MLS observations of exceptional ozone loss and unusual trace gas evolution in the 2019/2020 stratospheric polar vortex and comparisons with previous extreme winters

The Arctic stratospheric vortex in 2019/2020 was extremely strong, cold, and long-lasting. The 16-year record of Aura Microwave Limb Sounder (MLS) measurements of numerous trace gases provides a comprehensive view of transport and processes involved in chemical processing. Using these measurements, we show that chemical processing was critical to the observed record-low Arctic stratospheric ozone in spring 2020, and we compare polar processing with that in the previous winters with most ozone loss, 2015/2016 and 2010/2011. Chlorine activation and ozone depletion began earlier in 2019/2020 than in any previously observed winter, with evidence of chemical ozone loss starting in November; active chlorine then persisted as late into spring as it did in 2011. MLS measurements also show unusual long-lived trace gas abundances in the northern hemisphere middle stratosphere in fall, which, along with the exceptionally strong vortex, led to anomalous transport signatures compared to other winters. Further, MLS measurements are used to illustrate transport patterns of chemically processed air during the vortex breakup in spring.