Abstract. The quasi-biennial oscillation (QBO) is a quasiperiodic alternation between easterly and westerly zonal winds in the tropical stratosphere, propagating downward from the middle stratosphere to the tropopause with a period that varies from 24 to 32 months ( ∼ 28 months on average). The QBO wind oscillations affect the distribution of chemical constituents, such as ozone (O 3 ), water vapor (H 2 O), nitrous oxide (N 2 O), and hydrochloric acid (HCl), through the QBO-induced meridional circulation. In the 2015-2016 winter, radiosonde observations revealed an anomaly in the downward propagation of the westerly phase, which was disrupted by the upward displacement of the westerly phase from ∼ 30 hPa up to 15 hPa and the sudden appearance of easterlies at 40 hPa. Such a disruption is unprecedented in the observational record from 1953 to the present. In this study we show the response of trace gases to this QBO disruption using O 3 , HCl, H 2 O, and temperature from the Aura Microwave Limb Sounder (MLS) and total ozone measurements from the Solar Backscatter Ultraviolet (SBUV) Merged Ozone Data Set (MOD). Results reveal the development of positive anomalies in stratospheric equatorial O 3 and HCl over ∼ 50-30 hPa in May-September of 2016 and a substantial decrease in O 3 in the subtropics of both hemispheres. The SBUV observations show near-record low levels of column ozone in the subtropics in 2016, resulting in an increase in the surface UV index during northern summer. Furthermore, cold temperature anomalies near the tropical tropopause result in a global decrease in stratospheric water vapor.

[1] A major stratospheric sudden warming (SSW) strongly impacts the entire middle atmosphere up to the thermosphere. Currently, the role of atmospheric dynamics on polar ozone in the mesosphere-lower thermosphere (MLT) during SSWs is not well understood. Here we investigate the SSW-induced changes in the nighttime "secondary" (90-105 km) ozone maximum by examining the dynamics and distribution of key species (like H and O) important to ozone. We use output from the National Center for Atmospheric Research Whole Atmosphere Community Climate Model with "Specified Dynamics" (SD-WACCM), in which the simulation is constrained by meteorological reanalyses below 1 hPa. Composites are made based on six major SSW events with elevated stratopause episodes. Individual SSW cases of temperature and MLT nighttime ozone from the model are compared against the Sounding of the Atmosphere using Broadband Emission Radiometry observations aboard the NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The evolution of ozone and major chemical trace species is associated with the anomalous vertical residual motion during SSWs and consistent with photochemical equilibrium governing the MLT nighttime ozone. Just after SSW onset, enhanced upwelling adiabatically cools the polar region from 80 to 100 km and transports low H from below. These conditions promote a concentration increase in the secondary ozone layer. Subsequent downwelling from the lower thermosphere warms the MLT and enhances the descent of H from the thermospheric reservoir, thereby limiting the secondary ozone concentration increase. Negative correlation of secondary ozone with respect to temperature and H is more pronounced during winters with SSWs than during non-SSW winters.Citation: Tweedy, O. V., et al. (2013), Nighttime secondary ozone layer during major stratospheric sudden warmings in specified-dynamics WACCM,

The interannual variability of tropical lower stratosphere ozone and its connections to sea surface temperatures in the equatorial Pacific are examined using a combination of chemistry climate model simulations, satellite observations, and reanalyses. The model simulations and observations show large differences in the magnitude of interannual variability in ozone between northern tropic (NT; EQ‐18° N) and southern tropic (EQ‐18° S) during boreal summer but small differences in winter. The interannual variability during boreal summer is highly correlated with summer sea surface temperatures in the eastern and central Pacific Ocean and El Niño–Southern Oscillation (ENSO) events. Larger variability in NT ozone is primarily due to meridional advection, connected to the changes in the onset date and strength of the Asian summer monsoon anticyclone. The Asian summer monsoon anticyclone forms earlier in a season and tends to be stronger during cold (La Niña) events leading to more isentropic transport of ozone from the extratropics into the NT, with the reverse for warm (El Niño) events.

Observations from satellites and a ground‐based station are combined to construct a global data set for investigating the tertiary ozone maximum in the winter mesosphere for the period August 2004 to June 2017. These give a comprehensive picture of this ozone maximum in latitude, pressure, and time. The location of the tertiary ozone maximum shifts in latitude and pressure with the evolving season; the ozone peak occurs at lower latitude and higher pressure around the winter solstice. Highest average nighttime ozone concentrations and greatest degree of interannual variability are seen in late winter in the Northern Hemisphere (NH). The hemispheric differences and interannual variability in nighttime ozone are related to variations of temperature, H2O, and OH associated with dynamical activity. Elevated stratopause events in the NH winter are associated with transport of air that is depleted in H2O and enhanced in OH; photochemistry then leads to downward displacement of the altitude of maximum ozone and enhancement in the ozone amount. Transport by planetary waves in the NH extends the region of high ozone further from the pole and leads to longitudinal variations. The analysis shows that while the tertiary ozone maximum responds to a particular radiative situation as shown in previous studies, it is also the result of very dry air found in the winter polar mesosphere.