Factors contributing to the unusually low Antarctic springtime ozone in 2020– 2023

The 2020–2023 Antarctic spring seasons saw large ozone holes, significant ozone mass deficit, 38 and low polar cap total ozone compared to the second decade of the 21st Century, prompting 39 questions about the pace of ozone recovery over Antarctica. Here we use a stratospheric 40 composition reanalysis developed at the NASA Global Modeling and Assimilation Office, and 41 chemical ozone loss estimates derived from NASA’s Aura Microwave Limb Sounder 42 observations to identify the key factors contributing to these unusually large ozone holes. We 43 find that the below-average Antarctic ozone in each of the years of interest resulted from a 44 different combination of the following: anomalous initial polar vortex ozone content, chemical 45 ozone depletion, dynamical ozone resupply, and the size and geometry of the stratospheric 46 polar vortex, with dynamically-driven factors playing a key role. We also interpret our findings 47 in the broader context of ozone recovery, with a particular focus on September, the month 48 when signs of recovery are most evident. We find no evidence challenging the current 49 consensus that springtime Antarctic ozone is recovering in response to the implementation of 50 the Montreal Protocol and its amendments.