Subtropical and midlatitude ozone trends in the stratosphere: Implications for recovery

We present a comprehensive analysis of the trends of stratospheric ozone in the midlatitudes and subtropics. The analysis is performed using ground-based and space-based measurements over the light detection and ranging stations for the period 1985–2012. Also, trends are estimated for the zonal mean data made from a merged satellite data set, Global OZone Chemistry And Related trace gas Data records for the Stratosphere, over 1979–2012. The linear trends in stratospheric ozone are estimated using piecewise linear trend (PWLT) functions. The ozone trends during the increasing phase of halogens (before 1997) range from −0.2±0.08 to −1±0.07% yr−1 in the midlatitudes and −0.2±0.06 to −0.7±0.05 % yr−1 in the subtropics at 15–45 km, depending on altitude. In 1997–2012, the PWLT analyses show a positive trend, significantly different from zero at the 95% confidence intervals, toward ozone recovery in the middle- and low-latitude upper stratosphere (35–45 km), and the trends are about +0.5±0.07% yr−1 at midlatitudes and about +0.3± 0.05% yr−1 at subtropical latitudes. However, negative and insignificant trends are estimated in the lower stratosphere (15–20 km) over 1997–2012 in the midlatitudes, mainly due to the dynamics, as demonstrated by the large (50–60%) contributions from the quasi-biennial oscillation, El Niño–Southern Oscillation, and planetary wave activity to recent ozone changes. This suggests that the ozone changes are governed by the interannual variations in meteorology and dynamics of the regions; these factors will influence the recovery detection time and the behavior of the recovery path to pre-1980 levels.