GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L04709, doi:10.1029/2005GL025465, 2006 Monsoon drought over Java, Indonesia, during the past two centuries Rosanne D’Arrigo,1 Rob Wilson,2 Jonathan Palmer,3 Paul Krusic,1 Ashley Curtis,1 John Sakulich,1 Satria Bijaksana,4 Siti Zulaikah,5 and La Ode Ngkoimani6 Received 12 December 2005; revised 17 January 2006; accepted 19 January 2006; published 24 February 2006. [1] Monsoon droughts, which often coincide with El Nin˜o and Krusic, 2004], yet are scarce for Indonesia and the warm events, can have profound impacts on the populations tropics as a whole. A network of ring-width chronologies of Southeast Asia. Improved understanding and prediction has been derived from living teak (Tectona grandis L. F.) of such events can be aided by high-resolution proxy trees in Java and Sulawesi [D’Arrigo et al., 1994]. The climate records, but these are scarce for the tropics. Here development of these records follows pioneering efforts by we reconstruct the boreal autumn (October–November) Berlage [1931], who produced the first such record for Java. Palmer Drought Severity Index (PDSI) for Java, Indonesia In Java and Sulawesi, annual growth rings can form in teak (1787–1988). This reconstruction is based on nine ring- during the dry, east monsoon season which extends from width chronologies derived from living teak trees growing June to November [Hastenrath, 1987]. Here we use the on the islands of Java and Sulawesi, and one coral d18O teak records, along with one coral oxygen isotope series, to series from Lombok. The PDSI reconstruction correlates develop a reconstruction of a drought index for Java, significantly with El Nin˜o-Southern Oscillation (ENSO)- and describe its relation to past monsoon and ENSO related sea surface temperatures and other historical variability. and instrumental records of tropical climate, reflecting the strong coupling between the climate of Indonesia and the large scale tropical Indo-Pacific climate system. 2. Data and Methods Citation: D’Arrigo, R., R. Wilson, J. Palmer, P. Krusic, 2.1. Climate Data A. Curtis, J. Sakulich, S. Bijaksana, S. Zulaikah, and L. O. Ngkoimani (2006), Monsoon drought over Java, Indonesia, [4] We use the updated global monthly Palmer Drought during the past two centuries, Geophys. Res. Lett., 33, Severity Index (PDSI) data set (1870–2002) [Dai et al., L04709, doi:10.1029/2005GL025465. 2004] (http://www.cdc.noaa.gov; v. 2). An index of meteorological drought, the PDSI is a proxy for soil moisture and streamflow [Dai et al., 2004]. The PDSI is 1. Introduction based on temperature and precipitation data for global land areas, on a 2.5° grid. It was averaged over 5–10°S, 105– [2] Droughts can have devastating effects on the peoples 115°E, representing Java and vicinity (Figure 1). The of largely agrarian areas of monsoon Asia, including Javanese PDSI series are compared to several tropical Indonesia. Indonesian drought results from the failure of climate records, including El Nin˜o sea surface temperature the east monsoon, and tends to be linked to ENSO warm (SST) indices [Kaplan et al., 1998], the Southern Oscilla- events [Hastenrath, 1987; Harger, 1995]. During such tion Index (SOI) [Ropelewski and Jones, 1987], and station times the Walker Circulation weakens as the Indonesian records of precipitation for Jakarta, Indonesia (Global Low migrates eastward into the tropical Pacific, resulting in Historical Climate Network) [Peterson and Vose, 1997], drought over much of the country [Allan, 2000]. Indonesia and sea-level pressure (SLP) data for Darwin, Australia is, therefore, particularly relevant to the study of the Indo- [Basnett and Parker, 1997]. Pacific climate system [Hackert and Hastenrath,1986; Hastenrath,1987;Nicholls, 1995]. The correspondence between Indonesian climate and ENSO may be the most intense of any land area worldwide, with correlations between ENSO indices and Indonesian rainfall ranging from 0.7–0.8 for some periods [Moore, 1995; van Oldenborgh, 2002]. [3] Land-based rainfall proxies can considerably improve our understanding of past drought variability [e.g., Cook 1Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, New York, USA. 2School of Geosciences, University of Edinburgh, Edinburgh, UK. 3Canterbury, New Zealand. Figure 1. Map of Java and vicinity showing PDSI grid 4Department of Physics, Institut Teknologi Bandung, Bandung, (5-10°S-105-115°E) and tree-ring (blue, D’Arrigo et al. Indonesia. 18 5 [1994]) and coral d O records (pink, Moore [1995]) Department of Physics, State University of Jakarta, Jakarta, Indonesia. used as predictors in PDSI regression model over 1870– 6Department of Physics, Haluoleo University, Kendari, Indonesia. 1988 [Cook and Kairiukstis, 1990]. Values show Copyright 2006 by the American Geophysical Union. significant (90% C.L.) correlations between each proxy 0094-8276/06/2005GL025465$05.00 (at lag T and/or T+1) and Oct–Nov PDSI. L04709 1of4 L04709 D’ARRIGO ET AL.: MONSOON DROUGHT OVER JAVA, INDONESIA L04709 Table 1. Description of Teak Chronologies Used to Reconstruct Java PDSIa Site Name Site Code Latitude Longitude Full Period No of Series Mean r Period > 10 Series Individual Chronologies Randublatung RAN 7.06 S 111.22 E 1925–2004 13 0.47 1938–2004 Bekutuk BEK 7.07 S 111.22 E 1668–2004 20 0.40 1834–2004 Gubug Payung GUB 7.05 S 111.29 E 1864–2004 18 0.34 1879–2004 Donoloyo Cagar Alam DNLY 7.52 S 111.12 E 1714–2004 13 0.39 1746–2004 Pagerwunung Darupono DNRP 7.02 S 110.16 E 1776–2004 19 0.39 1820–2004 Klangon Natural Forest KLA 7.30 S 111.47 E 1707–2004 15 0.49 1812–2004 Saradan SAR 7.29 S 111.42 E 1689–2000 30 0.46 1812–2000 Bigin BIG 7.10 S 111.34 E 1839–1995 20 0.59 1853–1995 Muna MUN 5.30 S 123.00 E 1564–1995 39 0.42 1673–1994 Composite Series SAKLA SAKLA - - 1689–2004 45 0.46 1759–2004 RBGB RBGB - - 1668–2004 71 0.36 1834–2004 Coral Data Lombok Strait LOM 8.15 S 115.30 E 1782–1989 - - - aMean r = mean inter-series correlation derived from program COFECHA, and is an indication of common signal among tree-ring series within a chronology [Holmes, 1983]. Period > 10 series is an arbitrary cutoff for acceptable replication. SAKLA = composite of KLA and SAR; RBGB = composite of RAN, BEK, GUB and BIG. Series used in final reconstruction are: (tree rings): DNLY, DNRP, MUNA, SAKLA, and RBGB; (coral): Lombok d18O [Moore, 1995]. 2.2. Tree-Ring and Coral Data [7] The PDSI reconstruction explains 38% of the vari- [5] Nine individual Indonesian tree-ring series were used ance for the most replicated (1840–1988) period, dropping (Table 1 and Figure 1). Due to low replication at some to 26% for the least replicated (1787–1819) period locations, some sites were composited to develop more (Figure 2). Although these values are not very high, the robust time series (Table 1 and Figure 1). Some of these statistical results (Figure 2) demonstrate that the model is records were previously shown to correlate with ENSO indices [D’Arrigo et al., 1994]. A few of these chronologies were included in previous ENSO reconstructions [Stahle et al., 1998; Mann et al., 2000], and several of the teak series, along with coral records, were used to reconstruct Indonesian SSTs (R. D’Arrigo et al., Reconstructed Indonesian warm pool SSTs from tree rings and corals: Linkages to the Asian monsoon and ENSO, submitted to Paleoceanography, 2005, hereinafter referred to as D’Arrigo et al., submitted manuscript, 2005). The coral series is a monthly skeletal d18O record from Lombok Strait (composited to create an annual series), which also relates to local SSTs (Table 1 and Figure 1) [Moore, 1995; D’Arrigo et al., submitted manuscript, 2005]. After compositing some of the chronologies, 5 tree-ring series and 1 coral series were used to develop the reconstruction. 3. The Java PDSI Reconstruction [6] The October-November season optimized the PDSI Figure 2. (a) Reconstructed (black) and instrumental (red) signal in the combined proxies. These months fall within the Oct–Nov Java PDSI over 1787–1988 and 1870–2004, period when rainfall over Java is best linked to ENSO respectively. Reconstruction scaled to instrumental data [Haylock and McBride, 2001; van Oldenborgh, 2002; over the calibration period. Historical droughts identified Aldrian and Susanto, 2003]. Variability in precipitation from sea salt records (vertical bars, Quinn et al. [1978], over Java is due in part to orographic effects, and there is 1844–1976) and planter’s records and rain measurements a moisture gradient with drier conditions in eastern Java. (triangles, van Bemmelen [1916], 1833–1914) are high- This variability can complicate efforts to reconstruct lighted. White = reconstructed PDSI > 0; red = PDSI drought and pinpoints the need for more diverse proxy data between 0 and À3, blue = PDSI values < À3.0. Horizontal coverage. Our reconstruction is thus best representative of green dashed line highlights À3.0 PDSI level. (b) and conditions in central-east Java and vicinity. Details (c) Calibration and verification statistics for nested on reconstruction methodology are provided as auxiliary reconstruction models. aR2 = square of multiple correlation material1. coefficient adjusted for loss of degrees of freedom; CE = Coefficient of Efficiency. CE’s greater than zero indicate model skill [Cook and Kairiukstis, 1990]. ST = Sign test 1Auxiliary material is available at ftp://ftp.agu.org/apend/gl/ [Fritts, 1976] shows ratio of agreement/disagreement; all 2005GL025465. values are significant above 99% level. 2of4 L04709 D’ARRIGO ET AL.: MONSOON DROUGHT OVER