (Fig. 7), the southern one is the main ITCZ and the northern one is the sub ITCZ.
2. Both the heavy rainfall zone and the dense cloud zone are formed within the westerlies (Figs. 7_??_10). The main and sub ITCZs don't always correspond to the axes of these zones, but are located in the margins of these zones. Therefore, the main climatic factor which creates rainfall over Indonesia is the westerlies. In the region of the tropical easter-lies the amount of rainfall and clouds are generally small, which are more marked in the region of the tropical easterlies originate in the Southern Pacific Ocean (Figs. 7_??_10). Under any air streams the rainfall distribution is strongly affected by landforms, resulting in local orographic rainfall on the windward regions of mountains.
3. Indonesia may be divided into three regions by the rainfall distribution and the air streams; Java and Lesser Sunda, western Indonesia, and eastern Indonesia. The boundary between western and eastern Indonesia is the central mountains in Kalimantan. In Java and Lesser Sunda (A-I region in Fig. 2), the period when the westerlies dominate is short and the period when the tropical easterlies dominate is long. They have much precipita-tion under the westerlies (Fig. 7) and little precipitation under the tropical easterlies (Figs. 8_??_10). Therefore, the seasonal change of precipitation is marked. In western In-donesia (A-II region in Fig. 2), the westerlies dominate all the year round and they give much precipitation (Figs. 7_??_10). Therefore, the dry season is not so marked. In eastern Indonesia (B-I and B-II regions in Fig. 2), the tropical easterlies dominate almost all the year round and they don't give much precipitation except in the regions which have local orographic rainfall (Figs. 7_??_10).
