Reconstructing El Niño-Southern Oscillation
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RECONSTRUCTING EL NIÑO-SOUTHERN OSCILLATION EVIDENCE FROM TREE-RING, CORAL, ICE AND DOCUMENTARY PALAEOARCHIVES, A.D. 1525-2002. Joëlle L. Gergis A thesis submitted under the requirements for the degree of Doctor of Philosophy, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, Australia. April, 2006. EXECUTIVE SUMMARY El Niño-Southern Oscillation (ENSO) is the most important coupled ocean-atmospheric phenomenon to cause global climate variability on interannual time scales. Efforts to understand recent, apparently anomalous ENSO behaviour are hampered by the lack of long, high-quality climate records. While instrumental data generally covers the past 150 years, record length is insufficient for the assessment of past changes in the frequency, magnitude, and duration of ENSO. Here, multiproxy networks of high-resolution tree-ring, coral, ice and documentary records derived from eastern and western Pacific ENSO ‘centres of action’ are analysed (A.D. 1525-2002). Considerable improvements in ENSO reconstruction are achieved from expanding the use of records from the western Pacific. In particular, ~500 years of a continuous 3,722 year ENSO sensitive tree-ring record from New Zealand is introduced. Although extreme ENSO events are seen throughout a 478-year discrete event analysis, 43% of extreme, 20% of very strong and 28% of all protracted ENSO events occur within the 20th century. Principal component analysis was used to extend instrumental records of the Southern Oscillation Index (SOI) Niño 3.4 Sea Surface Temperature (Niño 3.4 SST) and a newly developed coupled ocean-atmospheric ENSO index (CEI) by 347 years. Significantly, of the three indices reconstructed here, CEI reconstructions were largely found to be the best predictors of ENSO. The results suggest that ENSO may be more effectively characterised using a coupled ocean-atmosphere index, particularly for December-May periods. Compared to the pre-instrumental period, the late 19th and early 20th centuries indicate a clear trend toward increased ENSO variability over the past 150 years. Significantly, spectral analysis of reconstructed indices reveals a marked change in the frequency and intensity of ENSO beginning ~A.D. 1850, coinciding with the end of the Little Ice Age and the boom in global industrialisation. This suggests that ENSO may operate differently under natural (pre-industrial) and anthropogenically influenced background states. This study asserts that recent ENSO variability appears anomalous in the context of the past five centuries. Given the considerable socio-economic impacts of ENSO events, future investigation into the implications an increasingly anthropogenically-warmed world may have on ENSO is vital. ACKNOWLEDGEMENTS First and foremost, thanks must go to ‘Team Dendro’ from the Tree-Ring Laboratory, and the School of Geography and Environmental Science at the University of Auckland, New Zealand, for hosting me in 2003 & 2004. This project would not have been possible without the training and support I gained from working with the ‘fellowship of the rings’; Anthony Fowler, Gretel Boswijk, Peter Crossley, Jonathan Palmer, Andrew Lorrey, Jenny Lux and Tim Martin. Thank you for all your efforts in making the ‘Aussie sheila’ feel welcome in NZ. I am particularly grateful to you all for participating in the community consultation required for my fieldwork and sharing the hours of ‘forest madness’ spent out there. Thanks for introducing me to the life of professional research. In particular, I am indebted to (Lord) Anthony Fowler for taking on the challenges involved with this project and mentoring me. I am most thankful for all your time, patience and good humour during those “NIÑO NIÑA NIÑO” conversations on both sides of the Tasman. I am appreciative of all those meticulous ‘spanners in the works’ that trained me in the rigors of good science. Many thanks for developing the tree-ring master chronologies (AGAU05a and AGAU05b) used in Chapter 4. It was a privilege working with you and outstanding Team Dendro on the landmark Kauri project. Sincere thanks to Richard Gilles and Janeen Collings (New Zealand Department of Conservation), Stephen King (Waipoua Forest Trust), Davey Paniora (Te Iwi O Te Roroa), Jeff Marsden (Panguru Development Trust), Rongo Bentson and Bobby Proctor (Te Iwi o Te Rarawa) for overseeing the collection Kauri tree-ring sampling process. On the sunny side of the ditch, particular thanks to Scott Mooney (UNSW), for his inspired introduction to the world of palaeoclimatology. I am grateful for your foresight, careful editing, financial assistance and guidance over the years. Thank you for taking a stand for geography existing at UNSW under such trying circumstances. I have appreciated all your time, patience and encouragement in pursuing my vision with this project. J. L. GERGIS I am fortunate to have crossed paths with Karl Braganza (National Climate Centre, Bureau of Meteorology, Melbourne) at CLIVAR in 2004. I have really appreciated all your time, online support and good humour in the final year of this project. I have learned much from your sharp insight into ENSO and the vision you have brought to interface of palaeoclimatology and dynamical studies. I gratefully acknowledge the scrupulous PCA reconstructions and advice on the material presented in Chapter 6. Many thanks to James Risbey (CSIRO, Hobart) for your interest and willingness to be involved in this cross-disciplinary project. I am very thankful for your support of my research and some of the spectral results presented in Chapter 6. Thank you to Penny Whetton (CSIRO, Melbourne) and Ian Rutherfurd (University of Melbourne) for access to the Nile Flood Record, Berlage Teak Chronology, North China Rainfall data, Erica Hendy (Lamont Doherty Earth Observatory, New York) for the Great Barrier Reef coral luminescence master chronology, Pavla Fenwick (Lincoln University, New Zealand) for Pink Pine tree-ring data, and Bruce Bauer (World Data Centre for Palaeoclimatology/National Climatic Data Centre Palaeoclimatology Branch) for data acquisition and assistance. I would also like to thank a number of academics who have provided me with advice and feedback during the course of my PhD; Mike Gagan, John Chappell, Erica Hendy, Simon Haberle, Michael Mann, Connie Woodhouse, Raj Kane, Takanori Horii, David Edwards, Gary Meyers, Anita Drumond, Manu Black, Jim Renwick, David Enfield, and David Meko. Above all I would like to acknowledge the friends and family that provided me with the personal support to get this one ‘over the line’. Endless thanks to my soul family; Emma Walter, Rebecca Schofield, Katy Alexander, Petra Lane, Charlie Honner and Juliette Di Moro for the unique support you have provided me on this journey. Thanks to all for making it over to New Zealand (and China!) to share precious time-out with me. I appreciate all the time spent bringing me back from my “I’m not a scientist” days. I am so fortunate to have such extraordinary people in my life; this PhD would not have been possible without you. Special thanks must go to Jennifer Ezzy for neighbourly support in the final stages of this project; always at the ready to drag me away from my desk for a reality check. Every thanks for your eleventh hour proof of the final manuscript and reassuring me that “I’m not having an ugly baby”! I would especially like to thank my family; Mum, Dad and Chris for all your moral support during this time. Particular thanks to my Mum for all your love and vi ACKNOWLEDGEMENTS encouragement on the ‘PhD-crisis hotline’, and my Dad for instilling ‘education is the key’ from an early age. Thanks to Chris for, ah…being Chris! Thank you all for putting up with my unavailability over the past few years. I look forward to making up for lost time. This project acknowledges financial assistance from a UNSW Faculty Research Grant ‘Late 20th Century El Niño-Southern Oscillation in the Context of Holocene Climate Change’ and the Royal Society of New Zealand’s Marsden Project ‘Reconstructing ENSO history from Kauri tree-rings’ (UOA Contract 108). I would also like to acknowledge generous funding from; i. The University of New South Wales for supporting my research with an Australian Postgraduate Award, 2002-2005. ii. CLIVAR for full financial assistance to participate in the 1st International CLIVAR Science Conference: Understanding and Predicting Our Climate System, Baltimore, USA, June 2004. iii. The European Geophysical Union and the International Research Center on El Niño (CIFFEN) for financial assistance to participate in the 1st International Alexander Von Humboldt Conference on the El Niño phenomenon and its global impact, Guayaquil, Ecuador, May 2005. iv. PAGES full finanical assistance to participate in the PAGES 2nd Open Science Meeting: Palaeoclimate, Environmental Sustainability and Our Future, Beijing, China, August 2005. v. The Australian Research Council, Monash University and the Bureau of Meteorology Research Centre for full financial assistance to participate in A Workshop on Earth System Models of Intermediate Complexity, Melbourne Australia, February 2006. Finally, I would like to thank my partner Josh Bassett for his steadfast love, patience and support through every stage of this task, including the move to New Zealand. I am eternally grateful for all your technical wizardry, heart-starter coffees and gentle reminders of life ‘outside of the PhD’. Thank you for providing me the great freedom and luxury of being fully consumed in the process of realising this project. vii PUBLICATION INFORMATION Chapter 2 appeared in full as: Gergis, J. and Fowler (2005), Classification of synchronous oceanic and atmospheric El Niño- Southern Oscillation (ENSO) events for palaeoclimate reconstruction, International Journal of Climatology 25: 1541–1565. I was the primary investigator and lead author of this paper and conducted all of the analyses presented therein. Material contained in Chapter 4 has appeared as: Gergis, J., Boswijk, G. and Fowler, A. (2005). An update of modern Northland Kauri (Agathis australis) tree-ring chronologies 1: Puketi State Forest.