Stratospheric ozone trends for 1984–2021 in the SAGE II–OSIRIS–SAGE III/ISS composite dataset

Abstract

Abstract. After decades of depletion in the 20th century, near-global ozone now shows clear signs of recovery in the upper stratosphere. The ozone column, however, has remained largely constant since the turn of the century, mainly due to the evolution of lower stratospheric ozone. In the tropical lower stratosphere, ozone is expected to decrease as a consequence of enhanced upwelling driven by increasing greenhouse gas concentrations, and this is consistent with observations. There is recent evidence, however, that mid-latitude ozone continues to decrease as well, contrary to model predictions. These changes are likely related to dynamical variability, but the impact of changing circulation patterns on stratospheric ozone is not well understood. Here we use merged measurements from the Stratospheric Aerosol and Gas Experiment II (SAGE II), the Optical Spectrograph and InfraRed Imaging System (OSIRIS), and SAGE III on the International Space Station (SAGE III/ISS) to quantify ozone trends in the 2000–2021 period. We implement a sampling correction for the OSIRIS and SAGE III/ISS datasets and assess trend significance, taking into account the temporal differences with respect to Aura Microwave Limb Sounder data. We show that ozone has increased by 2 %–6 % in the upper and 1 %–3 % in the middle stratosphere since 2000, while lower stratospheric ozone has decreased by similar amounts. These decreases are significant in the tropics (>95 % confidence) but not necessarily at mid-latitudes (>80 % confidence). In the upper and middle stratosphere, changes since 2010 have pointed to hemispheric asymmetries in ozone recovery. Significant positive trends are present in the Southern Hemisphere, while ozone at northern mid-latitudes has remained largely unchanged in the last decade. These differences might be related to asymmetries and long-term variability in the Brewer–Dobson circulation. Circulation changes impact ozone in the lower stratosphere even more. In tropopause-relative coordinates, most of the negative trends in the tropics lose significance, highlighting the impacts of a warming troposphere and increasing tropopause altitudes.