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The January 2022 eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano injected a large amount of water vapor into the mid‐stratosphere. This study uses model simulations to investigate the resulting stratospheric impacts out to 2031. Maximum radiatively‐driven model temperature changes occur in the Southern Hemisphere (SH) subtropics in April–May 2022, with warming of ∼1 K in the lower stratosphere and cooling of 3 K in the mid‐stratosphere. The radiative cooling combined with adiabatic cooling driven by the quasi‐biennial oscillation meridional circulation explains the near‐record cold anomaly observed in the SH subtropical mid‐stratosphere. Projected ozone responses maximize in 2023–2024 as the water vapor plume is transported globally throughout the stratosphere and mesosphere. The excess H2O increases the OH radical, causing a negative global ozone response (2%–10%) in the upper stratosphere and mesosphere due to increased odd hydrogen‐ozone loss, and a small positive ozone response (0.5%–1%) in the mid‐stratosphere due to interference of the NOx catalytic loss cycle by the additional OH. In the lower stratosphere, the excess H2O is projected to increase polar stratospheric clouds and springtime halogen‐ozone loss, enhancing the Antarctic ozone hole by 25–30 DU in 2023. Arctic impact is small, with maximum additional ozone loss of 4–5 DU projected in spring 2024. These responses diminish after 2024 to be quite small by 2031, as the excess H2O is removed from the stratosphere with a 2.5‐year e‐folding time. Given the year‐to‐year variability of the stratosphere, the magnitudes of these ozone responses may be below the threshold of detectability in observations. Plain Language Summary: Stratospheric ozone protects Earth's biosphere from harmful ultraviolet radiation, and along with water vapor, are key components in determining temperature and chemistry of the atmosphere. The January 2022 eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano in the South Pacific injected water vapor into the atmosphere, increasing stratospheric water vapor by 10%. In this study, we use computer simulations of the stratosphere to project how this additional water vapor changed temperature and ozone in the months and years following the eruption. The water vapor cooled the middle stratosphere (roughly 14–25 miles above Earth's surface) and warmed the lower stratosphere (6–14 miles above the surface), with the largest changes of 2–5° Fahrenheit in March–June 2022, several months after the eruption. The additional water vapor modified chemical processes that affect stratospheric ozone, leading to a projected 10%–15% enhancement in the Antarctic ozone hole. This ozone hole enhancement is estimated to have maximized in October 2023, almost 2 years after the eruption, due to the slow circulation of stratospheric water vapor from the subtropics to the polar region. These impacts are expected to diminish after 2024 as the excess water vapor is slowly removed from the atmosphere by natural processes. Key Points: The water vapor injection from the Hunga Tonga eruption impacts stratospheric temperature and ozoneLargest additional ozone depletion is estimated to occur in Antarctic spring 2023. The ozone response may not be detectable in observationsThe impacts are projected to gradually diminish after 2024 as the excess water vapor is removed from the stratosphere [ABSTRACT FROM AUTHOR]

Deep convection associated with tropical cyclones leads to stratosphere‐troposphere exchange (STE), which affects the upper‐tropospheric ozone concentrations in the vicinity of the cyclones. This study estimates the ozone enhancements over India due to the North Indian Ocean (NIO) cyclones‐driven STE. Indicators such as stratospheric fraction and potential vorticity calculated using the reanalysis data sets suggest that roughly 70% of the cyclones show anomalously high stratospheric intrusions. Aircraft observations over different locations across India also show elevated ozone concentrations in the mid‐to‐upper troposphere on cyclone days. Further, ozone and stratospheric ozone tracer concentrations from Goddard Earth Observing System‐Chemistry simulations and the Copernicus Atmosphere Monitoring Service reanalysis data sets show up to 40 ppb of excess upper tropospheric ozone over India, of which stratospheric ozone accounts for roughly 60%. Stratospheric intrusion due to the Bay of Bengal and the Arabian Sea cyclones affected the upper tropospheric ozone amounts over North and South India, respectively. The stratospheric ozone was observed to propagate downwards into the troposphere, often reaching ∼600 hPa and, in some cases, even the surface. Plain Language Summary: This study estimates the increase in the amount of ozone in the upper half of the lower atmosphere ("upper troposphere") during the North Indian Ocean (NIO) cyclones. Ozone in the upper atmosphere ("stratosphere") protects the Earth from harmful ultraviolet radiation but affects human and ecosystem health when present in the lower atmosphere ("troposphere"). Generally, the ozone in the lower atmosphere occurs due to chemical reactions of gases and sometimes due to the transport from the ozone‐rich upper atmosphere. However, there is a barrier between the upper and the lower atmospheres, also known as the tropopause. Using observations and atmospheric models, we study the penetration of ozone from the upper atmosphere to the lower atmosphere, during the NIO cyclones which disturb the separation barrier. We studied 20 cyclones and found that they disturb the barrier between the upper and the lower atmosphere significantly over India and a large amount of ozone moves between them, so much so that it almost doubles the ozone concentration in the upper half of the lower atmosphere. A warmer world in the future could lead to more cyclones which could increase ozone in the lower atmosphere. Key Points: Stratosphere‐troposphere exchange (STE) over India due to the North Indian Ocean cyclones is studied using reanalysis data and model outputEnhanced stratospheric airmass (∼20%) and ozone (∼40 ppb) were evident in the upper troposphere and lower stratosphereSTE due to the Bay of Bengal and Arabian Sea cyclones influence ozone amplification over northern and southern India, respectively [ABSTRACT FROM AUTHOR]

The Asian summer monsoon (ASM) as a chemical transport system is investigated using a suite of models in preparation for an airborne field campaign over the Western Pacific. Results show that the dynamical process of anticyclone eddy shedding in the upper troposphere rapidly transports convectively uplifted Asian boundary layer air masses to the upper troposphere and lower stratosphere over the Western Pacific. The models show that the transported air masses contain significantly enhanced aerosol loading and a complex chemical mixture of trace gases that are relevant to ozone chemistry. The chemical forecast models consistently predict the occurrence of the shedding events, but the predicted concentrations of transported trace gases and aerosols often differ between models. The airborne measurements to be obtained in the field campaign are expected to help reduce the model uncertainties. Furthermore, the large‐scale seasonal chemical structure of the monsoon system is obtained from modeled carbon monoxide, a tracer of the convective transport of pollutants, which provides a new perspective of the ASM circulation, complementing the dynamical characterization of the monsoon. Plain Language Summary: The Asian summer monsoon has been known as a weather system for centuries, but only in the recent decades has the system been recognized for its importance in atmospheric composition. Monsoon deep convection lofts near surface air to 15–17 km altitudes thus altering the chemical composition of the tropopause layer. The process also sends aerosols and chemically active trace gas species into the stratosphere where they affect climate through their impacts on ozone and aerosol radiative forcing. To understand the monsoon transport process and its impacts on climate system, a large airborne field campaign, the Asian summer monsoon Chemical and Climate Impact Project, was planned. This paper presents a set of results from precampaign model studies. These results serve as the hypotheses for the field investigation and provide guidance for its operational planning. Key Points: This model study is conducted in preparation for an airborne field campaign investigating the Asian monsoon transportResult shows that eastward eddy shedding of the anticyclone significantly alters upper tropospheric composition over the Western PacificCO seasonal distribution provides a chemical perspective of the monsoon system and sheds new light on monsoon dynamics and circulation [ABSTRACT FROM AUTHOR]

Trichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions increased during 2014–2017 relative to 2008–2012. In this study, we use a chemistry‐climate model to investigate the stratospheric impacts of potential CFC‐11 emissions continuing into the future. As a sensitivity test, we use a high CFC‐11 scenario in which the inferred 2013–2016 average emissions of 72.5 Gg/yr is sustained to year 2100. This increases equivalent effective stratospheric chlorine by 15% in 2100, relative to the WMO (2018) baseline scenario in which future emissions decay with a bank release rate of 6.4%/year. Consistent with recent studies, the resulting ozone response has a linear dependence on the accumulated CFC‐11 emissions, yielding global and Antarctic spring total ozone sensitivity per 1,000 Gg of −0.37 and −3.9 DU, respectively, averaged over 2017–2100. The deepened ozone hole reduces UV heating, causing a colder Antarctic lower stratosphere in spring/early summer. Through thermal wind balance, this accelerates the circumpolar jet which in turn alters planetary and gravity wave propagation through the Southern Hemisphere stratosphere, and modifies the Brewer‐Dobson circulation. Age of air in the high scenario is slightly younger than the baseline in the lower stratosphere globally during 2090–2099, with a maximum change of −0.1 years. Coupled atmosphere‐ocean model simulations show that the resulting greenhouse gas impact of CFC‐11 is small and not statistically significant throughout the troposphere and stratosphere. Key Points: Continuing trichlorofluoromethane (CFC‐11) emissions will cause additional future stratospheric ozone depletionLargest statistically significant ozone depletion occurs in the global and Antarctic spring total column, and global upper stratosphereAdditional ozone depletion due to continuing CFC‐11 emissions will cause small changes in stratospheric temperature and circulation [ABSTRACT FROM AUTHOR]

The Northern Hemisphere (NH) polar winter stratosphere of 2019/2020 featured an exceptionally strong and cold stratospheric polar vortex. Wave activity from the troposphere during December–February was unusually low, which allowed the polar vortex to remain relatively undisturbed. Several transient wave pulses nonetheless served to help create a reflective configuration of the stratospheric circulation by disturbing the vortex in the upper stratosphere. Subsequently, multiple downward wave coupling events took place, which aided in dynamically cooling and strengthening the polar vortex. The persistent strength of the stratospheric polar vortex was accompanied by an unprecedentedly positive phase of the Arctic Oscillation in the troposphere during January–March, which was consistent with large portions of observed surface temperature and precipitation anomalies during the season. Similarly, conditions within the strong polar vortex were ripe for allowing substantial ozone loss: The undisturbed vortex was a strong transport barrier, and temperatures were low enough to form polar stratospheric clouds for over 4 months into late March. Total column ozone amounts in the NH polar cap decreased and were the lowest ever observed in the February–April period. The unique confluence of conditions and multiple broken records makes the 2019/2020 winter and early spring a particularly extreme example of two‐way coupling between the troposphere and stratosphere. Plain Language Summary: Wintertime westerly winds in the polar stratosphere (from ∼15–50 km), known as the stratospheric polar vortex, were extraordinarily strong during the Northern Hemisphere winter of 2019/2020. The exceptional strength of the stratospheric polar vortex had consequences for winter and early spring weather near the surface and for stratospheric ozone depletion. Typically atmospheric waves generated in the troposphere spread outward and upward into the stratosphere where they can disturb and weaken the polar vortex, but tropospheric wave activity was unusually weak during the 2019/2020 winter. In addition, an unusual configuration of the stratospheric polar vortex developed that reflected waves traveling upward from the troposphere back downward. These unique conditions allowed the vortex to remain strong and cold for several months. During January–March 2020, the strong stratospheric polar vortex was closely linked to a near‐surface circulation pattern that resembles the positive phase of the so‐called "Arctic Oscillation" (AO). This positive AO pattern was also of record strength and influenced the regional distributions of temperatures and precipitation during the late winter and early spring. Cold and stable conditions within the polar vortex also allowed strong ozone depletion to take place, leading to lower ozone levels than ever before seen above the Arctic in spring. Key Points: The Arctic stratospheric polar vortex during the 2019/2020 winter was the strongest and most persistently cold in over 40 yearsLow tropospheric planetary wave driving and a wave‐reflecting configuration of the stratosphere supported the strong and cold polar vortexSeasonal records in the Arctic Oscillation and stratospheric ozone loss were related to the strong polar vortex [ABSTRACT FROM AUTHOR]

Abstract: The relative impacts of greenhouse gas (GHG) increase and stratospheric ozone depletion on stratospheric mean age of air in the 1960–2010 period are quantified using the Goddard Earth Observing System Chemistry‐Climate Model. The experiment compares controlled simulations using a coupled atmosphere‐ocean version of the Goddard Earth Observing System Chemistry‐Climate Model, in which either GHGs or ozone depleting substances, or both factors evolve over time. The model results show that GHGs and ozone depleting substances have about equal contributions to the simulated mean age decrease, but GHG increases account for about two thirds of the enhanced strength of the lower stratospheric residual circulation. It is also found that both the acceleration of the diabatic circulation and the decrease of the mean age difference between downwelling and upwelling regions are mainly caused by GHG forcing. The results show that ozone depletion causes an increase in the mean age of air in the Antarctic summer lower stratosphere through two processes: (1) a seasonal delay in the Antarctic polar vortex breakup that inhibits young midlatitude air from mixing with the older air inside the vortex, and (2) enhanced Antarctic downwelling that brings older air from middle and upper stratosphere into the lower stratosphere. [ABSTRACT FROM AUTHOR]

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH3CCl3) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom-up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long-lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH-SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long-term trend and emissions derived from the measured hemispheric gradient, the combination of HFC-32 (CH2F2), HFC-134a (CH2FCF3, HFC-152a (CH3CHF2), and HCFC-22 (CHClF2), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF. [ABSTRACT FROM AUTHOR]

Trichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions have been near constant or increasing since 2002. In this study, we use a two‐dimensional chemistry‐climate model to investigate the stratospheric ozone response to a range of future CFC‐11 emissions scenarios. A scenario with future emissions sustained at 10 gigagrams per year (Gg/year) above the baseline WMO (2018) A1 scenario results in minor additional global (90°S–90°N) ozone depletion of 0.13% by 2100, and a 1.5‐year delay in the global ozone recovery to 1980 levels, relative to the baseline. A scenario with 72.5 Gg/year (the 2013–2016 average) sustained to 2100 results in a substantial 15% increase in effective equivalent stratospheric chlorine and nearly 1% additional global ozone depletion by 2100, with a 7.5‐year delay in the recovery to 1980 global ozone levels, relative to the baseline. The ozone response averaged over time has a strong linear dependence on the cumulative amount of future CFC‐11 emissions under a wide range of scenarios. The resulting ozone response sensitivity gives a simple metric relating the time‐averaged ozone change to the cumulative CFC‐11 emissions. This sensitivity has an inverse dependence on future greenhouse gas concentrations (CO2, CH4, and N2O). For the medium Intergovernmental Panel on Climate Change Representative Concentration Pathway‐6.0 scenario, the sensitivity per 1,000 Gg of cumulative CFC‐11 emissions is −0.1% and −1% for global and Antarctic spring ozone, respectively. Plain Language Summary: Stratospheric ozone protects the Earth's biosphere from harmful ultraviolet radiation and is key in determining the radiative balance of the atmosphere. CFC‐11 is a man‐made chlorofluorocarbon, and its emissions and subsequent break down in the stratosphere result in ozone depletion. Because of this, production and consumption of CFC‐11 have been controlled under the Montreal Protocol, resulting in a rapid decline in emissions starting in the late 1980s. Recent studies show that CFC‐11 emissions have increased in recent years, at odds with expected declines caused by the Montreal Protocol controls. In this study, we examine how potential future CFC‐11 emissions will impact stratospheric ozone. The ozone response is proportional to the amount of CFC‐11 emitted, and the response is substantial for a future projection in which the increased emissions during 2013–2016 continue to 2100. This scenario will postpone the return of global ozone to 1980 levels from mid‐2052 to 2060 and causes additional 1% global ozone depletion by 2100, compared to the baseline. Although there is uncertainty in projecting future emissions, the ozone response is strongly dependent on the amount of CFC‐11 emissions accumulated over time, allowing for a simple metric relating the ozone depletion to the cumulative amount of emissions. Key Points: Continuing global CFC‐11 emissions at 2013–2016 average levels of 72.5 Gg/year will have a substantial impact on future stratospheric ozoneThe time‐integrated ozone depletion has a strong linear dependence on the cumulative amount of CFC‐11 emissionsThe sensitivity of the ozone response to CFC‐11 has a modest inverse dependence on future CO2, CH4, and N2O concentrations [ABSTRACT FROM AUTHOR]