Holocene mega-droughts and the seasonal structure of El Niño events in the Philippines
Rose D. Berdin1, Michael K. Gagan1, Fernando P. Siringan2 and Atsushi Suzulki3
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines
3 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
The Holocene climatic evolution of the Southeast Asian monsoon domain was largely controlled by opposing trends in the summer monsoon (weaker towards the present) and El Niño-Southern Oscillation (stronger towards the present). These trends have been attributed to orbitally driven changes in the seasonal cycle of insolation, which enhanced Asian monsoon rainfall and suppressed El Niños during the middle Holocene (Clement et al. 2000, Liu et al. 2000). However, new coral-based palaeoclimate reconstructions from the Philippines provide surprising evidence for abrupt shifts in climate and mega-droughts superimposed on this somewhat benign climatic picture (Gagan et al. 2006).
In this study, we examined the evolution of El Niños from 7,600 years ago to the present using annually resolved records of 18O/16O in a suite of fossil Porites corals from eastern Samar, Philippines. At Samar, coral 18O/16O reflects the combined effect of rainfall and sea surface temperature (SST) and pronounced positive 18O/16O anomalies relate to droughts and cooler SSTs during El Niños (Fig. 1a). The coral 18O/16O records show that long droughts (>3 years duration) were more prevalent in the middle Holocene, the longest of which lasted 14 years. These protracted droughts appear to be linked to multi-decadal climate fluctuations. The extraordinary duration of the mid-Holocene droughts may help explain why agricultural societies were not firmly established in the Philippines until after ~4,000 years ago.
To investigate the seasonal structure of the droughts in detail, we performed bi-weekly analysis of coral 18O/16O throughout the most prominent events (Fig. 1b). We compared coral 18O/16O anomalies at the start, peak and end of the four most recent El Niño events (1986/7, 1990-95, 1997/98 and 2002/03) with events recorded at 7.2 ka and 2.3 ka. Composite analysis of the droughts reveals significant changes in the onset and magnitude of the 18O/16O anomalies. Droughts associated with El Niño events in the modern coral record typically peak towards the end of the calendar year. In contrast, droughts at 7.2 ka and 2.3 ka commenced about 3-6 months earlier and ended in the middle of the calendar year within two months of the termination-time for present-day events.
In summary, the coral records show that the longest droughts occurred in the middle Holocene, yet peak anomalies on seasonal time-scales were largest in the late Holocene (at ~2.3 ka). This finding is consistent with a climate model (Clement et al. 2000) and palaeo-ENSO records (Moy et al. 2002) showing large amplitude and more frequent events in the late Holocene. Our results indicate that the magnitude, duration and seasonal structure of El Niño events have evolved substantially through the Holocene.
Figure 1. Holocene mega-droughts and the seasonal structure of El Niño events in the Philippines. (A) Evolution of El Niño events from 7,600 years ago (7.6 ka) to the present based on annually resolved 18O/16O anomalies in fossil corals from eastern Samar, Philippines. (B) Composite analysis of bi-weekly coral 18O/16O anomalies to reveal the seasonal structure of El Niño events at 7.2 ka, 2.3 ka, and the present.
Clement, A.C., Seager, R., and Cane, M.A., 2000. Suppression of El Niño during the mid-Holocene by changes in the Earth's orbit. Paleoceanography, 15: 731-737.
Gagan, M.K., Hantoro, W.S., Natawidjaja, D.H., Lough, J.M., Meyers, G.A., Liu, Z., Sieh, K., ARC Discovery Grant DP0663227 (2006-2010): The Indian Ocean Dipole, Australasian drought, and the great-earthquake cycle: Long-term perspectives for improved prediction.
Liu, Z., Kutzbach, J., and Wu, L., 2000. Modeling climate shift of El Niño variability in the Holocene. Geophysical Research Letters, 27: 2265-2268.
Moy, C.M., Seltzer, G.O., Rodbell, D.T., and Anderson, D.M., 2002. Variability of El Niño/Southern Oscillation activity at the millennial timescales during the Holocene epoch. Nature, 420: 162-165.