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

Abstract

Despite the recently reported beginning of a recovery in global stratospheric ozone (O 3 ), an unexpected O 3 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 O 3 decline. The SCIAMACHY observations show that the decrease in O 3 is accompanied by an increase in NO 2 . To reveal the causes of these observed O 3 and NO 2 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 SCIAMACHYobserved O 3 decrease and NO 2 increase in the tropical mid-stratosphere. The simulations suggest that the positive changes in NO 2 (around 7%decade-1) are due to similar positive changes in reactive odd nitrogen (NO y ), which are a result of a longer residence time of the source gas N 2 O and increased production via N 2 O+ O(1D). The model simulations show a negative change of 10%decade-1 in N 2 O 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 N 2 O allowing for more conversion to NO y , which enhances O 3 loss. During winter months (January-February) there are negative AoA changes, indicating faster N 2 O transport and less NO y production. Although the variations in AoA over a year result in a statistically insignificant linear change, nonlinearities in the chemistry-transport interactions lead to a statistically significant negative N 2 O change.