Dynamically controlled ozone decline in the tropical mid-stratosphere observed by SCIAMACHY

Galytska, Evgenia; Rozanov, Alexey; Chipperfield, Martyn P.; Dhomse, Sandip. S.; Weber, Mark; Arosio, Carlo; Feng, Wuhu; Burrows, John P.

Despite the recently reported beginning of a recovery in global stratospheric ozone (inline-formulaO3), an unexpected inline-formulaO3 decline in the tropical mid-stratosphere (around 30–35 km altitude) was observed in satellite measurements during the first decade of the 21st century. We use SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) measurements for the period 2004–2012 to confirm the significant inline-formulaO3 decline. The SCIAMACHY observations show that the decrease in inline-formulaO3 is accompanied by an increase in inline-formulaNO2.

To reveal the causes of these observed inline-formulaO3 and inline-formulaNO2 changes, we performed simulations with the TOMCAT 3-D chemistry-transport model (CTM) using different chemical and dynamical forcings. For the 2004–2012 time period, the TOMCAT simulations reproduce the SCIAMACHY-observed inline-formulaO3 decrease and inline-formulaNO2 increase in the tropical mid-stratosphere. The simulations suggest that the positive changes in inline-formulaNO2 (around 7 % decadeinline-formula−1) are due to similar positive changes in reactive odd nitrogen (inline-formulaNOy), which are a result of a longer residence time of the source gas inline-formulaN2O and increased production via inline-formulaN2O + O(inline-formula1D). The model simulations show a negative change of 10 % decadeinline-formula−1 in inline-formulaN2O that is most likely due to variations in the deep branch of the Brewer–Dobson Circulation (BDC). Interestingly, modelled annual mean “age of air” (AoA) does not show any significant changes in transport in the tropical mid-stratosphere during 2004–2012.

However, further analysis of model results demonstrates significant seasonal variations. During the autumn months (September–October) there are positive AoA changes that imply transport slowdown and a longer residence time of inline-formulaN2O allowing for more conversion to inline-formulaNOy, which enhances inline-formulaO3 loss. During winter months (January–February) there are negative AoA changes, indicating faster inline-formulaN2O transport and less inline-formulaNOy production. Although the variations in AoA over a year result in a statistically insignificant linear change, non-linearities in the chemistry–transport interactions lead to a statistically significant negative inline-formulaN2O change.

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Galytska, Evgenia / Rozanov, Alexey / Chipperfield, Martyn P. / et al: Dynamically controlled ozone decline in the tropical mid-stratosphere observed by SCIAMACHY. 2019. Copernicus Publications.

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