DOI: 10.25921/7aqa-qw06
title: NOAA/WDS Paleoclimatology - Maiana - Data

description: Today, the El Niño/Southern Oscillation (ENSO) system is the primary driver of interannual variability in global climate, but its long-term behaviour is poorly understood. Instrumental observations reveal a shift in 1976 towards warmer and wetter conditions in the tropical Pacific, with widespread climatic and ecological consequences. This shift, unique over the past century, has prompted debate over the influence of increasing atmospheric concentrations of greenhouse gases on ENSO variability. Here we present a 155-year ENSO reconstruction from a central tropical Pacific coral that provides new evidence for long-term changes in the regional mean climate and its variability. A gradual transition in the early 20th century and the abrupt change in 1976, both towards warmer/wetter conditions, co-occur with changes in variability. In the mid-late 19th century, cooler/drier background conditions coincided with prominent decadal variability; in the early 20th century, shorter-period (~2.9 years) variability intensified. After 1920, variability weakens and becomes focused at interannual time scales; with the shift in 1976, variability with a period of about 4 years becomes prominent. Our results suggest that variability in the tropical Pacific is linked to the region's mean climate, and that changes in both have occurred during periods of natural as well as anthropogenic climate forcing.
description: File includes columns for Year AD (bimonthly resolution = dec/jan, feb/mar, etc) and coral d18O (‰). The coral used in this study grew (and still does grow) at Maiana Atoll, Republic of Kiribati (1°N, 173°E). The colony lies at a depth of ~6m (low tide) and is about 4m in height. Annual growth rate is roughly 11mm/yr. Methods: Sampling and isotopic analysis: Cores (8.5 cm diameter) drilled from living coral colonies were slabbed and X-rayed to identify annual density bands and optimal sampling transects. Samples were drilled along the growth axis continuously at 1mm resolution. Stable isotopic analysis (d18O and d13C) of powdered samples was performed on a Micromass Optima with Isocarb automated preparation system at the University of Colorado's Institute for Arctic and Alpine Research. Analytical precision is =±0.08 d18O and =±0.05 d13C. The Maiana record is continuous except for a gap from 1901-1905, which we bridged through straight forward comparison and splicing of data from a coral record from nearby Tarawa Atoll. Age model: Seasonally varying d13C values and ephemeral annual density bands provide the basis for annual age assignment. Ages were adjusted subseasonally by comparison to major El Niño and La Niña anomalies in a regional rainfall index (available at http://tao.atmos. washington.edu/datasets/eqpacislandrainindex/); this procedure allows us to align major peaks and troughs that occur within the same calendar year with adjustments that are generally less than 3 months. Because corals grow at variable rates throughout the year, these small chronological adjustments are reasonable and customary (and do not affect estimates of multi-year variability). Near the base of the core, an age adjustment of 1 year was needed, suggesting an age uncertainty on the full record of ±1 year. The Maiana coral grew at a rate of 7-12 mm per year; we interpolated records to a common time step of bimonthly resolution for further analysis. Replication: The record from core MAI95-2-3 is well replicated by additional records of 65-70 years from Maiana and Aranuka, ~75 km southeast of Maiana, (JEC, FEU, and JTO, manuscript in preparation) and by a published 97-year record from Tarawa Atoll, ~30 km north of Maiana. Correlation coefficients among coral and instrumental data over the common period 1950-89 are consistently high (Maiana_correlations.txt), adding confidence to single-core reconstruction of regional variability in western Kiribati. We focus on the MAI95-2-3 record for further analysis because it extends the record of ENSO into pre-instrumental periods and avoids nonstationarity that may be introduced through inclusion of records that differ in length. The 4-coral average correlates slightly better with instrumental data than most individual coral records, but only Mai 2-3 extends the instrumental record of ENSO. Linear correlation coefficients demonstrate a strong agreement between the Maiana record and ENSO variability (Table 1), and decadal averages of the Maiana record correlate significantly over the past century (>95% confidence) with those from the Tarawa coral record and regional rainfall indices, substantiating the reality of these more subtle low-frequency signals (JEC, FEU, and JTO, manuscript in preparation).