Influence of Mean Climate Change on Climate Variability from a 155-year Tropical Pacific Coral Record

Influence of mean climate change on climate variability from a 155-year tropical Pacific coral record. Nature v.407, pp. 989-993, 26 Oct 2000.

F.E. Urban
Institute of Arctic and Alpine Research, University of Colorado.
J.E. Cole
Department of Geosciences, University of Arizona.
J.T. Overpeck
ISPE and Department of Geosciences, University of Arizona.

Today, the El Niņo/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.

Download the del 18O data and data description from this study from the WDC Paleo Archive.

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It was published in Nature v.407, pp. 989-993, 26 Oct 2000.

Fig. 1 (above)
Map of tropical Pacific, showing locations of instrumental and coral records used. Maiana Atoll is indicated by a star, and the warm pool is indicated by the 28 degree isotherm. Also shown are the regions used to define large-scale ENSO indices: the Niņo 3.4 SST region is outlined by a box, and the region used for the central Pacific rainfall index is shaded. The Multivariate ENSO Index includes information on surface air and ocean temperatures, wind, cloudiness, and sea level pressure from the full Pacific Ocean region shown in this map.

Fig. 2a (left)
Bimonthly records of tropical Pacific variability
from coral and instrumental data.
(A)Maiana coral d18O compared to the Multivariate ENSO index and Niņo 3.4 SST
Fig. 2b (left)
Bimonthly records of tropical Pacific variability
from coral and instrumental data.
(B)Expansion of these records over the 1950-95 period shows detailed correlation among these records.

Fig. 3 (below) Results of cross-spectral analysis between the normalized Maiana coral record and other normalized Pacific datasets. The Maiana variance (in red) is compared with a) Niņo 3.4 SST (1856-1992; in blue) and b) a coral record from Tarawa Atoll (1893-1989; in blue). Smooth red and blue lines are 90% confidence limits. Coherency between records is shown in green, with the 95% confidence level indicated. The Maiana record is coherent (>95% confidence) with both the Niņo 3.4 and the Tarawa records for all significant concentrations of variance at periods between 2 and 12 years (shaded). Maiana variance spectra differ between these panels because the interval of analysis differs; the univariate spectrum of the full-length Maiana time series (not shown) has significant features as described in the text.

Fig. 4 (right) Evolutionary spectral analysis results for a) Maiana and b) Niņo 3.4 SST records. Analysis was performed on 40-year segments of data overlapped by 4 years, using multitaper methods with red noise background assumptions. Colored regions show variance significant above the median background (>50%), and black contours enclose regions with variance significant at >90%. These spectra share several general features - significant 19th-century decadal variance, strong variance at 2.9 year periods in the early 20th century, attenuation of 2-4 year variance between 1920-1955, and the general progression from decadal towards higher frequency interannual variance over the length of the record. The low-frequency variance concentration in the final years of the coral record reflects the step-function change in 1976.

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27 Oct 2000