Interannual Rainfall Variability over the Eastern Maritime Continent

The authors investigate the interannual rainfall variability over the eastern maritime continent using station rainfall data in the Republic of Palau during 1923-2009 and Eastern Indonesia during 1973-2008. Two possible mechanisms are proposed to explain the interannual rainfall variability associated with the El Nino/Southern Oscillation over the surrounding region of the Banda and Arafura seas. During the El Nino developing year, rainfall starts to decrease, first in region close to the equator in summer than in 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 Nino years. The start of increasing rainfall is delayed. Cloud data show a convection-suppressed region from the east coast of the Kalimantan to the west coast of the New Guinea during July to November of El Nino 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 Nino years than other years. Another mechanism is the subsidence over the Banda and Arafura seas during the pre-monsoon of the Australian summer monsoon. Land-ocean thermal contrast between Australia and its surrounding seas is intensified during El Nino years. Shallow thermal convection produces subsidence over the Banda and Arafura seas, and extremely dry condition spreads over the surrounding region in October during El Nino years.