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Observations of March 2024 Arctic (63°N–90°N) total column ozone set a record high of 477 Dobson Units (DU) against the 1979–2023 satellite era time series. It was about 60 DU higher than average and 6 DU higher than the previous March 1979 471 DU record. Daily Arctic ozone was above average for every day in March 2024, and set record highs from 11–26 March 2024. Microwave Limb Sounder data show this record ozone anomaly was concentrated in the lower stratosphere (10–30 km). These record values developed over the 2023–2024 winter and can be associated with vertically propagating planetary‐scale wave events that caused significant stratospheric warmings. These wave events forced poleward and downward ozone advection into the lower stratosphere, leading to record column ozone levels. The above average levels persisted through August 2024 and across the northern hemisphere. Man‐made chlorofluorocarbons (CFCs) depleted the Earth ozone layer. The 1987 Montreal Protocol curbed CFC growth, but because CFCs have multi‐decadal lifetimes, Arctic ozone is not expected to recover back to 1980 levels until ∼2045. Current high CFC levels combined with persistent and cold polar vortices led to severe Arctic ozone spring depletion in 1997, 2011, and 2020. Contrary to expectations, March 2024 Arctic ozone showed a record high level, dramatically contrasting against the severe depletion events. Meteorological and ozone profile information show that the exceptional 2024 ozone was mainly found in the lowermost Arctic stratosphere, in association with record high lowermost stratospheric temperatures. The ozone levels incrementally increased during the 2023–2024 winter because of large‐scale weather systems that propagated from the troposphere into the stratosphere. Collectively, these weather systems also were at a record level, moving higher ozone concentrations from the mid‐latitudes and upper stratospheric into the Arctic region. This record high ozone would likely have not occurred if CFC levels had not begun slowly declining in response to the Montreal Protocol. Given the absence of high Arctic ozone since the 1970s, the March 2024 record high should be considered a positive harbinger of the future Arctic ozone layer. Arctic total column ozone in March 2024 set a record high for the 1979‐present periodPolar lower stratosphere temperatures also set a record high in March 2024 in the MERRA‐2 reanalysis dataA record amount of Rossby waves propagating upward from the troposphere caused the record total ozone and lower stratospheric temperature Arctic total column ozone in March 2024 set a record high for the 1979‐present period Polar lower stratosphere temperatures also set a record high in March 2024 in the MERRA‐2 reanalysis data A record amount of Rossby waves propagating upward from the troposphere caused the record total ozone and lower stratospheric temperature

The 1998–2016 ozone trends in the lower stratosphere are examined using the Modern‐Era Retrospective Analysis for Research and Applications Version 2 (MERRA‐2) and related National Aeronautics and Space Administration products. After removing biases resulting from step changes in the MERRA‐2 ozone observations, a discernible negative trend of −1.67 ± 0.54 Dobson units per decade (DU/decade) is found in the 10‐km layer above the tropopause between 20°N and 60°N. A weaker but statistically significant trend of −1.17 ± 0.33 DU/decade exists between 50°S and 20°S. In the Tropics, a positive trend is seen in a 5‐km layer above the tropopause. Analysis of an idealized tracer in a model simulation constrained by MERRA‐2 meteorological fields provides strong evidence that these trends are driven by enhanced isentropic transport between the tropical (20°S–20°N) and extratropical lower stratosphere in the past two decades. This is the first time that a reanalysis data set has been used to detect and attribute trends in lower stratospheric ozone. Stratospheric ozone shields the biosphere from harmful ultraviolet radiation and affects the Earth's radiative budget. Observational data show evidence that concentrations of ozone in the upper stratosphere have increased in the last 15 years. This is an expected result of the implementation of the Montreal Protocol and its amendments banning emissions of ozone depleting substances into the atmosphere. The evolution of stratospheric ozone is also impacted by climate change through its dependence on temperature and circulation, which can be different at different altitudes. These effects are less well understood. This study uses the National Aeronautics and Space Administration's data and computer models to analyze the long‐term changes in ozone since 1998. It is shown that the increase in the upper stratospheric ozone has been partially offset by a small but discernible decline of ozone concentrations in the lowermost stratosphere, in qualitative agreement with one recent study. A chemistry model simulation forced by meteorological data provides strong evidence that the primary mechanism driving this negative trend is an intensification of transport of ozone‐poor air from the Tropics into the extratropics, indicative of a systematic change in the lower stratospheric circulation between 1998 and 2016. The MERRA‐2 reanalysis shows negative ozone trends in the extratropical lower stratosphere between 1998 and 2016These ozone trends are likely a result of enhanced two‐way transport between the Tropics and the extratropicsThis study is the first to use bias‐corrected reanalysis ozone to assess and attribute vertically resolved ozone trends

Tropospheric trends in long‐lived source gases N2O and the chlorofluorocarbons cause trends in O3through changes in their reactive product gases. Transport affects the product gases because it controls the distribution of the long‐lived source gases. We find that large changes in tropical upwelling 10–5 hPa since 2012 have strengthened the northern branch of the upper stratospheric (UpS) transport circulation, dramatically altering the abundances of N2O and its odd nitrogen product gases, NOxand HNO3. Increased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterly winds at 10 hPa and above. We use simulations with and without time varying MERRA2 meteorology to quantify the impact of dynamical changes on O3loss via the NOxand ClOxcycles. We find that dynamical impacts on these cycles explain the mid‐stratospheric tropical O3increase and Arctic UpS O3decrease since 2005. Changes in the upper stratospheric circulation over the past decade have altered composition between 30 and 40 km altitude. In the tropics, faster ascent carried greater amounts of nitrous oxide (N2O), a gas produced at the surface, to the upper stratosphere. This increased the amount of the ozone‐destroying nitrogen radicals produced from N2O. Most of the extra nitrogen radicals traveled to the Arctic above ∼32 km, where they increased ozone destruction. Below the altitude where the radicals are produced (about 35 km), the faster tropical ascent carries reduced amounts of nitrogen radicals to the middle stratosphere (∼26 to 35 km), leading to an ozone increase. Observations by satellite instruments revealed changes in the composition. Model simulations showed that circulation change caused the composition changes that affected ozone. The circulation change was connected to changes in the tropical winds that circulate around the globe and change from easterly to westerly on a fairly regular basis. During the last 10 years, this long‐standing pattern changed. This could be variability or the start of a trend; it's too soon to tell. Either way, it complicates the identification and attribution of the ozone increases we expect due to international treaties that banned the use of manmade chlorocarbons. Tropical upwelling 10–5 hPa increased up to 30% from 2005 to 2021, increasing upper stratospheric N2O and reactive nitrogen productionIncreased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterlies at 10 hPa and aboveIncreased upwelling changed NOx‐driven O3losses, increasing tropical O3from 15 to 7 hPa while reducing Arctic O3from 15 to 2 hPa Tropical upwelling 10–5 hPa increased up to 30% from 2005 to 2021, increasing upper stratospheric N2O and reactive nitrogen production Increased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterlies at 10 hPa and above Increased upwelling changed NOx‐driven O3losses, increasing tropical O3from 15 to 7 hPa while reducing Arctic O3from 15 to 2 hPa

The quasi‐biennial oscillation (QBO) of tropical stratospheric winds was disrupted during the 2019/20 Northern Hemisphere winter. We show that this latest disruption to the regular QBO cycling was similar in many respects to that seen in 2016, but initiated by horizontal momentum transport from the Southern Hemisphere. The predictable signal associated with the QBO's quasi‐regular phase progression is lost during disruptions and the oscillation reemerges after a few months significantly shifted in phase from what would be expected if it had progressed uninterrupted. We infer from an increased wave‐momentum flux into equatorial latitudes seen in climate model projections that disruptions to the QBO are likely to become more common in future. Consequently, it is possible that in the future, the QBO could be a less reliable source of predictability on lead times extending out to several years than it currently is. The quasi‐biennial oscillation (QBO) consists of a regular switching between eastward and westward winds in the tropical stratosphere. The oscillation has persisted at least since its discovery in the 1960s, over which time its period averages about 28 months with some variability from cycle to cycle. Recently, during the Northern Hemisphere winters of 2015/16 and 2019/20, remarkable departures from this regular behavior occurred that have no precedent in the observational record. Both the 2015/16 and 2019/20 QBO disruptions occurred when large horizontal fluxes of momentum intruded into the tropics from higher latitudes. Using climate model projections, we find these horizontal fluxes are likely to increase in future, suggesting an increased future likelihood of QBO disruptions and a concomitant loss in QBO predictability. A second recent disruption of the quasi‐biennial oscillation (QBO) has occurredLarge momentum fluxes from the Southern Hemisphere made a substantial contribution to the 2019/20 disruptionIncreased equatorward momentum flux in climate model projections suggests QBO disruptions may become more likely in future A second recent disruption of the quasi‐biennial oscillation (QBO) has occurred Large momentum fluxes from the Southern Hemisphere made a substantial contribution to the 2019/20 disruption Increased equatorward momentum flux in climate model projections suggests QBO disruptions may become more likely in future

The Global Modeling and Assimilation Office (GMAO) has recently released a new version of the Goddard Earth Observing System (GEOS) Subseasonal to Seasonal prediction (S2S) system, GEOS‐S2S‐2, that represents a substantial improvement in performance and infrastructure over the previous system. The system is described here in detail, and results are presented from forecasts, climate equillibrium simulations, and data assimilation experiments. The climate or equillibrium state of the atmosphere and ocean showed a substantial reduction in bias relative to GEOS‐S2S‐1. The GEOS‐S2S‐2 coupled reanalysis also showed substantial improvements, attributed to the assimilation of along‐track absolute dynamic topography. The forecast skill on subseasonal scales showed a much improved prediction of the Madden‐Julian Oscillation in GEOS‐S2S‐2, and on a seasonal scale the tropical Pacific forecasts show substantial improvement in the east and comparable skill to GEOS‐S2S‐1 in the central Pacific. GEOS‐S2S‐2 anomaly correlations of both land surface temperature and precipitation were comparable to GEOS‐S2S‐1 and showed substantially reduced root‐mean‐square error of surface temperature. The remaining issues described here are being addressed in the development of GEOS‐S2S Version 3, and with that system GMAO will continue its tradition of maintaining a state‐of‐the‐art seasonal prediction system for use in evaluating the impact on seasonal and decadal forecasts of assimilating newly available satellite observations, as well as evaluating additional sources of predictability in the Earth system through the expanded coupling of the Earth system model and assimilation components. GMAO's New Seasonal Prediction Model and Assimilation shows substantial improvement in forecast skill over the previous version