Observed Hemispheric Asymmetry in Stratospheric Transport Trends From 1994 to 2018
S. E. Strahan, Dan Smale, A. R. Douglass, Thomas Blumenstock, James W. Hannigan, Frank Hase, Nicholas Jones, Emmanuel Mahieu, Justus Notholt, Luke D. Oman, Iván Ortega, Mathias Palm, Maxime Prignon, John Robinson, Matthias Schneider, Ralf Sussmann, Voltaire A. Velazco
Abstract
Abstract Total columns of the trace gases nitric acid (HNO 3 ) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air, a quantity that describes transport time scales in the stratosphere. Analyses of HNO 3 and HCl columns from the Network for the Detection of Atmospheric Composition Change panning 77°S to 79°N have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO 3 and HCl analyses combined with the age of air from a simulation using the MERRA2 reanalysis show that the Southern Hemisphere lower stratosphere has become 1 month/decade younger relative to the Northern Hemisphere, largely driven by the Southern Hemisphere transport circulation. The analyses reveal multiyear anomalies with a 5‐ to 7‐year period driven by interactions between the circulation and the quasi‐biennial oscillation in tropical winds. This hitherto unrecognized variability is large relative to hemispheric transport trends and may bias ozone trend regressions.