ENSO and Decadal Climate Variability: Evidence From Geochemical Analysis of Annually-Banded Corals in Papua New Guinea
Alexander W. Tudhope (Department of Geology & Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, Scotland, UK)
e-mail: Sandy.Tudhope@ed.ac.uk
Colin P. Chilcott (Dept. Geology & Geophysics, Edinburgh University)
Graham B. Shimmield (Dunstaffnage Marine Laboratory, PO Box 3, Oban, Scotland, UK)
ABSTRACT
Corals living on the north coast of Papua New Guinea record ENSO-related changes in sea surface temperature and rainfall in the oxygen isotopic composition of their skeletons. Two 110 year long, monthly-seasonal resolution δ18O records from living corals situated 150km apart are presented. Results of cross-spectral analysis of these records illustrates that they provide regionally-reproducible climatic information across a wide range of frequencies, from annual through interannual (ENSO) to decadal-interdecadal. Comparison of these records with instrumental indices of ENSO further illustrates the reliability of the coral δ18O timeseries as recorders of interdecadal variability in the strength of ENSO over the past century. A 95 year long bimonthly resolution record of δ18O from a 2,650 year old coral in the region is presented and compared to a modern coral record. This comparison suggests that ENSO-style climatic variations existed 2,650 years ago, but that ENSO cycles were, on average, weaker (lower amplitude) than the average of the past 110 years. In addition, the ENSO variability in the fossil coral displays a similar interdecadal modulation in strength to that displayed in the modern coral record. These results clearly demonstrate the potential for using massive corals to elucidate the nature of interdecadal and longer timescale variability in ENSO.
INTRODUCTION
Geochemical analysis of the skeletons of annually-banded massive corals has the potential to yield multi-century, monthly-resolution proxy records of sea surface temperature (SST) and sea surface salinity (SSS). Such records can contribute to the understanding of interannual-centennial climate variability by extending the instrumental record and by revealing the sensitivities of different modes of climate variability to changes in global climatic boundary conditions. This paper will illustrate some of these roles using examples from living and ancient corals from the north coast of Papua New Guinea.
CORALS AS ENVIRONMENTAL INDICATORS:
As corals grow, environmental changes are recrded in the stable isotopic and trace element composition of their aragonitic (CaCO3) skeletons. Individual colonies of some massive (dome-shaped) varieties of coral may live for several centuries, and, growing at 10-20mm/year, reach several metres in diameter. These attributes, combined with the presence of annual bands in the skeletons, makes corals excellent natural archives of high-resolution palaeoclimatic information.
Analysis of the stable oxygen isotopic composition of coral bands has proved to be a particularly robust tracer of the combined effects of changes in SST and seawater isotopic composition.
