Due to its geographical position, the meteorological condition of the archipelago is under the monsoon regime. During the west monsoon the northwesterly winds bring a lot of rain to the country, especially to the western part of the archipelago. During the east monsoon the southeasterly winds bring rain only to the eastern part of the islands, while the western islands experience dry period.
The monsoon regime influence strongly the oceanographic condition of the archipelago, not only at the surface but down to about 75 meter depth. The sea currents occurring in different seas, straits and gulfs in most port of the country reverse their direction twice annually and vary their strengths according to the monsoon wind.
The physical properties of the sea water such as temperature, salinity, light penetration, sound waves, etc., vary according to the monsoons; so do the chemical properties such as nutrients, organic carbon; and the biological condition such as primary production and chlorophyll, species abundance and distribution of the marine organisms.
In the subsurface and meso-layer of the seas of the archipelago, the oceanographic properties are governed by the Arlindo, namely the Pacific to Indian Througflow passing the archipelagic waters. The variability of the properties such as sound velocity, light transmission and turbidity, dispersal of organisms etc. to a significant extent are influenced by the variability of Arlindo. The most important to note is the variability of the amount of mass and heat that is connected to the global climate variability.
The deep and bottom layers of the sea basins within the archipelago is filled mostly with the deep water of the Pacific Ocean that eventually leave these basins out to the Indian Ocean. Therefore the oceanographic properties at these layers is much determined by the condition occurring in the Pacific Ocean, rather than the Indian Ocean.
The Monsoons
The primary driving force of the monsoon winds in Indonesia is the air pressure difference between Asia and Australia caused by the motion of the Inter-Tropical Low Pressure Zone and the Subtropical High Pressure Zone. During the boreal winter the southern hemisphere receive more solar radiation that heats the Astralian Continent more than the Southeast Asia Continent. As the consequence, the Tropical Low extends over Australia and the North Subtropical High extends southward to reach Southeast Asia Between the high of Aaia and the low of Australia, the monsoon winds develop during December, January and February. The direction of the winds is northeast in the part of the archipelago north of the equator, is north in the equator and northwest south of the equator. And the monsoon is called the northwest or west monsoon in Indonesia.
During the boreal summer of June, July, August, the reverse occurrs in the condition of solar radiation and heating, the locality of the atmonspheric pressures and the wind direction. The monsoon is called the east or southeast monsoon in Indonesia. The strength of the winds is 25 to 40 knots during west monsoon and 15 to 30 knots during the east monsoon.
