Ozone Anomalies Research Articles

ДИНАМІЧНІ УМОВИ ФОРМУВАННЯ ПРОСТОРОВИХ ЕКСТРЕМУМІВ ОЗОНОВОГО ШАРУ НАД ТЕРИТОРІЄЮ УКРАЇНИ

The paper examines the conditions for the formation of spatial extremes in total ozone content (TOC) over the territory of Ukraine caused by dynamic factors. The study used satellite observations of TOC and meteorological parameters (u,v components of wind and geopotential height) from the ERA5 reanalysis in the Northern Hemisphere. We describe the processes of air advection with significant TOC deviations and implement its classification into the main types. Seventy cases of spatial extremes were identified, 86% of which were observed under air advection with a western component. The intense westerly flow in the lower stratosphere is responsible for both the advection of air with high TOC (total ozone content) and its local formation. Under a well-developed polar vortex, most ozone extremes are transported by the main flow and reach the territory of Ukraine from the west and northwest, forming significant positive deviations. In this case, the polar vortex itself must be displaced into the Eastern Hemisphere for Ukraine to be closer to its outer boundary. When the integrity of the polar vortex is disrupted, it takes on a wavelike structure, leading to greater variability in the processes forming ozone extremes over Ukraine, including TOC advection from the north and local formation. With the breakdown of the polar vortex and the onset of a rapid TOC decrease in late March to April, the likelihood of positive ozone deviations from the north increases, though their recurrence does not exceed 7% of the total number of extremes. Significant negative TOC deviations spread over Ukraine during the period of seasonal minima under two conditions: advection from the northwest when the stratospheric polar vortex is absent (until November), and advection from the west in the early stages of vortex formation (in December). The established and described dynamic conditions for the formation of ozone layer extremes are important for extending the lead time in forecasting ozone anomalies over Ukraine.

Distinct Radiative and Chemical Impacts Between the Equatorial and Northern Extratropical Volcanic Injections

AbstractStratospheric volcanic aerosols can affect the global radiative balance and stratospheric composition. In this study, we analyze ensemble experiments with an interactive stratospheric aerosol microphysical general circulation model, designed to assess the climate forcing from large‐magnitude explosive eruptions in the tropics and northern extratropics. Previous studies have generally identified a lower radiative forcing from extratropical eruptions from the shorter stratospheric lifetime of volcanic sulfate aerosols. However, our study finds that both the shorter lifetime and lower effective radiative forcing (ERF) efficacy contribute to the lower ERF in the northern extratropical eruptions. The simulated 2‐year averaged ERF efficacy in northern extratropical eruptions is 22% lower than that in the equatorial eruptions due to the seasonal mismatch of peak stratospheric aerosol optical depth and solar radiation in the first year after a northern extratropical volcanic eruption. Additionally, equatorial eruptions accelerate the Brewer‐Dobson circulation (BDC), while northern hemispheric (NH) extratropical eruptions decelerate the BDC branch in NH and accelerate the BDC branch in southern hemisphere (SH), leading to different spatio‐temporal pattern of ozone anomalies. Consequently, dominated by the dynamical processes, equatorial eruption leads to ozone loss in tropics and increase in midlatitudes, while both northern extratropical summer and winter eruptions trigger ozone decrease in NH and increase in SH.

Record High March 2024 Arctic Total Column Ozone

AbstractObservations 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.

Beyond self-healing: stabilizing and destabilizing photochemical adjustment of the ozone layer

Abstract. The ozone layer is often noted to exhibit self-healing, whereby depletion of ozone aloft induces ozone increases below, explained as resulting from enhanced ozone production due to the associated increase in ultraviolet (UV) radiation below. Similarly, ozone enhancement aloft can reduce ozone below (reverse self-healing). This paper considers self-healing and reverse self-healing to manifest a general mechanism we call photochemical adjustment, whereby ozone perturbations lead to a downward cascade of anomalies in UV and ozone. Conventional explanations for self-healing imply that photochemical adjustment is stabilizing, damping perturbations towards the surface. However, photochemical adjustment can be destabilizing if the enhanced UV disproportionately increases the ozone sink, as can occur if the enhanced UV photolyzes ozone to produce atomic oxygen, which speeds up catalytic destruction of ozone. We analyze photochemical adjustment in two linear ozone models (Cariolle v2.9 and LINOZ), finding that (1) photochemical adjustment is destabilizing above 40 km in the tropical stratosphere and (2) self-healing often represents only a small fraction of the total photochemical stabilization. The destabilizing regime above 40 km is reproduced in a much simpler model: the Chapman cycle augmented with destruction of O and O3 by generalized catalytic cycles and transport (the Chapman+2 model). The Chapman+2 model reveals that photochemical destabilization occurs where the ozone sink is more sensitive than the source to perturbations in overhead column ozone, which is found to occur when the window of overlapping absorption by O2 and O3 is optically unsaturated, i.e., when overhead slant column ozone is below approximately 1018 molec. cm−2.

Large Ozone Hole in 2023 and the Hunga Tonga Volcanic Eruption

AbstractPolar stratospheric chemistry is highly sensitive to changes in water vapor content and temperature. We identified an unusual behavior of water vapor and temperature in the southern polar winter stratosphere in 2023. The relationships between the Hunga-Tonga eruption injection of water vapor (detected in the tropics) and its transport to SH high latitudes, temperature changes and ozone anomalies at southern high latitudes are discussed, as well as the roles of zonal wind and the meridional flux of zonal mean zonal momentum. These parameters exhibit a consistent pattern in anomalous year 2023. In the winter of 2023 in the Southern Hemisphere, an unexpected decrease in ozone levels and the emergence of an excessive ozone hole were observed. This event marked one of the deepest Antarctic ozone holes with the largest area since 2011. This appears to be associated with the Hunga Tonga eruption anomalous water vapor injection. This study highlights importance of water vapor for evolution of the Antarctic stratosphere.

How heating tracers drive self-lofting long-lived stratospheric anticyclones: simple dynamical models

Abstract. Long-lived “bubbles” of wildfire smoke or volcanic aerosol have recently been observed in the stratosphere, co-located with ozone, carbon monoxide, and water vapour anomalies. These bubbles often survive for several weeks, during which time they ascend through vertical distances of 15 km or more. Meteorological analysis data suggest that this aerosol is contained within strong, persistent anticyclonic vortices. Absorption of solar radiation by the aerosol is hypothesised to drive the ascent of the bubbles, but the dynamics of how this heating gives rise to a single-sign anticyclonic vorticity anomaly have thus far been unclear. We present a description of heating-driven stratospheric vortices, based on an axisymmetric balanced model. The simplest version of this model includes a specified localised heating moving upwards at fixed velocity and produces a steadily translating solution with a single-signed anticyclonic vortex co-located with the heating, with corresponding temperature anomalies forming a vertical dipole, matching observations. A more complex version includes the two-way interaction between a heating tracer, representing the aerosol, and the dynamics. An evolving tracer provides heating which drives a secondary circulation, and this in turn transports the tracer. Through this two-way interaction an initial distribution of tracer drives a circulation and forms a self-lofting tracer-filled anticyclonic vortex. Scaling arguments show that upward velocity is proportional to heating magnitude, but the magnitude of peak quasigeostrophic vorticity is O(f) (f is the Coriolis parameter) and independent of the heating magnitude. Estimates of vorticity from observations match our theoretical predictions. We discuss 3-D effects such as vortex stripping and dispersion of tracer outside the vortex by the large-scale flow, which cannot be captured explicitly by the axisymmetric model and are likely to be important in the real atmosphere. The large O(f) vorticity of the fully developed anticyclones explains their observed persistence and their effective confinement of tracers. To further investigate the early stages of formation of tracer-filled vortices, we consider an idealised configuration of a homogeneous tracer layer. A linearised calculation reveals that the upper part of the layer is destabilised due to the decrease in tracer concentrations with height there, which sets up a self-reinforcing effect where upward lofting of tracer results in stronger heating and hence stronger lofting. Small amplitude disturbances form isolated tracer plumes that ascend out of the initial layer, indicative of a self-organisation of the flow. The relevance of these idealised models to formation and persistence of tracer-filled vortices in the real atmosphere is discussed, and it is suggested that a key factor in their formation is the time taken to reach the fully developed stage, which is shorter for strong heating rates.

Chemistry Contribution to Stratospheric Ozone Depletion After the Unprecedented Water‐Rich Hunga Tonga Eruption

AbstractFollowing the Hunga Tonga–Hunga Ha'apai (HTHH) eruption in January 2022, stratospheric ozone depletion was observed at Southern Hemisphere mid‐latitudes and over Antarctica during the 2022 austral wintertime and springtime, respectively. The eruption injected sulfur dioxide and unprecedented amounts of water vapor into the stratosphere. This work examines the chemistry contribution of the volcanic materials to ozone depletion using chemistry‐climate model simulations with nudged meteorology. Simulated 2022 ozone and nitrogen oxide (NOx = NO + NO2) anomalies show good agreement with satellite observations. We find that chemistry yields up to 4% ozone destruction at mid‐latitudes near ∼70 hPa in August and up to 20% ozone destruction over Antarctica near ∼80 hPa in October. Most of the ozone depletion is attributed to internal variability and dynamical changes forced by the eruption. Both the modeling and observations show a significant NOx reduction associated with the HTHH aerosol plume, indicating enhanced dinitrogen pentoxide hydrolysis on sulfate aerosol.

Ozone Anomalies in Dry Intrusions Associated With Atmospheric Rivers

AbstractAs a result of their important role in weather and the global hydrological cycle, understanding atmospheric rivers' (ARs) connection to synoptic‐scale climate patterns and atmospheric dynamics has become increasingly important. In addition to case studies of two extreme AR events, we produce a December climatology of the three‐dimensional structure of water vapor and O3 (ozone) distributions associated with ARs in the northeastern Pacific from 2004 to 2014 using MERRA‐2 reanalysis products. Results show that positive O3 anomalies reside in dry intrusions of stratospheric air due to stratosphere‐to‐troposphere transport (STT) behind the intense water vapor transport of the AR. In composites, we find increased excesses of O3 concentration, as well as in the total O3 flux within the dry intrusions, with increased AR strength. We find that STT O3 flux associated with ARs over the NE Pacific accounts for up to 13% of total Northern Hemisphere STT O3 flux in December, and extrapolation indicates that AR‐associated dry intrusions may account for as much as 32% of total NH STT O3 flux. This study quantifies STT of O3 in connection with ARs for the first time and improves estimates of tropospheric ozone concentration due to STT in the identification of this correlation. In light of predictions that ARs will become more intense and/or frequent with climate change, quantifying AR‐related STT O3 flux is especially valuable for future radiative forcing calculations.

Fingerprints of the COVID-19 economic downturn and recovery on ozone anomalies at high-elevation sites in North America and western Europe

Abstract. With a few exceptions, most studies on tropospheric ozone (O3) variability during and following the COrona VIrus Disease (COVID-19) economic downturn focused on high-emission regions or urban environments. In this work, we investigated the impact of the societal restriction measures during the COVID-19 pandemic on surface O3 at several high-elevation sites across North America and western Europe. Monthly O3 anomalies were calculated for 2020 and 2021, with respect to the baseline period 2000–2019, to explore the impact of the economic downturn initiated in 2020 and its recovery in 2021. In total, 41 high-elevation sites were analyzed: 5 rural or mountaintop stations in western Europe, 19 rural sites in the western US, 4 sites in the western US downwind of highly polluted source regions, and 4 rural sites in the eastern US, plus 9 mountaintop or high-elevation sites outside Europe and the United States to provide a “global” reference. In 2020, the European high-elevation sites showed persistent negative surface O3 anomalies during spring (March–May, i.e., MAM) and summer (June–August, i.e., JJA), except for April. The pattern was similar in 2021, except for June. The rural sites in the western US showed similar behavior, with negative anomalies in MAM and JJA 2020 (except for August) and MAM 2021. The JJA 2021 seasonal mean was influenced by strong positive anomalies in July due to large and widespread wildfires across the western US. The polluted sites in the western US showed negative O3 anomalies during MAM 2020 and a slight recovery in 2021, resulting in a positive mean anomaly for MAM 2021 and a pronounced month-to-month variability in JJA 2021 anomalies. The eastern US sites were also characterized by below-mean O3 for both MAM and JJA 2020, while in 2021 the negative values exhibited an opposite structure compared to the western US sites, which were influenced by wildfires. Concerning the rest of the world, a global picture could not be drawn, as the sites, spanning a range of different environments, did not show consistent anomalies, with a few sites not experiencing any notable variation. Moreover, we also compared our surface anomalies to the variability of mid-tropospheric O3 detected by the IASI (Infrared Atmospheric Sounding Interferometer) satellite instrument. Negative anomalies were observed by IASI, consistent with published satellite and modeling studies, suggesting that the anomalies can be largely attributed to the reduction of O3 precursor emissions in 2020.

The effects of gravity waves on ozone over the Tibetan Plateau

The ozone valley over the Tibetan Plateau (TP) has an important effect on global weather and climate. As a significant source of atmospheric gravity waves (GWs), TP is also a key area of global stratosphere-troposphere exchange (STE), yet the exploration of the causes of low ozone values over the TP has been scarce from the perspective of GW. In this paper, we used the hyperspectral Atmospheric Infrared Sounder (AIRS) data to extract GWs and deep convection over the TP and analyzed the statistical characteristics of ozone for corresponding events. The results show that: (1) The GWs observed by AIRS mainly occur in the mid-latitude region, with a maximum frequency of >10%, and their intensity varies with longitude. Three large-value regions have been identified on the coast of Southeast Asia, Central America, and Central Africa. (2) Most deep convection occurs in the Intertropical Convergence Zone (ITCZ), with three large value centers in the East Asian monsoon region, Central America and Central Africa. Among them, the Indian monsoon region, the Northwest Pacific region to the east of the Philippines, and the western side of the South China Sea (SCS) are the large-value areas within the East Asian monsoon region, with the maximum occurrence frequency exceeding 15%. (3) TP is a region with a large frequency of convective gravity waves. In most areas of the southern foothills of the TP, the frequency of GWs related to convection is 30%–50%. Almost all deep convection over the TP and its adjacent areas are accompanied by GWs, with a frequency of 60–90%. (4) The composite analysis results show that the occurrence of GWs or deep convection is accompanied by negative anomalies of ozone in most areas of the southern foothills of the TP, indicating a decrease in the integrated column amount of ozone. (5) By analyzing the anomaly distribution of meteorological elements such as cloud fraction, cloud top height, tropopause height, and outgoing longwave radiation (OLR), the results show a dual effect in the presence of GWs or deep convection. The effect involves not only an increased generation of clouds, conducive to convection but also elevated cloud top height, promoting the formation of high clouds, some even beyond the tropopause. In addition, the occurrence of GW or deep convection can also affect the tropopause height, strengthen convective activity, and facilitate the upward transfer of low-concentration ozone from the lower layer to the upper layer, thereby causing a decrease in the integrated column amount of ozone.

Role of Southern Hemisphere sudden stratospheric warming in altering the intensity of Brewer-Dobson circulation and its impact on stratospheric ozone and water vapor

The changes in the intensity of Brewer-Dobson circulation (BDC) in the record-breaking Southern Hemisphere sudden stratospheric warming (SH SSW) event in 2019 are investigated in the present study. Wave driving was observed to be at its highest in 2019 SH SSW, despite the event being classified as a minor warming event. The Eliassen-Palm flux divergence, indicative of wave driving, was observed to be increasing before the SSW, and subsiding afterwards. The stratospheric circulation followed the wave driving and intensified along the warming episode. The resulting stratospheric distribution from Aura Microwave Limb Sounder showed a negative ozone anomaly (∼20%) in the tropical lower stratosphere and a downward propagating positive anomaly in the Southern Hemisphere polar stratosphere around the central dates. The water vapor mixing ratio showed an increase of more than 25% in the polar lower stratosphere and 10% in the tropical upper stratosphere. A cooling of about 5 K was observed in the tropical middle stratosphere. The results indicate that SH SSW can change the intensity of the stratospheric circulation which in turn affects the ozone and water vapor transport from the tropics to the pole.

Impact of the Hunga Tonga volcanic eruption on stratospheric composition.

The explosive eruption of the Hunga Tonga-Hunga Ha'apai (HTHH) volcano on 15 January 2022 injected more water vapor into the stratosphere and to higher altitudes than ever observed in the satellite era. Here, the evolution of the stratospherically injected water vapor is examined as a function of latitude, altitude, and time in the year following the eruption (February to December 2022), and perturbations to stratospheric chemical composition resulting from the increased sulfate aerosols and water vapor are identified and analyzed. The average calculated mass distribution of elevated water vapor between hemispheres is approximately 78% Southern Hemisphere (SH) and 22% Northern Hemisphere in 2022. Significant changes in stratospheric composition following the HTHH eruption are identified using observations from the Aura Microwave Limb Sounder satellite instrument. The dominant features in the monthly mean vertical profiles averaged over 15° latitude ranges are decreases in O3 (-14%) and HCl (-22%) at SH midlatitudes and increases in ClO (>100%) and HNO3 (43%) in the tropics, with peak pressure-level perturbations listed. Anomalies in column ozone from 1.2-100 hPa due to the HTHH eruption include widespread O3 reductions in SH midlatitudes and O3 increases in the tropics, with peak anomalies in 15° latitude-binned, monthly averages of approximately -7% and +5%, respectively, occurring in austral spring. Using a 3-dimensional chemistry-climate-aerosol model and observational tracer correlations, changes in stratospheric composition are found to be due to both dynamical and chemical factors.

On the pattern of interannual polar vortex–ozone co-variability during northern hemispheric winter

Abstract. Stratospheric ozone is important for both stratospheric and surface climate. In the lower stratosphere during winter, its variability is governed primarily by transport dynamics induced by wave–mean flow interactions. In this work, we analyze interannual co-variations between the distribution of zonal-mean ozone and the strength of the polar vortex as a measure of dynamical activity during northern hemispheric winter. Specifically, we study co-variability between the seasonal means of the ozone field from modern reanalyses and polar-cap-averaged temperature at 100 hPa, which represents a robust and well-defined index for polar vortex strength. We focus on the vertically resolved structure of the associated extratropical ozone anomalies relative to the winter climatology and shed light on the transport mechanisms that are responsible for this response pattern. In particular, regression analysis in pressure coordinates shows that anomalously weak polar vortex years are associated with three pronounced local ozone maxima just above the polar tropopause, in the lower to mid-stratosphere and near the stratopause. In contrast, in isentropic coordinates, using ERA-Interim reanalysis data, only the mid- to lower stratosphere shows increased ozone, while a small negative ozone anomaly appears in the lowermost stratosphere. These differences are related to contributions due to anomalous adiabatic vertical motion, which are implicit in potential temperature coordinates. Our analyses of the ozone budget in the extratropical middle stratosphere show that the polar ozone response maximum around 600 K and the negative anomalies around 450 K beneath both reflect the combined effects of anomalous diabatic downwelling and quasi-isentropic eddy mixing, which are associated with consecutive counteracting anomalous ozone tendencies on daily timescales. We find that approx. 71 % of the total variability in polar column ozone in the stratosphere is associated with year-by-year variations in polar vortex strength based on ERA5 reanalyses for the winter seasons 1980–2022. MLS observations for 2005–2020 show that around 86 % can be explained by these co-variations with the polar vortex.

Arctic stratosphere changes in the 21st century in the Earth system model SOCOLv4

Two ensemble simulations of a new Earth system model (ESM) SOCOLv4 (SOlar Climate Ozone Links, version 4) for the period from 2015 to 2099 under moderate (SSP2-4.5) and severe (SSP5-8.5) scenarios of greenhouse gas (GHG) emission growth were analyzed to investigate changes in key dynamical processes relevant for Arctic stratospheric ozone. The model shows a 5–10 K cooling and 5%–20% humidity increase in the Arctic lower–upper stratosphere in March (when the most considerable ozone depletion may occur) between 2080–2099 and 2015–2034. The minimal temperature in the lower polar stratosphere in March, which defines the strength of ozone depletion, appears when the zonal mean meridional heat flux in the lower stratosphere in the preceding January–February is the lowest. In the late 21st century, the strengthening of the zonal mean meridional heat flux with a maximum of up to 20 K m/s (∼25%) in the upper stratosphere close to 70°N in January–February is obtained in the moderate scenario of GHG emission, while only a slight increase in this parameter over 50 N–60 N with the maximum up to 5 K m/s in the upper stratosphere and a decrease with the comparable values over the high latitudes is revealed in the severe GHG emission scenario. Although the model simulations confirm the expected ozone layer recovery, particularly total ozone minimum values inside the Arctic polar cap in March throughout the 21st century are characterized by a positive trend in both scenarios, the large-scale negative ozone anomalies in March up to −80 DU–100 DU, comparable to the second lowest ones observed in March 2011 but weaker than record values in March 2020, are possible in the Arctic until the late 21st century. The volume of low stratospheric air with temperatures below the solid nitric acid trihydrate polar stratospheric cloud (PSC NAT) formation threshold is reconstructed from 3D potential vorticity and temperature fields inside the stratospheric polar vortex. A significant positive trend is shown in this parameter in March in the SSP5-8.5 scenario. Furthermore, according to the model data, an increase in the polar vortex isolation throughout the 21st century indicates its possible strengthening in the lower stratosphere. Positive trends of the surface area density (SAD) of PSC NAT particles in March in the lower Arctic stratosphere over the period of 2015–2099 are significant in the severe GHG emission scenario. The polar vortex longitudinal shift toward northern Eurasia is expected in the lower stratosphere in the late 21st century in both scenarios. The statistically significant long-term stratospheric sulfuric acid aerosol trend in March is expected only in the SSP5.8-5 scenario, most probably due to cooler stratosphere and stronger Brewer–Dobson circulation intensification. Both scenarios predict an increase in the residual meridional circulation (RMC) in March by the end of the 21st century. In some regions of the stratosphere, the RMC enhancement under the severe GHG scenario can exceed 20%.

Ozone Changes Due To Sudden Stratospheric Warming‐Induced Variations in the Intensity of Brewer‐Dobson Circulation: A Composite Analysis Using Observations and Chemical‐Transport Model

AbstractWe quantify the changes in the intensity of Brewer‐Dobson Circulation (BDC) during sudden stratospheric warming (SSW) and its impact on the tropical stratospheric thermal structure and ozone distribution by composite analysis using observations and a chemical‐transport model. An increase in the planetary wave activity and enhancement in BDC intensity before the central date of SSW is noticed. A positive ozone anomaly is observed in the tropical upper stratosphere. The tropical lower stratosphere shows a cooling (1–2 K) and negative ozone anomaly (∼0.1 ppmv) after ∼10 days from the central date. The polar stratosphere experiences a positive ozone anomaly, whereas the upper stratosphere shows ozone depletion due to the downwelling of NOx‐rich mesospheric air. The cold‐point tropopause temperature shows a cooling of ∼0.5 K for major warming which in turn dries the lower stratosphere.

Decadal changes in the relationship between Arctic stratospheric ozone and sea surface temperatures in the North Pacific

Using multi-source observations and ERA-Interim reanalysis data, this study found that the relationship between Arctic stratospheric ozone in March and North Pacific sea surface temperature (SST) in April shows dramatic decadal changes, and it significantly weakens since the 2000s. By enhancing the stratospheric circulations, a decreased Arctic stratospheric ozone favors tropospheric positive Arctic Oscillation (+AO)-like anomaly during PRE eras (1979–1999), with positive geopotential height (GH) anomalies over high-latitude Asia. The stratospheric ozone anomaly also favors the Asian positive GH anomalies via modulating high cloud and its related longwave radiation. According to geostrophic wind relationships, the Asian positive GH anomalies are accompanied by easterly anomalies over midlatitude Asia in late March, which further extend eastward and thereby induce easterly anomalies over the midlatitude North Pacific. The North Pacific easterly anomalies result in negative North Pacific Oscillation (–NPO)-like anomaly via vorticity forcing of zonal wind and the interactions between synoptic-scale eddies and mean flow, forcing significant SST anomalies. However, during POST eras (2000–2019), firstly, the +AO-related positive GH anomalies over high-latitude Asia are relatively weak, accompanied by weak easterly anomalies over midlatitude Asia according to geostrophic wind relations, which induce weak easterly anomalies over the North Pacific. Secondly, a westerly anomaly occurs over the midlatitude North Pacific, which partly offsets the easterly anomalies from Asia, weakening the North Pacific easterly anomalies. The weak easterly anomalies further induce weak –NPO and SST anomalies during the POST eras. Our analysis further indicates that the North Pacific westerly anomalies and weakening of the positive GH anomalies over high-latitude Asia associated with ozone are related to the interference of the westerly phase of quasi-biennial oscillation and the weakened stratospheric ozone anomaly over high-latitude Asia, respectively.

Enhancement of Arctic surface ozone during the 2020–2021 winter associated with the sudden stratospheric warming

Surface ozone is an important pollutant causing damage to human health and ecosystems. Here, we find that the Arctic surface ozone during the 2020–2021 winter was evidently enhanced after the sudden stratospheric warming (SSW) onset based on reanalysis data and simulations of a state-of-the-art chemistry-climate model. Further analysis suggests that this enhancement of Arctic surface ozone is primarily a result of the strengthening of the stratosphere-to-troposphere transport associated with the SSW. It is found that the SSW leads to more ozone in the Arctic stratosphere and enhanced downward transport with SSW-related downdraft. The 2021 SSW may also lead to positive anomalies in surface ozone in the northern midlatitudes, which are associated with cold air outbreaks. Our results indicate that the SSW not only affects the weather and climate in the troposphere but may also affect the surface air quality.

Quantifying the drivers of surface ozone anomalies in the urban areas over the Qinghai-Tibet Plateau

Abstract. Improved knowledge of the chemistry and drivers of surface ozone over the Qinghai-Tibet Plateau (QTP) is significant for regulatory and control purposes in this high-altitude region in the Himalayas. In this study, we investigate the processes and drivers of surface ozone anomalies (defined as deviations of ozone levels relative to their seasonal means) between 2015 and 2020 in urban areas over the QTP. We separate quantitatively the contributions of anthropogenic emissions and meteorology to surface ozone anomalies by using the random forest (RF) machine-learning model-based meteorological normalization method. Diurnal and seasonal surface ozone anomalies over the QTP were mainly driven by meteorological conditions, such as temperature, planetary boundary layer height, surface incoming shortwave flux, downward transport velocity and inter-annual anomalies were mainly driven by anthropogenic emission. Depending on region and measurement hour, diurnal surface ozone anomalies varied over −27.82 to 37.11 µg m−3, whereas meteorological and anthropogenic contributions varied over −33.88 to 35.86 µg m−3 and −4.32 to 4.05 µg m−3 respectively. Exceptional meteorology drove 97 % of surface ozone non-attainment events from 2015 to 2020 in the urban areas over the QTP. Monthly averaged surface ozone anomalies from 2015 to 2020 varied with much smaller amplitudes than their diurnal anomalies, whereas meteorological and anthropogenic contributions varied over 7.63 to 55.61 µg m−3 and 3.67 to 35.28 µg m−3 respectively. The inter-annual trends of surface ozone in Ngari, Lhasa, Naqu, Qamdo, Diqing, Haixi and Guoluo can be attributed to anthropogenic emissions in 95.77 %, 96.30 %, 97.83 %, 82.30 %, 99.26 % and 87.85 %, and meteorology in 4.23 %, 3.70 %, 2.17 %, 3.19 %, 0.74 % and 12.15 % respectively. The inter-annual trends of surface ozone in other cities were fully driven by anthropogenic emission, whereas the increasing inter-annual trends would have larger values if not for the favorable meteorological conditions. This study can not only improve our knowledge with respect to spatiotemporal variability of surface ozone but also provide valuable implications for ozone mitigation over the QTP.

Effects of Arctic ozone on the stratospheric spring onset and its surface impact

Abstract. Ozone in the Arctic stratosphere is subject to large interannual variability, driven by both chemical ozone depletion and dynamical variability. Anomalies in Arctic stratospheric ozone become particularly important in spring, when returning sunlight allows them to alter stratospheric temperatures via shortwave heating, thus modifying atmospheric dynamics. At the same time, the stratospheric circulation undergoes a transition in spring with the final stratospheric warming (FSW), which marks the end of winter. A causal link between stratospheric ozone anomalies and FSWs is plausible and might increase the predictability of stratospheric and tropospheric responses on sub-seasonal to seasonal timescales. However, it remains to be fully understood how ozone influences the timing and evolution of the springtime vortex breakdown. Here, we contrast results from chemistry climate models with and without interactive ozone chemistry to quantify the impact of ozone anomalies on the timing of the FSW and its effects on surface climate. We find that ozone feedbacks increase the variability in the timing of the FSW, especially in the lower stratosphere. In ozone-deficient springs, a persistent strong polar vortex and a delayed FSW in the lower stratosphere are partly due to the lack of heating by ozone in that region. High-ozone anomalies, on the other hand, result in additional shortwave heating in the lower stratosphere, where the FSW therefore occurs earlier. We further show that FSWs in high-ozone springs are predominantly followed by a negative phase of the Arctic Oscillation (AO) with positive sea level pressure anomalies over the Arctic and cold anomalies over Eurasia and Europe. These conditions are to a significant extent (at least 50 %) driven by ozone. In contrast, FSWs in low-ozone springs are not associated with a discernible surface climate response. These results highlight the importance of ozone–circulation coupling in the climate system and the potential value of interactive ozone chemistry for sub-seasonal to seasonal predictability.

Zonally asymmetric influences of the quasi-biennial oscillation on stratospheric ozone

Abstract. The quasi-biennial oscillation (QBO), as the dominant mode in the equatorial stratosphere, modulates the dynamical circulation and the distribution of trace gases in the stratosphere. While the zonal mean QBO signals in stratospheric ozone have been relatively well documented, the zonal (longitudinal) differences in the QBO ozone signals have been less studied. Using satellite-based total column ozone (TCO) data from 1979 to 2020, zonal mean ozone data from 1984 to 2020, three-dimensional (3-D) ozone data from 2002 to 2020, and ERA5 reanalysis and model simulations from 1979 to 2020, we demonstrate that the influences of the QBO (using a QBO index at 20 hPa) on stratospheric ozone are zonally asymmetric. The global distribution of stratospheric ozone varies significantly during different QBO phases. During QBO westerly (QBOW) phases, the TCO and stratospheric ozone are anomalously high in the tropics, while in the subtropics they are anomalously low over most of the areas, especially during the winter–spring of the respective hemisphere. This confirms the results from previous studies. In the polar region, the TCO and stratospheric ozone (50–10 hPa) anomalies are seasonally dependent and zonally asymmetric. During boreal winter (December–February, DJF), positive anomalies of the TCO and stratospheric ozone are evident during QBOW over the regions from North America to the North Atlantic (120∘ W–30∘ E), while significant negative anomalies exist over other longitudes in the Arctic. In boreal autumn (September–November, SON), the TCO and stratospheric ozone are anomalously high from Greenland to Eurasia (60∘ W–120∘ E) but anomalously low in other regions over the Arctic. Weak positive TCO and stratospheric ozone anomalies exist over the South America sector (90∘ W–30∘ E) of the Antarctic, while negative anomalies of the TCO and stratospheric ozone are seen in other longitudes. The consistent features of TCO and stratospheric ozone anomalies indicate that the QBO signals in TCO are mainly determined by the stratospheric ozone variations. Analysis of meteorological conditions indicates that the QBO ozone perturbations are mainly caused by dynamical transport and also influenced by chemical reactions associated with the corresponding temperature changes. QBO affects the geopotential height and the polar vortex and subsequently the transport of ozone-rich air from lower latitudes to the polar region, which therefore influences the ozone concentrations over the polar region. The geopotential height anomalies associated with QBO (QBOW–QBOE) are zonally asymmetric with clear wave number 1 features, which indicates that QBO influences the polar vortex and stratospheric ozone mainly by modifying the wave number 1 activities.