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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.
According to Ault et al. (2013), "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 be impossible to successfully complete, since they indicate (1) "dec-cen variability in the tropical Pacific may be more prominent than instrumental records alone reveal," and (2) it is even less clear in that region "how external influences and internal processes generate variability at these timescales."

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.

In discussing their findings, the four U.S. researchers report that on decadal to multi-decadal timescales, the variability in the three types of data sets 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 "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 "the time series of the model and the reconstruction do not agree with each other." And last of all, they state CMIP5 model results and the reconstructed low-pass time series "mostly range from being uncorrelated to anti-correlated to each other."

In light of such, in the concluding sentence of their paper's abstract, Ault et al. write "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." Such uncertainty must be adequately addressed before model projections can be taken seriously.

Additional References
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.

Archived 4 February 2014