The intrinsic ultra-long periodicity of the Southern Annular Mode
Abstract
Variability of the Southern Hemisphere extratropical circulation is dominated by the Southern Annular Mode (SAM), which arises from interactions of eddies and anomalous zonal-mean zonal winds. Past observational and theoretical studies have mostly suggested that SAM lacks any periodicity, consistent with the turbulent and thus chaotic nature of the extratropical troposphere. Here, we show, using observations and a hierarchy of climate models, that SAM has a 150-day periodicity. This periodicity is robustly detectable in power spectra and principal oscillation patterns (also known as dynamic mode decomposition, DMD) of the daily zonal-mean zonal winds, 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 SAM, suggesting that the periodicity is tied with a more complex interaction of eddies and zonal wind anomalies, as the latter propagate from low to high latitudes (the so-called propagating regime of the annular modes). Given that SAM's structure and timescale are often used as metrics to evaluate climate models and the well-known influence of SAM's variability on the Southern Hemisphere weather and climate, the potentially broader implications of this periodicity are discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.A54G..01H