Differential Credibility of Climate Modes in CMIP6 ESM
All climate projections at the global, regional or local scale are contingent on the credible representation of internally forced climate modes. We objectively characterize that credibility for 58 CMIP6 ESM realizations in terms of; (i) the spatial patterns and intensity of the modes, (ii) their probability distributions (variability), (iii) their power spectra and periodicities.
Internal climate variability (forced by air-sea interactions) is a major confounding factor in the detection and attribution of greenhouse gas forced climate change. New metrics for quantifying differential credibility of Earth System Model (ESM) representation of modes of nature climate variability are introduced and applied to output from 11 ESM (58 realizations in total). This work enables ESM selection for downscaling and illustrates areas for ESM improvement.
All climate projections at the global, regional or local scale are contingent on the credible representation of internally forced climate modes. These modes are patterns of repeated atmospheric and/or oceanic variability. This work quantitatively evaluates the fidelity with which size of these dominant climate modes are represented in current-generation global models. The six modes are; the Northern Annular Mode (NAM), Southern Annular Mode (SAM), Pacific-North American pattern (PNA), El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families, and specific ESM realizations relative to the ERA5 reanalysis. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climate modes. Substantial variability in skill scores manifests across realizations from individual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM-PNA first-order interaction and underestimate the NAM-AMO connection. These results suggest that the choice of ESM and ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near term.