Signatures of Anomalous Transport in the 2019/2020 2 Arctic Stratospheric Polar Vortex

The exceptionally strong and long-lived Arctic stratospheric polar vortex in 2019/2020 resulted in large transport anomalies throughout the fall-winter-spring period from vortex development to breakup. These anomalies are studied using Aura MLS N2O, H2O, and CO long-lived trace gas data, ACE-FTS CH4 data, and meteorological and trace gas fields from reanalyses. Anomalies are strongest throughout the winter in the lower through middle stratosphere (from about 500K through 700K), with record low (high) departures from climatology in N2O and CH4 (H2O). CO shows extreme high anomalies in midwinter through spring down to about 550K. Descent rates, vortex confinement, and trace gas distributions in the preceding months indicate that early-winter anomalies in N2O and H2O arose primarily from entrainment of air with already-anomalous values into the vortex as it developed in fall 2019 followed by descent of those anomalies to lower levels within the vortex. Trace gas anomalies in midwinter through the late vortex breakup in spring 2020 arose primarily from inhibition of mixing between vortex and extravortex air because of the exceptionally strong and persistent vortex. Persistent strong N2O and H2O gradients across the vortex edge demonstrate that air within the vortex and its remnants remained very strongly confined through late April (mid-May) in the middle (lower) stratosphere. These results are important for understanding the evolution of trace gas distributions, which affects both polar chemical processing and radiative pro40 cesses related to climate.