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

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.