In this study, we will discuss the analysis of power spectra as part of a space-time diagnosis of modes of extratropical variability (Northern Annular Mode-NAM, the North Atlantic Oscillation-NAO, the Pacific North America pattern-PNA, the Southern Annular Mode-SAM, and the Pacific Decadal Oscillation-PDO). In addition to assessing the overall shape of the power spectra, we consider the skill in representing variability in four separate time bins: interdecadal (over 10 year), interannual (2 to 10 year), quasi-annual (10 month to 2 year), and sub-annual (2 to 10 month).The metrics are applied to all available Historical simulations from CMIP5 and CMIP6. For each model, we average data across initial condition realizations and/or the time bins to reduce sampling uncertainty and improve the signal-to-noise ratio to assess if statistically significant periodicities exist in the data. Where possible, the metrics and significance are assessed relative to a variety of statistical models of the noise.

Acknowledgement: This work was supported by the Office of Science (BER), U.S. Department of Energy through Lawrence Livermore National Laboratory contract DE-AC52-07NA27344.