Natural Climate Variability on Decade-to-Century Time Scales

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Short Records Document Spatial Patterns of ENSO

Major dynamic features of ENSO variability include SST and upwelling anomalies in the eastern Pacific, and rainfall and wind anomalies in the western Pacific. Coral records can be targeted to address variability in each of these parameters through careful site and tracer selection. Figure 3 (from Cole et al., 1992) compares six short (20-30 yr) monthly-to-seasonal-resolution records from corals that grew at sites spanning the equatorial Pacific: the Galapagos Islands (1°S, 89°W), Tarawa (1°N, 172°E), and Bali (8°S, 115°E). In the eastern Pacific, Galapagos corals yield records of d¹⁸O, Cd/ Ca, and Ba/Ca that track variations in upwelling and SST (Shen et al., 1987; McConnaughey, 1989; Lea et al., 1989). In Tarawa corals, d¹⁸O primarily monitors monthly variability in local rainfall, a consequence of the displaced Indonesian Low (Cole and Fairbanks, 1990; Cole et al., 1993), and Mn/Ca variations reflect wind reversals associated with especially strong ENSO warm extremes (Shen et al., 1992b). In Bali, the migration of the Indonesian Low causes drought during ENSO warm extremes; coral d¹⁸O shows moderate

Figure 3

Comparison of short proxy records from corals that grew at three sites across the Pacific. (From Cole et al., 1992; reprinted with permission of Cambridge University Press.) All records are presented in units of standard deviation. Dark shading denotes recognized ENSO warm extremes (Quinn and Neal, 1992); lighter shading indicates conditions similar to the warm phase of ENSO at individual sites. These records target distinct components of the ENSO system; together they indicate both the Pacific-wide extent of ENSO extremes and the potential for the reconstruction of subtle patterns of spatial variability in ENSO. Periods of anomalies similar to ENSO warm phases but not recognized by Quinn and Neal (1992) occur in most records in 1963 and 1979-1980.

sensitivity to these fluctuations but probably includes a significant SST component as well.

Overall, these records monitor warm phases of ENSO consistently between sites. Cool phases of ENSO are best recorded by the Galapagos records and by the Tarawa d¹⁸O history. The Mn/Ca record at Tarawa appears to indicate changes in westerly wind intensity associated with strong warm phases, although it may not track variations in the intensity of easterly winds, and the Bali record does not show cool-phase anomalies consistent with the other coral records or with large-scale indices of Pacific climate.

Comparison of these records reveals patterns of spatial variability that are confirmed by concurrent instrumental records. For example, during 1979-1980, most of the Pacific experienced conditions similar to a weak ENSO warm anomaly. Although this year is not generally recognized as a major event (sensu Quinn et al., 1987), other studies have documented unusually warm SSTs and rainfall in the central Pacific during that time (Donguy and Dessiers, 1983). In 1977, the warm extreme of 1976 intensified in the western Pacific but decayed more rapidly into cool-phase conditions in the eastern Pacific. Variations in intensity of anomalies are also reflected in the coral data and generally confirmed by climatological data. In 1963 and 1969, for example, weak anomalies in the Tarawa d¹⁸O record are not concurrent with anomalies in the Mn/Ca record, which is consistent with observations of rainfall increases but no wind reversals at Tarawa. Cole et al. (1992) discuss these patterns in greater detail. This proxy intercomparison demonstrates that coral paleoclimatic tracers monitor large-scale ENSO anomalies across the entire Pacific basin, both warm and cool, and that they can also discern relatively subtle patterns of spatial variability and anomaly intensity in specific climatic and oceanographic parameters.

Calibration of Geochemical Tracers in a Galapagos Coral

In Galapagos corals, several tracers hold promise for reconstructing the SST and upwelling variability that define the eastern Pacific signature of ENSO. Shen et al. (1992a) measured parallel records of Cd/Ca, Ba/Ca, Mn/Ca, d¹⁸O, and d¹³C at quarterly resolution from a coral that grew at Punta Pitt, Isla San Cristobal, on the eastern side of the Galapagos archipelago. These records span the period 1936 through mid-1982. With the exception of Mn/Ca, all tracers show high correlations with SSTs measured at Puerto Chicama, Peru; for seasonal records, R falls between 0.51 and 0.65, and for annual averages, R ranges from 0.61 to 0.73. Even higher correlations are found between the tracers and the shorter SST record from Academy Bay, Galapagos, with seasonal R values ranging from 0.57 to 0.79 and annual values falling between 0.72 and 0.90. These tracers monitor slightly different aspects of the eastern Pacific ENSO signal: d¹⁸O tracks SST variability, Ba and Cd reflect the surface

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