The Southern Annular Mode in 6th Coupled Model Intercomparison Project Models
Olaf Morgenstern
Abstract
Abstract I analyze trends in the Southern Annular Mode (SAM) in CMIP6 simulations. For the period 1957–2014, simulated linear trends are generally consistent with two observational references but seasonally in disagreement with two other representations of the SAM. Using a regression analysis applied to model simulations with interactive ozone chemistry, a strengthening of the SAM in summer is attributed nearly completely to ozone depletion because a further strengthening influence due to long‐lived greenhouse gases is almost fully counterbalanced by a weakening influence due to stratospheric ozone increases associated with these greenhouse gas increases. Ignoring such ozone feedbacks would yield comparable contributions from these two influences, an incorrect result. In winter, trends are smaller but an influence of greenhouse gas‐mediated ozone feedbacks is also identified. The regression analysis furthermore yields significant differences in the attribution of SAM changes to the two influences between models with and without interactive ozone chemistry, with ozone depletion and GHG increases playing seasonally a stronger and weaker, respectively, role in the chemistry models versus the no‐chemistry ones.