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Modelling Decadal to Centennial Climate in the Equatorial Pacific
Ault, T.R., Deser, C., Newman, M. and Emile-Gray, J. 2013. Characterizing decadal to centennial variability in the equatorial Pacific during the last millennium. Geophysical Research Letters 40: 3450-3456.

The authors write that "characterizing decadal-to-centennial ("dec-cen") climate fluctuations in the tropical Pacific is critical to understanding how that region may evolve with human-induced climate change." This task, however, may well be impossible to successfully complete, since they indicate that (1) "dec-cen variability in the tropical Pacific may be more prominent than instrumental records alone reveal," and that (2) it is even less clear in that region "how external influences and internal processes generate variability at these timescales."

What was done
To explore this issue in more detail, Ault et al. evaluated "the magnitude of tropical Pacific dec-cen variability in an extensive suite of data sets including [1] instrumentally-based products, [2] climate model simulations from the Climate Model Intercomparison 5 (CMIP5) archive, and [3] a newly published ensemble of paleoclimate reconstructions (Emile-Geay et al., 2013a,b)," which they subsequently refer to as EG13a,b.

What was learned
First of all, the four U.S. researchers report that on decadal to multi-decadal timescales, the variability in the three types of data sets they employed in their analyses is "consistent with the null hypothesis that it arises from 'multivariate red noise' generated from a linear inverse model of tropical ocean-atmosphere dynamics." Second, they say that "on centennial and longer timescales, both a last millennium simulation performed using the Community Climate System Model 4 (CCSM4) and the paleoclimate reconstructions have variability that is significantly stronger than the null hypothesis," and that "the time series of the model and the reconstruction do not agree with each other." And last of all, they indicate that CMIP5 model results and the reconstructed low-pass time series "mostly range from being uncorrelated to anti-correlated to each other."

What it means
In the concluding sentence of their paper's abstract, Ault et al. write that "these findings imply that the response of the tropical Pacific to future forcings may be even more uncertain than portrayed by state-of-the-art models because there are potentially important sources of century-scale variability that these models do not simulate."

Emile-Geay, J., Cobb, K.M., Mann, M.E. and Wittenberg, A.T. 2013a. Estimating central equatorial Pacific SST variability over the past millennium. Part 1: Methodology and validation. Journal of Climate 26: 2302-2328.

Emile-Geay, J., Cobb, K.M., Mann, M.E. and Wittenberg, A.T. 2013b. Estimating central equatorial Pacific SST variability over the past millennium. Part 2: Reconstructions and implications. Journal of Climate 26: 2329-2352.

Reviewed 29 January 2014