The 2019/2020 Arctic stratospheric polar vortex was exceptionally strong and long-lived, leading to the largest chemical ozone loss on record in the Arctic. The extremely strong vortex influenced transport via both radiative changes that control vertical motions and differences in mixing into/out of the vortex. This paper elucidates the transport mechanisms responsible for anomalies in observed long-lived trace gases.
Long-lived trace gas data from the Aura Microwave Limb Sounder (MLS), along with meteorological fields from a NASA reanalysis (MERRA-2) and a new chemical reanalysis that assimilates MLS data (M2-SCREAM), are used to diagnose transport processes. In early 2019/2020 winter, anomalously high H2O and low N2O inside the vortex arose from inclusion of pre-existing trace gas anomalies into the developing vortex, followed by descent. In spring, vortex trace gas anomalies arose from prolonged strong confinement within the exceptionally robust polar vortex.
Transport and vortex confinement are key factors affecting chemical ozone loss in the polar vortex, and knowledge of transport is critical to quantifying that loss. N2O and H2O are also strong greenhouse gases, so anomalies in these species in the lower stratosphere may significantly impact climate. MLS measurements of these long-lived trace gases provide a uniquely complete global daily record for quantifying transport in the stratosphere.
The record-breaking strength and persistence of the Arctic stratospheric vortex in the spring of 2020 caused unprecedented anomalies in long-lived trace gases measured by Aura MLS. The boundary of the stratospheric polar vortex (denoted by black contours in the figure) represents a transport barrier that inhibits mixing between air confined within it and lower-latitude air. Large positive anomalies (differences from climatology) in the gradient of scaled potential vorticity (top row) signify strong confinement within a robust vortex, which persisted longer in 2020 than in any other Arctic spring in the last 40 years. Because of the remarkably impermeable and long-lived vortex, continuing confined descent brought enhanced abundances of CO (2nd row) from the mesosphere down to the middle stratosphere, leading to large positive CO anomalies in spring 2020. Anomalously high H2O (3rd row) and low N2O (4th row) inside the vortex also lingered into spring. The vortex developed in fall 2019 in an environment with extremely strong pre-existing H2O and N22O anomalies throughout most of the Northern Hemisphere that appeared following a sudden stratospheric warming in early January 2019. The exact mechanisms giving rise to the highly unusual pervasive (subtropics to midlatitudes) and persistent (spring to fall) trace gas anomalies are currently under investigation.
The Aura Microwave Limb Sounder (N2O, H2O, CO, O3)
NASA’s Modern Era Retrospective-analysis for Research and Applications (MERRA-2)
MERRA-2 Stratospheric Composition Reanalysis using Aura MLS (M2-SCREAM)
The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (registration required)
Technical description of figure:
Adapted from Figure 7 of Manney et al. (2022). Time series on the 700 K isentropic surface (about 27–28 km) of anomalies from the 2005–2020 climatology of (top to bottom) gradients in scaled potential vorticity (sPV) from MERRA-2 with respect to equivalent latitude and detrended CO, H2O, and N2O from MLS version 5 data, shown for the 2010/2011 through 2019/2020 winters. Black overlays are scaled PV contours of 1.4 and 1.8 × 10−4 s−1, demarking the vortex edge region. High sPV gradient anomalies, especially in spring 2020, indicate a stronger and longer-lasting stratospheric polar vortex. MLS trace gas measurements show high CO and H2O and low N2O anomalies in 2019/2020 that are unique in the approximately 18-year Aura MLS record.
Scientific significance, societal relevance, and relationships to future missions:
This analysis showed the interplay of several different transport processes that resulted in anomalous trace gas transport in 2019/2020. Global daily fields of multiple long-lived trace gases with different vertical and horizontal gradients (such as N2O, H2O, and CO in the stratosphere) are invaluable for diagnosing transport processes that (1) directly affect ozone in the polar vortex by transporting higher ozone from above into the region of chemical depletion and by keeping ozone-depleted air confined within the vortex, and (2) can have important radiative effects (through changes in the distribution of trace gases) in the upper troposphere through lower stratosphere, which will evolve as the climate changes. While future missions include daily global stratospheric ozone measurements, no global daily measurements of H2O and N2O similar to those from Aura MLS will be available from currently scheduled missions, deeply compromising our ability to diagnose and quantify transport processes that affect stratospheric ozone (and thus surface ultraviolet exposure) and climate.
Manney, G.L., Millán, L.F., Santee, M.L., Wargan, K., Lambert, A., Neu, J.L., et al. (2022). Signatures of anomalous transport in the 2019/2020 Arctic stratospheric polar vortex. J. Geophys. Res.: Atmos., 127, e2022JD037407. https://doi.org/10.1029/2022JD037407.