HARRY H. HENDON
NOAA–CIRES Climate Diagnostics Center, Boulder, Colorado
(Manuscript received 9 November 2001, in ﬁnal form 28 October 2002)
Relationships between Indonesian rainfall and Indo-Paciﬁc sea surface temperatures (SSTs) and circulation anomalies are investigated using observations for 1951–97. Indonesia receives signiﬁcant rainfall year-round but experiences a wet season that peaks in January and a dry season that peaks in August. Dry season rainfall anomalies are spatially coherent, strongly correlated with SST, and tightly coupled to El Nino–Southern Oscillation (ENSO) variations in the Paciﬁc basin. Drought conditions typically occur during El Nin˜o, when SSTs surrounding Indonesia are cool and the Walker circulation is weakened, resulting in anomalous surface easterlies across Indonesia. The opposite tends to occur during La Nina. Broadscale Indonesian rainfall and SST anomalies tend to not persist from the dry season into the wet season. Rainfall in the heart of the wet season tends to be uncorrelated with SST and spatially incoherent.
Seasonally varying feedback between Indonesian SST, winds, and rainfall explains the growth, persistence, and coherence of the local anomalies during the dry season and their decay or change in sign once the wet season commences. During the dry season anomalous surface easterlies, remotely driven by warm SSTs in the central Paciﬁc during El Nino, act to increase local wind speed, cooling the ocean surrounding and to the east of Indonesia and thereby increasing the anomalous SST gradient across the Paciﬁc. Hence, local rainfall and the Walker circulation are further reduced. Once the wet season commences and the climatological surface winds across Indonesia shift from southeasterly to northwesterly, the anomalous surface easterlies now act to reduce the wind speed. The initial cold SST anomaly is damped, reducing the negative rainfall anomalies and surface easterlies. The opposite scenario occurs during La Nina.
Indonesian rainfall variations during the dry season are also coupled to the development of an anomalous zonal SST gradient in the equatorial Indian Ocean. This anomalous gradient is strongly related to ENSO and is most prominent during the dry season. Once the wet season commences, the entire Indian Ocean tends to have the same-signed SST anomaly (positive during El Nin˜o and negative during La Nin˜ a). Development and decay of this anomalous zonal SST gradient in the Indian Ocean is promoted by seasonally varying air–sea interaction in the eastern Indian Ocean in response to ENSO conditions in the Paciﬁc. The eastern Indian Ocean SST changes are driven largely by induced surface heat ﬂux variations (primarily changes in latent heat ﬂux and net shortwave radiation). Biennial variations in the Indonesian region may also be induced by this seasonally varying air–sea interaction associated with ENSO.