Observed Hemispheric Asymmetry in Stratospheric Transport Trends from 1994-2018

Total columns of the trace gases nitric acid (HNO3) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air (AoA), a quantity that describes transport timescales in the stratosphere. The transport circulation controls the distribution of lower stratospheric (LS) O3 as well as that of key trace gas families inorganic chlorine and odd nitrogen (Cly and NOy) that produce radicals that control ozone (O3) at all altitudes of the global stratosphere. Changes in the transport circulation can alter how long manmade ozone depleting substances will stay in the atmosphere and therefore the rate of recovery of ozone. To credibly project how the O3 layer will evolve requires that models be able to produce variability and differences in hemispheric behavior similar to observations.
Analyses of HNO3 and HCl columns from the Network for the Detection of Atmospheric Composition Change (NDACC) from 9 stations spanning 77oS to 79oN have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO3 and HCl analyses combined with AoA from a simulation using the MERRA2 reanalysis show that the southern hemisphere (SH) lower stratosphere has become 1 month/decade younger relative to the northern hemisphere, largely driven by the SH transport circulation. These results are in contrast to several model studies that have predicted that Antarctic ozone hole recovery will cause an increase in the SH AoA.
The analyses also reveal variability with a 5-7 yr period driven by interactions between the annual cycle of the circulation and the Quasi Biennial Oscillation (QBO) in tropical winds. This previously unrecognized periodic variability is much larger than hemispheric transport trends and is likely to cause bias in trends calculated using shorter records, e.g., trend studies using satellite data records of less than 15 years. The NDACC data records span 4 cycles of this variability and are able to identify a small but significant trend in the transport circulation against a background of large variability.