Comment on egusphere-2025-6549

<strong class="journal-contentHeaderColor">Abstract.</strong> The representation of stratospheric transport in Chemistry-Climate Models (CCMs) is key for accurately reproducing and projecting the evolution of the ozone layer and other radiatively relevant trace gases. We evaluate stratospheric transport in CCMs that have participated in three model intercomparison initiatives (CCMVal-2, CCMI-1, and CCMI-2022) over the last ~15 years using modern satellite datasets and reanalyses. Key long-standing<em><span style="font-style: normal;"> model biases persist across generations, with some worsening in recent simulations.</span></em><span style="font-style: normal;"> </span><em><span style="font-style: normal;">Transport remains overly fast in the models, </span></em><em><span style="font-style: normal;">with</span></em><em><span style="font-style: normal;"> a </span></em><em><span style="font-style: normal;">global</span></em><em><span style="font-style: normal;"> mean age of air young bias </span></em><em><span style="font-style: normal;">of ~</span></em><em><span style="font-style: normal;">1 year </span></em><em><span style="font-style: normal;">for</span></em><em><span style="font-style: normal;"> the </span></em><em><span style="font-style: normal;">CCMI-2022</span></em><em><span style="font-style: normal;"> median.</span></em><span style="font-style: normal;"> </span><em><span style="font-style: normal;">It is argued that t</span></em><em><span style="font-style: normal;">his </span></em><em><span style="font-style: normal;">bias </span></em><em><span style="font-style: normal;">could be </span></em><em><span style="font-style: normal;">associated with </span></em><em><span style="font-style: normal;">too fast</span></em><em><span style="font-style: normal;"> tropical upwelling in the lower stratosphere, insufficient horizontal mixing and/</span></em><em><span style="font-style: normal;">or</span></em><em><span style="font-style: normal;"> excessive vertical diffusion.</span></em> In the springtime southern polar stratosphere, the final warming is delayed (~3 weeks), downwelling is underestimated (~25 %), and the depth of the ozone minimum is overestimated (~10 DU) on average in the most recent models. The tropopause is too high in all generations, and the tropical cold point tropopause is too warm in the latest generation (~1<span>&ndash;</span>2 K). Long-term trends in transport and over 1980<span>&ndash;</span>1999 are consistent across model generations and highlight the crucial role of ozone depletion in contributing to accelerate the Brewer-Dobson circulation and delaying the southern polar vortex breakdown.