Arctic Ozone Depletion in 2019/20: Roles of Chemistry, Dynamics and the Montreal Protocol.

We use a three‐dimensional chemical transport model and satellite observations to investigate Arctic ozone depletion in winter/spring 2019/20 and compare with earlier years. Persistently, low temperatures caused extensive chlorine activation through to March. March‐mean polar‐cap‐mean modeled chemical column ozone loss reached 78 DU (local maximum loss of ∼108 DU in the vortex), similar to that in 2011. However, weak dynamical replenishment of only 59 DU from December to March was key to producing very low (<220 DU) column ozone values. The only other winter to exhibit such weak transport in the past 20 years was 2010/11, so this process is fundamental to causing such low ozone values. A model simulation with peak observed stratospheric total chlorine and bromine loading (from the mid‐1990s) shows that gradual recovery of the ozone layer over the past 2 decades ameliorated the polar cap ozone depletion in March 2020 by ∼20 DU. Plain Language Summary: Ozone depletion in the polar stratosphere is caused by chlorine and bromine species which are activated by low temperatures. Chlorine and bromine are transported to the stratosphere following the surface emission of ozone‐depleting substances (ODSs). While springtime ozone depletion in the Antarctic is almost always large, it is much more variable in the Arctic due to warmer temperatures and more disturbed stratospheric dynamics. Using a three‐dimensional atmospheric chemical transport model and satellite observations, we show that the very low ozone columns observed in March 2020 were a consequence of large chemical destruction and weaker‐than‐normal replenishment by transport. These very low ozone levels are, by some measures, record values despite 2 decades of decreasing stratospheric chlorine and bromine through controls of the Montreal Protocol. Had the meteorology of 2019/20 occurred 2 decades ago, the ozone loss would have been notably larger. The Arctic stratospheric dynamics for 2019/20 are extreme relative to the past 2 decades but fit a compact relationship that links column ozone variations over Arctic and Antarctic winters. Key Points: Large mean Arctic (>63°N) chemical ozone destruction in 2019/20 of 78 DU, similar to other extreme cold winters in the past 2 decadesAnomalously weak wintertime dynamical replenishment of only ∼60 DU contributed strongly to the very low observed ozone column in MarchOzone recovery caused 20 DU less mean Arctic ozone loss in March 2020 than would have occurred with stratospheric halogens at 1995 levels [ABSTRACT FROM AUTHOR]