PNNL

Abstract

Variability of the Southern Hemisphere extratropical circulation is dominated by the Southern Annular Mode (SAM), whose timescale is extensively used as a key metric in evaluating state-of-the-art climate models. Past observational and theoretical studies suggest that the SAM lacks any internally generated (intrinsic) periodicity. Here, we show, using observations and a climate model hierarchy, that the SAM has an intrinsic 150-day periodicity, consistent with the predictions of a new theoretical model. This periodicity is robustly detectable in the power spectra and dynamical mode decomposition of the zonal-mean circulation, and in hemispheric-scale precipitation and ocean surface wind stress. We further propose a new metric for evaluating climate models, and show that previously reported shortcomings and improvements in simulating SAM’s variability connect to the models’ ability in reproducing this periodicity. We argue that this periodicity should be considered in evaluating climate models and understanding the past, current, and projected Southern Hemisphere climate variability.