Figure 5: (a) The interannual variations of total column ozone in the interior of polar vortex (units: DU), (b) polar vortex area (units: 106 km2), and (c) polar vortex strength (units: m/s) in September (black curve), October (blue curve), and SO averaged (red curve), respectively. (d) The polar vortex breakup time (units: day) for the period 1979–2016. (e)–(h) Time series of detrend and normalized ozone, vortex area, strength, and breakup time index during the period 1979–2016. Values less than −1.0 (blue line) or more than +1.0 (red line) are used to distinguish the vortex area, smaller/larger; strength, weaker/stronger; and breakup time earlier/later years, respectively.