Evidence from IASI of a speeding up in stratospheric O3 recovery in the Southern Hemisphere contrasting with a decline in the Northern Hemisphere

Abstract : In this paper, we present the global fingerprint of recent changes in the mid-upper stratospheric (MUSt; >25hPa) ozone (O3) in comparison with the lower stratospheric (LSt, 150-25 hPa) O3 derived from the first 10 years of the IASI/Metop-A satellite measurements (January 2008-December 2017). The IASI instrument provides vertically-resolved O3 profiles with very high spatial and temporal (twice daily) samplings, allowing to monitor O3 changes in these two regions of the stratosphere. By applying multivariate regression models with adapted geophysical proxies on daily mean O3 time series, we discriminate anthropogenic trends from various modes of natural variability, such as the El Niño/Southern Oscillation-ENSO. The representativeness of the O3 response to its natural drivers is first examined. One important finding relies on a pronounced contrast between a positive LSt O3 response to ENSO in the extra-tropics and a negative one in the tropics, with a delay of 3 months, which supports a stratospheric pathway for the ENSO influence on lower stratospheric and tropospheric O3. In terms of trends, we find an unequivocal O3 recovery from the available period of measurements in winter/spring at mid-high latitudes for the two stratospheric layers sounded by IASI (>~35°N/S in the MUSt and >~45°S in the LSt) as well as in the total columns at southern latitudes (>~45°S) where the increase reaches its maximum. These results confirm the effectiveness of the Montreal protocol and its amendments, and represent the first detection of a significant recovery of O3 concurrently in the lower, in the mid-upper stratosphere and in the total column from one single satellite dataset. A significant decline in O3 at northern mid-latitudes in the LSt is also detected, especially in winter/spring of the northern hemisphere. Given counteracting trends in LSt and MUSt at these latitudes, the decline is not categorical in total O3. When freezing the regression coefficients determined for each natural driver over the whole IASI period but adjusting a trend, we calculate a significant speeding up in the O3 response to the decline of O3 depleting substances (ODS) in the total column, in the LSt and, in a lesser extent, in the MUSt, at high southern latitudes over the year. A significant acceleration of the O3 decline at northern mid-latitudes in the LSt and in the total column is also highlighted over the last years. That, specifically, needs urgent investigation for identifying its exact origin and apprehending its impact on climate change.
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Catherine Wespes, Daniel Hurtmans, Simon Chabrillat, Gaétane Ronsmans, Cathy Clerbaux, et al.. Evidence from IASI of a speeding up in stratospheric O3 recovery in the Southern Hemisphere contrasting with a decline in the Northern Hemisphere. Atmospheric Chemistry and Physics Discussions, European Geosciences Union, In press, ⟨10.5194/acp-2019-206⟩. ⟨insu-02101539⟩

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