The impact of sudden stratospheric warmings on Arctic ozone depletion

Arctic spring O3 depletion is highly variable and cannot be diagnosed using observations of March mean column O3 because dynamically driven O3 transport during winter is also variable. It is therefore difficult to attribute a change in Arctic O3 to a decline in ozone depleting substances (ODSs). After the polar vortex breaks down, ozone-depleted air impacts life in the midlatitudes by increasing spring surface UV. Assessing the impact of depletion there in the context of ozone's natural variability is challenging.

Using NASA Aura Microwave Limb Sounder (MLS) ozone observations and simulations of the Global Modeling Initiative (GMI) chemistry and transport model with and without heterogeneous chemical reactions, we quantify Arctic ozone depletion and seasonal O3 transport (resupply) from 2005-2015. The simulations use Modern-Era Retrospective Analysis for Research and Applications (MERRA) meteorological fields. We find that the maximum annual O3 depletion averaged over the polar cap (63-90N equivalent latitude) depends on the number of days with lower stratospheric vortex temperatures below the threshold for chlorine activation. Ozone depletion results for this 11 year period fall into two groups: 1) low loss years, 12-22 Dobson Units (DU), that experienced a sudden stratospheric warming (SSW) and 2) high loss years, 30-54 DU, with no SSW. Sudden stratospheric warmings affect the severity of Arctic ozone depletion by limiting the duration of low vortex temperatures. Seasonal resupply of O3 is variable but is not strongly correlated with chemical loss during this period. High loss years have twice the impact on the midlatitudes in spring compared to SSW years.

The occurrence of SSWs is responsible for large interannual variability in Arctic ozone depletion and its midlatitude impacts. While ODS levels remain elevated, variations in SSW frequency may give the appearance of an Arctic O3 trend.