The decrease in mid-stratospheric tropical ozone since 1991

Abstract. While global stratospheric O₃ has begun to recover, there are localized regions where O₃ has decreased since 1991. Specifically, we use measurements from the Halogen Occultation Experiment (HALOE) for the period 1991–2005 and the NASA Aura Microwave Limb Sounder (MLS) for the period 2004–2013 to demonstrate a significant decrease in O₃ near ~ 10 hPa in the tropics. O₃ in this region is very sensitive to variations in NO_y, and the observed decrease can be understood as a spatially localized, yet long-term increase in NO_y. In turn, using data from MLS and from the Atmospheric Chemistry Experiment (ACE), we show that the NO_y variations are caused by decreases in N₂O which are likely linked to long-term variations in dynamics. To illustrate how variations in dynamics can affect N₂O and O₃, we show that by decreasing the upwelling in the tropics, more of the N₂O can photodissociate with a concomitant increase in NO_y production (via N₂O + O(¹D) → 2NO) at 10 hPa. Ultimately, this can cause an O₃ decrease of the observed magnitude.