Calibrations of Bermuda corals against large-scale SST- and SLP-pattern time series and implications for climate reconstructions

Abstract

Proxy records from stony reef corals are established tools for reconstructing climate features. However, validation procedures for proving the calibrations are often insufficient or even lacking. We address this problem by the establishment and validation of statistical regression models for the reconstruction of large-scale climate patterns from local coral records (upscaling). We apply the upscaling technique to the annual growth rates of two coral species from Bermuda, representing a period of about 140 years. The coral records are related to reanalyzed sea surface temperature (SST) and sea level pressure (SLP) fields. In the western North Atlantic, the correlation maps between the SSTs and the single coral records and their leading principal component show similar structures. On the basis of regression patterns for limited areas, upscaling models are established. Validation of such models for the SST yields an explained variance of 30% of the low-pass-filtered data using the first principal component of both corals. For the detrended SLP time series, representing an almost hemispheric SLP pattern, 21% of the interannual variability is explained by means of only one coral. Thus different corals from neighboring sites represent similar climate signals, however, to slightly varying degrees, leading to different reconstruction skills. Problems remain in reconstructing long-term trends. We conclude that reconstructions from multiple corals are not a priori predominant to those from single corals. A calibration to what extent a single coral represents a climate parameter should be the first step before combining multiple corals as the basis for a reconstruction.