The Meteorology and Geophysics Agency (BMG) said large waves that have been pounding the southern coastlines of Java, Sumatra and Bali since Thursday have weakened, but are expected to continue for another week.
The agency said the waves were caused by persistent winds from the Indian Ocean coinciding with the arrival of the lunar tide.
The head of the maritime meteorology information subsection at the BMG, Suratno, said Monday high atmospheric pressure in the southern part of the world, especially in the Indian Ocean, and low atmospheric pressure in the northern part of the world, especially in India and Japan, had caused strong southerly winds, which subsequently had caused massive waves around Indonesia.
"High atmospheric pressure causes only normal winds of between 25 to 30 knots, but because this is persistent, it has caused massive waves," Suratno told The Jakarta Post after a closed-door meeting to discuss the phenomenon.
Representatives from the Agency for the Assessment and Application of Technology, the Coordinating Body for the Survey and National Charting Development Board and the Office of the Coordinating Minister for People's Welfare attended the meeting.
Suratno said the waves were elongated, similar to the waves of a tsunami, although were less powerful.
"The massive waves are not as powerful as those of a tsunami but the condition has been exacerbated by the swelling of southern coastal waves caused by the arrival of the lunar tide on May 16.
"We have observed that the moon's gravitational pull on the water along the southern coasts of Sumatra, Java and Bali is at its maximum.
"This is a superposition which causes very high waves, especially in coastal areas where the sea bottom is shallow," Suratno said.
The fluctuation in the size of waves caused by the wind had been detected as early as May 15, but reached its peak on May 20, he said.
"A superposition like this won't last long. It is already subsiding now," Suratno said.
Suratno dismissed the idea the large waves were a new phenomenon in Indonesia.
"We have experienced waves like this before, but this time it has been particularly bad as superposition like this rarely happens," Suratno said.
However, he said the BMG had not predicted the waves would be so big or cause such widespread destruction.
He said the BMG regularly provided information on the weather to the mass media and local administrative bodies, but this obviously did not prevent the occurrence of disasters.