Preprints
https://doi.org/10.5194/egusphere-2025-666
https://doi.org/10.5194/egusphere-2025-666
21 Feb 2025
 | 21 Feb 2025

Southern Annular Mode Persistence and Westerly Jet: A Reassessment Using High-Resolution Global Models

Ting-Chen Chen, Hugues Goosse, Matthias Aengenheyster, Kristian Strommen, Christopher Roberts, Malcolm Roberts, Rohit Ghosh, Jin-Song von Storch, and Stephy Libera

Abstract. This study evaluates the performance of high-resolution (grid sizes of 9–28 km for the atmosphere; 5–13 km for the ocean) global simulations from the EERIE project in representing the persistence of the Southern Annular Mode (SAM), a critical driver of Southern Hemisphere climate variability. Using the decorrelation timescale of the SAM index (τ), we compare EERIE coupled and atmosphere-only (AMIP) simulations with CMIP6 and ERA5 datasets. EERIE coupled simulations improve the long-standing biases in SAM persistence, especially in early summer, with τ values of 9–17 days compared to CMIP6’s 9–32 days. This improvement generally correlates with a more accurate climatological jet latitude (λ0) distribution in EERIE simulations than in CMIP6, but such a correlation is not robust within EERIE AMIP simulations with a well-represented jet location, suggesting other factors in play. With prescribed SSTs, EERIE AMIP show even smaller biases in both τ and λ0 than EERIE coupled runs, highlighting the critical role of SST representation. Using the same AMIP model, finer grids (9 km vs. 28 km) can further reduce τ, but the underlying cause remains unclear, likely because of potential compensation between different processes. Sensitivity experiments filtering ocean mesoscale features in SST boundary conditions suggest that mesoscale processes enhance SAM persistence by ~2 days in early summer, though this effect is clear in ensemble means at 28 km but not in the single 9-km runs. We also show that the atmospheric eddy feedback strength is a better indicator than λ0 to infer the SAM persistence, although the metric alone does not fully explain the τ differences across SST scenarios. These findings underscore the interplay of dynamic processes influencing SAM persistence and offer insights for advancing global climate model performance.

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Journal article(s) based on this preprint

21 Oct 2025
Southern Annular Mode persistence and westerly jet: a reassessment using high-resolution global models
Ting-Chen Chen, Hugues Goosse, Cécile Davrinche, Stephy Libera, Christopher Roberts, Matthias Aengenheyster, Kristian Strommen, Malcolm Roberts, Rohit Ghosh, and Jin-Song von Storch
Weather Clim. Dynam., 6, 1179–1193, https://doi.org/10.5194/wcd-6-1179-2025,https://doi.org/10.5194/wcd-6-1179-2025, 2025
Short summary
Ting-Chen Chen, Hugues Goosse, Matthias Aengenheyster, Kristian Strommen, Christopher Roberts, Malcolm Roberts, Rohit Ghosh, Jin-Song von Storch, and Stephy Libera

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-666', Anonymous Referee #1, 27 Mar 2025
  • RC2: 'Comment on egusphere-2025-666', Anonymous Referee #2, 03 Apr 2025
  • AC1: 'Comment on egusphere-2025-666', Stephy Libera, 23 May 2025

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-666', Anonymous Referee #1, 27 Mar 2025
  • RC2: 'Comment on egusphere-2025-666', Anonymous Referee #2, 03 Apr 2025
  • AC1: 'Comment on egusphere-2025-666', Stephy Libera, 23 May 2025

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Stephy Libera on behalf of the Authors (06 Jul 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (07 Jul 2025) by Amy Butler
RR by Anonymous Referee #1 (18 Jul 2025)
RR by Anonymous Referee #2 (24 Jul 2025)
ED: Publish subject to minor revisions (review by editor) (28 Jul 2025) by Amy Butler
AR by Stephy Libera on behalf of the Authors (14 Aug 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (15 Aug 2025) by Amy Butler
AR by Stephy Libera on behalf of the Authors (22 Aug 2025)  Manuscript 

Journal article(s) based on this preprint

21 Oct 2025
Southern Annular Mode persistence and westerly jet: a reassessment using high-resolution global models
Ting-Chen Chen, Hugues Goosse, Cécile Davrinche, Stephy Libera, Christopher Roberts, Matthias Aengenheyster, Kristian Strommen, Malcolm Roberts, Rohit Ghosh, and Jin-Song von Storch
Weather Clim. Dynam., 6, 1179–1193, https://doi.org/10.5194/wcd-6-1179-2025,https://doi.org/10.5194/wcd-6-1179-2025, 2025
Short summary
Ting-Chen Chen, Hugues Goosse, Matthias Aengenheyster, Kristian Strommen, Christopher Roberts, Malcolm Roberts, Rohit Ghosh, Jin-Song von Storch, and Stephy Libera
Ting-Chen Chen, Hugues Goosse, Matthias Aengenheyster, Kristian Strommen, Christopher Roberts, Malcolm Roberts, Rohit Ghosh, Jin-Song von Storch, and Stephy Libera

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Short summary
The Southern Annular Mode (SAM) is a key driver of Southern Hemisphere climate variability, but global models often overestimate its persistence in summer. Using high-resolution models, we show this bias can be reduced, along with some improvements in jet latitude and likely a better-resolved eddy-mean flow feedback. Controlled experiments reveal the potential roles of sea surface temperature biases and ocean mesoscales, underscoring the complex mechanisms shaping SAM persistence.
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