While global stratospheric O3 has begun to recover, there are localized regions where O3 has decreased since 1991. O3 in the mid-stratosphere is very sensitive to nitrogen chemistry, with increased NOy resulting in decreased O3. We show how the observed O3 changes in the tropical mid-stratosphere can be caused by long-term variations in dynamics. These variations result in a decrease in N2O, an increase in NOy, and a resulting decrease in O3.