| 06 May 2025
Quantifying the contribution of transport to Antarctic springtime ozone column variability
Abstract. Quantifying chemical and dynamical drivers of Antarctic ozone variability remains important while stratospheric chlorine levels gradually reduce and the ozone hole recovers in response. While chemistry dominates the formation of the ozone hole in September, the role of dynamics grows as the spring season progresses. To improve our ability to characterize the dynamical impacts on Antarctic total column ozone (TCO), we use MLS/Aura observations of carbon monoxide to trace the path of an air parcel that originates in the mesosphere and descends into the springtime polar vortex. We define a new metric, the Mesospheric Parcel Altitude (MPA), which measures the altitude of the descending mesospheric air parcel at the end of October. The MPA is highly correlated with October TCO and functions as a diagnostic tool, capturing the dynamical state of the inner-vortex. Based on the MPA, we classify October ozone holes from 2004–2024 into three mesospheric descent types (Strong, Regular, and Weak) and provide a formula to estimate the magnitude of horizontal ozone transport (poleward of 70° S and between 17–27 km) during a given October. A higher MPA (>26.5 km) indicates Weak descent, reduced ozone transport, and a larger, longer-lived ozone hole. A lower MPA (<25 km) indicates Strong descent, increased ozone transport, and a smaller, shorter-lived ozone hole. When the MPA is used as a proxy for polar cap TCO, approximately 63% of the observed variance during October is explained by the metric.
The paper by Kessenich et al. introduces a novel way of using mesospheric CO as a diagnostic for transport affecting the Antarctic stratospheric polar vortex in September and October. They show that CO descent in polar night correlates with many quantities and particularly with the re-supply of ozone from lower latitudes in Antarctic spring and the depth of the Antarctic ozone hole in October / late spring. The paper contains fresh and novel ideas and does a great job presenting and supporting them. It is relevant for the field and very much suited for publication in ACP.
I have only one suggestion: If possible, it would be helpful to have a conceptual picture / figure that shows how the mesospheric descent and the transport into the polar region are connected. Fig. 6 goes a little bit into that direction, but a conceptual picture early on, would probably help for easier understanding of the paper, which then goes into quite a lot of technical detail. Apart from that: a great and well-written paper. Congratulations!