Evidence from IASI of a speeding up in stratospheric O<sub>3</sub> recovery in the Southern Hemisphere contrasting with a decline in the Northern Hemisphere

  • Wespes C
  • Hurtmans D
  • Chabrillat S
  • et al.
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<p><strong>Abstract.</strong> In this paper, we present the global fingerprint of recent changes in the mid-upper stratospheric (MUSt; &amp;gt;&amp;thinsp;25&amp;thinsp;hPa) ozone (O<sub>3</sub>) in comparison with the lower stratospheric (LSt, 150&amp;ndash;25&amp;thinsp;hPa) O<sub>3</sub> derived from the first 10 years of the IASI/Metop-A satellite measurements (January 2008&amp;ndash;December 2017). The IASI instrument provides vertically-resolved O<sub>3</sub> profiles with very high spatial and temporal (twice daily) samplings, allowing to monitor O<sub>3</sub> changes in these two regions of the stratosphere. By applying multivariate regression models with adapted geophysical proxies on daily mean O<sub>3</sub> time series, we discriminate anthropogenic trends from various modes of natural variability, such as the El Niño/Southern Oscillation – ENSO. The representativeness of the O<sub>3</sub> response to its natural drivers is first examined. One important finding relies on a pronounced contrast between a positive LSt O<sub>3</sub> 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 O<sub>3</sub>. In terms of trends, we find an unequivocal O<sub>3</sub> recovery from the available period of measurements in winter/spring at mid-high latitudes for the two stratospheric layers sounded by IASI (&amp;gt;&amp;sim;35&amp;deg;&amp;thinsp;N/S in the MUSt and &amp;gt;&amp;sim;45&amp;deg;&amp;thinsp;S in the LSt) as well as in the total columns at southern latitudes (&amp;gt;&amp;sim;45&amp;deg;&amp;thinsp;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 O<sub>3</sub> concurrently in the lower, in the mid-upper stratosphere and in the total column from one single satellite dataset. A significant decline in O<sub>3</sub> 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 O<sub>3</sub>. 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 O<sub>3</sub> response to the decline of O<sub>3</sub> 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 O<sub>3</sub> 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.</p>




Wespes, C., Hurtmans, D., Chabrillat, S., Ronsmans, G., Clerbaux, C., & Coheur, P.-F. (2019). Evidence from IASI of a speeding up in stratospheric O&lt;sub&gt;3&lt;/sub&gt; recovery in the Southern Hemisphere contrasting with a decline in the Northern Hemisphere. Atmospheric Chemistry and Physics Discussions, 1–46. https://doi.org/10.5194/acp-2019-206

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