During the El Niño developing year, rainfall starts decreasing close to the equator earlier in summer than off-equatorial region in autumn, such as in mainland Palau. Eastern Indonesia has a dry season during boreal summer in general, and the dry season becomes a significantly drier condition during El Niño years.
One of the mechanisms for these changes is the air-sea interaction over the Banda and Arafura seas. Intensification of seasonal southerly wind associated with the western North Pacific monsoon decreases sea-surface temperature over the Banda and Arafura seas, and convection is more suppressed during El Niño years than usual. Another mechanism is the subsidence over the Banda and Arafura seas during the premonsoon of Australian summer monsoon. Land-ocean thermal contrast between Australia and its surrounding seas is intensified during El Niño years. Shallow thermal convection produces subsidence over the Banda and Arafura seas, and extremely dry condition spreads over the surrounding region of the Banda and Arafura seas in October during El Niño years.