DOI: 10.25921/j078-dh26
title: NOAA/WDS Paleoclimatology - Mentawai - d18O Data and DMI Reconstruction

description: The interplay of the El Nino Southern Oscillation, Asian monsoon and Indian Ocean Dipole (IOD) drives climatic extremes in and around the Indian Ocean. Historical and proxy records reveal changes in the behaviour of the El Nino Southern Oscillation and the Asian monsoon over recent decades. However, reliable instrumental records of the IOD cover only the past 50 years, and there is no consensus on long-term variability of the IOD or its possible response to greenhouse gas forcing. Here we use a suite of coral oxygen-isotope records to reconstruct a basin-wide index of IOD behaviour since AD 1846. Our record reveals an increase in the frequency and strength of IOD events during the twentieth century, which is associated with enhanced seasonal upwelling in the eastern Indian Ocean. Although the El Nino Southern Oscillation has historically influenced the variability of both the IOD and the Asian monsoon, we find that the recent intensification of the IOD coincides with the development of direct, positive IOD-monsoon feedbacks. We suggest that projected greenhouse warming may lead to a redistribution of rainfall across the Indian Ocean and a growing interdependence between the IOD and Asian monsoon precipitation variability.
description: Monthly record of oxygen isotope ratios (d18O) from 3 coral cores in the Mentawai Island arc, western Indonesia, and reconstructed Indian Ocean dipole mode index (DMI). The Mentawai Islands are located ~200 km offshore of Sumatra in western Indonesia (~0ºS, 98º30'E), within the eastern region used to define the Indian Ocean DMI. A 2.7 m long core (PG01-A-2, 0º08'S, 98º31'E, 6m. water depth) was extracted from a dead modern Porites sp. coral in June 2001 using an underwater hydraulic drill-rig. This coral colony is part of a fringing reef system approximately 400 m offshore of Penang Island, a small coral cay in the northern Mentawai Island chain. A second core was collected from a dead modern Porites sp. coral in 1.2 m water depth at a fore-reef slope offshore of Tanabala Island (TN99-A-4; 0º24'S, 98º31'E; 29 cm long core collected July 1999). A third core was sampled from a live Porites sp. coral growing in ~6 m water depth on the fore-reef slope of a fringing reef on Tanamasa Island (TM01-A-10; 0º23'S, 98º33'E; 33 cm long core collected June 2001).The individual coral records span 1858-1992 (PG01-A-2), 1990-1996 (TN99-A-4), and 1993-1997 (TM01-A-10). Adjustments of +0.27‰ for the TN99-A-4 coral and -0.09‰ for the TM01-A-10 coral were applied to match the mean d18O values of these shorter records to the PG01-A-2 d18O record. Following these adjustments, the periods of overlap between the individual d18O time series were averaged to produce the Mentawai coral d18O record. Anomalies were calculated by removing the average seasonal cycle of d18O. The approximately 7 month gap in the Mentawai record in 1915/1916 is due to a boring in the PG01-A-2 coral skeleton at this point. The Mentawai coral d18O record has significant correlations with local SST in a range of gridded SST products including the Reynoldsv2 blended ship, buoy and satellite SST record (rJul-Nov = -0.90, p = 0.0000, 1982-1997), the HadISST compilation (rJul-Nov = -0.58, p = 0.0000, 1958-1997), and the ERSSTv3 SST dataset based on ICOADS ship measurements (rJul-Nov = -0.42, p = 0.0000, 1880-1997). The coral DMI was derived from coral d18O anomaly records from the Mentawai Islands (this study; Supplementary Table 1), Bali (Charles et al 2003), and Seychelles (Charles et al 1997, Pfeiffer and Dullo 2006). The filtered DMI was calculated using the method described by Saji et al.(1999), where variability at periods longer than 7 years has been removed. The unfiltered DMI retains IOD variability at periods longer than 7 years. Both coral DMIs have been normalised over the 1960-1990 interval.