Tertiary‐Ozone‐Maximum‐Like Mesospheric Ozone Increases Driven by Solar Eclipses: Sensitivity Experiments on WACCM‐X

Abstract

Solar eclipses offer natural experiments to isolate the photochemical response of mesospheric ozone to solar radiation. Using the WACCM‐X model, we performed mechanism‐oriented sensitivity experiments for the 14 December 2020 eclipse to quantify the tertiary‐ozone‐maximum‐like (TOM‐like) response. Simulations reveal that ozone at 0.0973 hPa (∼60–65 km) increases by ∼80% near maximum obscuration, anti‐correlated with a 50% reduction in active hydrogen (HOx = H + OH + HO ₂ ). We explicitly decouple this enhancement into two physical processes: suppression of HOx‐catalyzed ozone loss and repartitioning of odd oxygen (Ox = O ₃  + O). The suppression of water vapor photolysis limits HOx production, effectively braking the catalytic loss of Ox. On the other hand, the diminishing photolysis rates within the Chapman cycle drive a rapid repartition from atomic O to ozone. This mechanism contributes an additional ozone increase of ∼0.12 ppm/hr. Furthermore, the model reconciles apparent observational discrepancies at high solar zenith angles (SZAs), where ozone changes remain observable but HOx changes become indistinguishable from observational noise. Our results show that under weak background photolysis at high‐SZA ranges, the relative HOx reduction naturally attenuates from ∼50% to ∼37%. The weakened change derived from simulation, compounded by background noise, explain the undetectable HOx changes from observation under high‐SZA ranges, whereas the ozone signature remains robust.