Abstract
AbstractOne common approach to investigating past changes in El Niño–Southern Oscillation (ENSO) amplitude is through quantifying the variance of ENSO‐influenced proxy records. However, a component of the variance of all such proxies will reflect influences that are unrelated to the instrumental climatic indices from which modern ENSO amplitudes are defined. The unrelated component of proxy variance introduces a fundamental source of uncertainty to all such constraints on past ENSO amplitudes. Based on a simple parametric approach to modeling this uncertainty, we present guidelines for the magnitudes of proxy variance change required to robustly infer the following: (i) any change at all in ENSO amplitude and (ii) a change in ENSO amplitude that exceeds the plausible range of unforced variability. It is noted that more extreme changes in proxy variance are required to robustly infer decreases, as opposed to increases, in past ENSO amplitude from modern levels.
Highlights
A common problem of interest to paleoclimatic studies is whether the amplitudes of phenomena observed in the modern system, such as the El Niñ o–Southern Oscillation (ENSO), have changed through time and if so, whether these changes exceed the range of unforced variability
Based on a simple parametric approach to modeling this uncertainty, we present guidelines for the magnitudes of proxy variance change required to robustly infer the following: (i) any change at all in ENSO amplitude and (ii) a change in ENSO amplitude that exceeds the plausible range of unforced variability
Constraints on past changes in ENSO amplitude derived from proxy variance contain a further fundamental source of uncertainty, arising from the component of proxy variance that is unrelated to the instrumental indices from which modern ENSO amplitudes are defined
Summary
A common problem of interest to paleoclimatic studies is whether the amplitudes of phenomena observed in the modern system, such as the El Niñ o–Southern Oscillation (ENSO), have changed through time and if so, whether these changes exceed the range of unforced variability. The present study uses a simple parametric approach to exploring this uncertainty, with the aim of developing practical guidance on the magnitudes of proxy variance change required to make robust inferences about past changes in ENSO amplitude. The results and their implications are illustrated using the practical examples provided by one set of ENSO-relevant proxy records [Cobb et al, 2003]. Insofar as the statistical requirements of the simple parametric model used to generate these results are met, they may be applied to the relationship between any climatic index and any other time series, be that a single-proxy record, a multiproxy synthesis, or an instrumental record from another location
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