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Seasonal Characteristics of the Indian Ocean Dipole During the Holocene Epoch

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Seasonal Characteristics of the Indian Ocean Dipole During the Holocene Epoch Vol 445 | 18 January 2007 | doi:10.1038/nature05477 LETTERS Seasonal characteristics of the Indian Ocean Dipole during the Holocene epoch Nerilie J. Abram1{, Michael K. Gagan1, Zhengyu Liu2,3,4, Wahyoe S. Hantoro5, Malcolm T. McCulloch1 & Bambang W. Suwargadi5 The Indian Ocean Dipole1,2 (IOD)—an oscillatory mode of IOD sector results in anomalous cooling of SSTs and intense coupled ocean–atmosphere variability—causes climatic extremes drought. An unambiguous record of these eastern IOD SST and and socio-economic hardship throughout the tropical Indian rainfall anomalies can potentially be provided through coupled mea- Ocean region1–5. There is much debate about how the IOD inter- surements of Sr/Ca (a proxy for SST10) and oxygen isotope ratios acts with the El Nin˜o/Southern Oscillation (ENSO) and the Asian (d18O; a proxy for SST and changes in rainfall/evaporation11)in monsoon, and recent changes in the historic ENSO–monsoon rela- corals from the Mentawai Islands. Furthermore, great palaeo- tionship6 raise the possibility that the properties of the IOD may earthquakes along the Mentawai island arc12 have frequently uplifted also be evolving. Improving our understanding of IOD events and the fringing reefs surrounding the Mentawai Islands, leaving indi- their climatic impacts thus requires the development of records vidual fossil Porites colonies well-preserved in growth position. This defining IOD activity in different climatic settings, including pre- historic times when ENSO and the Asian monsoon behaved dif- ferently from the present day. Here we use coral geochemical a 30° N +4 records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events +2 over the past 6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of 0° 0 strong surface ocean cooling, together with droughts that peaked –2 later than those expected by El Nin˜o forcing alone. Climate model SST anomaly (°C) simulations suggest that this enhanced cooling and drying was the 25° S –4 result of strong cross-equatorial winds driven by the strengthened 30° E 130° E Asian monsoon of the middle Holocene. These IOD–monsoon b connections imply that the socioeconomic impacts of projected 1 future changes in Asian monsoon strength may extend throughout 0 Australasia. –1 (°C) The IOD, also termed the Indian Ocean Zonal Mode, involves a –2 reversal of the sea surface temperature (SST) gradient and winds SST anomaly –3 across the equatorial Indian Ocean from their climatological state. 300 200 Climate simulations suggest that IOD events can be initiated by –0.1 El Nin˜o conditions that decrease the thermocline depth in the 100 Indonesian throughflow region, and cause an eastward migration 0.0 0 –100 of the Pacific Walker circulation cell (which encourages atmospheric (‰) 0.1 7,8 O anomaly –200 subsidence over the eastern Indian Ocean) . Simulations also indi- 18 0.2 ∆δ –300 cate that IOD events can result from processes internal to the Indian Rainfall anomaly (mm) Ocean region, through intensified Hadley circulation of the Asian– 1993 1994 1995 1996 1997 1998 1999 Australian monsoon, which strengthens the southeasterly tradewinds Year and promotes upwelling of cooler water in the equatorial eastern Figure 1 | IOD climate anomalies. a, SST anomalies during November of the Indian Ocean7,8. However, the ability to test these simulations is 1997 IOD event13, when anomalous cooling in the east and warming in the restricted by the spatial limitations and brevity of high-quality his- west produced a reversal of the equatorial SST gradient across the Indian torical climate records (Supplementary Fig. 1) and the scarcity of Ocean. Boxes mark the eastern and western sectors used to define the Dipole 2 proxy records4,9, particularly in the eastern IOD region. Mode Index , and the white circle shows the location of the Mentawai High-resolution reconstructions of past climate in the tropical Islands. b, In the Mentawai Islands the strong IOD events of 1994 and 1997 were characterized by cool SST anomalies (black curve)13 and drought (grey ocean regions can be obtained through the analysis of geochemical bars)29. These distinct IOD SST and rainfall anomalies are preserved, tracers in the skeletons of massive Porites sp. corals. The coral reefs of respectively, in coral Sr/Ca–SST (red curve) and Dd18O (blue curve) the Mentawai Islands, western Indonesia, lie within the eastern anomalies11. Coral time series between July 1993 and February 1997 are Indian Ocean sector used to define the Dipole Mode Index of IOD based on the average of two coral records, with error bars showing the activity2 (Fig. 1a). During strong IOD events, upwelling in the eastern difference between the coral records for each monthly data point. 1Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia. 2Center for Climatic Research, University of Wisconsin- Madison, 1225 W. Dayton Street, Madison, Wisconsin 53706, USA. 3Earth Environment Institute, Chinese Academy of Science, Xi’an 710075, China. 4The Ocean University of China, Qingdao 266003, China. 5Research and Development Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Bandung 40135, Indonesia. {Present address: British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, UK. 299 © 2007 Nature Publishing Group LETTERS NATURE | Vol 445 | 18 January 2007 allows us to examine the ocean–atmosphere signatures of individual climate in the Indian Ocean region2,5,14,15, consistent with strong IOD IOD events for slices of time since ,6,500 calibrated years before events. Indeed, 1961, 1994 and 1997 correspond with the three stron- present (BP). gest droughts in the rainfall record of the Padang-Tabing station in To test the suitability of Mentawai corals for reconstructing IOD western Indonesia (1u S, 100u E), which was established as part of an SST and rainfall anomalies, Sr/Ca and d18O were analysed at high extensive network of Indonesian rainfall stations following the devas- (,monthly) resolution in two modern corals from the Mentawai tating drought of 1877 (ref. 15). The modern coral records suggest Islands (Supplementary Methods). Both corals were collected in that strong IOD events occur in both El Nin˜o (1877, 1994 and 1997) open, fore-reef slope environments on fringing reef systems, at water and non-El Nin˜o (1961) years, consistent with climate simulation depths of 1.2 m (coral TN99-A-4) and ,6 m (coral TM01-A-10; results7,8. IOD events also appear to exist over a wide range of mon- Supplementary Table 1). Both provide a record of the strong IOD soon conditions, with the 1877 IOD event corresponding to a year of event in 1994. The TM01-A-10 coral record also extends into the extreme monsoon failure, while the 1961 IOD occurred during the 1997 IOD event, when the Mentawai reefs experienced widespread heaviest monsoon in the nearly 200-year-long all-India monsoon coral mortality4. The coral Sr/Ca–SST anomalies reliably reconstruct rainfall index16. Hence, understanding how the IOD, ENSO and both the timing and magnitude of IOD cooling during 1994 and Asian monsoon systems interact has important implications for the 1997, as documented by blended buoy, ship and satellite SST predictability of climate extremes in the Indian Ocean region. records13 (Fig. 1b, Supplementary Figs 1 and 2). The coral records During the early to middle Holocene, broad-scale strengthening of of residual d18O(Dd18O), determined by using the Sr/Ca–SST the Asian summer monsoon occurred owing to increased Northern records to remove the temperature component of coral d18O mea- Hemisphere summer insolation17. At the same time, ENSO variabil- sured for the same samples, reflect changes in the d18O composition ity was suppressed18,19 and a more La Nin˜a-like mean state existed of sea water that are due to variations in the surface–ocean balance of across the tropical Pacific Ocean20. Thus, mid-Holocene fossil corals rainfall and evaporation11. Comparison with station and gridded from the Mentawai Islands provide a good opportunity to examine rainfall data shows that the Mentawai coral Dd18O records replicate how externally forced increases in Asian monsoon strength (which the droughts in western Indonesia that accompanied the 1994 and have been predicted for the coming century21–23) may influence the 1997 IOD events (Fig. 1b). IOD system. To do this, reconstructions of four individual IOD The distinct cool/dry IOD signal in the Mentawai coral Sr/Ca and events from the middle Holocene (6,570–4,160 calibrated years BP; d18O records indicates that corals from the eastern upwelling region Fig. 2) were averaged to produce a composite IOD event for a period can be used as an independent proxy for strong IOD activity. Further of strong Asian monsoon forcing (Fig. 3, Supplementary Fig. 3, analysis of a 140-year-long coral record from the Mentawai Islands Supplementary Table 2). For comparison, a composite IOD event has identified two more exceptionally strong IOD signals in 1961 and was also constructed using four late Holocene IOD events (from 1877 AD4 (Supplementary Fig. 3). Both years were times of extreme 2,180 calibrated years BP to the present), when the Asian monsoon weakened and El Nin˜o events were relatively strong and frequent. Comparison of the composite SST anomalies shows that, on aver- a age, the magnitude of cooling during strong eastern IOD upwelling –9.0 15 O (‰) events is similar for the strong El Nin˜o and strong monsoon scenarios 18 δ –8.5 10 (Fig. 3; 23.9 6 0.2 uC and 24.3 6 0.6 uC, respectively). The timing of –8.0 peak IOD cooling also appears to have remained essentially constant –7.5 5 with respect to the seasonal cycle of SST.
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