The Intrinsic 150‐Day Periodicity of the Southern Hemisphere Extratropical Large‐Scale Atmospheric Circulation

Abstract

The variability of the Southern Hemisphere (SH) extratropical large‐scale 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. This periodicity is robustly detectable in the power spectra and principal oscillation patterns (aka dynamical mode decomposition) of the zonal‐mean circulation, and in hemispheric‐scale precipitation and ocean surface wind stress. The 150‐day period is consistent with the predictions of a new reduced‐order model for the SAM, which suggests that this periodicity is associated with a complex interaction of turbulent eddies and zonal wind anomalies, as the latter propagate from low to high latitudes. These findings present a rare example of periodic oscillations arising from the internal dynamics of the extratropical turbulent circulations. Based on these findings, we further propose a new metric for evaluating climate models, and show that some of the 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.

The Role of Cloud Radiative Effects in the Propagating Southern Annular Mode

“…Recent studies from a multi‐EOF perspective indicate that the single‐EOF approach does not fully capture the annular mode of the zonal jet variability (LH21, Lubis & Hassanzadeh, 2023; Smith et al., 2024). Our cross‐EOF analysis and the DMD analysis based on two EOFs may provide a more comprehensive representation of the spatiotemporal characteristics and the predictability of the annular mode, especially when it is in a propagation regime.…”

“…Lubis and Hassanzadeh (2023) performed similar DMD analysis on several reanalysis data and data simulated by the latest climate models. It is straightforward to show that the corresponding continuous system of has complex eigenvalues, corresponding to a propagating DMD mode, if the entries in the matrix K satisfy the criterion: $$${\left({k}_{11}-{k}_{22}\right)}^{2}+4{k}_{12}{k}_{21}< 0,$$$ which turns out to be in the same form as the necessary and sufficient condition for propagation introduced in Lubis and Hassanzadeh (2021).…”

“…On the other hand, growing evidence indicates that the leading mode of the mid‐latitude westerly jet variability in both hemispheres should be interpreted as a propagation mode (Sheshadri & Plumb, 2017; Lubis & Hassanzadeh, 2021, LH21 hereafter; Lubis & Hassanzadeh, 2023; Smith et al., 2024) and better represented by a principal oscillation pattern (POP, von Storch et al., 1988; Penland, 1989). Therefore, in addition to the conventional single EOF‐based feedback analysis, we also perform a Dynamical Mode Decomposition (DMD, Schmid & Sesterhenn, 2008; Bai et al., 2017) analysis based on the first two EOFs of the SH zonal wind to identify the leading POP and to quantify the cross‐EOF interactions.…”